Synthesis and characterization of some amine complexes of bromocarboxyboranes and bromo(methoxycarbonyl)boranes

Abstract

Amine-carboxyboranes [A.BH 2COOH; A = Me 3N, Et 3N, quinuclidine (Q)] are readily decarbonylated with bromine in dichloromethane to produce amine-dibromoboranes (A.BHBr 2). The formation of A.BHBr 2 is explained by fast loss of CO from the acid bromides A.BH(Br)COBr generated because of the action of HBr. In proton-acceptor solvents (such as H 2O), however, only substitution takes place giving a chiral boron atom containing A.BH(Br)COOH and/or A.BBr 2COOH. Also, bromocarboxyborane complexes can be prepared easily using bromination with N-bromosuccinimide (NBS). The products are rather stable in water under acidic conditions but bases induce fast decarbonylation followed by complete decomposition. Amine(methoxycarbonyl)boranes A.BH 2COOMe (A = Me 3N, Et 3N and Q) are conveniently synthesized, with good yields, in methanol by treatment with a cation exchange resin (H +) as a catalyst. The bromo derivatives A.BH 2- nBr n COOMe ( n = 1, 2) have been prepared by treatment of amine complexes of BH 2COOMe with bromine or NBS, or by esterification of the bromocarboxylic derivatives. In addition, these bromo compounds can be readily obtained in a one-pot reaction from A.BH 2COOH with bromine; when esterification proceeds unexpectedly fast in parallel with the bromination. The structures of the new derivatives were substantiated by elemental analyses and IR, UV, 1H NMR and 11B NMR spectroscopic methods.