Fall Organic Chemistry Experiment #7

Transcription

Fall Organic Chemistry Experiment #7
Fall Organic Chemistry Experiment #7
Diels-Alder Reaction
Suggested Reading
Jones Section 12.12 (pages 587-598) and Section 20.4 (pages 1133-1139)
Introduction
Previously, you were introduced to the theory and practice of NMR spectroscopy. In this
experiment, you will be introduced to some of the more modern and advanced techniques
involving NMR that enable chemists to deduce the structure of more complex molecules. In
particular, these techniques are especially useful in the structural elucidation of natural products
and biological molecules.
You should already be familiar with standard one-dimensional (1D) proton and carbon
NMR spectroscopy. In today’s experiment, you will be introduced to three new techniques: (a)
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C DEPT spectroscopy; (b) two-dimensional (2D) COSY spectroscopy; and (c) 2D HETCOR
spectroscopy. DEPT stands for distortionless enhancement by polarization transfer. The DEPT
technique has been developed to distinguish among the different types of carbons in a molecule
(CH, CH2, CH3). In a DEPT spectrum there are two types of signals (upward pointing and
downward pointing). The upward pointing signals arise from either CH or CH3 carbons, whereas
the downward pointing signals arise from CH2 groups. A saturated carbon (one which does not
contain a hydrogen) does not show a signal in the DEPT spectrum. An example spectrum is
shown below.
Note that there are four signals in the fully decoupled 13C spectrum for 2-butanol (one signal for
each chemically nonequivalent carbon). In the DEPT spectrum for 2-butanol, there are three
upwardly pointing signals, two arising from CH3 groups and one from a CH group. The single
downwardly pointing signal (at 32 ppm) arises from the lone CH2 group. It should be apparent
that the DEPT technique provides an excellent method for the sorting of 13C signals into methyl,
methylene, methine, or quaternary carbons.
Pulsed NMR spectroscopy has enabled the chemist to devise experimental methods for the
solution of complex organic structures. Traditional one-dimensional techniques do not allow the
observer to discern complicate patterns when peak regions overlap. Such a situation normally
defies elucidation by the interpreter. However, two-dimensional NMR techniques have arisen to
allow for more definitive peak assignments and structural elucidation. A two-dimensional
spectrum has two chemical shift axes and a third axis corresponding to peak intensity (a 1D plot
has one shift axis and one intensity axis). The resultant spectrum when viewed from the side
appears as a “mountain range”. Normally, however, we view the spectrum from the top in a form
called a contour plot (see below). These contour plots (when correctly analyzed) show the
correlation between neighboring nuclei in the molecule. The most common method involves a
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H-1H shift correlation; that is, protons that are coupled to one another. This technique is called
“correlation spectroscopy” or COSY. An example of a COSY spectrum is shown below.
Remember, we view the plot from above (top view). Along the x-axis is a one-dimensional
proton NMR of ethyl vinyl ether. Along the y-axis is another one-dimensional proton NMR of
ethyl vinyl ether. All of the dots (peaks) that arise inside the box (contour plot) are significant
peaks that will yield important structural information.
Notice that some peaks lie along a diagonal while others are off of the diagonal. The
peaks along the diagonal simply represent the individual peaks one each 1D proton NMR. The
important peaks are the ones that are not along the diagonal. These peaks are the cross peaks.
The cross peaks arise from pairs of protons that are splitting one another (that is, they are coupled
or correlated). How do we use this information? The first thing we need to do is find the peaks
along the diagonal and draw a line through them. Then, we can start at any cross peak (let’s use
A) and draw a straight line vertically (up) to the diagonal and horizontally (over) to the diagonal.
Notice that we have intersected the diagonal at two different peaks (1.1 ppm and 3.8 ppm). The
protons giving rise to the peak at 1.1 ppm are from the terminal methyl group (labeled “a”). The
protons giving rise to the peak at 3.8 ppm are from the methylene group (labeled “b”). Therefore,
we can conclude that the cross peak A results from the correlation (coupling) of the methyl group
protons with the neighboring methylene protons (as expected). The other cross peaks can be
analyzed in a similar fashion.
A second 2D NMR technique that also involves nuclear correlation is HETCOR or
heteronuclear correlation spectroscopy. The appearance of a HETCOR spectrum is very similar
to the COSY spectrum. Each axis along the perimeter represents a 1D spectrum. However, in
HETCOR, one axis contains a 1D proton NMR and the other axis contains a 1D carbon NMR.
The resultant HETCOR spectrum, therefore, indicates the coupling between protons and the
carbon to which they are attached. For example, a HETCOR spectrum of 2-methyl-3-pentanone
is shown below.
Notice that a diagonal set of peaks does not exist in the example spectrum. A straight
line is drawn from each cross peak to the x and y-axes. Notice in our example that cross peak A
arises from the correlation of the carbon peak at 5 ppm on the x-axis and the proton peak at 0.9
ppm on the y-axis. We would conclude that the terminal methyl protons (of type “a”) are located
on the carbon at the 5 ppm chemical shift.
