Michael addition closed transition state model

Compared to the aldol addition, the stereochemical scheme is complicated by the fact that the Michael acceptor may not always and not exclusively adopt the -configuration as shown in 421 but also as Z-diastereomer. The effect of this isomerism has been addressed in a fundamental contribution of Corey and Peterson, which is also one of the first applications of an auxiliary-based stereoselective Michael addition. The chiral lithium enolate 425 that was generated from the propionic ester 424 of phenylmenthol by deprotonation was assumed to adopt the enolate in fcr /is-configuration, in accordance with Ireland s model (cf Section 2.1). The reaction of the enolate with ( )- and (Z)-methyl crotonate led to the Michael products sy/i-426 and a f/-427, respectively. The Michael addition to ( )-crotonate was faster at low temperatures than that of the (Z)-diastereomer and provided higher chemical yields as well as syw-anti-selectivity and induced stereoselectivity. A closed, eight-membered transition state model 428 has been proposed that plausibly explains the opposite stereochemical outcome depending on the double-bond configuration of the Michael acceptor. As the rear side is shielded by the bulky 2-phenyl-2-propyl substituent, the attack of both croto-nates occurs at the Si-face of the enolate 425. Whereas Si-face of ( )-crotonate is selected for the addition of the enolate, the attack to (Z)-crotonate occurs predominantly from the e-face (Scheme 4.92) [206]. 

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