3,3,4,4,5,5-Hexamethyl-1,2-bis(methylene)

3,3,4,4,5,5-Hexamethyl-1,2-bis(methylene)

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GCIT A21, 373-423
Vol. 121, No. 8, August, 1991
GAZZETTA
CHIMICA
. ^ j o
ITALIANA
An International Journal of Chemistry
Published by
Societä Chimica Italiana
Viale Liegi 48, 00198 Roma, Italy
_
5
6
Q
3
GAZZETTA CHIMICA IT ALIANA
an International J o u r n a l of Chemistry
published by
Societä C h i m i c a Italiana, V i a l e L i e g i 48, 00198 R o m a , Italy
Editor:
Fausto CALDERAZZO, Universitä d i Pisa
Editorial Board:
Angelo ALBERTI, C.N.R., I.Co.C.E.A., Ozzano E . , B o l o g n a
R o b e r t o AMBROSETTI, C.N.R., I.C.Q.E.M., Pisa
E n r i c o BACIOCCHI, Universitä d i R o m a
Paolo BELTRAME, Universitä d i M i l a n o
G i a n c a r l o BERTI, Universitä d i Pisa
C l a u d i o BIANCHINI, C.N.R., I.S.S.E.C.C., Firenze
Sergio CABANI, Universitä d i Pisa
L u i g i CASSAR, Italcementi S.p.A., Bergamo
M a r i o CIAMPOLINI, Universitä d i Firenze
P a o l o CORRADINI, Universitä d i N a p o l i
Alessandro DONDONI, Universitä d i Ferrara
M a r i o FARINA, Universitä d i M i l a n o
B a r b a r a FLORIS, II Universitä d i R o m a
M a r c o FoÄ, H i m o n t Italia, Centro Ricerche, Novara
Dante, GATTESCHI, Universitä d i Firenze
Alberto GHIRLANDO, Universitä d i P a r m a
M a u r o GRAZIANI, Universitä d i Trieste
G i n o LUCENTE, Universitä d i R o m a
C l a u d i o LUCHINAT, Universitä d i Bologna
Ugo MAZZUCATO, Universitä d i Perugia
M a r i o NARDELLI, Universitä d i P a r m a
G i a n l u c a NASINI, C.N.R., Centro S.S.O.N., P o l . d i M i l a n o
Piero PAOLETTI, Universitä d i Firenze
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Ugo ROMANO, E n i C h e m Synthesis, M i l a n o
Raffaello ROMEO, Universitä d i Messina
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V i t t o r i o CRESCENZI, Universitä d i R o m a
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Jean-Marie L E H N , Universite de Strasbourg
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Läszlö MARKO, H u n g a r i a n Acad. of Sciences Veszprem
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Ilya I. MOISEEV, Academy of Sciences, M o s c o w
Fernando MONTANARI, Universitä d i M i l a n o
H e i n r i c h N O E T H , Universität München a n d C h e m . B e r .
George A. OLAH, University of Southern California
Wolfgang VON PHILIPSBORN, Universität Zürich
Jacqueline SEYDEN-PENNE, Universite de Paris S u d
a n d B u l l S o c . C h i m . Fr.
M a l c o l m G . H . WALLBRIDGE, University of W a r w i c k a n d
/. C h e m . S o c , D a l t o n
Trans.
H e l m u t WERNER, Universität Würzburg
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G A Z Z E T T A C H I M I C A ITALIANA, Vol. 121, No. 8, August 1991
C O N T E N T S
Research Report
HERBERT
MAYR,
JANUSZ
BARAN a n d ULRICH W . H E I G L
3,3,4,4,5,5-hexamethyl-l,2-bis(methylene)cy-
clopentane: a novel
probe
for the study of cy-
cloaddition mechanisms
373
Articles
LUISA BENATI, PIER CARLO
Boron
MONTEVECCHI
SPAGNOLO
benzenesulphenanilide w i t h alkynes. A further
insight into the reactivity of the resulting t h i i r e n i u m
i o n intermediates
a n d PIERO
trifluoride-promoted
reaction
of
p-nitro-
383
ALBERTO ARNONE, GLANLUCA
Isolation and structure e l u c i d a t i o n o f d o r e m o n e A ,
NASINI, ORSO VAJNA DE PAVA
a
and LORENZO CAMARDA
from ammoniac g u m resin
387
ANGELO LIGUORI, GIOVANNI
ROMEO, GIOVANNI SINDONA
Competitive formation of tetrahydro-1,3-oxazines b y
i s o x a z o l i d i n i u m salts ring-opening r e a c t i o n
393
new
spiro-sesquiterpenoidic chroman-2,4-dione
a n d NICOLA U C C E L L A
SULTAN ABU-ORABI, ADNAN
Dipolar cycloaddition reactions of organic bis-azides
ATFAH,
w i t h some acetylenic Compounds
IBRAHIM
JIBRIL,
397
FAKHRI MARII a n d A M E R A L SHEIKH ALI
FABIO BENEDETTI, SILVANO
BOZZINI, SILVANA FATUTTA,
Reactivity of secondary f u n c t i o n a l i z e d
towards electrophilic diazenes
enamines
401
MIRELLA FORCHIASSIN, PATRIZIA N l T T I , G l U L I A N A P l T A C C O ,
a n d CLAUDIO RUSSO
LIBERATO CARDELLINI, L U C E -
Single electron - transfer. Reactions of i n d o l i n o n i c
DIO GRECI, J O H N M . T E D D E R
aminoxyls w i t h d i a z o n i u m salts
.
