3, 3 -dimethylplatin acyclobutanes Produces Bis(tri

3, 3 -dimethylplatin acyclobutanes Produces Bis(tri

J. Am. Chem.Soc.1982.104.3601-3607
3601
ReductiveEliminationof a Carbon-CarbonBondfrom
Bis(trialkylphosphine
acyclobutanes
)-3,3-dimethylplatin
ProducesBis(trialkylphosphine)platinum(0)
and
-Dimethylcyclopropaner
1, 1
Robert DiCosimo2 and George M. Whitesides*
Contrtbution from the Department of Chemistry, MassachusettsInstitute of Technology,
Cambridge, Massachusetts 02139. ReceiuedOctober 26, 1981
Abstract The thermaldecompositions
(1), bis(triisopropylphosphine)of severalplatinacyclobutanes-bis(triethylphosphine)(2), and bis(tricyclohexylphosphine)-3,3-dimethylplatinacyclobutane
(3)-have been examinedin cyclohexanesolution.
Decomposition
of I proceeds
platinummetal,neopentane,
heterogeneously:
it produces
l,l-dimethylcyclopropane,
and ethylene
and seemsto be autocatalytic.Decompositions
conductedusingreactionmixturescontainingmercury(0)proceedhomogeneously
and producepredominatelyl,l-dimethylcyclopropane;any
platinummetal producedis apparentlyamalgamatedby the Hg(0)
and renderedcatalyticallyinactive.Thermaldecompositions
of 2 and 3 proceedhomogeneously,
evenin the absence
of Hg(0),
and yield 1,1-dimethylcyclopropane,
bis(trialkylphosphine)platinum(0),
and a small quantityof neopentane.The ratesof
decomposition
are decreased
by additionof trialkylphosphine,
and a studyof this dependence
of rate on concentration
of added
phosphinefor 2 indicatesthat onephosphinedissociates
platinum(ll) intermediateprior to reductive
to producea three-coordinate
eliminationof 1,1-dimethylcyclopropane.
The Arrheniusactivationparametersfor decomposition
of 2 and 3 are respectively
E^=46*3kcalmol-r,logA=23+2andE^=42*,2kcalmol-r,logA=21 +1. Substitutionofdeuteriumforhydrogen
in the triisopropylphosphine
groupsof 2 producesno changein the rate of reaction. We cannotdistinguishbetweenmechanisms
in which reductiveeliminationof 1,1-dimethylcyclopropane
occursdirectlyfrom the three-coordinate
intermediateand those
in which oxidativeadditionof a C-H bond to platinum by cyclometalationof the phosphineproducesa Pt(IV) intermediate
in a step precedingrate-limiting reductiveeliminationof 1,1-dimethylcyclopropane.
Heterogeneousreactionsof hydrocarbonswith transition metals
(cracking, reforming, hydrogenation)involve cleavingand reforming C-H and C{ bonds,and are believedto proceedthrough
metallacyclicand surface-alkylintermediates.3Although studies
of the mechanismsof theseheterogeneous
reactionshave been
difficult, the studiesof related reactionsin homogeneous
transition-metalsystemshave providedexamplesof C-H and C-C
p-hydrideelimination
bondcleavageand formationby metathesis,a
and insertionof olefinsinto metal-hydrogenbonds,s'6
oxidative
addition and reductiveelimination of C-H7'8 and C-C bonds,e-13
and cyclometalation.ra-16This paper is concernedwith the
S c h e m eI . R c e et i o n o t ' T r i i s o p r o p y l p h o s p h i n ew i t h
t 1 .5 - C ) ' c l o o eat d r c n c)d i n c o p e n t y l p l a t i n u n rI()l ( L = l - P r ,P ; Y i e l d s
L l a s c do n t 1 . 5 - ( ' rc l o o c t a d i e n e ) d i n e o p e n t y l p l a t i n u m ( l I ) )
'
-
1 1 . 5 - 6 Q ! ) P r t Ht " c ( csH. ,r )) ;z
-
55"C
i'*'
t5^
I
lt.' '
urerl(
36%
unidanlifiad
+
+ LzPr(o)
plaiinum
*5O%
producls
- 25%
| ,,.,o.uo,.
drynrrr
I
2) c6Hr2
|
rze.c
I
+ 2n
tO6%
+
(cqbc(cH2DL +
(cH3)3ccHzcH2c(cH3)5
+ LzPr(O)
lo
5)Dcrloro
* ctcH3L
26%
(CH3)3CCHzCHzC(CH3)3
80%
26%
| 55"c
+i5h
A
( I ) Supportedby the National ScienceFoundation,Grant CHE 8012722.
(2) ChevronFellow, 1980-1982.
(3) Clarke,J. K. A.; Rooney,J. J. Adu. Catal.1976,25,125-183.Sinfelt,
J. H. Science(Washington,D.C.) 1977, i,95,641-646.
(4) Grubbs, R. H.; Hoppin, C. R. "/. Am. Chem. Soc. 1979, I|t,
1499-1508. Leconte,M.; Basset,J. M. Ann. N.Y. Acad. Sci. 1980. -l-13.
1 6 5 - 1 8 7 .R a p p e A
, . K . ; G o d d a r dW
, . A . ; I I I J . A m . C h e m .S o c . 1 9 8 01, 0 2 ,
5ll4-5115.
(5) Ozawa, F.; Ito, T.; Yamamoto, A. "f. Am. Chem. Soc. 1980, 102,
6457-6463. Komiya, S.; Yamamoto,A.; Yamamoto,T. Chem.Lett.1978,
t 2 7 3 - t 2 76 .
(6) McCarthy,T.J.; Nuzzo,R. G.; Whitesides,
G. M. J. Am. Chem. Soc.
1 9 8 1 , 1 0 3 , 3 3 9 6 - 3 4 0 3W
. h i t e s i d e sG, . M . ; G a a s c h ,J . F . ; S t e d r o n s k yE. . R .
rbid. 1972. 94. 52s8-527o.
(7) Abis, L.; Sen, A.; Halpern, J. J. Am. Chem. Soc. 1978, 100,
2915-2916.
(8) CrockerC
, .; ErringtonR
, . J . ; M a r k h a m , R . ; M o l t o n ,C . J . ; O d e l l ,K .
J . ; S h a w ,B . L . J . A m . C h e m . , S o c1. 9 8 0 ,1 0 2 , 4 3 7 3 - 4 3 7 9 .S i m p s o nS
, . J.;
Turner, H. W.; Andersen,R. A. Ibid. 1979,101,7728-'1729.
(9) Ruddick,J.D.;Shaw, B. L. J. Chem.Soc.A 1969,2969-2970.Clark,
H . C . ; M a n z e r ,L . E . I n o r g . C h e m . 1 9 7 3 ,1 2 , 3 6 2 - 3 6 8 . B r o w n , M . P . ;
Puddenphatt,R. J.; Upton, C. E. E. J. Chem. Soc.,Dalton Trans. 1974,
2457-2465. Al-Essa,R. J.; Puddephatt,R. J.; Thompson,P. J.; Tipper, C.
F . H . " r .A m . C h e m .S o c . 1 9 8 0 .1 0 2 . 7 5 4 6 - 7 5 5 3 .
(10) Tamaki,A.; Magennis,S. A.; Kochi, J.K. J. Am. Chem. Soc.1974,
96,614 148. Komiya,A.; Albright,T. A.;Hoffmann,R.;Kochi, J.K.Ibid.
1976. 98.7255-'7265.
( l l ) G i l l i e , A . ; S t i l l e ,J . K .
" r . A m . C h e m .S o c . 1 9 8 0 ,t 0 2 . 4 9 3 3 - 4 9 4 1 .
Moravskiy,A.; Stille, J. K. Ibid. 1981,103,4182-4186.
(12) Ito, T.; Tsuchiya,H.; Yamamoto,A. Bull. Chem.Soc.Jpn.1977, 50,
t3t9-r327.
(13) Smith, G.; Kochi, J. K. J. Organomet.Chem. 1980,t98, 199-214.
(14) Foley,P.; DiCosimo,R.; Whitesides,G. M. J. Am. Chem.Soc. 1980,
102,6713-6725. Moore,S. S.; DiCosimo,R.; Sowinski,A. F.; Whitesides,
G. M. rbid.. t981. 103.948-949.
c(cH3L +
2 tPrP
'
(cH3)3c(cHzD)
27%
2770
tOO%
36%
Scheme II. Reaction of Tricyclohexylphosphine with
( I , 5 - C y c l o o c t a d i e n e ) d i n e o p e n t y l p l a t i n u m ( I I()L = c - H x . P ; Y i e l d s
B a s e do n ( 1 , 5 - C y c l o o c t a d i e n e ) d i n e o p e n t y l p l a t i n u m ( I I ) )
c(cH3[ + (cH3)3ccHzcHzc(cH3)3
Z Cyrp
+
| .-t, ^
t".
