Kaimakliotis and Fry
that the increase in HOMO energy arises by interaction
between the filled carbon-silicon σ-bond and the nearby
π-system. We argue here for a similar interaction, but
involving the rear lobe of the C-Si σ bond. There is
precedent, both experimental and theoretical, for this
hypothesis. Shiner and co-workers have reported a silicon
γ-effect upon solvolysis of alkyl sulfonates, in which
sulfonates bearing a trialkylsilyl group at the γ-position
ionize substantially faster that those lacking this sub-
stituent.21-23 This neighboring group effect exhibits a
pronounced stereoelectronic preference, which is opposite
to that expected in the absence of participation by
silicon: the equatorial cis-brosylate 10 ionizes 89 times
faster than the axial trans-isomer 11 in 2,2,2-trifluoro-
ethanol,21 whereas equatorial sulfonates ionize 3-4 times
slower than equatorial sulfonates.24 High-level ab initio
computations showed that stabilization of the incipient
positive charge in such ionizations is maximal when the
developing p-orbital is aligned collinearly with the rear
lobe of the carbon-silicon bond.23
F IGURE 4. (a) Black triangles: HOMO energy of 1-phenyl-
2-trimethylsilylethane (4) computed at the AM1 level as a
function of Ψ, the C3-C2-C1-Si dihedral angle. (b) Open
diamonds: total energy of 4 computed at the AM1 level as a
function of Ψ, the C3-C2-C1-Si dihedral angle. The dihedral
angle Φ is fixed at 90° throughout.
(b) the phenyl and silyl groups were shown to be anti (Ψ
) 180°) and the side chain is perpendicular to the
benzene ring (Φ ) 90°) in both compounds. AM1 calcula-
tions were used to explore the conformational surface of
4. This was not feasible with 3d because of its larger
number of rotatable bonds, but its conformational prop-
erties had been explored in detail previously by molecular
mechanics.15 Taken together, the molecular mechanics,
AM1, and ab initio computations point to two clear
conclusions: 4 and 3d exist, at a conservative estimate,
close to 90% in the anti conformation and this conforma-
tion has an ionization potential that is lower by about
4.3 kcal (0.187 eV) than that of the other low-energy
conformation. We then ask what property of the anti
conformation accounts for the relative ease with which
it gives up an electron. A significant part of the answer
to this question can be seen in Figure 4: the HOMO
orbital of 4 is highest in energy when the silicon atom is
anti to the benzene ring, i.e., in the molecule’s most
highly populated conformation. This conclusion in itself
simply begs the question. Rather, it is more appropriate
to ask what it is specifically about the anti conformation
(Ψ ) 180°) that drives up the energy of the HOMO and
thus allows it to give up an electron more readily than
for any other value of Ψ. We believe that interaction
between the highest filled orbital of the benzene ring (via
C3, the ipso carbon) and the rear lobe of the carbon-
silicon bond is the cause of the increase in the HOMO
energy. This is illustrated in structure 9. We refer to this
phenomenon as the “anodic γ-aryl effect” although similar
situations should arise with other appropriately oriented
π and silicon groups, even when the groups are separated
by a longer chain or where a π-system other than benzene
is located at the γ-position.
Mech a n ism of F or m a tion of Tr iflu or oa cetoxy
Ester 5. Formation of rearranged ester 5 is excellent
evidence for initial generation of a cation (14) R to the
ester carbonyl group. By analogy to mechanisms previ-
ously postulated for the anodic oxidation of silanes of type
1a and 1b,1,2,6,18,25 we suggest the following sequence of
events for the anodic conversion of 3d to 5. Initial
removal of an electron from 3d by the electrode takes
place from conformation A to afford a radical cation (3d +•
)
in which most of the charge is carried by the benzene
ring, particularly by the ipso carbon (C3); electron density
is supplied by the carbon-silicon bond via overlap of its
rear lobe with the π-system. Nucleophilic attack upon the
silicon atom of the radical cation by solvent (probably by
an addition-elimination mechanism) would afford radi-
cal 13; anodic oxidation of 13 at the very positive
electrode potential being employed would then afford 14.
Rearrangement of 14 to 15 by hydride migration and
(19) Spartan ‘02 for Macintosh; Wave Function, Inc.: Irvine, CA.
(20) Bernardi, F.; Bottoni, A.; Garavelli, M. Quant. Struct.-Act.
Relat. 2002, 21, 128.
(21) Shiner, V. J ., J r.; Ensinger, M. W.; Kriz, G. S.; Halley, K. A. J .
Org. Chem. 1990, 55, 653-661.
Effects analogous to this have been reported previ-
ously. Yoshida has observed that in silanes in which one
carbon atom separates the π-system and the silicon atom,
e.g., benzyl and allyl silanes, the HOMO energy is highest
and ionization potential lowest when the carbon-silicon
bond is perpendicular to the plane of the π-system, i.e.,
parallel to the p-orbitals of the π-system.18 It is believed
(22) (a) Ensinger, M. W.; Shiner, V. J ., J r. Stud. Org. Chem.
(Amsterdam) 1987, 31, 41-58. (b) Adcock, W.; Coope, J .; Shiner, V.
J ., J r.; Trout, N. A. J . Org. Chem. 1990, 55, 1411-1412.
(23) Davidson, E. R.; V. J . Shiner, J . J . Am. Chem. Soc. 1986, 108,
3135-3137.
(24) Winstein, S.; Holness, N. J . J . Am. Chem. Soc. 1955, 77, 5562.
9896 J . Org. Chem., Vol. 68, No. 26, 2003