Transmetalation silyl enol ether formation

If the electrophile is a vinyl triflate, it is essential to add LiCl to the reaction so that the chloride may displace triflate from the palladium o-complex. Transmetallation takes place with chloride on palladium but not with triflate. This famous example illustrates the similar regioselectivity of enol triflate formation from ketones to that of silyl enol ether formation discussed in chapter 3. Kinetic conditions give the less 198 and thermodynamic conditions the more highly substituted 195 triflate. 

Preparation of enones from saturated ketones by Pd(II)-promoted dehydrosilyla-tion via silyl enol ethers was reported by Ito. Transmetallation of the silyl enol ether of cyclohexanone 519 with Pd(OAe)2 gives the oxo-TT-allylpalladium complex 520 (Pd enolate), which undergoes -H elimination to afford cyclohexenone. BQ is used as an oxidant of Pd(0) [211], However, the enone formation can be carried out using a catalytic amount of Pd(OAc)2 in DMSO under oxygen without other oxidants at room temperature. Also aldehyde 521 is converted to unsaturated aldehyde 522 via silyl ether in DMSO [4],... 

The cleavage of silyl enol ethers by Lewis acidic metal salts may also be categorized as a transmetallation. Kuwajima and coworkers have shown that silyl enol ethers react with titanium tetrachloride under formation of the corresponding titanium enolates 174, which were characterized by NMR spectroscopy (Scheme 2.51). Whereas (Z)-enolsilanes of acyclic ketones retain their configuration in the titanium enolates, the ( )-silyl enol ethers lead to a mixture of... 

Mukaiyama aldol reactions of silyl enol ethers are generally rationalized by a Lewis acid activation of the carbonyl group by in situ formation of a complex that reacts with the silyl enol ether or the silyl ketene acetal [99,167]. Transmetallation mechanisms according to which silicon is replaced under formation of a metal enolate have been discussed as well for catalytic versions of the reaction [168], in particular for late transition metals [169]. 

---