407
a n d JOHN C. W A L T O N
RINALDO POLI a n d B E T H E .
OWENS
Synthesis a n d structure of b i s ( l , 2 - b i s d i p h e n y l phosphinoethane)hexahalodimolybdenum(III),
M o X ( d p p e ) (X=C1, B r , I)
2
6
. 413
2
Communication
LORENZO DE NAPOLI, DANIELA
Synthesis
MONTESARCHIO,
GENNARO
cleotides
PlCCIALLI, ClRO
SANTACROCE
of cyclic
branched
oligodeoxyribonu.
419
a n d ANNA M E S S E R E
Additions and corrections
423
Gazzetta
373
C h i m i c a I t a l i a n a , 121, 1991 — P u b l i s h e d by Societä C h i m i c a I t a l i a n a
R E S E A R C H REPORT
3,3,4,4,5,5-HEXAMETHYL-1,2-BIS(METHYLENE)CYCLOPENTANE:
A NOVEL PROBE FOR T H E STUDY OF CYCLOADDITION MECHANISMS (*)
H E R B E R T M A Y R (°), JANUSZ BARAN a n d ULRICH W . H E I G L
I n s t i t u t für C h e m i e , M e d i z i n i s c h e Universität zu Lübeck, R a t z e b u r g e r Allee
160, D - 2 4 0 0 Lübeck 1, G e r m a n y
S u m m a r y — 3,3,4,4,5,5-hexamethyl-l,2-bis(methylene)cyclopentane, 1, w h i c h is readily accessible v i a
electrophilic a c y l a t i o n a n d a l k y l a t i o n reactions, incorporates a p l a n a r s-cis-fixed 1,3-diene system, the n o n t e r m i n a l positions of w h i c h are sterically shielded. Therefore, C o m p o u n d 1 s h o w s a relatively great tendency
to undergo 1,4-additions instead of 1,2-additions. W i t h d i h a l o c a r b e n e s , a 1,2 over 1,4 a d d u c t r a t i o of o n l y
2.3-2.7 is observed, a n d diphenylketene undergoes (4+2)-cycloadditions across the C C - a n d the C O - d o u b l e
b o n d . T h e r m a l , non-catalysed d i m e r i s a t i o n of 1 gives a m i x t u r e of [4+2]- a n d [4+4]-cyclodimer, b o t h p r o d u c t s
a r i s i n g t h r o u g h intermediate d i r a d i c a l s . T h e r e a c t i o n of 1 w i t h 1 , 3 - d i p h e n y l a z a l l y l l i t h i u m affords the [4+3]cycloadduct 12 as the m a i n p r o d u c t . B e n z o n i t r i l e oxide a n d 1 c o m b i n e w i t h f o r m a t i o n of the regulär [3+2]c y c l o a d d u c t 15 a n d the o x i m e 16, w h i c h are e x p l a i n e d t h r o u g h intermediate d i r a d i c a l s . C , N - D i p h e n y l n i t r o n e
reacts w i t h 1 a n d other 1,3-dienes to give the o r d i n a r y [3+2]-cycloadducts (e.g. 21, 22) as w e l l as the [4+3]cycloadducts 26 a n d 29-31, a g a i n i n d i c a t i n g the i n t e r m e d i a c y of d i r a d i c a l s . P o s s i b i l i t i e s to use 1 as a probe
for concertedness are discussed.
INTRODUCTION
SCHEME 2 -
1,3-DIENES U S U A L L Y
GIVE
1,2-ADDUCTS W I T H
K E T E N E S , A L L Y L A N I O N S , 1,3-DIPOLES A N D
A d d i t i o n reactions to 1,3-dienes c a n lead to the
formation of 1,2- and/or 1,4-adducts (scheme l ) .
CARBENES,
1,3-DIENES
*2
y
SCHEME 1
CX \
' R C = C = 0
2
+
A
2
— B
A
A
1,2-adduct
1,4-adduct
If A - B represents a cycloaddition partner, one of
the two reaction modes is usually highly preferred.
Dienophiles, for instance, like maleic anhydride,
acrylates, etc., undergo 1,4-additions a n d lead to the
formation of cyclohexenes (Diels-Alder reaction) .