150'c
]
t501
85%
tr,s- cooter[cHzc(cH3)J
z rsh
unidonlificd
u.erl(
+ LzPr(o)
+
rsr
II
| 2h
Y
\./
44%
+
4O/o
I 3) DCr/DzO
|
Y
C(CH.).
t26%
(cH3)t(cH2D)2
+ (cH3)3c(cHzD)
(CH3)3ccHzCHzC(CH3)3
+ L.Pl(O)
/5y"
producls
*
I l ) G v o c u o t too
dryncss
I
| 2) C6Hr2
I rzs.c
A
plol Inum
-15%
44%
39c4
44%
too%
mechanismof carbon-carbonbond formation by reductiveelimination from platinum. A previousstudy of the thermal decom( 1 5 ) A n d e r s e nR
, . A.;JonesR
, . A . ; W i l k i n s o nG
, . " / . C h e m .S o c . D
, alton
Trans. 1978, 466-453. Andersen,R. A.; Jones,R. A.; Wilkinson. G.:
H u r s t h o u s eM, . B . ; M a l i k , K . M . A . J . C h e m .S o c . ,C h e m .C o m m u n . 1 9 7 7 ,
283-285.
Reprintedfrom the Journalof the AmericanChemicalSociety,1982,104,3601.
Copyright @ 1982by the AmericanChemicalSocietyand reprintedby permissionof the copyrightowner.
D i ( ' o . s i n t r tt t n i l l l h t t r , i i d e s
3602 J. Am. Chem. Soc., Vol. 104, No. 13, 1982
indicated that reductive
positionof platinum(II) metallacyclesrT
elimination of a C-C bond took place only after the oxidative
addition of a halogenatedsolventmolecule(methylenechloride)
to platinum(Il) had occurred and generateda platinum(IV) intermediate. In nonhalogenatedsolvents,only B-hydride elimination
of alkene was observed. Here we describea study of the mechanism of thermal decompositionof bis(trialkylphosphine)-3,3dimethylplatinacyclobutanesto bis(trialkylphosphine)platinum(0)
in hydrocarbonsolvents. The reand 1,1-dimethylcyclopropane
action proceedsby an initial dissociationof one of the trialkylphosphine ligands and generation of a three-coordinateplatinum(II) intermediate. The large Arrhenius preexponentialfrequency factors that are observedsuggestthat the reductive elimination of l,l-dimethylcyclopropaneis rate limiting. The possibility that oxidativeaddition of a C-H bond of the coordinated
phosphineto PI(II) to form a PI(IV) intermediate preceedsreductive elimination of dimethylcyclopropanecan neither be proved
nor disproved.
Results
Preparation of Bis(triisopropylphosphine)-3,3-dimethylplatinacyclobutane (2) and Bis(tricyclohexylphosphine)-3'3-dimethylplatinacyclobutane(3). Platinacyclobutanes2 and 3 were
isolatedin lg%oand lTVoyields,respectively,as air-stable,white
crystalline solidsfrom the reaction of 2 equiv of the corresponding
trialkylphosphinewith dineopentyl(1,5-cyclooctadiene)platinum(II) in cyclohexaneat 50 oC and were characterizedby a comelementalanalysis,
binationof rH and t'Pl'Hl NMR spectroscopy,
and chemical reactions. Addition of DCI to a solutionof 2 in
cyclohexanegeneratedI equiv of neopentanewhich was determined by GC/MS analysisto be >-95Vo(CH3)2C(CH2D)2;repeating the procedure with 3 produced I equiv of neopentane
which was >94Vo(CHr)2C(CHzD)2.
Unlike the reaction of 2 equiv of triethylphosphinewith ( 1,5( I I ), which cleanly produces
cyclooctadiene)dineopentylplatinum
[), rathe corresponding
dineopentybis(triethylphosphine)platinum(I
reaction of (1,5-COD)PtICH2C(CHr)r]r with triisopropylphosphine(SchemeI) or tricyclohexylphosphine(SchemeII) gave
a complex reaction mixture containing no dineopentylbis(tricompounds(as determinedby ItPltHl
alkylphosphine)platinum(Il)
N M R s p e c t r o s c o p y )I.n s t e a d ,a b i s ( t r i a l k y l p h o s p h i n e ) - 3 . 3 - d i platinum( 0 )
and bis(trialkylphosphine)
methylplatinacyclobutane
complexwere producedas the major platinum-containingcompounds,along with neopentaneand 2,2,5.5-tetramethylhexane.
Oxidativeadditionof an allylic C-H bond of 1,5-cyclooctadiene
to platinum(Il) is undoubtedlythe sourceof someof the hy'drogen
reductiveelimination
atomswhich participatein the subsequent
of a C-H bond to form neopentane;a l5-20Vc yield of 1,3,5cyclooctatrienewas obtained from the reaction of triisopropylphosphinewith ( 1,5-cyclooctadiene)dineopentylplatinum(II).
The marked difference in the products of the reaction of
( 1,5-cyclooctadiene)dineopentylplatinum(II)
and triisopropylphosphine,tricyclohexylphosphine,and triethylphosphinemay be
due to the larger stericbulk of the first two of thesephosphines.r8
A square-planarplatinum(II) speciescomposedof two neopentyl
groupsand two stericallydemandingtrialkylphosphineligands
is probably very unstable. The displacementof 1,5-cyclooctadiene
by a bulky phosphinecould producea highly reactive.coordinatively unsaturated 14-electronplatinum(II) specieswhich could
oxidatively add a C-H bond of a neopentylgroup, cyclooctadiene.
or phosphinealkyl group to yield the observedproducts.
Thermal Decomposition of Bis(trialkylphosphine)-3,3-dimethylplatinacyclobutane. Products. The thermal decomposition
of a cyclohexane solution of bis(triethylphosphine)-3,3-di(l) in a sealedtube at 157 oC produces
methylplatinacyclobutane
( 16) Miyashita,A.; Grubbs,R. H. Tetrahedronlzrt. 1981,22, 1255-1256.
(17) McDermott,J. X.;White. J. F.; Whitesides,
G. M. J. Am. Chem.Soc.
1976, 98, 6521-6528. Young, G. B.; Whitesides,G. M. Ibid. 1978, 100,
5 8 0 8 - s 8l s .
(18) The phosphinecone angles for triethylphosphine,triisopropylphosphine,
are I 32, 160,and 170o,respectively:
and tricyclohexylphosphine
T o l m a n ,C . A . C h e m . R e u . 1 9 7 7 , 7 7 , 3 1 3 - 3 4 8 .
I
a.
,i\r
"\;
\
to
o
()
1\\ . ' -
i
, J,
\3\
\T
o\
o u
U.3
\r
\.\"\
\
oo
\ \\ \
\
\\
a
, J ,
o
oto2c30
T i m e( m i n )
of 2 (1, tr),
plotsof the thermaldecompositions
Figurel. First-order
2 - ( L D ) (: a ) . a n d3 ( o . o ) i n c y c l o h e x aantel l 6 o C . S o l i ds y m b o l s
are
opcnsymbols
b) t'PlrHl \\{R spcctroscop\:
datatrbtained
represent
b a s e do n G L C d a t a
a h e t e r o g e n e o ur se a c t i o nm i x t u r c c o n t . r i n i n gn c o p L - n t i . l nl .el ' d i m e t h y l c y c l o p r o p a n e ,t h y l e n e ,a n d p l a t i n u m m c t a l . i a T h e
formation of a platinum mirror and collodial platinum was
p r o b a b l y d u e t o t h e d i s p r o p o r t i o n a t i o n o f b i s (t r i e t h y l formed by the reductiveeliminationof
phosphine)platinum(0),
I , l - d i m e t h y l c y c l o p r o p a n fer o m I , t o p r o d u c e t r i s ( t r i e t h y l a )n d p l a t i n u m m e t a l . I n f a c t , w h e n t h e
phosphine)platinum(0
of 1 was repeatedat 178 oC in the presenc€
thermaldecomposition
o f H g ( 0 ) , t h e r e a c t i o nm i x t u r e r e m a i n e dh o m o g e n e o u s .l , l were producedin a ratio
and neopentane
Dimethylcyclopropane
of 94:6 throughout the thermolysis. No other hydrocarbon
productswere observedby GLC when the reactionwas repeated
in isooctaneor 2,2,4.4-tetramethylpentane.
"Pl'Hl NMR spectroscopy revealedonly the production of free triethylphosphine
as I was consumed,and no (EtrP)rPtowas observed,indicating
that platinum metal was amalgamatedwith mercury and rendered
inert. The small percentageof neopentaneproduced in the
presenceof Hg(O) indicatesthat its formation in larger quantities
in the absenceof Hg(O) may be due to reactions catalyzedbv the
platinum metal which is formed.
