Azetidine–Borane Complexes: Synthesis, Reactivity, and Stereoselective Functionalization

The present study reports, for the first time, the synthesis and structural features of azetidine-borane complexes, as well as their reactivity in lithiation reactions. A temperature-dependent stereoselectivity has been disclosed in the reaction of borane with N-alkyl-2-arylazetidines, allowing for a stereoselective preparation of azetidine-borane complexes 2 and 3. A regioselective hydrogen/lithium permutation, at the benzylic position, was observed in lithiation reactions of complexes possessing a syn relationship, between the ring proton and the BH3 group. In contrast, scarce or no reactivity was noticed in complexes lacking such a stereochemical requirement. The configurational stability of the lithiated intermediates has also been investigated, in order to shed some light on the stereoselectivity of the lithiation/electrophile trapping sequence. Calculations helped in supporting experimental observations, concerning structure and reactivity of these azetidine-borane complexes. Data suggest that the BH3 group could promote the lithiation reaction likely by an electrostatic complex induced proximity effect. Interestingly, a new synthetic strategy for the synthesis of N-alkyl-2,2-disubstituted azetidines has been developed.