Carbenes, ketenes, allyl a n d azallyl anions as w e l l as
1,3-dipoles a n d related reactants generally a d d to a
2 u n i t of the 1,3-diene , thus exhibiting a strong
preference for 1,2-addition (scheme 2). W h i l e 1,4additions of carbenes a n d ketenes are o r b i t a l
symmetry allowed processes, the corresponding
reactions of allyl anions, 1,3-dipoles, a n d 1,3-dienes
represent ( ^ s + TC4 ) processes, a n d are therefore
thermally forbidden reactions .
A s s u m i n g that less t h a n 0.5% of side products are
not usually detected d u r i n g conventional product
analyses, the failure to observe 1,4-adducts i n
reactions of 1,3-dienes w i t h the c y c l o a d d i t i o n
partners s h o w n i n scheme 2 indicates the activation
energies for the 1,4-additions to be at least 3 kcal
2
3
n
m o H higher t h a n those of the observable 1,2additions. It is of theoretical interest to learn whether
the barrier for the elusive 1,4-additions is just slightly
greater (-3-5 k c a l m o H ) t h a n that for the observed
1,2-additions o r whether there is a huge difference
between the activation energies of the two processes
(scheme 3).
s
3
(*) D e d i c a t e d to Professor Jürgen S a u e r o n the o c c a s i o n of his
60th b i r t h d a y . L e c t u r e presented at the VT Conference o n P r a c t i c e
a n d T h e o r y of P e r i c y c l i c R e a c t i o n s , F i r e n z e , Italy, M a y 24-25,1990.
(°) T o w h o m correspondence s h o u l d be addressed.
S C H E M E 3 - H O W B A D A R E 1,4-ADDITIONS? W H A T IS T H E B A R R I E R
1,4-ADDITIONS?
A E > 3 kcal mol"
a
1
FOR
374
H . Mayr,
J. B a r a n a n d U. W. H e i g l
There are two ways to improve the chance of
observing 1,4-additions: One can either make the 1,4additions more attractive by fixing the 1,3-diene i n
an s - c i s conformation, o r one c a n retard the 1,2additions by attaching bulky substituents at C-2 a n d
C-3 of the 1,3-diene (scheme 4).
not yield a n a d d i t i o n product (sterically shielded
double bond!), but w i t h excess b r o m i n e at r o o m
temperature the bisallyl b r o m i d e 3 is generated
selectively . Treatment w i t h m a g n e s i u m yields the
title C o m p o u n d 1 i n 6 2 % yield f r o m 2 .
7
5
SCHEME
- SYNTHESIS OF HEXAMETHYL-1,2-BIS(METHYLENE)CYCLO-
7
PENTANE
SCHEME 4
1,4-additions are favoured i n s-c/s-fixed dienes
cndocyclic
exocyclic
n = 1,2
n = 1,2,3
1,2-additions are retarded by b u l k y groups i n 2- a n d 3-position
3
The title C o m p o u n d 1 incorporates b o t h features,
and is, therefore, an ideal candidate for 1,4-additions
(scheme 5).
1
The structurally related 3,3,6,6-tetramethyl-l,2bis(methylene)cyclohexane c a n be synthesised i n
only 3 steps by the sequence s h o w n i n scheme 8 » .
5
9
SCHEME 8
SCHEME 5
Facile approach possible
(^4)
Sterically hindercd
( 2)
K
SYNTHESIS AND PROPERTIES OF T H E TITLE
COMPOUND
Several years ago we have elaborated a n efficient
access to highly substituted cyclopentenes v i a [3 +2]cycloaddition reactions (scheme 6). Hexa- to octamethyl-substituted cyclopentenes have been synthesised by the ZnCl -catalysed reaction of allyl chlorides
with alkenesl
+
The spectroscopic properties of 1 are not abn o r m a l . Its U V - m a x i m u m is almost identical to that
of the non-methylated
l,2-bis(methylene)cyclopentane, a n d the slight l o w e r i n g of the ionisation
Potential by the methyl groups c a n be attributed to
CC-hyperconjugation (scheme 9 ) > .
Analogous
effects by b r a n c h i n g i n allylic p o s i t i o n have been
observed i n the photoelectron spectra of acyclic
alkenes .
J 0
n
72
S C H E M E 9 - COMPARISON OF UV-
AND PE-SPECTROSCOPIC D A T A
7 0
'
7
7
2
S C H E M E 6 - C Y C L O P E N T Y L C A T I O N S via
[3 + 2 ] - C Y C L O A D D I T I O N S O F A L L Y L
+
CATIONS WITH A L K E N E S
W"m)
250
246
IP .i(eV)
8.73
8.4
V
This reaction type is the key-step i n the synthetic
sequence outlined i n scheme 7, w h i c h is selfexplanatory. Octamethylcyclopentene, 2, is the only
intermediate of this sequence, w h i c h has been
purified; it is obtained i n 5 8 % yield f r o m acetyl
chloride a n d trimethylethylene . A less favourable
access to the intermediate tetramethylallyl alcohol,
w h i c h we had elaborated i n the initial period of this
project^, has recently been published by L a m b e r t
and Z i e m n i c k a - M a r c h a n t . B r o m i n a t i o n of 2 does
Neat 1 c a n be stored for months at <5 °C i n a
nitrogen atmosphere. W h e n a drop of it is exposed
to atmospheric oxygen at ambient temperature for
24 h , crystals grow out of the l i q u i d w h i c h have been
identified as a n oligomeric peroxide w i t h H N M R
singlets at 6 0.88, 1.10 a n d 4.72 p p m , a n d C N M R
resonances at 8 21.27 (q), 24.72 (q), 46.69 (s), 49.45
(s), 67.83 (t) a n d 142.64 p p m (s) .
