Scheme 38. 4-Centered transition state showing the rearrangement of P-ketosilanes

Oxidative addition of the 16-electron manganese center on the silyl ether leads to silyl-manganese complex III. Attack on III by an incoming alcohol (ROH) leads to complex TV. Pseudorotation gives all four possible structures of IV. Complex IV, having a free proton, 

Pathway A is a dissociation of the silyl ketal from the metal center forming a metal-dihydrogen complex V and the unsymmetrical silyl ketal product. Once this product is formed, it cannot get reprotonated because the metal-dihydrogen complex V is not sufficiently acidic to reprotonate silyl ketal. Hydrogen gas then dissociates from the metal center aided by the silane to regenerate the catalyst (III). 

Pathways B and C describe the process that could occur to form the symmetrical silyl ketal. Formation of silyl ketal-metal complex VII could occur in one step via an associated pathway B, or in two steps via a dissociative pathway C. Pathway B could occur by attack on the pentacoordinated silyl ketal-metal complex IV by alcohol ROH, forcing the dissociation R OH leading to complex VII. The associative pathway would precede through a hexacoordinated silicon species. Hexacoordinated silicon compounds have been shown to be very reactive.36,23d The choice of the alcohol that dissociates would be determined by electronic and/or steric factors. 

All these processes, except for dissociation of the silyl ketal from the metal center, are presumed reversible. The dotted arrows signify the possible reactions of R OH that is present in low concentration. 

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Scheme 38. 4-Centered transition state showing the rearrangement of p-ketosilanes.

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