Certainly, each of these examples has not demonstrated the power of two-dimensional
techniques for solving structures. However, you would be hard pressed to solve the structure of
complex synthetic and biological molecules without employing one or both of these techniques.
In fact, you will likely find that these techniques will be very helpful in elucidating the structure
of your unknown in today’s experiment.
Chemistry
In 1927, Otto Diels and Kurt Alder obtained a compound from the reaction of eucalyptus
oil with maleic anhydride. Upon crystallization they described the product as forming "grosse
glasglanzende Krystalle von ungewohnlicher Schonheit (what’s the German translation here?).
Little did they know that their discovery would result in one of the most famous and widely used
organic reactions in history. The reaction is useful because of its high yield, production of sixmember rings, carbon-carbon bond forming potential, and stereospecificity. In 1950, they
received the Nobel Prize for their work.
The Diels-Alder reaction is an example of a cycloaddition reaction between two molecular
components: the diene and the dienophile. The diene consists of a molecule containing a
conjugated double bond system. The dienophile is typically an alkene. Therefore, a total of 6 pi
electrons are involved in the reaction mechanism --- 4 pi electrons from the diene and 2 pi
electrons from the dienophile. Thus, the Diels-Alder reaction is a [4+2] cycloaddition reaction.
An example is shown below.
Some very interesting (and, indeed, groundbreaking chemistry) followed the discovery by
Diels and Alder. In the 1960's, Robert Woodward (Harvard), Roald Hoffmann (Cornell), and
Kenichi Fukui (Kyoto) each proposed a molecular orbital explanation for the Diels-Alder
reaction. The resultant theory is called the "Conservation of Orbital Symmetry" or "Frontier
Orbital Theory". Again, the work resulted in all three men receiving the Nobel Prize in
chemistry.
Objective
In today's experiment, you will perform a recapitulation of the classic reaction by Diels
and Alder) in which a terpenoid (natural product essential oil) will be used as the diene and
maleic anhydride will function as the dienophile. There are many examples of essential oils from
natural products (e.g. eucalyptus oil) that contain conjugated diene or triene components.
Certainly, eucalyptus oil is not the only example of an essential oil containing a diene or a triene.
Other examples include:
limonene -- lemony odor in lemons, oranges, and other fruits
-myrcene -- fragrance and flavor of bay leaves (also present in hops)
-ocimene -- found in Javanese oil of basil
-phellandrene -- found in ginger grass, cinnamon, and star anise
-phellandrene -- found in lemon oil and peppermint oil
-terpinene -- found in the essential oils of cardamom, marjoram, and coriander.
As is typical of conjugated dienes from essential oils, they can be readily separated
because they have different chemical properties from the other constituents found in the oil. Our
objective is to form a Diels-Alder adduct that will separate from the mixture as a crystalline
product. You should be able to derive its structure from mp analysis, IR, 1H-NMR, 13C-NMR, and
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H-1H COSY.
Procedure
Physical properties addendum:
diene oil
MW = 136.2 g/mol
b.p. = 171-178 °C
d = 0.838 g/mL
melting points of maleic anhydride adducts of some common natural product dienes:
-myrcene
33-34 °C
-terpinene
60-61 °C
-phellandrene
126-127 °C
allo-ocimene
83-84 °C
Take a sample vial of one of the diene oils from the hood. Weigh the vial and using 10
mL of anhydrous diethyl ether transfer the oil from the vial to a 25 or 50 mL round-bottom flask.
Reweigh the vial and determine the mass of crude diene oil being used in the reaction. Determine
the mass and molar amount of diene in your sample of oil based upon the fact that the oil contains
50% of the diene component.
Calculate the amount of maleic anhydride required to react with the diene based upon the
fact that the diene and dienophile react in a 1:1 equivalency ratio. Determine the theoretical yield
of the Diels-Alder reaction based upon the calculated mass of starting diene. Add the maleic
anhydride to the flask and gently reflux for 45-60 minutes. While the flask is still warm, transfer
the reaction mixture to a small beaker. Cover the beaker with a watch glass and let it cool to
room temperature. Once at room temperature, transfer for further cooling to an ice bath.
Collect the adduct by vacuum filtration and wash the crystals on the filter paper with 10
mL of COLD petroleum ether. Recrystallize the adduct from dry methanol being sure to avoid
prolonged boiling. Dry the adduct and determine its mass, % yield, and melting point. Prepare a
100 mg sample for NMR analysis in the usual fashion using CDCl3/TMS as the NMR solvent.
Obtain the following spectroscopic data on your own:
IR (thin film)
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H-NMR
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C-NMR
Submit your NMR sample to Dr. Timm for:
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C-DEPT
H-1H COSY
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