Synthesis of a Nonracemic C₂‑Symmetric Tetrahydro-1,4-azaborine and Evaluation of Hydroboration Enantioselectivity

Tetrahydro-1,4-azaborines were accessed by hydroboration of N,N-diprenyl­toluene­sulfonamide 4. Conversion to the methylborinates 11 and 12 followed by heating with l-alanine and crystallization afforded (R,R,S)-13 (27%). Reduction of borinic acid (R,R)-18 with Soderquist’s KH* gave (R,R)-19, and hydride abstraction by TMSCl in the presence of alkenes resulted in hydroboration, 84–86% ee for (Z)-alkenes, but (E)-alkenes or 1,1-disubstituted alkenes gave <5% ee.