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Benjamin Sepe, Chem 213
Synthetic #1 FFR
A Diels Alder Puzzle
Introduction
The Diels-Alder reaction, detected in 1950 by Otto Diels and
Kurt Alder, is a
Nobel Chemistry prize-winning discovery that has been a source
of useful synthetic
products since its unearthing 62 years ago. The mechanism, often
referred to as
diene synthesis is very versatile, and the compounds it has
helped to develop
range from dyes and textiles to coatings.1One example of a
compound that the
Diels-Alder mechanism is responsible for synthesizing is
Chlordan, a strong
insecticide.1Another reason that this mechanism is such a
significant force in
chemical synthesis is its ability to create complex polycyclic
rings, synthetic
products that were almost impossible to produce easily before
1950.3One of the
particular reasons that this mechanism is of such great use to
synthetic chemists is
that is allows extremely complex molecules to be constructed
from relatively simple
starting materials and with careful consideration to
stereochemistry throughout the
duration of the process.3
This being said, an important governing principal of chemical
synthesis that
one must consider when attempting to understand the Diels-Alder
mechanism is the
difference between thermodynamic and kinetic control.5At their
most basic, these
controls are ways of determining the outcome of a particular
combination of
reactants. A kinetic product, for instance, is typically formed
as a result of lowest
activation energy in a reaction, whereas thermodynamic controls
are formed when
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enough energy is put into the system to over come the influence
of activation
energies and yield a different product.
In this experiment, Furan and Maleic Anhydride were synthesized
to one of
two cyclic adducts, either an exoor endoadduct, each with unique
properties. The
relative geometries of these two products can be understood from
Figure 1., below.
Figure 1.Endo/Exo adducts of Furan and Maleic Anhydride6
The mechanism, depicted below in Scheme 1, is simple. The
reaction is a one
step concerted reaction between the conjugated diene of Furan
and the double bond
on the ring of Maleic Anhydride, resulting in a polycyclic
adduct.2, 4 There are no
intermediates in this cyclic redistribution, however there is
still much discussion
about whether or not the reaction is synchronous.4This is to
say, that despite being
accepted as concerted, the equivalency of the new C-C bonds is
still undecided.
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SCHEME 1.Mechanism of Furan and Maleic Anhydride to Exo
Adduct
The Diels-Alder reaction is a simple reaction with numerous
applications in
industry. The purpose of this lab is to investigate the
Diels-Alder reaction as a way
of creating a polycyclic adduct from reagents Furan and Maleic
Anhydride.
Additionally, the reaction is an interesting expose into
relationship of endoversus
exoproduct formation and thermodynamic versus kinetic controls.
Moreover, upon
completion the product of this synthesis can be quantified and
analyzed using NMR
instrumentation.
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Experimental
Exo/Endo-Adduct.Maleic Anhydride (0.593 g, 3.57 mmol) and
Anhydrous Ether (5
mL) were added to a 10 mL Erlenmeyer flask. The solution was
warmed until Maleic
Anhydride completely dissolved. Upon returning to room
temperature, Furan (0.50
mL) was added. The Erlenmeyer flask was stoppered, sealed with
Parafilm, and
allowed to sit for 45 hours. Upon completion of the reaction,
the crystals were
collected and dried to yield a translucent crystalline product
(0.473 g, 47.07 %)
mp 110-115 oC; 1H NMR (60 MHz, CDCl3) (ppm) 6.575-6.544 (t, 2H),
5.459-4.425
(t, 2H), 3.153 (s, 2H); 1H NMR (400 MHz, CDCl3) (ppm) 6.5850 (s,
2H), 5.469 (s,
2H), 3.1846 (s, 2H); 13C NMR (400 MHz, CDCl3) (ppm) 169.80,
136.91, 136.42,
102.89, 102.87, 101.70, 101. 68, 99.91. 99.90, 98.11, 82.14,
77.26-76.62, 48.63.
Results and Discussion
The synthesis of the adduct product for this lab utilized the
Diels-Alder
mechanism on the conjugated pi bonds of Furan and the single
cyclic pi bond of
Maleic Anhydride to form a polycyclic compound. The
redistributed adduct
synthesized can be of either the endo or exo formation, as
determined by the
relative orientation of the Maleic Anhydride component to Furans
ether
component.