H e a t i n ga s e a l e dt u b e c o n t a i n i n ga 0 . 0 8 M s o l u t i o no f 2 " r 3
reactionwhich produ,,"-'l
in cvclohcraneresultedin a homogeneous
a) o r b i s ( t r i c y c l c
e i t h e rb i s lt r i i s o p r o plip h o s p h i n e ) p l a t i n u m ((O
h e x i ' l p h o s p h i n e ) p l a t i n u n r( 5
0 ) . r e s p e c t i v e l y1, , 1 - d i m e t h y l o c l o p r o p a n e a. n d a s m a l lq u a n t i t ! ' o fn e o p e n t a n(ee q l ) . l e T h e
rrPr\
tfrt
+[
L.Pr(o)
* c(cHrL (r)
?498%2%
3594%7%
nature of the reactionand the high yieldsof l,lhomogeneous
can be attributed to the stability of the
dimethylcyclopropane
complexeswhich are formed.20
bis(trialkylphosphine)platinum(0)
The sourceof the two hydrogenatoms which are required in the
productionof neopentanehave not beenpositivelyidentified,but
they did not originate from the solvent,since the thermal deWhen
compositionof 2 in C6D11producedonly neopentane-d6.
a c y c l o h e x a n es o l u t i o no f 2 - ( L D ) : ( L D = [ ( C D : ) z C D ] 3 P )w a s
heatedfor 0.5 h at 178 oC, the neopentaneproducedwas 80%
ds, l57odr, and47ad2,indicatingsometransferof hydrogenatoms
originallypresentin the phosphineligandto the platinacyclobutane
ring to form neopentane.
( 1 9 ) T h e c o r r e s p o n d i nngi c k e la n a l o g u eb i s ( t r i p h e n y l p h o s p h i n e ) - 3 , 3 - d i is believedto be in equilibriumwith an olefin-comethylnickelacyclobutane,
in tolueneupon heating
ordinatednickel-carbenecomplexand decomposes
o
C
t o g i v e e t h y l e n e( l 5 E o ) ,2 - m e t h y l p r o p e n(e2 6 V o ) ,l , l - d i m e t h y l at 49
c y c l o p r o p a n e( 4 7 5 0 ) ,3 - m e t h y l - 1 - b u t e n(e6 % ) , a n d 2 , 2 - d i m e t h y l p r o p a n e
(6Vo\.t6
( 2 0 ) T h e p r e p a r a t i o na n d r e a c t i v i t i e os f a v a r i e t yo l ' b i s ( t e r t i a r yp h o s , .:
p h i n e ) p l a t i n u m ( 0c)o m p l e x ehsa v eb e e nr e p o r t e d :O t s u k a ,S . ; Y o s h i d a T
M a t s u m o t o ,M . ; N a k a t s u ,K . J . A m . C h e m . ' S o c .1 9 1 6 . 9 8 , - 5 8 5 0 - 5 8 5 8 .
YoshidaT
. . : O t s u k a .S . I b i d . 1 9 7 7 , 9 9 , 2 1 3 4 - 2 1 4 0 .
J. A m. C hem.S oc.,V ol .104,N o. 13, 1 982 3603
Elimination of a C-C Bond from Platinacyclobutanes
<--2-(f
)2
2-LHuo
a
a\
/
ll
)
2-(g )2
+
/ z- t-Ht-o
{
\__
B
o
o.or
of 2 on the
Figure2. Dependence
of the rateof thermaldecomposition
Rate constantswere
of addedtriisopropylphosphine.
concentration
solutionby 3tPltHfNMR specmeasured
at 126.5oC in cyclohexane
troscopy.
Kinetics. The thermal decompositionof I in cyclohexane(in
the presenceof Hg(0)) was followed by monitoring the appearan€
of l,l-dimethylcyclopropaneby GLC. This reaction followed
first-orderkinetics(ftoua= 1.2 x l0-s s-r, 178 oC) through the
first half-life of the reaction. Free triethylphosphinewas also
generatedas the reactionprogresseddue to amalgamationof ft(O)
by Hg(O). Becauseadded trialkylphosphinewas found to inhibit
the rate of decompositionof 2 or 3 (vide infra) and becausethe
effect of Hg(0) on the mechanismof thermal decompositionof
I is unknown,further investigationof this reactionwas deferred
i n f a v o r o f a n e x a m i n a t i o no f t h e s i m p l e rt h e r m o l i s i so f 2 a n d
much louer temperatures
3, which proceededhomogeneously'at
than those necessary'
for the decompositionof l.
Ratesof decompositionof 2 and 3 were determinedbi' follo*ing
by ''Pl'Hf NMR specthe disappearance
of platinacyclobutane
troscopy and by monitoring the appearanceof I ,l -dimethylcyclopropaneby GLC (Figure 1). Rates determinedby using
these techniqueswere indistinguishable.Reaction rates were
independentof the initial concentrationof platinacyclobutane
(from 2.4 x l0-2 M to 8.0 x l0-2 M), and the disappearance
of
2 or 3 and appearanceof I ,l -dimethylcyclopropanewere both first
order in platinacyclobutaneto greater than 90Vodecomposition.
to
or tricyclohexylphosphine
Addition of triisopropylphosphine
cyclohexanesolutionsof the correspondingplatinacyclobutane
inhibited the reaction,but the yieldsof l,l-dimethylcyclopropane
and neopentanewere unchanged. An examination of the dependenceof the rates of decompositionof 2 (0.08 M) in the
presenceof added triisopropylphosphine(0.005-0.040 M) yielded
rate constantswhich obeyedthe empirical rate eq 2 (Figure 2).
-dt2l
tz
/dt = k"b*d[2][((cH3)2cH)rp1-orr+o
Q)
The inhibition in the decomposition produced by added trialkylphosphineis not due to an associativeprocess(eq 3) since
L z P r T* t #
r-.erf,{
i
2-(gf).----
0.02 0.03 0.o4
[ i e r , c ]( M )
(3)
the 3rPltHl NMR spectraof the reaction mixtures containing
various concentrationsof added phosphineshowedno compounds
other than platinacyclobutaneand free phosphine.
Phosphinedissociationwas demonstratedto occur in a preequilibrium rather than rate-determining step by heating a cyclohexanesolution of 2 (0.07 M) and 2-(LD)z (0.07 M, LD =
oC and following the rate of exchangeof
[(CD3)2CD]rP)at 50
phosphinesby 3tPltHl NMR spectroscopy.Completeequilibration
of deuteratedand nondeuteratedphosphinesamong 2,2-LHLD,
and 2-(LD)2 was observedafter 8 h, with no detectabledecom(Figure 3).
positionof 2 to bis(triisopropylphosphine)platinum(0)
Analysis of the temperature dependenceof thermolysesof 2
and 3 over the range 90-120 oC yielded Arrhenius activation
parametersof Cu = 46 + 3 kcal mol-r, log A = 23 + 2 and E^
A
az-(gb
j
Y
ii
'/-\
\=_--^----:
*---')
33
34
3?
d(ppm)
F i g u r e3 . 3 ' P l ' H I N M R s p e c t r a( 1 0 1 . 3M H z ) o f a c y c l o h e x a n es o l u t i o n
o f 2 - ( L H ) , ( 0 . 0 7 M ) a n d 2 - ( L D ) r ( 0 . 0 7 M ) , A , a n d a f t e r h e a t i n ga t 5 0
oC for 8 h to produce an equilibrium mixture of 2-(LH)r, 2-(LD)2, and
2-LHLD, B. No decompositionto form l,l-dimethylcyclopropane and
L2Pt0 was observed. (re5Pt satellites are not shown; the underlined
phosphine(Ltttol; correspondsto the peak(s) indicated.)
"'
]
I
'=
t*..
"'o'a..
2'8
2'5
,tjl,,o' "*-'r'''
of 2 (o) and
Figure4. Arrheniusplotsfor the thermaldecompositions
by 3'Pl'HlNMR spectroscopy.
3 (r) determined
= 42 *,2kcal mol-r, log A = 2l *. l, respectively(Figure 4).
These activation parametersare similar in magnitude to those
reported for the thermal decompositionof bis(2,2-dimethylI) complexes,which produce
aklyl)bis(triethylphosphine)platinum(I
Ia The
platinacycloalkanes
and hydrocarbon.
bis(triethylphosphine)
rate-limiting step for the cyclometalationreactionsis believedto
be reductiveeliminationof alkane from a Pt(lV) intermediate,
basedon the large Arrhenius preexponentialfrequencyfactor and
the observationof a kinetic deuterium isotopeeffect (kHlkD ,=
3) when deuterium is substitutedfor hydrogenin the alkyl groups.
To examinethe possibilitythat the thermal decompositionof 2
might proceedin a similar manner via a rate-limitingoxidative
C-H
addition or reductiveeliminationof a triisopropylphosphine
bond, the rate of decompositionof 2 and 2-(LD\z at I l6 oC were
compared (Figure 1); a knlko = 0.98 + 0.04 was obtained,
indicating the absenceof a rate-limiting oxidative addition or
reductiveelimination of a C-H bond of the coordinatedphosphine.
