The reduction of α-silyloxy ketones using phenyldimethylsilyllithium

Phenyldimethylsilyllithium reacts with acyloin silyl ethers RCH(OSiMe3)COR8 to give regiodefined silyl enol ethers RCH=C(OSiMe2Ph)R9, and hence by hydrolysis ketones RCH(2)COR10, The yields can be high but are usually moderate. The mechanism of this reduction is established to involve a Brook rearrangement (Scheme 6) rather than a Peterson elimination (Scheme 1). Although the mechanism appears to be the same in each case, the stereochemistries of the silyl enol ethers 9 are opposite in sense in the aromatic series (R=Ph, Scheme 7) and the aliphatic series (R=cyclohexyl, Scheme 8), with the major aromatic silyl enol ether being the thermodynamically less stable isomer E-PhCH=C(OSiMe2Ph)Ph E-9aa, and the major aliphatic silyl enol ether being the thermodynamically more stable isomer Z-c-C6H11CH=C(OSiMe2Ph)-c-C6H11 Z-9ba. This is a consequence of anomalous anti-Felkin attack in the aromatic series. The reaction with the silyl ether Bu'CH(OSiMe3)COPh 13b is normal in giving Z-Bu'CH=C(OSiMe2Ph)Ph Z-38 (Scheme 11), but reduction of the silyl ether 8a with lithium aluminium hydride is also anti-Felkin giving with high selectivity the meso diol PhCH(OH)CH(OH)Ph39, The reaction between phenyldimethylsilyllithium and the acyloin silyl ether 8d (R=Bu') does not give the ketone Bu'CH2COBu', but gives instead the anti-Felkin meso diol Bu'CHOHCHOHBu'40 also with high selectivity (Scheme 12). Silyllithium and some related reagents react with trifluoromethyl ketones 46 and 48 to give alpha,alpha-difluoro silyl enol ethers 47 and 49 (Scheme 14).