Imines reactions with allyl organometallic reagents

The purpose of this review is to provide a summary (through to the end of 1988) of the uncatalyzed reactions of type I and type III allyl organometallics with C X electrophiles. Most of the examples involve aldehydes and ketones, but the reactions of allyl organometallics with imines are also covered. Because the focus of this review is on selectivity and synthetic efficiency, this review is not intended to be as comprehensive as an Organic Reactions chapter or a Chemical Reviews article. Rather, we have attempted to define and illustrate the factors that influence stereoselectivity, to provide access to the most pertinent literature, and, most importantly, to provide a basis for selection of an allyl organometallic reagent for application in specific synthetic problems. 

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. 

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. 

This book chapter is limited to Lewis acid-mediated reactions, and does not discuss the important field of Lewis base-mediated allylations, nor does it describe the reactions of allylsilanes with other electrophiles such as epoxides, imines, and allyl-X (X = -Cl, -OR, -OAc). The SaJcurai reaction has been covered under different forms in reviews focusing on The Stereochemistry of the Sakurai reaction , Intramolecular Addition Reactions of Allylic and Propargylic Silanes ," Selective Reactions Using Allylic Metals , Catalytic Enantioselective Addition of Allylic Organometallic Reagents to Aldehydes and Ketones , and Modem Carbonyl Chemistry . ... 

Nitrones have a more reactive C=N bond toward nucleophilic addition compared to imines. In spite of this fact, there have been only a limited number of studies on the nucleophilic addition reactions of nitrones, particularly organometallic reagents.352-355 During the last decade, research related to reactions of nitrones with zinc-containing reagents was essentially focused on (i) dialkylzinc-assisted alkynylations356-358 and vinylations359 of nitrones, (ii) catalytic asymmetric nucleophilic additions to the C=N bond,360-364 and (iii) nitrone allylations by allylzinc halides.365,366... 

Initially, we have considered the A -aluminum imines which may be obtained by DIBAH reduction of nitriles at -78 C in THF. Treatment of the aluminum imines thus obtained with 2 equivalents of alkyl or allyl lithium and benzyl and allyl magnesium chloride at room temperature fw 48 hours leads, after hydrolysis, to the expected primary amines CTable 4). The reaction appears to be quite general. In fact, aromatic as well as olizable aliphatic aluminum imines react with a variety of organometallic reagents to afford good to racellent yields of primary amines after hydrolysis. The stoichiometry of the... 

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