!
1 3
9
5
6
SCHEME
10
Study
375
of c y c l o a d d i t i o n m e c h a n i s m s
C o m p o u n d 1 undergoes n o r m a l Diels-Alder
reactions w i t h dienophiles. T h e relative reactivities,
given i n scheme 11, show that the m e t h y l groups i n
1 exert only a s m a l l influence o n the 1,4-reactivity
of l .
7 7
independent of the concentration of l . T h i s
Observation Supports the assertion that the 1,4adducts, like the 1,2-adducts, are produced v i a free
carbenes. I n accord w i t h this conclusion, s i m i l a r
product ratios were observed w h e n the dihalocarbenes were generated f r o m H C B r . W h e n 11,11dibromo-l,6-methano[10]-annulene was used as a
carbene precursor, 1 gave 57-65% of 1,4-adduct,
i n d i c a t i n g that n o w a complexed carbene was
reacting (scheme 13) .
3
SCHEME
11 - A T T A C K T O T H E T E R M I N A L
M E T H Y L E N E GROUPS
1,3-DIENE IS O N L Y S L I G H T L Y A F F E C T E D B Y T H E M E T H Y L
CH3O2C
OF T H E
SUBSTITUENTS
CO2CH3
6
CH3O2C—
CO2CH3
* /*CH =0.14
H
3
CH3O2C
CO2CH3
+
:CBr
2
*H/*CH = 9.1
3
3
H
CH3O2C
H
KETENES
CO2CH3
Ketenes react w i t h 1,3-dienes to give 3v i n y l c y c l o b u t a n o n e s ' . Stepwise [4+2]-cycloaddit i o n reactions across the C = 0 double b o n d take place
w h e n donor(alkoxy o r trimethylsiloxy)-substituted
1,3-dienes are c o m b i n e d w i t h alkyl-, aryl- o r haloketenes , a n d w h e n the electron-deficient bis(trifluoromethyl)ketene reacts w i t h buta-1,3-diene . O n l y
in reactions w i t h heterodienes (e.g. a,ß-unsaturated
ketones a n d imines), the CC-double b o n d of
diphenylketene h a d been reported to act as a
dienophile .
3
CARBENES
1,3-dienes usually react w i t h singlet carbenes i n
a 1,2-fashion to give v i n y l c y c l o p r o p a n e s . H o m o 1,4-additions to n o r b o r n a d i e n e a n d i n t r a m o l e c u l a r
1,4-additions i n the synthesis of benzvalenes are
among the few cases, w h i c h show a different
reactivity pattern. B i c k e l h a u p t observed 1-3% of 1,4adducts i n reactions of dichlorocarbene to 1,2bismethylenecycloalkanes (ring size 5-8) a n d 4-19%
of 1,4-adducts i n the corresponding reactions of
d i b r o m o c a r b e n e . Scheme 12 shows that the ratio
(l,4-/l,2-adducts) is considerably increased by
adding m e t h y l groups. A s s u m i n g the rate of the 1,4additions to be unaffected by the methyl groups, one
can derive that the methyl groups raise the barriers
for the 1,2-additions by 1.5-1.7 kcal m o H , w i t h the
consequence that 1 gives the highest p r o p o r t i o n of
1,4-adduct i n intermolecular carbene additions
reported u p to n o w .
73
74
75
78
79
20
27
SCHEME
14
76
77
S C H E M E 12 - C O M P E T I N G
1,2-
:CX,
A N D 1,4-ADDITIONS O F C A R B E N E S
x
x
A
A
cC -cxx
x = a
2
X = Br
4
Bickelhaupt
6
3
c = c=o
>
Ph
W h e n 1 is c o m b i n e d w i t h diphenylketene, the
[4+2]-cycloadducts 5 a n d 6 are produced i n a 1:1
ratio, a n d 6 represents the only cyclohexenone w h i c h
has been formed i n a Diels-Alder-reaction of a
ketene . Since cases w i t h concomitant formation of
vinylcyclobutanones a n d cyclohexenones are not
k n o w n , a n d kinetic data for the reaction described
in scheme 14 are not available either, the difference
of the activation energies of [2+2]- a n d [4+2]cycloadditions for reactions of ketenes w i t h ordinary
1,3-dienes cannot yet be estimated.
