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(−31 to −37 e.u) than the measured values (−9 to −24 eu,
Tables 4 and 5). As a result, the computed free energy barriers
(ΔG⧧) tend to be dominated by their −TΔS⧧ components, and
are considerably larger than the experimental ΔG⧧ values. The
different physical phases used as references for the calculations
(idealized gas phase) and the experiments (condensed liquid
phase) undoubtedly play a role in the discrepancy. The
reduction in solution reaction entropies, relative to their
(hypothetical) gas phase values, is presumably related at least
partially to translational and rotational motions becoming more
restricted in the condensed phase.44 It appears from a simple
comparison of computed and calculated values that the
reduction in ΔS⧧ due to different reaction phases could amount
to ∼15−25 eu, but there does not appear to be a dependable
way of applying a “condensed phase” correction to the
computed results, and our current inability to rationalize the
variability and trends in the measured ΔS⧧ values further
complicates progress in this matter.
We have proposed that precoordination of the carbene to the
alkene may occur in some cases with the formation of weakly
bound precursor complexes, potentially promoting a change in
the carbene-alkene resting state and influencing both activation
energy and entropy parameters.21 Unfortunately, as yet we have
been unable to convincingly demonstrate the viability of this
proposal by experiment or by computation for the present
carbene-alkene sets, even though we have clearly documented
the formation of stable complexes from carbenes interacting
with, e.g., aromatic solvent molecules.45 However, we would
not anticipate significant complex formation to occur for a
highly stabilized carbene such as (MeO)2C.
We continue to actively investigate the fundamental reasons
for the discrepancies between measured and calculated
activation parameters in carbene reactions. Although the
present discussion has focused on computations using the
B3LYP set of hybrid functionals, the discrepancies between
computed and measured activation parameters remain when
other functionals21 or even wave function-based methods are
employed. In the Supporting Information (Table S-4) we
briefly present additional results for the activation parameters
pertaining to the (MeO)2C/ClACN cycloaddition reaction,
derived from computations with a number of different
exchange-correlation functionals as well as MP2 and CCSD(T)
techniques. Whereas the activation enthalpies obtained span
approximately a 10 kcal/mol range, the activation entropies
span a much narrower range of only 5 eu and are always much
more negative than the observed value. Surely, the potential
energy surfaces governing reactions of carbenes are exceedingly
intricate.46,47 Perhaps a resolution to most of the disagreements
can be found from detailed considerations of the dynamics in
these reactions47 and studies of reaction trajectories instead of
(or in addition to) classical transition state theory and potential
energy surface calculations.7
pattern and the noted discrepancies between computed and
measured activation parameters are currently lacking.
9. EXPERIMENTAL SECTION
Diazirines. Preparative details have been published in full for
diazirines 1−6. For 3,3-dichlorodiazirine (1), see ref 15b. For 3-chloro-
3-fluorodiazirine (2), see the Supporting Information of ref 16a. For
3,3-difluorodiazirine (3), see the Supporting Information of ref 17. For
3-chloro-3-methoxydiazirine (4), see refs 18 and 48. For 3-fluoro-3-
methoxydiazirine (5), see ref 19. For 3,3-dimethoxydiazirine (6), see
ref 49.
Activation Parameters. Activation parameters are collected in
Tables 3−5. The table notes provide references to the original data for
CCl2, ClCF, CF2, ClCOMe, and FCOMe. The (MeO)2C data are
described above.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures S-1−S-10, NMR spectrum of E,Z-8, Tables S-1−S-4,
computational details, geometries, and energies of relevant
species. This material is available free of charge via the Internet
AUTHOR INFORMATION
Corresponding Author
■
Present Addresses
†School of Chemistry & Chemical Engineering, Guangxi
University, 100 Daxue Road, Nanning, 530004, China.
‡Undergraduate exchange student from The University of
Manchester, Manchester, England.
ACKNOWLEDGMENTS
We are grateful to the National Science Foundation and to the
Petroleum Research Fund for financial support.
■
REFERENCES
■
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8. CONCLUSION
In general, over the six carbenes of Table 1 and the six alkenes
of Table 2, the evolutionary trends in Ea and ΔH⧧ expressed in
Tables 3−5 parallel expectations based on consideration of
ΔEstab, ΔεE, and ΔεN. However, the evolutionary behavior of
ΔS⧧ is neither as regular nor as predictable, resulting in some
disorder to otherwise anticipated trends in ΔG⧧. In fact, there is
some evidence of the operation of reciprocal behavior of ΔH⧧
and ΔS⧧. Compelling explanations for this counterintuitive
849
dx.doi.org/10.1021/jo2023558 | J. Org. Chem. 2012, 77, 843−850