Asymmetric Allylation with the CAB Catalyst

The CAB-catalyzed reactions of aldehydes with allyl- and crotylsilanes are summarized in Table 11-28 [288, 309], These data show that the CAB-catalyzed crotylation reactions with crotylsilanes 123b and 460 are highly diastereo- and en-antioselective, while the corresponding allylation reactions with allylsilanes 131 occur with somewhat lower enantioselectivity. 

Marshall subsequently demonstrated that the efficiency of the crotylation reactions catalyzed by CAB catalyst 459b could be improved by using the more reactive crotylstannanes and employing two equivalents of (CF3C0)20 to aid catalyst turnover [310]. In a comparative study of the catalytic asymmetric allylation and crotylation reactions of cyclohexane carboxaldehyde with allyl- and crotylstannanes 98 and 10, Marshall demonstrated the complementarity between the BINOL-Ti(0-i-Pr)4 catalyst 451 and the CAB catalyst 459b (Table 11-29). 

In a demonstration of the synthetic utility of the CAB-catalyzed crotylation reaction, Marshall synthesized the commonly used polypropionate adducts 106 and 

107 (Eqs. (11.48) and (11.49)) [310]. While adduct ent-l06c, obtained with 98 2 diastereo-selectivity, results from the matched crotylation of ent-91c with crotyl-stannane 10, the anti,syn adduct 107c, obtained in 90 10 selectivity, is the result of a mismatched double asymmetric crotylation reaction. However, with these aldehydes, a stoichiometric amount of the CAB catalyst is necessary for synthetic efficiency. 

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