Discussion
The thermal decompositionof bis(trialkylphosphine)-3,3-di(0)
to bis(trialkylphosphine)platinum
methylplatinacyclobutane
can be explainedby one of two
and 1,1-dimethylcyclopropane
possiblemechanisms. Either mechanismis supportedby the
observationthat dissociationof trialkylphosphineand formation
of a coordinativelyunsaturatedl4-electronPt(II) speciesis required for the reaction to occur. The large Arrhenius preexponential factor (log A ,= 2l-23) is also consistentwith a mechanism
DiCosimo and Whitesides
3604 J. Am. Chem.Soc.,Vol. 104,No. 13, 1982
S c h e m eI I I . P r o p o s e dM e c h a n i s mf o r T h e r m a l D e c o m p o s i t i o n o f
i ne W h i c h a
Bis(trialkylphosphine)-3,3-dimethylplatinacyclobutan
C-C Bond Is Reductively lrliminated from a Pt(ll) Intermediate
tI
L
I
+
L2Pf(O)
<-
[r-.t
Lt'l
1
I l\-r
L2Pf(O)
-
--[\
l";o*--el 14rcorzmor
aG+-ioxcotzn{ot \*l
/
\
AG
+L+ X
L-rer]{tr)
S c h e m e[ V . P r o p o s e dM e c h a n i s mt o r T h e r m a l D e c o m p o s i t i o n o f
B i s ( t r i a l k y l p h o s p h i n e ) - 3 , 3 - d i m e t h y l p l a t i n a c y c l o b u t a ni ne W h i c h a
C-C Bond Is ReductivelyEliminated from a Pt(lV) Intermediate
-[.**]
i>*ox
-
1
.__r,af
*,
''...vz\=_ [r_r,111
V\J
L/'
- - -a\-
-
+
C( CH - ) ,
X
+L:-[il;tr] +L
- R e o c t i oC
n o o r d i n o t-e- +
[.:;''.1F;i"-J
+ fl+
L2Pt(O)
+
L
in which three particles(phosphine,I,l-dimethylcyclopropane,
and the remainingPt moiety) are generatedin the transitionstate
from one particle in the ground state.
One mechanismof reactionwould involvedissociationof trialkylphosphine,rate-limiting reductiveeliminationof a C-C bond
and LPto,and recombination
to produce1,1-dimethylcyclopropane
of LPto with dissociatedphosphine(SchemeIII). Dissociation
of a phosphineligand prior to reductiveelimination of a C-C bond
from a three-coordinateintermediatehas beenreportedfor thermal
and ArNiMeL2.2r
decompositions
of LAu(CH3)2R,L2Pd(CH3)2,
In the absenceof a polar, coordinatingsolvent,the L2Pd0formed
after reductive elimination of ethanedisproportionatesto metallic
palladium and LaPd0,and for Au(III), the LAu(I)R species
produced upon reductive elimination of R2 undergoesfurther
intermolecular decomposition to produce Au metal and free
phosphine;in contrast,the thermal decompositionof ArNiMeL2
in cyclohexaneor benzeneproceedshomogeneouslyto produce
L2Ni0 as a stable product. Thus, the production of LPIO (an
intermediatewhich is probably highly unstable)in a nonpolar
solventsuch a cyclohexanewould have to be followed by rapid
recombinationwith dissociatedphosphineto preventthe formation
of platinum metal.
An alternatemechanismcould involveinitial dissociationof
trialkylphosphine,oxidativeaddition of a C-H bond of the remaining coordinatedphosphineto producea Pt(lV) intermediate,
rate-limiting reductiveelimination of 1,I -dimethylcyclopropane,
and recombinationof dissociatedphosphinewith reductiveelimination of the phosphineC-H bond from the remaining Pt(lI)
intermediateto produce L2Pt0 (Scheme IV). The absenceof a
kinetic deuterium isotopeeffect upon substitutionof ((CD3)2CD)rP
for ((CH3)2CH)jP in the thermolysisof 2 (Z-(LD)r) indicatesonly
that the oxidativeaddition of a phosphineC-H bond, if it does
occur, does not take place in the rate-limiting step. A number
of reactionshave been reported in which the oxidative addition
of a trialkylphosphineC-H bond to Pt(II) has resultedeither in
the formation of a cyclometalatedphosphinePt(II) compound
via a Pt(IV) intermediatez2or in the exchangeof 2H for rH in
the alkyl groups of coordinated phosphines(in reactions performed
in D2O/CH3COOD).23 The observationof incorporationof 2H
into the ca. l%oneopentaneproduced from the thermal decom( 2 1 ) F o r L A u R 3 , L = P h r P a n d R = C H : , C 2 H 5 ,n - C 3 H 7i,- C 3 H 7 ,a n d
t-C4He.ro For L2Pd(CHr)2, L = PPh3 and PPh2CH3and L2 =
For ArNiMeLz, L = Et3P and Ar = o-tolyl.r3
Ph2PCH2CH2PPh2.rr
(22) Goel,R. G.; Montemayor,R. G. Inorg. Chem.1977,16,2183-2186.
Cheney,A. J.; Mann, B. E.; Shaw,B. L.; Slade,R. M. ,r. Chem.Soc.A 1971,
, . D . ; M a r k h a m ,R . ; S h a w ,B . L . ;
, . A . ; E m p s a l lH
3 8 3 3 - 3 8 4 2A
. l - S a l e mN
Weeks,B. J. Chem.Soc.,Dalton Trans.1979,1972-1982.
(23) Kiffen, A. A.; Masters,C.; Raynand,L. ,/. Chem. Soc.,Dalton Trans.
1975.853-857.
the
diagramillustrating
coordinate
Figure5. Gibbsfreeenergy-reaction
or
of activationfor reactionol I with neopentane
relativefreeenergies
andprodfor 0.1 M reactants
to L2Pt0(TA^Scalculated
decomposition
ucts).
positionof 2-(LD)r might be due to a phosphinecyclometalation
reactionin which the reductiveelimination of a C-2H bond instead
of a C-C bond occurs a small percentageof the time. Since
reductiveeliminationof a C-C bond from Pt(lV) is well docuand reports of this same reaction for Pt(ll) are few,25
mentede'2a
the possibilityof cyclometalationof a trialkylphosphineto produce
a P t ( l V ) i n t e r m e d i a t ef r o m w h i c h r e d u c t i v ee l i m i n a t i o no f l . l could occur cannotbe ignored,though no
dimethylcyclopropane
concreteevidencehas been produced by this investigationto
i n d i c a t es u c h a r e a c t i o nd o e st a k e p l a c e .
Pt(ll) compounds
Our study'of the reactionsof homogeneous
has beendirectedtoward achievingthe oxidativeaddition of an
unactivatedC-H bond of a hydrocarbonsolventto a homgeneous
platinum species,so that comparisonsof the mechanismsof the
C-H bond activationreactions
and heterogeneous
homogeneous
can be made. We havenow completedthe study of one particular
of dineopentylbissystem,that of the thermal decompositions
(trialkylphosphine)platinum(lI)and its product, bis(trialkyl(eq 4). One may ask
phosphine)-3,3-dimethylplatinacyclobutane
. 1 ^ l r e- ,r l n L 2 P i ( O )
-e \+
,pr'I
+
'''
c(cHrL
-
;+
r
+
(4)
x
why the reactionproceedsonly to the right, with no observable
back reactionvia oxidativeadditionof a C-H bond of neopentane
or the solvent. The resultsof this investigationcan be used to
constructan argumentwhich outlinesthe problemsof trying to
*unactivated"C-H bondsof hydrocarbonsolvents
oxidativelyadd
t o h o m o g e n e o upsl a t i n u m ( l l ) s p e c i e s .
For the reaction(EtrP)2Pt[CH2C(CH3)
tlz-- I * neopentane
= 3.8 x 16-2t-t'll this rate corresponds
to a AG+
(178 oC), Trcatd
- 30 kcal mol-I. Two particlesare producedfrom one in the
reaction,resultingin a fAS(translation)= l5-17 kcal mol-r (178
oC,0.l M reactantsand products).26The sumsof bond energies
in the reactantand productsare the sameto a first approximation
(neglectingring strain, vide infra), and one can make the sim(24) The reactionof cyclohexane
of I or 2 with bromineor iodine
solutions
a t 2 5 o C r e s u l t si n t h e i m m e d i a t er e d u c t i v ee l i m i n a t i o no f 1 , 1 - d i m e t h y l and formationof trans-L2PtX,(X = Br, I); this reactionmost
cyclopropane
likely proceedsthrough a Pt(lV) intermediate,from which reductiveelimination of a C-C bond is rapid.
(R = C6H5,
of cis-L2PIR2
(25) The thermaldecomposition
of solidsamples
Lr = dppe,dppm)in the absence
p-CH3C6Ha;
L = P(CuH5)r,P(p-CH3coHa)rl
of solventresultedin the concerted,intramolecularreductiveeliminationof
were found to increaseupon addition of added
R2; rates of decomposition
phosphine:Braterman,P. S.; Cross,R. J.; Young,G. B. "/. Chem. Soc.,Dalton
T r a n s . 1 9 7 6 ,1 3 0 6 - 1 3 1 4 .G l o c k l i n g ,F . ; M c B r i d e ,T . ; P o l l o c k ,R . J . I . " / .
C h e m . S o c . ,C h e m .C o m m u n . 1 9 7 36, 5 0 .
(26) Pageand Jenckshavecalculatedthat for a reactionwhich produces
two particlesfrom one L\S ry 10.5kcal mol-r at 300 K: Page,M. l. Angew.
Chem.,Int. Ed. Engl. 1971,16,449-459. Page,M. I.; Jencks,W. P. Proc.
, .;
N a t l . A c a d . S c i .( l . S . A . 1 9 7 1 , 6 8 , 1 6 7 8 - 1 6 8 3 .I b e r s ,J . A . ; D i C o s i m o R
Whitesides,G. M. Organometallics1982,1, 13.