22
[4+4]-CYCLODIMERISATION OF 1
W h e n 1 is heated at temperatures above 80 ° C ,
d i m e r i s a t i o n takes place w i t h formation of the DielsA l d e r d i m e r 8 a n d the [4+4]-cyclodimer 9 . T h e
product ratio s h o w n i n scheme 15 reflects kinetic
c o n t r o l since b o t h 8 a n d 9 have been found to be
persistent at 150 ° C . W h i l e 8 m a y be produced b y a
concerted process ( ^ s +
o r by cyclisation of a n
intermediate (e.g. 7), o r b i t a l s y m m e t i y r u l e s require
a stepwise m e c h a n i s m to account for the formation
of 9.
23
et
al,
1985
L a m b e r t et a l investigated the reaction of 1 w i t h
C B r (from P h H g C B r ) at different diene c o n centrations a n d found the product ratio to be
2
Ph
3
376
SCHEME
H . Mayr,
J. B a r a n a n d U. W. H e i g l
SCHEME
15
16
2-azallyl a n i o n s u n d e r g o [3"+2]-cycloadditions
and isoprene
Dimcrization o f the Diene 1 in Toluene
w i t h butadiene
R
R
2.
R=
H.CH
N
H,0
Ph
„
H
3
Th. Kauffmann,
1971
[3~+2]-cycloadditions also w i t h s-cis-fixed dienes
#
But:
150 ° C
2.
H 0
2
Temperature Effect on Product Ratio ( 1 b a r )
T/°C
80
100
125
150
[8]/[9]
3.4
3.7
3.9
4.2
Pressure Effect on Product Ratio ( 7 9
p/bar
[8]/[9]
3.4
500
6000
3.1
3.5
N l T R I L E OXIDES
C)
9000
4.0
Since 1,3-dipoles incorporate the 4 electron - 3
centre 7t-orbital characteristic for allyl anions, the
isolation o f 12 encouraged us also to look for [4+3]cycloadducts w i t h 1,3-dipoles. N i t r i l e oxides have
first been selected, since their reaction w i t h arylacetylenes has been k n o w n to yield isoxazoles a n d
oximes c o n c o m i t a n t l y . The intermediate f o r m a t i o n
of diradicals o r zwitterions has been inferred f r o m
this O b s e r v a t i o n .
29
F r o m the product ratio determined at different
temperatures (scheme 15) one can calculate AS* for
the f o r m a t i o n of 8 to be 5 entropy units less negative
than A S * f o r the formation o f 9. Since concerted
processes are usually characterised by m o r e
negative
activation entropies than analogous
stepwise
m e c h a n i s m s , this finding is an argument for 8 being
formed t h r o u g h a n intermediate. F r o m the influence
of pressure o n rate a n d product ratio, activation
volumes o f - 1 5 . 8 (for 8) a n d -15.5 c m m o H (for 9)
have been determined. These values are to be
c o m p a r e d w i t h the reaction volume o f -51.2 c m
m o H for the formation of 8. Since i n ordinary DielsAlder-reactions, the activation volumes are closely
s i m i l a r to the reaction v o l u m e s , the strong
discrepancy between A V * a n d AV° indicates that 8
like 9 is p r o d u c e d by a stepwise pathway. I n analogy
to related s t u d i e s , the d i r a d i c a l 7 appears to be a
reasonable intermediate.
3037
SCHEME
17
e e
N
R - C E N - 0
NOH
+
24
3
3
25
26
2-AZALLYL ANIONS
H - C =C -
= C-Ar
Ar
W h e n benzonitrile oxide, 14, was generated f r o m
b e n z o h y d r o x a m o y l chloride a n d t r i m e t h y l a m i n e i n
diethyl e t h e r
i n the presence of diene 1, the [3+2]cycloadduct 15, the oxime 16, a n d the bisadduets
17 a n d 18 were p r o d u c e d . Evidence f o r the
f o r m a t i o n o f a [4+3]-cycloadduct has n o t been
obtained. As indicated i n scheme 18, treatment of
15 w i t h benzonitrile oxide, 14, affords 18 (not 17),
while 17 is p r o d u c e d f r o m the reaction o f 16 w i t h
14.
37a
32
SCHEME
The 1,3-diphenylazallylanion 10 has been reported
to react w i t h 1,3-butadiene a n d isoprene w i t h
exclusive f o r m a t i o n o f the 3-vinylpyrrolidine, 1 1 .
Analogous reactions w i t h 10 have also been observed
w i t h several 5-cis-fixed dienes (scheme \ 6 ) .
W h e n the sterically hindered diene 1 was treated
w i t h 1,3-diphenylazallyllithium, 10, Compound 12
was the o n l y eyeloadduet isolated after h y d r o l y s i s .
U n d e r c e r t a i n conditions, 12 is aecompanied by the
aeyclic adduet 13, suggesting a stepwise process
being responsible for the formation o f 12.
