Synthesis of B- and P-Heterocycles by Reaction of Cyclic Acetals and Ketals with Borinium and Phosphonium Ions.

Tricoordinated cyclic boron cations result from gas-phase ion/molecule reactions of dicoordinated borinium ions with neutral acetals and ketals and thiazolidine. The reaction, which proceeds via initial cationic binding to a heteroatom followed by a consecutive ring-opening and ring-reclosing process, resembles the Eberlin transacetalization of acylium ions (Eberlin, M. N.; Cooks, R. G. Org. Mass Spectrom. 1993, 28, 679). The cyclic structure of the tricoordinated boron cation is demonstrated by tandem mass spectrometry and further confirmed by comparison with authentic cyclic tricoordinated boron cations. The five-membered cyclic boron cations dissociate by ethylene oxide loss to thus reform the reactant-dicoordinated borinium ion; the six-membered boron cations fragment instead by ethylene loss. Consistent with the proposed mechanism, the ion/molecule reaction efficiency falls in the order CH3OB+C2H5 > CH3OB+OCH3 ≫ CH3B+CH3; i.e., the higher the nucleophilicity of the borinium ion, the higher the reacti...