Elimination of a C-C Bond from Platinacyclobutanes
plifying assumptionthat AH lu 0 and that AG for the reaction
is primarily controlled by entropic effects. For the subsequent
reactionof I * LrPt0 * l,l-dimethylcyclopropane(178 oC, with
- 1.2 x l0-5 s-r. The actual reaction
Hg(0)), /robsd
rate could
be up to 102faster than observed,sincefree triethylphosphineis
producedduring the reactiondue to amalgamationof Pt(0) with
Hg(0) and may inhibit the rate of decompositionas was observed
for thermolysesof 2 and 3. This range of rate constantscorrespondsto a AG+ = 33-37 kcal mol-I.27 From this information
one can construct a reaction coordinatediagram which illustrates
the relative free energiesof activation for the forward and back
reactionsof 1 (Figure 5); AG+(back)- AGt(forward) ry 8-14
kcal mol-r.
The oxidativeaddition of a C-H bond of a solventmolecule
to I should be more favorablethan reaction with neopentane
becauseof its greater relativeconcentration(eq 5); AS = R ln
L,)ttx
o'rM
+ , f,-.._7t
*
)tt-.tl*
) ,f l,
L-/\LLtYi
'l
l= ,<
)ti ) + c(cHrl (5)
,l:,X,,
- R ln 100 - 9.1 eu. The hypothetical
[solvent]/[neopentane]
reactionof I with solventshouldthereforehave ZAS = 4.1 kcal
mol-r (178'C) more favorablethan reactionof I with neopentane
b u t s t i l l h a v ea l G * = 4 - 1 0 k c a l m o l r g r e a t e r t h a n t h a t f o r
r e d u c t i v ee l i m i n a t i o no f l . l - d i m e t h l l c y c l o p r o p a n e
from 1.
T h i s e n t i r e a r g u m e n t i s o n l l ' q u a l i t a t i v ei n t h a t i t i g n o r e s
changesin bond strengthsand strain energiesin proceedingfrom
reactantsto products and that it is based on very qualitative
estimatesof hS. Entropicfactorsaloneare not necessarily
the
dominant contributorsto lG for thesereactions: steric crowding
in (EtrP),Pt[CH,C(CHr)r]: is relievedupon lossof neopentane,28
and ring strainin the platinacyclobutane
which is producedis small
but significant(<5 kcal mol-r greaterthan an analogousplatinacyclopentanel.lr':'r
Although the relativeimportanceof entropic
and enthalpiccontributionsin determiningthe free energyof these
reactionshave not beenestablished,the aboveargument suggests
that the highly favorablechange in entropy in producing two
particlesfrom one, both for the productionof I and for its subsequentdecomposition,is significantin determiningthe direction
of the reaction.
Experimental Section
GeneralData. .{ll reactions
involvingorganometallic
compounds
were
performedunder argon or prepurifieddinitrogenby using standard
anaerobic
techniques.r0
The procedures
followedfor performingsealed
tubereactions
havebeendescribed
previously.ra
Boilingpointsareuncorrected.\{elting pointsweredetermined
on a Thomas-Hoover
Ap3'P[tHlNMR spectra
paratusandareuncorrecred.
wererecorded
at 36.2
and 101.3\{Hz on JeolFX-90Qand BrukerWP-250spectrometers.
respectivelr.
Spectra
arereported
at 36.2MHz unless
otherwise
noted,
andchemicalshiitsarereported
relativeto external85%phosphoric
acid
( 2 7 ) A t t h i ss a m et e m p e r a t u r(e1 7 8 ' C ) , t h e f r e ee n e r g yo f a c t i v a t i o nf o r
decomposition
of 2 or 3 is calculatedto be AGr = 26-2i kcal mol-l. A
comparisonof the free energiesof activationfor additionof a C-H bond to
2 or 3 with their thermaldecomposition
to L2Pt0and l,l-dimethylcyclopropane
is more difficult to constructthan for I, since the correspondingdineopentylbis(trialkl'lphosphine)platinum(II)
compoundscould not be prepared
in order to determineAG for formationof the platinacvclobutanes.
(28) Steric crowdingabout transitionmetal'alkylstias beenreportedto
resultin a decreaseof metal-carbonbond energiesby l9-20 kcal mol-lfor
I M ( C H 2 C ( C H r ) r ) r ] ( M = T i , Z r , o r H f w i t h r e s p e ct o s t e r i c a l l yl e s sd e , . F . ; P a t i l ,D . S . ; P a d l e yJ,. B . J . C h e m .
m a n d i n ga l k y l s u b s t i t u e n(tLsa p p e r tM
S o c . ,C h e m .C o m m u n . l 9 7 5 .8 3 0 - 8 3 1 ) . D i f f e r e n c eisn c o b a l tn - n r o n v la n d
cobaltneopentylbondenergies
of -7 kcal mol t havebeenobserved
in'iertain
organocobaltcomplexes(Tsou,T.-T.; Loots,M.; Halpern,J. J. Am. Chem.
Soc. 1982, 104, 623-624).
(29) The ring strain in the platinacyclobutane
moiety of platinum(lV)
(e.g.,PtCl2(C:Ho)(bpy))hasbeenestimatedat -12 kcal mol-rby
complexes
comparisons
of heatsof decomposition
measuredby usingdifferentialscanning
calorimetry (Puddephatt,R. J. Coord. Chem. Reu. 1980, 33, 149-1941.
Comparisonof this estimatewith that derivedfor similar platinum(II) complexeslamay not be appropriate,sincestericcrowding,bond strengths,and
electronicstructuresof the two complexesmay be different.
(30) Shriver, D. F. 'The Manipulation of Air-SensitiveCompounds";
McGraw-Hill: New York, 1969.
J. Am. Chem. Soc., Vol. 104, No. 13, 1982
3605
(downfield shifts positive). 'H NMR spectra were recordedat 90 and
250 MHz, respectively,on thesesame spectrometersand are reportedat
90 MHz unlessotherwisespecified. Chemical shifts are reported relative
to internal tetramethylsilane. Mass spectra were recorded on a Hewl e t t - P a c k a r d5 9 9 0 4 G C / M S a t a n i o n i z i n gv o l t a g eo f 7 0 e V . D i e t h y l
ether and tetrahydrofuran were distilled from disodium benzophenone
dianion under argon. Cyclohexaneand cyclopentanewere stirred over
concentratedsulfuric acid for 48 h, washed with saturated sodium bicarbonatesolutionand then water, dried with phosphoruspentoxide,and
distilled under argon. Triethyl phosphatewas distilled and stored under
a r g o n . D C I ( 3 8 7 0 i n D 2 O , 9 9 V oD , S t o h l e r ) , t r i i s o p r o p y l p h o s p h i n e
(Strem), and tricyclohexylphosphine(Strem) were used as received.
D i n e o p e n t y l1( , 5 - c y c l o o c t a d i e n e ) p l a t i n ruam
and bis(triethylphosphine)3 , 3 - d i m e t h y l p l a t i n a c y c l o b u t a n( le) r a w e r e p r e p a r e da s p r e v i o u s l yd e scribed. A published procedure was used to prepare 2-bromopropane-d1
from 2-propanol-d3(Stohler, 99.5VoD).3t Organoplatinum compounds
were storedin tightly sealedvials in the dark at -15 oC and were freshly
recrystallizedbefore performing thermolyses.
Tris(isopropyl-d7)phosphine. To a 500-mL round-bottomed flask
e q u i p p e dw i t h s t i r r i n g b a r , a d d i t i o n f u n n e l , a n d r e f l u x c o n d e n s e rw a s
a d d e d 7 . 0 g ( 0 . 2 9 m o l ) o f m a g n e s i u mm e t a l a n d 2 0 0 m L o f e t h e r . T h e
a d d i t i o n f u n n e l w a s c h a r g e d w i t h a s o l u t i o no f 3 l g ( 0 . 2 4 m o l ) o f 2 b r o m o p r o p a n e - dirn 3 0 m L o f e t h e r . T h i s s o l u t i o nw a s a d d e dd r o p w i s e
w i t h s t i r r i n g a t a r a t e s u f f i c i e n tt o m a i n t a i ng e n t l er e f l u x . T h e m i x t u r e
w a s s t i r r e d i o r 3 h a f t e r t h e a d d i t i o n w a s c o m p l e t ea n d t h e n f i l t e r e d
t h r o u g h a p l u g o f g l a s sw o o l t o y i e l d 2 3 0 m L o f 0 . 9 7 M ( i s o p r o p y l - d r ) m a g n e s i u mb r o m i d e ( 0 . 2 2 m o l , 9 4 q " l i e l d ) .