R - C - C
18
2 7
2 8
25
16
<>
7%
17
<»*>
(3*)
S t u d y of c y c l o a d d i t i o n
W h i l e the reactions of nitrile oxides w i t h aromatic
rc-systems have previously been reported to yield
s m a l l amounts of o x i m e s , Compounds 16 a n d 17
are the first oximes that have been p r o d u c e d f r o m
benzonitrile oxide 14 a n d a non-aromatic C C doublebonded dipolarophile. The analogous reaction of 14
w i t h the non-methylated bismethylenecyclopentane
4 proceeds w i t h exclusive f o r m a t i o n of the regulär
1,3-dipolar c y c l o a d d i t i o n product 19 (scheme 19) .
33
377
mechanisms
SCHEME
21
A s s u m p t i o n : o x i m e is f o r m e d v i a a d i r a d i c a l , i s o x a z o l i n e is f o r m e d
b y a concerted c y c l o a d d i t i o n pathway.
32
SCHEME
19
Ph
If one assumes the oxime 16 to be generated by
intramolecular
hydrogen
abstraction
in
an
intermediate diradical, the different behaviour of
methylated a n d non-methylated bis(methylene)cyclopentane (schemes 18 a n d 19) c a n be rationalised
in two ways.
SCHEME
20
A s s u m p t i o n : o x i m e a n d [3+2]-cycloadduct are f o r m e d t h r o u g h a n
intermediate diradical.
expected to affect only the barrier of
the
cycloaddition, thus giving rise to the f o r m a t i o n of
15 a n d 16. W h i l e m e t h y l Substitution affects the ratedetermining Step i n scheme 21, it only affects the
product-determining step i n scheme 20. Therefore,
kinetic experiments s h o u l d allow these
two
possibilities
to
be
differentiated. C o m p e t i t i o n
experiments (CC1 , 20.5 °C) showed that 1 is 26 times
less reactive t h a n the non-methylated C o m p o u n d 4.
T h i s value implies that the c y c l o a d d i t i o n of
benzonitrile oxide w i t h bis(methylene)cyclopentane
4 does not profit highly f r o m concertedness. If the
oxime 16 is produced t h r o u g h a n intermediate
diradical, the «energy of concert» for the f o r m a t i o n
of 19 must be less t h a n 3 kcal m o H .
4
34
SCHEME
22
Ph
l
One a s s u m p t i o n (scheme 20) is that b o t h products,
o x i m e a n d [3+2]-cycloadducts, are p r o d u c e d t h r o u g h
intermediate diradicals. If the barrier for cyclisation
is considerably lower t h a n the barrier for hydrogen
transfer i n the non-methylated C o m p o u n d , the
exclusive f o r m a t i o n of the cycloadduct takes place
(scheme 20, left). The methyl groups i n 1 s h o u l d
increase the barrier for cyclisation, w h i l e the b a r r i e r
for hydrogen transfer s h o u l d hardly be affected.
T h u s , the activation energies for the competing
reactions become similar, a n d a mixture of products
is formed (scheme 20, right).
A c c o r d i n g to the second a s s u m p t i o n (scheme 21)
o n l y the oxime 16 arises f r o m a diradical, w h i l e a
concerted c y c l o a d d i t i o n m e c h a n i s m accounts for the
f o r m a t i o n of the isoxazolines. N o w , the exclusive
f o r m a t i o n of a cycloadduct (scheme 21, left) w o u l d
be due to the fact that the activation energy for the
concerted cycloaddition reaction is considerably
l o w e r t h a n that for the f o r m a t i o n of the d i r a d i c a l .
I n t r o d u c t i o n of the methyl groups can n o w be
23
24
25
26 (21%)
27 (4%)
28 (-10%)
378
H.
Mayr,
J. B a r a n and U. W.
S C H E M E 24 - [ 4 + 3 J - C Y C L O A D D U C T S F R O M 20
NlTRONES
T h e reaction of C,N-diphenylnitrone, 20, w i t h 1
affords several types of products, as s h o w n i n scheme
2 2 > . F o r the formation of the spiranes 21 a n d 22,
a concerted cycloaddition m e c h a n i s m as well as a
stepwise pathway w i t h formation of a n intermediate
(e.g. 23) has to be considered. I n contrast, the
f o r m a t i o n o f the [4+3]-cycloadduct 26 b y a concerted
process is orbital symmetry forbidden . If a n
intermediate w i t h zwitterionic character were
involved, trapping w i t h the solvent ethanol s h o u l d
be p o s s i b l e . Since the yields of the cycloadducts
were very s i m i l a r i n benzene, toluene, d i m e t h y l
sulphoxide, acetonitrile a n d ethanol, we suggested
the
intermediacy
of the d i r a d i c a l 23. T h i s
intermediate may also account for the formation of
27 a n d 28 since intramolecular hydrogen transfer,
as discussed for the reactions of nitrile oxides, might
give the hydroxylamine 24, a potential precursor of
27 a n d 28.
Is the formation of a n intermediate d i r a d i c a l a
special property of o u r m o d e l C o m p o u n d 1, o r is the
i s o l a t i o n o f 26-28 a n i n d i c a t i o n that reactions of
nitrones w i t h 1,3-dienes generally involve d i r a d i c a l
intermediates?
