Mechanism and Selectivity of Methyl and Phenyl Migrations in Hypervalent Silylated Iminoquinones

Chlorosilanes R­(X)­(Y)­SiCl (R = Me,
Ph; X, Y = Me, Ph, Cl) have
been reported to react with Pb­(ONOQ)2 (ONOQ = 3,5-di-tert-butyl-1,2-quinone-(3,5-di-tert-butyl-2-oxy-1-phenyl)­imine) to give five-coordinate
(X)­(Y)­Si­(ON­[R]­O), in which the R group has migrated from silicon to
nitrogen. This migration is intramolecular, as confirmed by the lack
of crossover between (CH3)3SiCl and (CD3)3SiCl in their reaction with Pb­(ONOQ)2. Reaction of PhSiMeCl2 takes place with
high kinetic stereoselectivity to produce isomer Ph­(Cl)­Si­(ON­[Me]­O)
in which the phenyl is axial in the trigonal bipyramid, which subsequently
isomerizes to the thermodynamic isomer with axial chlorine. This indicates
that migration takes place preferentially from the stereoisomer of
the octahedral intermediate, κ3-Ph­(CH3)­(Cl)­Si­(ONOQ), in which the phenyl and methyl groups are
mutually trans, indicating that the observed complete selectivity
for methyl over phenyl migration is due to intrinsic differences in
migratory aptitude. DFT calculations suggest that migration takes
place from this isomer not because it undergoes migration faster than
other possible stereoisomers, but because it is formed most rapidly,
and migration occurs faster than isomerization.