The deuteratedphosphinewas preparedaccordingto a variation of a
published procedure.r2 To a flame-dried, argon-filled 500-mL roundbottomedflask equippedwith stirring bar and addition funnel was added
6.1 mL (9.6 g, 0.070 mol) of freshly distilledphosphoroustrichloride and
1 0 0 m L o f e t h e r . T h e a d d i t i o n f u n n e l w a s c h a r g e dw i t h t h e G r i g n a r d
solution describedabove,which was then added dropwise with stirring
u n d e r a r g o n a t - 1 0 o C . A f t e r t h e a d d i t i o n w a s c o m p l e t e ,t h e m i x t u r e
was refluxed for 2 h and cooled to room temperature. Degassed,distilled
w a t e r ( 1 0 m L ) w a s a d d e d. s l o w l vb y s y r i n g ew i t h s t i r r i n g , f o l l o w e db y
50 mL of degasse2
d N H C I a n d 6 6 m L o f d e g a s s e2
d N NH4OH which
d i s s o l v e dt h e s a l t s f r o m t h c r e a c t i o na n d l e f t t w o p h a s e sc o n t a i n i n gn o
s o l i d s . T h e o r g a n i cl a y e r w a s t r a n s l e r r e db y c a n n u l at o a n a r g o n - f i l l e d
f l a s k c o n t a i n i n gc a . 2 0 g o f a n h y ' d r o u ss o d i u m s u l f a t e ,t h e r e m a i n i n g
a q u e o u sl a y e r w a s s t i r r e d w i t h a n a d d i t i o n a l 1 5 0 m L o f e t h e r , a n d t h e
o r g a n i c l a l e r s w e r e c o m b i n e da n d f r a c t i o n a l l yd i s t i l l e d u n d e r a r g o n a t
a t m o s p h e r i cp r e s s u r e .T h e f r a c t i o nb o i l i n ga t 1 7 0 - 1 7 5 o C ( l i t . 3 0b p 1 7 5
o C ) w a s c o l l e c t e da n d f i l t e r e d u n d e r a r g o n t h r o u g h a 0 . 5 - c m X
2.0-cm
c o l u m n o f a l u m i n a ( W o e l m , A c t i v i t y I ) t o y i e l d 5 . 3 g ( 4 2 V oy i e l d ) o f
3
r
P
{
r
H
l
tris(isopropy'l-dr)phosphine:
N M R ( C o D o )6 1 7 . 3 .
Reaction of Dineopentyl(1,5-cyclooctadiene)platinum(II) with Triisopropylphosphineand Tricyclohexylphosphine. Into a flame-dried, argon-filled0.5-mL thermal decompositiontube was weighed l5 mg (0.034
m m o l ) o f d i n e o p e n t y l ( 1 , 5 - c y c l o o c t a d i e n e ) p l a t i n u m ( lt lh) e
; tube was
capped with a rubber septum and flushed with argon. To the tube was
t h e n a d d e d b y s y r i n g e0 . 1 0 m L ( 0 . 0 6 8 m m o l ) o f a 0 . 6 8 M s o l u t i o no f
triisopropylphosphine
in cyclohexaneor 0.25 mL (0.068 mmol) of a0.27
M s o l u t i o no f t r i c y c l o h e x y l p h o s p h i ni e
n c y c l o h e x a n ef,o l l o w e db y 0 . 1 5
m L o f a 0 . 1 0 2 M s o l u t i o no f c y c l o p e n t a n e( G L C i n t e r n a l s t a n d a r d )i n
c y c l o h e x a n e .T h e c o n t e n t so f t h e t u b e w e r e d e g a s s e db y t h r e e f r e e z e p u m p - t h a w c v c l e s .a n d t h e t u b e w a s s e a l e du n d e r v a c u u m ( 0 . 0 1 t o r r ) .
T h e t u b e u ' a s h e a r e da t - 5 0o C f o r l 5 h , t h e n f r o z e n i n l i q u i d n i t r o g e n ,
and openedunder argon,and the reactionmixture examinedby GLC for
neopentane
a n d 2 , 2 , 5 , 5 - t e t r a m e t h y l h e x abnye u s i n ga l O - f t , I f s - i n .2 0 V o
S E - 3 0 o n C h r o m o s o r bP c o l u m n a n d f o r 1 , 5 - c y c l o o c t a d i e naen d 1 , 3 , 5 c y c l o o c t a t r i e n eu s i n g a 1 0 - f t , t / r - i n . 1 5 V oi 3 , p ' - o x v d i p r o p i o n i t r i l oe n
C h r o m o s o r bP c o l u m n . T h e y i e l d so f n e o p e n t a n e , 2 , 2 , 5 , 5 - t e t r a m e t h y l h e x a n e ,1 , 5 - C O D ,a n d 1 , 3 . 5 - c y c l o o c t a t r i e nwee r e 8 0 V c ,3, 6 V o 8, 0 V o ,a n d
l4Vo, resfi,ctively, for reaction with triisopropylphosphineand 85Vc,,l5Tc.
30Vo and 37o, respectively,for reaction with tricyclohexylphosphine.
A d d i t i o n o f 0 . 1 0 m L o f c o n c e n t r a t e dH C I t o t h e s e r e a c t i o n m i x r u r e s
producedan additional 53 and 83Voyield of neopentanefrom the reactions with triisopropylphosphineand tricyclohexylphosphine,respectively,
a c c o u n t i n gf o r a l l t h e n e o p e n t y lg r o u p so r i g i n a l l y o n p l a t i n u m .
Repeating the aboveprocedureand heating the tube an additional 2
h at 126 oC after l5 h at 50 oC produced l,l-dimethylcvclopropaneand
additional neopentanein 26Voand 26Vayields, respectively,for reaction
( 3 1 ) C o u l s o n ,E . J . ; G e r r a r d ,W . ; H u d s o n ,H . R . J . C h e m .S o c . 1 9 6 5 ,
2364-2369.
(32) Lorenz,H. J.; Zintl, A. R.; Volker,F. (DeutscheAdvanceProduction
G . m . b . H . )U S 3 4 9 9 0 3 9( C l . 2 6 0 - 6 0 6 . 5C; 0 7 F ) , M a r 3 , 1 9 7 0 A
, ppl.Nov l0
1 9 6 6( C A 7 2 P l 3 2 9 6 7 s ) ; N e t hA
. p p l . 6 , 6 1 4 , 9 4 (5C l . C 0 7 F ) , A p r 2 4 , 1 9 6 7 ;
G e r . A p p l . O c t 2 1 , 1 9 6 6( C A 6 1 P 1 0 8 7 5 7 2 ) .
3 6 0 6 J . A m . Chem So
. c .,Vo l .1 0 4 ,N o . 1 3 ,1 9 8 2
with triisopropylphosphineand in 44Voand 4l%oyields for reaction with
tricyclohexylphosphine.The yield of l,l-dimethylcyclopropane correspondsto the fraction of the reaction of trialkylphosphine with dineopentyl( l, 5-cyclooctadiene)platinum(II)which producesplatinacyclobutane, determined by heating a reaction mixture at 50 oC for 15 h, removing all volatile componentsunder vacuum, redissolvingthe remaining
oil in dry cyclohexane,and adding 0.10 mL of 38VoDCI in D2O. The
yields of (CH3)2C(CHzD)z produced in this manner for reactionsusing
triisopropylphosphineand tricyclohexylphosphinewere 27Voand 39Vo,
respectively(SchemesI and II), as determined by GC/MS analysison
a 2o-ft,'/e-in. 3VoUCW-98 on Chromosorb P column.
Bis(triisopropylphosphine)-3,3-dimethylplatinacyclobutane(2 nd 2(LD)z). Into each of three l2-in. x 0.5-in. medium-wall thermal decomposition tubes was weighed 0.89 g (2.0 mmol) of dineopentyl(1,5cyclooctadiene)platinum(Il); the tubes were capped with a rubber septum
and flushed with argon. To each tube was then added by syringe l0 mL
of dry, degassedcyclohexaneand 0.83 mL (0.64 g, 4.0 mmol) of triisopropylphosphine;the tubes were degassedby three freeze-pump-thaw
cyclesand sealedundervacuum (0.01 torr). The tubes were heated at
50 oC for l8 h, then frozen in liquid nitrogen, opened,and capped with
rubber septa under argon. The reaction mixtures were transferred by
cannula to a flame-dried. argon-filled 100-mL round-bottomed flask
equippedwith stirring bar, and the solventwas removed under vacuum
with stirring. The remaining yellow-brownsolid was dissolvedin 50 mL
of degassedacetone and the solution cooled to -78 oC. The yellow
precipitate was collectedby suction filtration and redissolvedin 25 mL
of degassedacetone. The volume of the solution was reduced by rapidly
flowing argon over the solution at -78 oC. The fine, white crystalline
precipitate was collected by suction filtration and remaining acetone
removedunder vacuum to yield 0.68 g (19%) of 2: mp 126-129 oC dec:
I P I ' H I N M R ( c o D o )d 3 3 . 0( r ,
' H N M R ( c 6 D 6 )6
=
"/n_p r83l Hz);
0 . 8 6 ( 4 H , t , " / R _ H= ' 1 5 . 2 H 2 , J p a l = 1 0 . 3H z , P t C H r ) . 1 . 0 5( 3 6 H . d
= l l . 7 H z , J = 6 . 8 H z , ( C 1 1 r ) 2 C H P ) ,1 . 4 9 ( 6 H . b s . C of d, "/p-11
( C H r ) : ) , 2 . 1 7 ( 6 H , s e p t e t ,J = 7 . 3 H z , C H P ) ; I R ( K B r ) 2 9 9 0 - 2 7 - 5 0( v s
b r ) , 1 4 5 0( s ) , 1 3 7 0( s ) , 1 3 5 0( s ) , 1 3 4 0( m ) , 1 2 3 5( s ) , 1 0 3 8( s ) , 1 0 1 9( s ) .