3 5
Heigl
AND «NORMAL»
3 6
DIENES
29 (9%)
Ph
le
3
20
F h
I
51
O
>
N
" (
30 (3%)
Ph Ph
37
S C H E M E 23 - R E L A T I V E R A T E C O N S T A N T S F O R T H E R E A C T I O N S O F C , N D I P H E N Y L N I T R O N E W I T H 1,3-DIENES
Ph
Dipolarophile
e
o '
N
V
Ph
(Toluene, 80° C)
H
krel
AA<? / k c a l m o l
1.0
0.0
0.033
2.4
0.13
1.4
0.42
0.6
2
1989
C o n s i d e r i n g the small reactivity
difference
between 1 a n d 4 (scheme 23) a n d following the same
line of arguments employed for the discussion of
schemes 20 a n d 21, we came to the conclusion, that
cycloadditions of diphenylnitrone 20 w i t h ordinary
1,3-dienes also cannot profit highly from c o n certedness, i . e . , the appearance of intermediates has
to be generally considered. Therefore, a detailed
analysis o f the products formed from 20 a n d several
other 1,3-dienes has been carried out.
A p a r t from the n o r m a l [3+2]-cycloadducts, the
[4+3]-cycloadducts 29, 30 a n d 31 were formed i n
l o w yields from bis(methylene)cyclopentane, 4, 2phenyl-1,3 -butadiene, a n d 2,3 -dimethyl-1,3 -but a-
(3%)
diene, respectively. Since their f o r m a t i o n t h r o u g h
concerted mechanisms is o r b i t a l symmetry forbidden, the intermediacy of diradicals is a g a i n
deduced. The following section shows that these
intermediates are not thermally equilibrated.
W h e n the spirane 21 is heated at 100 °C,
decomposition w i t h f o r m a t i o n of unidentified h i g h
m o l e c u l a r weight products takes place. After 60 h ,
more t h a n half of the material is lost a n d o n l y 6 %
of the [4+3]-cycloadduct 26 is observable (expt. 1,
scheme 25). U n d e r the same conditions, 22, a
diastereomer of 21, selectively rearranges i n t o 26
(expt. 2, scheme 25). C o m p o u n d 26 is stable u n d e r
these conditions (expt. 3, scheme 25).
The b o t t o m block of scheme 25 shows that the
relative t h e r m o d y n a m i c stability of [3+2]- a n d [3+4]cycloadducts is reversed, w h e n R = H instead o f
R = C H . N o w , three quarters of 21' a n d 22' r e m a i n
unaffected w h e n heated at 100 °C for 7 1 h (expts.
4 a n d 5, scheme 25). I n contrast to 26, the
non-methylated [4+3]-cycloadduct 26' rearranges
into the spiranes 21' a n d 22' w i t h h i g h preference
for the latter stereoisomer, the one w h i c h is p r o d u
ced i n lower yield d u r i n g the cycloaddition (expt.
6, scheme 25). T h e greater rate of the 2 6 ^ 2 2 (and
2 6 ' ^ 22') isomerisations compared with the 2 6 ^ 2 1
(and 2 6 ' ^ 21') isomerisations clearly shows that the
intermediate
diradicals
are
not
thermally
equilibrated, i . e . , internal rotations are not fast
compared w i t h r a d i c a l combinations.
A rationalisation for the stereoselectivities o f the
rearrangements is given i n scheme 25. Let us assume
that cleavage of the C - 0 b o n d i n 21 a n d 22 is
associated w i t h a rotation of the planar nitroxide
fragment to give the diradicals 23a a n d 23b,
respectively, w i t h an^'-alignment of the t w o p h e n y l
groups. I n 23b, the nitroxide oxygen is close to the
C H - t e r m i n u s of the allylic radical, a n d c y c l i s a t i o n
to yield 26 v i a a boat-like transition State requires
only s m a l l geometric reorganisations. The conf o r m e r
23a, o n the other h a n d , has to undergo rotations
a r o u n d b o n d a o r b o n d b before cyclisation c a n give
26. These rotations are obviously slow, so that side
reactions are taking place a n d the rearrangement 21
—» 26 hardly takes place. T h e reverse order o f
arguments (principle of m i c r o s c o p i c reversibility)
c a n be used to explain the stereoselective
rearrangement 26' —» 22'.
3
1
C0 Et
Baran, Mayr,
H
2
379
S t u d y of c y c l o a d d i t i o n m e c h a n i s m s
S C H E M E 25
- THERMOLYSIS
OF T H E CYCLOADDUCTS
R
IN
TOLUENE
2
N-Ph
R = CH
3
21
6
0
12
a
40
Expt 2
a
0
88
1
95
a
R= H
b
Expt 4
b
Expt 5
Expt 6
b
C
22*
74
2
6
2
8
74°
7
20°
74
b) Composition after 71 h at 100° C;
SCHEME
Ph
CH
HjCC^C-
c'
•o
2
'CN
NC
Huisgen, Mioston, Langhals
Ph
.N .
