TRANSITION-STATE OF THE SILICON-DIRECTED ALDOL REACTION - AN AB-INITIO MOLECULAR-ORBITAL STUDY

The transition state structure for the silicon-directed aldol reaction between the enol silane 1 and formaldehyde producing 3-(silyloxy)prop anal (2) is located at the MP2/6-31G level of theory. A boatlike six- membered ring with a pentavalent trigonal bipyramid silicon species is described. The formaldehyde oxygen occupies the apical position with the Si-O bond length of 2.01 Angstrom. The enol silane oxygen assumes the equatorial orientation with the Si-O bond distance of 1.81 Angstro m. The calculated Si-O bond length is 1.69 Angstrom in the starting en ol silane 1 and 1.68 Angstrom in the product 2. The distance between t he bond-forming sp(2) carbons of 1 and formaldehyde is 2.07 Angstrom i n the transition state. A secondary kinetic isotope effect of k(H)/k(D ) = 0.80 was calculated, which is in excellent agreement with the expe rimental value of 0.76.