1.3- Silyl migrations configuration

Evidence for the mainly SN2 pathway for the silyl migration was obtained from reactions of enantioenriched mesylates (Eq. 9.153). The connfigurations of the alle-nylboronate intermediates were deduced from their reactions with cyclohexanecar-boxaldehyde to afford the anti products of known configuration (Eq. 9.154). It is assumed that these reactions proced by way of a cyclic transition state. 

Since the ring was formed by silyl migration, we believe the configuration with respect to the P=C double bond did not change. This assumption is supported by the lp nmr data described in the next section. 

The 1,3-silyl migrations from C to C occurred with inversion of configuration at silicon and with a rather high activation enthalpy for the 1,3-silyl migration of a-methylallyltrimethylsilane studied by Kwart and Slutsky, AH =47.7 kcalmol-1 and AS = —6.2 calmol-1 K-1297. On the other hand, 1,3-silyl migrations in ketosilanes... 

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... 

In the presence of catalytic amounts of Pd(0), silicon-substituted vinyloxiranes can rearrange into the corresponding ot-silyl- 3,y-unsaturated aldehydes (Scheme 9.34) [151]. Treatment of 80 with Pd(OAc)2 and P(OPh)3 results in the formation of 7t-allylpalladium complex 81. Bond rotation to give 82, followed by migration of the silyl moiety, affords aldehyde 83, which is trapped in situ to provide the Felkin-Anh product 84. The reaction proceeds with retention of configuration and the ee of the starting material is retained in the product. The size of the silicon substituents is critical for the outcome of the reaction, as is the choice of ligands on palladium. 

The hydrogen migration in the second step will not change the configuration of the chiral silyl ligand. 

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