CrC
R C
2
This
work
I n w h i c h cases c a n C o m p o u n d 1 be used as a
mechanistic probe? Scheme 11 has s h o w n that 1
shows a n o r m a l „ 4 g reactivity, i . e . , this C o m p o u n d
w i l l not exhibit special effects for reactions, w h i c h
n o r m a l l y take place i n 1- a n d 4 - p o s i t i o n of a 1,3diene. Let us, therefore, consider cycloadditions, for
w h i c h the simultaneous attack to positions 1 a n d 4
is o r b i t a l symmetry forbidden, a n d w h i c h usually
employ a 2 - u n i t of a 1,3-diene. W h e n we n o w
compare the reactivity of 1 a n d 4, the m e t h y l groups
t=0).
p r o d u c t s of the r e a c t i o n (a-
27
Methylation
affects rate
Methylation
affects products
Conclusion
a
no
no
None
b
no
yes
Stepwise mechanism with 4 and 1
c
yes
no
Concerted mechanism with 4 and 1
d
yes
yes
Concerted mechanism with 4, stepwise
mechanism with 1
26
Radical stabilising groups at
the intermediate's termini
and/or
at
Compare the reactivity of 4 and 1:
3 8
Ion stabilising groups at
the intermediate's termini
c) Starting material (=100
can influence rate
d, scheme 27).
Several new types of cycloadditions have been
realised by u s i n g the sterically hindered 1,3-diene as
a c y c l o a d d i t i o n partner. C o m p l e m e n t i n g Huisgen's
w o r k o n stepwise 1,3-dipolar cycloadditions v i a
zwitterionic intermediates (scheme 2 6 ) , we have
found that 1,3-dipolar cycloadditions may also o c c u r
stepwise, if the t e r m i n i of the potential intermediate
carry radical-stabilising groups.
2
0
C
26'
C
CONCLUSION
R C*
C
21'
a) Composition after 60 h at 100° C;
n
22
Expt l
Expt 3
SCHEME
26
CASE a
If Compounds 4 a n d 1 exclusively give 1,2-adducts
w i t h s i m i l a r rates, this may be due to a stepwise
m e c h a n i s m o r to a concerted m e c h a n i s m w i t h a n
early transition State, w h i c h does not experience a n
extra steric strain by the methyl groups. N o
mechanistic c o n c l u s i o n c a n be d r a w n f r o m the
c o m p a r i s o n of 4 a n d 1.
CASE b
If Compounds 4 a n d 1 give different products w i t h
s i m i l a r rates, one c a n conclude that the m e t h y l
groups only affect the product-determining step, not
the rate-determining step. T h i s w i l l be the case i f
b o t h dienes reacted t h r o u g h a n intermediate (scheme
28). However, the Observation of different p r o d u c t s
formed w i t h s i m i l a r rates is also compatible w i t h a
concerted c y c l o a d d i t i o n of 4 w h e n the energy of
380
H . M a y r , J. B a r a n a n d U. W. H e i g l
S C H E M E 28 - E N E R G Y P R O F I L E S F O R T H E R E A C T I O N S O F 4 ( L E F T ) A N D
1 ( R I G H T ) W I T H C Y C L O A D D I T I O N P A R T N E R S T H A T U S U A L L Y P R E F E R 1,2ATTACK
Case b
m a n y Systems w i l l not provide a clear yes/no answer.
W e believe, however, that a large r e a c t i v i t y
difference
b e t w e e n 1 a n d 4 is a r e l i a b l e i n d i c a t i o n t h a t a c o n c e r t e d
m e c h a n i s m is o p e r a t i n g w i t h o r d i n a r y d i e n e s .
It m a y seem, as i f a n y cycloaddend w i t h b u l k y
substituents at one e n d (e.g.
tert-butyl-substituted
ethylenes) c o u l d serve as a n analogous m e c h a n i s t i c
probe. T h i s is not the case, however, since o r d i n a r i l y
a strong r e d u c t i o n of rate caused b y a b u l k y
substituent m a y either indicate the increase of the
b a r r i e r of the concerted process o r signify that the
cyclisation of a reversibly p r o d u c e d intermediate is
slowed d o w n b y steric effects. T h e latter possibility
can be excluded i n reactions w i t h C o m p o u n d 1: I n
a potential intermediate, there is always one n o n shielded allylic p o s i t i o n (the t e r m i n a l C H - g r o u p ) ,
w h i c h c a n be attacked i n the c y c l i s a t i o n step. S i n c e
concerted 1,2-additions are also discussed i n several
hydrogenation a n d oxidation reactions, further areas
of a p p l i c a t i o n for 1 are conceivable. T h e convenient
access described i n scheme 7 encourages further
experiments.
2
Case c
We
thank W . Hellebrandt for experimental
assistance
a n d the Deutsche
Forschungsgemeinschaft a n d the F o n d s der C h e m i s c h e n Industrie
for f i n a n c i a l support.
Received February l l t h 1991
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