posted on 2007-02-16, 00:00authored byChiong Teck Wong, Ming Wah Wong
Ab initio calculations (MP2/6-311+G**//B3LYP/6-31G*) were employed to investigate the mechanism
of metal chloride-promoted Mukaiyama aldol reaction between trihydrosilyl enol ether and formaldehyde.
The metal chlorides considered include TiCl4, BCl3, AlCl3, and GaCl3. In contrast to the concerted pathway
of the uncatalyzed aldol reaction, the Lewis acid-promoted reactions favor a stepwise mechanism. Three
possible stepwise pathways were located. The lowest energy pathway corresponds to a simultaneous
C−C bond formation and a chlorine atom shift in the first (rate-determining) step. This process is calculated
to have a low activation barrier of 12 kJ mol-1 for the TiCl4-promoted reaction. The alternative [2+2]
cycloaddition and direct carbon−carbon bond formation pathways are energetically competitive. BCl3,
AlCl3, and GaCl3 are predicted to be efficient catalysts for the silicon-directed aldol reaction as they
strongly activate the formaldehyde electrophile. Formation of a stable pretransition state intermolecular
π−π complex between enol silane and the activated formaldehyde (CH2O···MCln) is a key driving
force for the facile metal chloride-promoted reactions.