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

Amine-carboxyboranes [A.BH₂COOH; A = Me₃N, Et₃N, quinuclidine (Q)] are readily decarbonylated with bromine in dichloromethane to produce amine-dibromoboranes (A.BHBr₂). The formation of A.BHBr₂ 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 H₂O), however, only substitution takes place giving a chiral boron atom containing A.BH(Br)COOH and/or A.BBr₂COOH. 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.BH₂COOMe (A = Me₃N, Et₃N 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.BH₂-nBrⁿ COOMe (_n = 1, 2) have been prepared by treatment of amine complexes of BH₂COOMe 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.BH₂COOH 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.