Allyl additions allylborane reagents

Carbonyl Allylation and Propargylation. Boron complex (8), derived from the bis(tosylamide) compound (3), transmeta-lates allylstannanes to form allylboranes (eq 12). The allylboranes can be combined without isolation with aldehydes at —78°C to afford homoallylic alcohols with high enantioselectivity (eq 13). On the basis of a single reported example, reagent control might be expected to overcome substrate control in additions to aldehydes containing an adjacent asymmetric center. The sulfonamide can be recovered by precipitation with diethyl ether during aqueous workup. Ease of preparation and recovery of the chiral controller makes this method one of the more useful available for allylation reactions. 

The first example of enantioselective allylation of an azomethine function was reported in 1995 by Itsuno and co-workers [42a]. These researchers studied the addition of preformed chirally modified allylboranes to N -(trimethylsilyl)ben-zaldehyde imine (5a) (<2 g of imine, ca. 0.27 M). Of the wide range of chirally modified allylboron reagents reported in the literature, the use of chiral allylbo-ronates 42a-c and 5-allyldialkylborane 43 were logical first choices given their utility in the enantioselective addition to carbonyl substrates (Scheme 20). 

A particularly useful borane is the chiral reagent (-)-B-allyl(diisopinocampheyl)borane.l 3 Although the chemistry of acyl addition to aldehydes and ketones to give alcohols will not be formally discussed until Section 8.4.C, the addition of this chiral allylborane to carbonyl compounds will be presented here. In a simple example taken from Smith s synthesis of (-)-macrolactin A,n 156 was treated with (-)-B-... 

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