9 1 5 ( m ) , 8 7 2 ( s ) , 7 5 7( m ) , 6 3 2 ( v s ) , 5 1 9( c ) c m - r . A n a l . ( C 2 r H 5 2 P 2 P t )
C. H.
Compound 2-(LD)zwas prepared in the same manner as describedfor
2 , u s i n g 0 . 9 4 m L ( 0 . 7 2 g , 4 . 0 m m o l ) o f t r i s ( i s o p r o p y l - d 7 ) p h o s p h i fnoer
each 0.89 g (2.0 mmol) of dineopentyl(1,S-cyclooctadiene)platinum(l
I)
( 2 l V o y i e l d ) : m p 1 2 9 . 0 - 1 3 0 . 5" C d e c ; 3 t P l t H f N M R ( C o D o )6 3 1 . 8 ( t ,
tH
/n-p = 1834 Hz);
NMR (CoDe)6 0.85 (4 H, t, "/ps_s= 76.2H2, Jpal
= 9 . 8 H z , P I C H 2 ) , 1 . 4 9 ( 6 H , b s , C ( C H 3 ) 2 ) ;I R ( K B r ) 2 9 2 0 ( s ) , 2 8 6 0
( s ) , 2 8 2 0( m ) , 2 2 0 5( m ) , 2 1 9 0( s ) , 2 1 3 0( m ) , 2 1 0 0( m ) , 2 0 6 0( m ) , 1 4 5 0
( m ) , 1 3 6 0( m ) , l 3 l 0 ( m ) , 1 0 5 5( m ) , l 0 l 8 ( s ) , 8 0 5 ( m ) , 7 8 5 ( m ) , 5 9 5
cm-r. Anal. (C23HroDorPrPt)C, P.
Bis(tricyclohexylphosphine)-3,3-dimethylplatinacyclobutane
(3). Into
each of three l2-in. x 0.5-in. medium-wall thermal decompositiontubes
w a s w e i g h e d0 . 6 0 g ( 1 . 4 m m o l ) o f d i n e o p e n t y l ( 1 , 5 - c y c l o o c t a d i e n e ) p l a t inum(Il); the tubes were cappedwith a rubber septum and flushed with
argon. To each tube was added 8.0 mL (2.8 mmol) of a 0.34 M solution
of tricyclohexylphosphinein cyclohexane. The tube walls were washed
d o w n w i t h a n a d d i t i o n a l2 . 0 m L o f c y c l o h e x a n ea. n d t h e r e s u l t i n gs o l u tions were degassedby two freeze-pumpthaw cyclesand sealedunder
vacuum (0.01 torr). The tubes were heated at 50 oC for I 5 h and then
opened under argon and the reaction mixtures transferred by cannula to
an argon-filled 50-mL round-bottomedflask equippedwith stirring bar.
The volatile componentsof the reaction mixture were removed under
vacuum while being heated at 40-50 oC, and the remaining red-brown
oilwas redissolvedin 150 mL of degassedacetoneand cooledto -10 oC
for l2 h. The pale yellow solid which precipitated was collected by
suction filtration and dissolvedin ca. 100 mL of THF. The resulting
solution was filtered through a 0.5-pm millipore filter and the filtrate
reduced in volume to l0 mL by rotary evaporation at25 "C.. Acetone
(50 mL) was added,and the resulting solution was cooledto -78 oC. The
white, crystalline solid which precipitated was collected by suction filtration and remaining solvent removed under vacuum to yield 0.56 g
( l l E o ) o f 3 : m p 1 5 5 - 1 5 6 o C ( d e c ) ; 3 ' P N M R ( C o D o )6 2 2 . 3 ( t , , / n - p =
1 7 9 7 H z ) : ' H N M R ( C e D o )6 0 . 9 8 ( 4 H , m . , / n - H = 7 5 . 1 H 2 , P t C H r ) ,
1 . 2 7( 1 8 H , m , C H 2 ) , 1 . 5 8( 6 H , s , C ( C H 3 ) 2 ) ,1 . 5 9 - 1 . 9 9( 3 0 I { , b m ) ,
2 . 1 4( 1 8 H , . ) ; I R ( K B r ) 2 9 9 0 - 2 8 7 0( v s b r ) , 2 8 4 5 ( v s ) , 1 4 4 3( s ) , 1 3 4 5
( m ) , 1 2 9 5( m ) , 1 2 6 8( m ) , 1 1 7 2( m ) , 1 1 2 2 ( m ) . 1 0 0 2( m ) , 8 4 5 ( s ) , 7 2 9
( m ) c m - r . A n a l . ( C 4 r H 7 6 P 2 P IC) , H . P .
Thermal Decomposition of Bis(triethylphosphine)-3.3-dimethylplatinacyclobutane(l) in the Presenceof Hg(0). lnto each of nine
flame-dried, argon-filled 0.5-mL medium-wall thermal decomposition
tubes was placed 0.40 g of triply distilled mercury. The tubes were
cappedwith rubber septaand flushed with argon, and then 0.10 mL of
a solution of 0.150 g (0.299 mmol) of freshly recrystallizedI dissolved
i n 1 . 5 0 m L o f a 0 . 1 0 2 l v { s o l u t i o no f c y c l o p e n t a n ei n c y c l o h e x a n ew a s
DiCosimo and Whitesides
added to each tube by syringe. The constrictionin each tube was rinsed
w i t h 0 . 0 3 m L o f c y c l o h e x a n e .T h e c o n t e n t so f t h e t u b e sw e r e d e g a s s e d
a n d t h e t u b e ss e a l e du n d e r v a c u u m ( 0 . 0 1 t o r r ) . T h e t u b e sw e r e h e a t e d
for predeterminedintervalsat 178 oC. then frozenin liquid nitrogen,and
o p e n e da n d t h e r e a c t i o nm i x t u r e s a n a l y z e db y G L C f o r p r o d u c t i o no f
l,l-dimethylcyclopropaneT
. h e o b s e r v e dr a t e c o n s t a n tf o r t h e r e a c t i o n
was measuredfrom data collectedthrough I half-life of the reaction;/c"M
= 1 . 2 x l 0 - 5 s - r . A t t h e c o m p l e t i o no f t h e r e a c t i o n t, h e y i e l d so f I , l dimethylcyclopropaneand neopentanewere 92Voand 970,respectively.
Thermal Decompositionof 2 and 3. Into a 5-mm NMR tube was
w e i g h e d3 0 m g ( 0 . 0 5 2 m m o l ) o f 2 ; t h e t u b e w a s c a p p e dw i t h a r u b b e r
septum and flushed with argon. To the tube was added by syringe 0.50
m L o f a 0 . 1 0 8 M s o l u t i o no f c y c l o p e n t a n ei n c y c l o h e x a n et;h e c o n t e n t s
of the tube were degassed,and the tube was sealedunder vacuum. The
tube was then heated for 2 h at 126 oC and the pale yellow, homogeneous
r e a c t i o nm i x t u r e e x a m i n e db y r r P l ' H l N \ ' l R . N o s t a r t i n g m a t e r i a l r e mained, and the only product observedwas bis(triisopropylphosphine)p l a t i n u m ( 0 ) ] 36 7 3 . 2 ( t , l : 4 : 1 , , / n - p = 4 2 2 9 H z ) ( l i t . 3 a( o c t a n e )6 7 2 . 8
("In-p = 4202H2)). The tube was openedunder argon and the reaction
mixture examinedby GLC on a l0-ft, I L-in.20% SE--10on Chromosorb
P c o l u m n . w h i c h i n d i c a t e dt h e f o r m a t i o n o f l , l - d i m e t h y l c y c l o p r o p a n e
(99Vo)and neopentane(2Vo).
Repeating the above procedurewith 30 mg (0.036 mmol) of 3 prod u c e d b i s ( t r i c y c l o h e x y l p h o s p h i n e ) p l a t i n u m (a0s) t h e s o l e p r o d u c t b y
3 ' P l r H l N M R : 6 6 2 . 5 ( t , l : 4 : 1 , , I n - p = 4 2 2 9H z ) ( l i t . r a( h e p t a n e )6 6 3 . 8
("/n-p = 422'7Hz)). Analysis of the pale yellow, homogeneousmixture
by GLC indicatedthe formation of l,l-dimethylcyclopropane(94Vo)and
n e o p e n t a n e( 7 % ) .
