Unique structural characteristics and reactivities caused by the less‐polar Si−C double bond in the 4‐silatriafulvene 1 were revealed by spectroscopic studies and X‐ray structural analysis. Compound 1 was isolated by applying a synthetic procedure using a sila‐Peterson‐type reaction.
The structures and reactions of 4-silatriafulvene were studied in
detail by ab initio molecular orbital
theory. At the equilibrium structure, 4-silatriafulvene shows
severe bending about the SiC double bond. A
planar structure having two conjugated double bonds (an SiC and a
CC double bond in the cyclopropenyl
moiety) is the transition state for flopping between the two degenerate
bent structures; the activation energy
is within 1 kcal/mol at MP2/6-311++G** + ZPE. It is
concluded that 4-silatriafulvene is balanced by two
stabilizing factors, aromaticity of the cyclopropenyl moiety and SiC
double-bond formation. The remarkably
low reactivity of a 4-silatriafulvene toward water that was found
experimentally is reproduced theoretically by
comparing the transition structures and activation energies with those
of silaethene + water. Intramolecular
isomerization from 4-silatriafulvene to silacyclobutadiene via
cyclopropenylsilylene is even competitive with
the water-addition reaction.
The structure and aromaticity of several 4-silatriafulvene derivatives are studied using correlated ab initio
MO calculations. Electronegative substituents on silicon have been found to stabilize a nonplanar structure
around the formal SiC double bond, while π-electron acceptors planarize the molecule. To assess the
aromaticity, the geometry index ΣCC, the nucleus-independent chemical shift (NICS), and the Bird indices
are employed. Appropriate substituents on 4-silatriafulvene to enhance the aromatic character have been shown
to cause the significant decrease of the SiC π-bond strength. In cyclopropenylidene-1-silacyclopentadiene
derivatives, both the three- and five-membered rings have low aromaticity, irrespective of the pyramidality
of the molecular skeleton. Remarkably high aromaticity of the three-membered rings have been found in
cyclopropenylidene-2-silaallene and 1-cyclopropenylidene-3-cyclopentadienylidene-2-silaallene.
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