Addition by Enolate Equivalents

Conditions for effecting conjugate addition of neutral enolate equivalents such as silyl enol ethers in the presence of Lewis acids have been developed and are called Mukaiyama-Michael reactions. Trimethylsilyl enol ethers can be caused to react with electrophilic alkenes by use of TiCl4. These reactions proceed rapidly even at -78° C.308 

Initial stereochemical studies suggested that the Mukaiyama-Michael reaction proceeds through an open TS, since there was a tendency to favor anti diastereoselec-tivity, regardless of the silyl enol ether configuration.312 

The stereoselectivity can be enhanced by addition of Ti(0-i-Pr)4. The active nucleophile under these conditions is expected to be an ate complex in which a much larger Ti(0- -Pr)4 group replaces Li+ as the Lewis acid.313 Under these conditions, the syn anti ratio is dependent on the stereochemistry of the enolate. 

Silyl acetals of thiol esters have also been studied. With TiCl4 as the Lewis acid, there is correspondence between the configuration of the silyl thioketene acetal and the adduct stereochemistry.314 L-Isomers show high anti selectivity, whereas Z-isomers are less selective. 

Fukuzumi, T. Okamoto, K. Yasui, T. Suenobu, S. Itoh, and J. Otera, Chem. Lett., 667 (1997). 

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