Borane with benzyl bromide

The asymmetric arylation or alkylation of racemic secondary phosphines catalyzed by chiral Lewis acids in many cases led to the formation of enantiomerically enriched tertiary phosphines [120-129]. Chiral complexes of ruthenium, platinum, and palladium were used. For example, chiral complex Pt(Me-Duphos)(Ph)Br catalyzed asymmetric alkylation of secondary phosphines by various RCH2X (X=C1, Br, I) compounds with formation of tertiary phosphines (or their boranes) 200 in good yields and with 50-93% ee [121]. The enantioselective alkylation of secondary phosphines 201 with benzyl halogenides catalyzed by complexes [RuH (/-Pr-PHOX 203)2] led to the formation of tertiary phosphines 202 with 57-95% ee [123, 125]. Catalyst [(R)-Difluorophos 204)(dmpe]Ru(H)][BPh4] was effective at asymmetric alkylation of secmidaiy phosphines with benzyl bromides, whereas (R)-MeOBiPHEP 205/dmpe was more effective in the case of benzyl chlorides (Schemes 65, 66, and 67) [125—127]. 

Benzylic halides are reduced very easily using complex hydrides. In a-chloroethylbenzene lithium aluminium deuteride replaced the benzylic chlorine by deuterium with inversion of configuration (optical purity 79%) [537]. Borane replaced chlorine and bromine in chloro- and bromodiphenylme-thane, chlorine in chlorotriphenylmethane and bromine in benzyl bromide by hydrogen in 90-96% yields. Benzyl chloride, however, was not reduced [5iSj. Benzylic chlorine and bromine in a jy/n-triazine derivative were hydrogeno-lyzed by sodium iodide in acetic acid in 55% and 89% yields, respectively [5i9]. 

Alcohols are converted to alkyl iodides by reaction with A A-diethyl-aniline/borane/iodine. Reductive iodination is observed when this system reacts with ketones or with carboxylic acids934,935. Direct reduction of aromatic aldehydes to benzyl bromide is reported by Le Corre and coworkers, who have brominated benzaldehydes and acetophenones in presence of a trimethylamine/borane complex and have obtained benzyl bromides and a-bromoethylbenzenes, respectively936. See also Reference937. 

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