One interesting facet of these particular experimental reagents
is that they do
not follow the usual mechanism of the Diels-Alder reaction,
which typically favors
the kinetic endoadduct, instead opting for the less favorable
thermodynamic exo
adduct.2The reasoning for this unexpected product is based on
the low
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thermodynamic stability of Furan [4 + 2] adducts, which induces
a retro-Diels-Alder
reaction that opts for the more thermodynamically stable adduct
(exo) over the
normally synthesized kinetically stable adduct (endo).2 The
geometric instability
can be better understood by the relationship between these two
adducts seen in
Figure 1.
The specific adduct was identified by melting point analysis.
This is a useful
analysis in the identification of the product as the endo or exo
geometry because the
most significant difference between the two is their melting
point. The endo adduct
has melting point of 80-81 OC, and the exo adduct has a melting
point 114 OC. Upon
analysis, it was determined that the isolated product had a
melting point of 110-115
OC. This gave reasonable insight into the identity of the
isolated product as the exo
adduct, though further analysis was needed to identify the
structure of this
particular stereoisomer.
1H NMR further characterized the product. 1H NMR analysis is an
invaluable
tool in determining the proton arrangement of a compound under
question.
Specifically, when analyzing the NMR spectrum of the exo adduct,
a few important
qualities are notable that provide evidence of successful adduct
formation from
Furan and Maleic Anhydride, found in supplementary Figures 1
& 2. The first is a set
of two protons at 6.571 ppm, which splits into a triplet,
telling of the shifted single
alkene bond formed in the concerted reaction. The second sign
that this is the
appropriate product is the two protons on the ether from Furan
found in the
polycyclic compound, represented by a triplet at 5.453 ppm.
Lastly, potentially the
most important peak in the NMR spectra falls at 3.164 ppm with
an integration
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value of 2, and is clearly indicative of the protons at the
joining site of Furan and
Maleic Anhydride that are subjected to the ester from Maleic
Anhydride. It is
important to note that these peaks for all protons in the exo
adduct are subjected to
a strong downfield shift because of the inductive effect from
numerous
electronegative oxygen atoms in the compound. This is the reason
why some of the
peaks fall at higher ppm values than expected.
Another tool to characterize the structure of the product is 13C
NMR, similar
to 1HNMR, is used in determining the carbon skeleton of a
compound. The
conversion of Furans double bond to a single alkene bond is an
important part of
this particular Diels-Alder reaction. In Figure 3, this single
alkene is featured as the
strong peak at 136.9 ppm, within the typical range of 110-150
ppm for C=C bonds.
Another bond that is noticeable is the ester carbon-to-carbon
bond that shows up as
the peak at 169.8 ppm. Additionally, peaks at 82.1 and 77 ppm
represent ether
bonds. Lastly, the peak at 48.6 is indicative of the C-C bond
adjacent to the C=C of
the alkene discussed previously.
Melting point and NMR analysis confirmed the identity of my
product as the
exo adduct of Furan and Maleic Anhydride. The two forms of
analysis also indicated
that the products were relatively pure and contained
inconsequential amounts of
undesired starting material or impurity. The yield of the exo
adduct crystals was
47.07%. Some of the reasons that this product was held in only
about half he
expected yield include known issues in preparing this specific
Diels-Alder product,
or possibly even an insufficient amount of reaction time
allocated. These issues
could arise from common retro Diels-Alder reactions or the
inflexible 48 hours
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between lab periods, respectively. In the future, if given the
opportunity to repeat
this experiment, I would consider allowing the solution to
precipitate crystals for
longer than the time between lab periods.
The Diels-Alder reaction has been a useful synthetic tool since
it was
developed in 1950. Its applications transcend numerous fields
and products,
namely for its ease of synthesis and its ability to make
polycyclic rings. In this
experiment, Furan and Maleic Anhydride were fused to a
polycyclic exo adduct
product using the Diels-Alder reaction with relatively good
yield. MP and NMR
spectroscopy analyzed the products in order to confirm their
identities.
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References
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