Mukaiyama-Michael reactions mechanisms

In SiCl4-mediated Mukaiyama-Michael reactions, an electron-transfer mechanism is proposed for the case in which ketene silyl acetals bearing less hindered silyl substituent are used as substrates.342-344 As shown in Scheme 82, ketene silyl acetals having more substituents at the /3-position are much more reactive. 

The mechanistically quite similar double Michael reaction of 2-silyloxy dienes and enones has been shown recently to go via a similar two-step mechanism rather than a Diels-Alder cycloaddition S. Kobayashi, Y. Sagawa, H. Akamatsu and T. Mukaiyama, Chem. Lett., 1988, 1777. 

Mukaiyama variant of the Michael reaction and Michael additions of 1,3-diketones to 2-oxo-3-butenoate esters. However, these examples have always involved activation of bidentate electrophiles by Cu(II) followed by the addition of a weak nucleophile to the resultant complex. The attempts to employ bis(oxazoline)copper(II) complexes to catalyze a classical Michael reaction with p-ketoesters and monodentate enones are precedented however, racemic products were obtained in such cases. Interestingly, the Michael reaction developed in our studies is most likely to proceed via reversed activation (Scheme 11). Thus, we proposed that Cu(II) complex H chelates the enol form of p-ketoester 12a, and the resultant chiral enol complex 22 undergoes addition to electrophile (5) to provide 23. It should be noted that the precise mechanism of this reaction and particularly the step for the addition of 22 to 5 to provide 23 are yet to be investigated. 

Metal compounds possessing Lewis acid character are often used in the Michael addition reaction, and the methodology is reasonably applied to the asymmetric reaction in the presence of chiral ligands. The mechanism could involve either purely Lewis acidic activation of the Michael acceptor or generation of new orga-nometallic species by the transmetalation or C-H activation, although they were not clear in many cases. The system of Sn(OTf)2 and chiral (S)-diamine developed by Mukaiyama promoted the asymmetric addition of trimethylsilyl enethi-olate 68 to P-arylvinyl ketones (Scheme 13) [70, 71]. The diamine-coordinated tin enolate was considered to be involved, and slow addition of 68 was essential to inhibit the racemate formation process. 

A possible mechanism for the catalytic [2+2] cycloaddition reaction catalyzed by Tf2NH is depicted in Scheme 4.9. The Mukaiyama-type Michael addition of silyl enol ether to enoate catalyzed by silyl triflic imide aHbrds the corresponding silyl ketene acetal, and then it proceeds successively to the intramolecular silyl oxonium carbon to... 

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