Chiral reactions with allyl organometallics

Some examples of combined syn-anti and diastereofacial selectivity employing /V-n-propyl- and N-isopropyl-aldimines, derived from a-phenylpropionaldehyde, and crotyl-9-BBN, -magnesium and -zirconium reagents have been reported by Yamamoto et al. Cram selectivity, which is observed in the analogous reactions of these chiral imines with allyl organometallics (see Section 4.3.2.1.2i), is preserved as ratios of Cram anti-Cram products are consistently about 8 1. Anti selectivity is also observed but the ratios do not exceed 7 3. The weak anti selectivity parallels that observed in reactions of crotyl-9-BBN with branched a-alkylaldimines. Since syn-anti selectivity is influenced more by the a-substituent than by the A-substituent of the aldimine, more synthetically useful levels of combined syn-anti and diastereofacial selectivity might be expected in other series of a-substituted aldimines. 

Table 9.6 Reaction of chiral iminoesters with allylic organometallic compoimds [19] Allylmetal...

Further reactions of allyl organometallics with a-alkoxyaldimines 1, prepared from (S)-2-(methoxymcthoxy)propionaldehyde and (R)- and (S)-l-phenylethylamine, illuminate the difference in the influence of the nitrogen chiral auxiliary and the x-alkoxy center7. 

The reaction of allyl organometallics (8) with achiral aldimines (9) is the simplest combination of reactants, stereochemically, and affords (except in the case of formaldehyde imines) homoallylamines (10) containing one stereocenter (equation 2). These reactions, which typify the general scope of allyl organo-metallic-imine reactions are surveyed in Table 2. Reactions of allyl-borane, -titanium and -aluminum reagents are not shown in Table 2, only because they have been reported in reactions with chiral imines and are discussed in Sections 4.3.2.1.2 and 4.3.2.2. 

As the results show, the chirality of the a-alkoxy center, as well as the type of allyl metal employed, are the two most important determinants for the stereochemical outcome of the reaction. In other words, the 1,2-asymmetric induction combined with the right choice of the allyl organometallic overrides the influence of the chiral nitrogen substituent. 

Allylic organometallics modified at the metal center by chiral adjuvants add to aldehydes and ketones to provide optically active homoallylic alcohols. This process has been described for reagents containing boron, tin and chromium metal centers. Gore and coworkers have shown that a chromium-mediated addition reaction of allylic bromides to simple aldehydes that uses a complex of lithium N-methyl-norephedrine and chromium(Il) chloride occurs with modest (6-16% ee) enantioselectivity (equation 61, Table 8). 

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