Kinetics of Thermal Decompositionof 2, 2-(LD')2,and 3. Into each of
eight flame-dried,argon-filled 5-mm NMR tubes was weighed 23.0 mg
( 0 . 0 4 0 m m o l ) o f f r e s h l y r e c r y s t a l l i z e d2 . T h e t u b e s w e r e c a p p e dw i t h
rubber septaand flushedwith argon. To each tube was addedby syringe
0 . 5 0 m L o f a 0 . 1 6 M s o l u t i o no f t r i e t h y l p h o s p h a t ei n c y c l o h e x a n et;h e
tubesu'eredegassedb1'threefreeze-pumpthaw cyclesand sealedunder
vacuum 100l torr). Duplicatethermolysesat four different temperatures
w e r e p e r f o r m e ds i n r u l t r n e o u s l yf,o l l o w i n gt h e d i s a p p e a r a n coef s t a r t i n g
m a t e r i a lb \ r r P l r H i \ \ 1 R s p e c t r o s c o pt lh r o u g ha t l e a s t2 h a l f - l i v e s .T h e
o b s e n ' e dr a t e c o n s t a n t s( a v e r a g eo f t w o r u n s ) f o r t h e r m o l y s eosf 2 a t 9 8 . 5 ,
I I 1 . 0 . I 1 6 . 5 .a n d l : - 50 ' C w e r ek o M = - i 7 0 . 3 9 4 . t { ' 1 . 6a.n d 4 0 1 X l 0 s
s-r, respectively'.
R e p e a t i n gt h e a b o v ep r o c e d u r eu s i n g I 0 . 0 m g ( 0 . 0 I I I m m o l ) o f 3 i n
eachtube, and heatingthe tubesat 50 oC lor 5 min to conrpletelydissolve
t h e p l a t i n u m c o m p l e x b e f o r e b e g i n n i n gt h e k i n e t i c r u n s , g a v e t h e i o l l o w i n g r a t e c o n s t a n t s( a v e r a g eo f t w o r u n s ) a t 9 0 . 5 , 9 8 . 0 . 1 0 9 . 5 ,a n d
1 1 6 . 0o C : k o M = 2 . 4 1 , 7 . ' 7 14, 4 . 8 ,a n d 1 0 5 X l 0 5 s - r , r e s p e c t i v e l y .
The rate of productionof I ,l -dimethylcyclopropane
from the thermal
decompositioo
n f 2 , 2 - ( L D ) 2 ,a n d 3 w a s m e a s u r e db y G L C . I n a t y p i c a l
procedure.93.6 mg (0. 160 mmol) of 2 was dissolvedin 1.0 mL of a 0.102
M s o l u t i o no f c y c l o p e n t a n ei n c y c l o h e x a n ea n d t h e r e s u l t i n gs o l u t i o n
d i l u t e d t o 2 . 0 m L w i t h c y c l o h e x a n ei n a 2 . 0 - m L v o l u m e t r i c f l a s k . T o
e a c h o f n i n e 0 . 5 - m L m e d i u m - w a l l t h e r m a l d e c o m p o s i t i o nt u b e s w a s
added 0.20 mL of this standardsolution,and the walls of the tubes were
washeddown with an additional0.l0 mL of cyclohexane.The tubeswere
degassed
t h r e e t i m e s a n d s e a l e du n d e r v a c u u m ( 0 . 0 1 t o r r ) , t h e n h e a t e d
a t 1 1 6 . 5 o C f o r p r e d e t e r m i n e di n t e r v a l s ,a n d o p e n e d ,a n d t h e y i e l d o f
I,l-dimethylcyclopropanewas determined. The observedrate constants
f o r 2 a n d 2 - ( L D ) za t I 1 6 . 5 o C w e r e k o b s =
d 8.20and 8.39 x l0-a s 1,
r e s p e c t i v e l ya, n d t h e r a t e c o n s t a n tf o r 3 a t I 1 6 . 0 o C w a s l o b s d- 9 . 8 8 x
l0-as-r.
Thermal Decomposition of 2 and 3 in the Presence of Added Trialkylphosphine.Cyclohexanesolutions0.0075,0.0125,0.025,0.050,and
0 . 1 0 M i n t r i i s o p r o p y l p h o s p h i nwee r e p r e p a r e db y d i l u t i n g 0 . 0 7 5 , 0 . 1 2 , s ,
0 . 2 5 , 0 . 5 0 , a n d L 0 m L o f a 0 . 2 0 M s t o c k s o l u t i o no f t h e p h o s p h i n ei n
c y c l o h e x a n et o 2 . 0 m L i n v o l u m e t r i cf l a s k su n d e r a r g o n .a n d t h e n 0 . 2 0
m L o f e a c h o f t h e s es o l u t i o n sw a s a d d e d b y s y r i n g et o a 5 - m m N M R
tube containing0.30 m[- of a cyclohexanesolution0.I 33 M in 2 and 0.l6
M in triethyl phosphate. The tubes were degassedand sealed under
v a c u u m a n d t h e n h e a t e da t 1 2 6 . 5o C , a n d t h e r a t e o f d i s a p p e a r a n coef
2 was measuredrelativeto triethyl phosphateinternal standardby 3rP[tHf
NMR spectroscopy.The observedrate constantsfor reaction mixtures
80 mM in 2 and 3.0.5.0, 10,20,and 40 mM in triisopropylphosphine
w e r e k o b ' d= 1 1 9 ,6 1 . 2 , 3 3 . 4 , 2 1 . 8a, n d 8 . 7 9 x l 0 - s s - r ,r e s p e c t i v e l y .
(3 3) Reactionof the remainingbis(triisopropylphosphine)platinum(0)
with
I equiv of methyl iodide resultedin the immediateproductionof trans-(iP r r P ) 2 P t ( I ) C H 3m
: p 1 7 8 - 1 8 0o C d e c ;3 ' P { r H }N M R ( C 6 D 6 )6 2 8 ( t , " / n _ p=
) 0 . 7 ' 7( 3 H , t , / R _ H= 8 8 . 9H z , P I C H 3 ) ,1 . 2 3( 3 6
2 8 0 1H z ) ; ' H N M R ( C e D o 6
H , d o f d , / p - s = 1 3 . 7 H z . J = 6 . 8 H z , ( C I I 1 ) 2 C H P ) ,2 . 9 5 ( 6 H , m ,
( C H r ) 2 C l l P ) . A n a l . ( C r e H 4 5 l P 2 PCt ), H , I .
(34) Mann, B. E.; Musco,A. J. Chem.Soc.,Dalton Trans.1980,776-785.
3607
Thermal decompositionof a cyclohexanesolutionof 3 (0.024 M) and
t r i c y c l o h e x y ' l p h o s p h i(n0e. 0 2 8 M ) a t I l 6 o C a s d e s c r i b e da b o v ey i e l d e d
ft4b.d- 5.93 x l0-5 s-r, comparedto FM = 105 X l0-5 s-r in the absence
oi addedphosphine.
PhosphineExchangeof 2 with 2-(LD\2. A degassedcyclohexanesolution of 2 (86 mM), Z-(LD)z(86 mM), and triethyl phosphate(16 mM)
was heatedfor 8 h at 50 oC in a 5-mm NMR tube. Examination of the
r e s u l t i n gs o l u t i o nb y I ' P [ ' H ] N M R ( C e H , z , 1 0 1 . 2 8M H z ) r e v e a l e dt h e
=
p r e s e n c eo f t h e f o l l o w i n gc o m p o u n d s :2 , 6 3 3 . 4 5 ( t , l : 4 : 1 ,
"/p,_p l8l6
H z ) ; 2 - ( L D ) 2 , 6 3 2 . 1 6( t , l : 4 : 1 , J p r _ p= 1 8 2 0 H z ) ; 2 - L H L D , a : l . s + 1 d o f
t, "Iq-r = l8l8 Hz, Jp-p = 4 Hz). The doublet of triplets for LD of
}-LHLD could not be resolved from the signal for 2-(LD)z due to the
b r o a d n e s so f t h e p e a k . N o b i s ( t r i i s o p r o p y l p h o s p h i n e ) p l a t i n u m (w0a) s
observed,and heating the solution for an additional l6 h at 50 "C prod u c e d n o c h a n g ei n t h e d i s t r i b u t i o no f p r o d u c t s .
Registry No. l, 70620-74-5; 2, 81602-78-0;2-(LD)2, 8l 602-79-I ; 3,
8 I 6 0 2 - 8 0 - 4 ; t r i s ( i s o p r o p y l ) p h o s p h i n e6,4 76 - 3 6 - 4 ; l , I - d i m e t h y l c y c l o propane, I 6 30-94-0; neopentane,463-8 3- I ; 2,2,5,5-tet"ramethylhexa
ne,
l 0 7 l - 8 1 - 4 11 , 3 , 5 - c y c l o o c t a t r i e n1e8,7 1 - 5 2 - 9t;r i s ( i s o p r o p y l - d r , ) p h o s p h i n e ,
8 I 5 8 3 - 5 8 - 6 ; i s o p r o p y l - d 7b r o m i d e , 3 9 0 9 1 - 6 3 - 9 ; I , 5 - C O D , I I l - 7 8 - 4 ;
[ ( i - P r ) 3 P ] 2 P t6, 0 6 4 8 - 7I - 7 ; ( 1 , 5 - C O D ) P t [ C H 2 C ( C H 3 ) : ] r7, 5 I 0 I - I 9 - 8 ;
( C H 3 ) 2 C ( C H z D ) 2 ,7 2 1 1 0 - 0 9 - 9 ;( C H 3 ) 3 C ( C H r D ) , 4 7 4 1 - 9 4 - ! C y . P ' ,
2 6 2 2 - 1 4 - 2(;C y j P ) 2 P t ,5 5 6 6 4 - 3 3 - 0 .