Correlations between Carbene and Carbenium Stability: Ab Initio Calculations on Substituted Phenylcarbenes, Nonbenzenoid Arylcarbenes, Heteroatom-Substituted Carbenes, and the Corresponding Carbocations and Hydrogenation Products†

Краткое содержание

Introduction
Carbenes, including the relatively stable heteroatom carbenes,
are important objects of study from both theoretical
and practical points of view. The chemistry of singlet
carbenes ranges from the electrophilic to the nucleophilic
side of the electronic supply and-demand spectrum1 and
includes recent applications as ligands in organometallic
catalysis of a number of useful synthetic processes,2 of which
† This paper is dedicated to the memory of Yvonne Chiang Kresge
(1) Jones, M., Jr.; Moss, R. A. In Reactive Intermediate Chemistry; Moss,
R. A., Platz,M. S., Jones, M., Jr., Eds.; Wiley-Interscience: New York, 2004;
Chapter 7.
(2) (a) Herrmann, W. A.; K€ocher, C. Angew. Chem., Int. Ed. Engl. 1997,
36, 2162. (b) Warkentin, J. In Advances in Carbene Chemisty; Brinker, U. H.,
Ed.; JAI Press Inc.: Stamford, CT, 1998; Vol. 2, pp 245-295. (c) Arduengo,
A. J.III. Acc. Chem. Res. 1999, 32, 913. (d) Herrmann, W. A. Angew. Chem.,
Int. Ed. 2000, 41, 1290. (e) Bourissou, D.; Guerret, O.; Gabbaie, F. P.;
Bertrand, G. Chem. Rev. 2000, 100, 39. (f ) Alder, R. W. In Carbene
Chemistry; Bertrand, G., Ed.; FontisMedia S. A.; Marcel Dekker, Inc.;
New York, 2002; pp 153-176.
Downloaded by AUSTRIA CONSORTIA on July 6, 2009
Published on June 17, 2009 on http://pubs.acs.org | doi: 10.1021/jo9006464
B J. Org. Chem. Vol. XXX, No. XX, XXXX
JOCArticle Gronert et al.
C-C bond-forming cross-coupling reactions provide an
example.3a A very recent application of N-heterocyclic
carbenes is their function as stabilizing electron-donating
ligands in the formation of a metalloaromatic, octahedral
array of six gallium atoms.3b Protonation at the central
carbon of a singlet carbene gives another important reactive
intermediate, a carbenium ion. The ease of the reverse
process, loss of a proton from a carbenium center, corresponds
to the “R-acidity” of the carbenium ion, a variant of
the well established “β-acidity” where loss of a proton
fu
ishes an alkene. Factors affecting the stability of carbenium
ions can also affect that of singlet carbenes, notably the
electron-donating ability of attached groups.1
In a previous study, experimental and computational
work were combined to explore structural effects on the
R-acidity of carbenium ions, including methyl+, other small
alkyl+, benzyl+, and methoxylated carbocations.4 Equilibrium
constants for hydrolysis of the carbocations (pKR),
hydration of the singlet carbenes (pKH2O), and the R-acidity
of the carbocations (pKa) appropriate to the aqueous phase
were obtained. From a linear plot of pKH2O vs pKR the
effects of structural changes on carbene and carbocation
stability were compared. Similarly, computed values for the
hydride ion affinities of the carbocations (HIAs) and heats of
hydrogenation of the singlet carbenes (ΔHHYDROG) allowed
the same comparison to be made for the gas phase. Although
the slopes and intercepts of the two plots were quite different,
the quality of the correlations was similar and both showed
sharp deviations of the points for the methoxylated compounds,
which indicated that the attachedCH3Ogroup has a
larger stabilizing effect on the carbene than on the carbocation
compared with attached H, alkyl, and phenyl groups.
We conjectured that this result is a general one for groups
possessing an electron pair on the atom directly attached to
the carbene or carbenium center. In this work, we look at all
such row 2 and row 3 heteroatom substituents. To establish a
baseline measure of electronic effects, we have extended our
study to include 23 meta- and para-substituted phenylcarbenes
and benzyl cations. Additionally, we examine three
nonbenzenoid, aryl substituents: cycloheptatrienyl+, cyclopentadienyl
-, and cyclopropenyl+ fu
ishing carbenes 1-3.
We note at the outset the work of Geise and Hadad who
reported a DFT computational study of the singlet-triplet
gap for phenylcarbene and a variety of ortho-, meta-, and
para-substituted phenylcarbenes.5 The same group has also
examined substituent and solvent effects, experimentally
and by computation, on the singlet-triplet gap for a series
of phenylcarbomethoxycarbenes6 and 2-naphthylcarbomethoxycarbenes.
7 Also relevant is a recent DFT study in
which steric factors affecting the singlet-triplet gap for
mono- and diarylcarbenes were investigated.8
Methods and Results
All structures were built and optimized at HF/3-21 or HF/
6-31G(d) levels using the MacSpartan Plus software package.
9 Conformational preferences were established at these
or higher levels, including G3MP2. All geometry optimizations
were completed using the G3MP2 method. The
GAUSSIAN 03 quantum mechanical package was used for
all the higher level calculations.10 All structures reported
here represent electronic energy minima except for structures
identified as transition states, each of which has one imaginary
frequency.
G3MP2 enthalpies and free energies at 298 K for the
substituted phenyl compounds, in hartrees, are tabulated
in the Supporting Information, Tables S1 (meta compounds)
and S2 (para compounds). These tables also list the calculated
point groups for all compounds. Reaction enthalpies
and free energies are found in Tables 1 (meta) and 2 (para).
The corresponding data for other sets, including the nonbenzenoid
aryl compounds, are found in Table S3 and Table
S4, and the heteroatom carbene data are in Table S4 and
Table S5.
Substituted Phenyl Compounds: Geometries. In the carbene
structures, the carbenic C-H bond is invariably coplanar
with the ring to within 0.5
, usually to within 0.1
. It was
necessary to investigate alte
ate conformations for a number
of the carbenes because the rotational orientation of the
substituent group relative to the carbenic C-H bond is a
variable. For all of the meta carbenes, the C-H bond is
directed away from the substituent. However, there are other
rotational possibilities for some of the substituents. We
report in the tables the results only for the most stable of
the calculated conformers. Conformational preferences and
other geometric features found by us agree well with those of
Geise and Hadad.5 Conformational enthalpy differences are
small, less than 0.2 kcal/mol.