salts B and C showed an interesting reversal in reactivity
dependent on the solvent employed in the reaction.10
In methanol, all catalysts (A-D) greatly favor the oxida-
tion pathway (producing 8a) as observed by GC analysis
(Figure 3). Reactions with nonprotic solvents (THF, CH2Cl2,
G(2df,p) single-point energy corrections.13 These species
were reoptimized in both MeOH and CH2Cl2 using the SM8
solvation model14 at the M06L/MIDI!(6D)/Auto level to
provide free energies of solvation. Composite energies were
determined as the gas-phase enthalpy plus the solvation
correction (Figure 4). All calculations were performed with
Figure 3. Dependence of oxidation process on solvent polarity.
PhMe) resulted in mixtures of both methyl butyrate from
the homoenolate pathway and methyl crotonate from the
redox pathway. As the polarity of the solvent decreases,
homoenolate products are increasingly favored, although the
variation is not substantial (e.g., entries 6-8 and 10-12 and
Figure 3). Degassing of the solvent to preclude oxidation
via an interaction of the intermediates with molecular oxygen
showed no significant effect on the ratios of 6a to 8a.
While the observed GC yields of methyl butyrate (6a) were
poor in all cases (<25%), the ratio of products (6a/8a) is the
critical data and indicates (a) which azolium structures favor
each product and (b) how solvent hydrogen-bonding capabil-
ity impacts the outcome.11 The difference in the observed
ratio of products originates from partial suppression of the
oxidation pathway in nonpolar solvents.
Figure 4. Calculated enthalpies for NHC-catalyzed pathways.
a locally modified version of Gaussian 03 (University of
Minnesota).15
Collectively, the experimental and computational data
indicate a weak solvent effect at play in the homoenolate
pathway, while the oxidation pathway is strongly disfavored
by nonpolar solvent. The importance of solvent polarity in
carbene-catalyzed Umpolung and oxidation reactions can be
described in relation to the mechanisms proposed in Figure
2. From the initial addition intermediate (2) proton transfer
For a systematic elucidation of the experimental results,
the competitive formation of 3 (∆H2) or 7 (∆H5) from 2
was probed for the carbenes of A and B using density
functional calculations.12 In the formation of 7, crotonalde-
hyde (1a) was used as the oxidant, giving 7 plus the alkoxide
of crotyl alcohol. A single explicit methanol molecule was
complexed with each reactant or product (placed at the most
favorable position) to mimic the likely solution environment.
Enthalpies at 298 K for 1, 2, 3, 7 and the alkoxide of crotyl
alcohol were determined in the gas phase using the M06L/
MIDI!(6D)/Auto level of theory, with M06-2X/6-311+
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