1.3- Silyl migrations in allylsilanes

This type of thermal 1,3-silyl migration is mechanistically very interesting, because two typical intramolecular 1,3-silyl migrations in allylsilanes 190 (equation 118) and /3-ketosilanes 191 (equation 119) were shown to occur concertedly but with different stereochemistry at the silicon center1. 

Two transition structures with a retention (192, TSret) and an inversion (193, TSinv) configuration (Figure 2) were optimized for 1,3-silyl migration in allylsilane at HF/6-31G, MP2/6-31G and DFT/6-31G levels. The TSjnV 193 was found to be a distorted trigonal bipyramid (TBP) around the silicon with the two allylic carbons at the equatorial positions different from the TS illustrated by Kwart and Slutsky297,302, while 192 has a distorted square pyramid (SP) structure around silicon. Analysis of the orbital interaction in the transition states showed that the major stabilization of 193 was caused by the MO interaction as predicted by the Woodward-Hoffmann rules, while the major stabilization in 192 was ascribed to the subjacent orbital control. 192 was more stable than 193 at... 

Comparison of the retention transition states for 1,3-silyl migrations in allylsilane and formylmethylsilane... 

Remarkable differences in the retention transition structures between the 1,3-silyl migrations in formyhnethylsilane and the 1,3-silyl migrations in allylsilane are revealed by detailed analysis of geometries, natural bond orbitals, and the Laplacian of the electron density. The presence of BCPs and RCPs was monitored and some typical properties were evaluated at the BCPs. 

Allylsilanes 12 react with a-ketoesters 174 to give [3 + 2] cycloadducts 175 in 50-85% yields through 1,2-silyl migration (equation 73)59,228. Use of an allylic trimethylsilane produces an allyl alcohol by-product through a competing silyl elimination process. It can be circumvented by use of a bulkier silicon-containing reagent, such as the... 

The [3 + 2] reaction of an allylsilane with an enone was proposed to proceed via a regiospecific electrophilic substitution (cf 80) of the enone at C-3 of the allylsilane followed by a cationic 1,2-silyl migration (equation 57). At the yyw-clinal transition state (81), the carbonyl group of the unsaturated ketone was assumed to occupy an endo orientation in relation to the allylsilane142. The transition state was thought to be favorable due to minimization of the charge separation and to possible secondary orbital stabilization. 

Another approach leading to pyrrolidine 210 via [3 + 2] cycloaddition is the reaction of A-protected a-amino aldehydes with allylsilanes in the presence of a catalytic amount of BF3 OEt2 (equation 172). No 1,2-silyl group migration occurs in these annulation processes313. 

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