Borane and alane reductions of bulky N,N′-diaryl-1,3-diimines: structural characterization of products and intermediates in the diastereoselective synthesis of 1,3-diamines

Abstract

Two new C₂-symmetric diazaborinanes were prepared by diastereoselective intramolecular dihydroboration of bulky 1,3-diamines, the remarkably stable l-[HB(2,6-Prⁱ₂-C₆H₃NCHMe)₂CMe₂], from which it was not possible to isolate free diimine, and the less bulky l-[HB(2-Prⁱ-C₆H₄NCHMe)₂CMe₂], which yielded l-(2-Prⁱ-C₆H₄NHCHMe)₂CMe₂ on acid work up. The BH₃ reductions were highly diastereoselective for l-products (de > 95%). Use of AlCl₃/LiAlH₄ mixtures in diethyl ether gave lower (de ≈ 75%) and opposite selectivity, yielding predominantly u-(2,6-Prⁱ₂-C₆H₃NHCHMe)₂CMe₂ upon work up, via a u-[H₂Al(2,6-Prⁱ₂-C₆H₃)NHCHMeCMe₂CHMeN(2,6-Prⁱ₂-C₆H₃)] intermediate in a two-step reduction. All products were characterized crystallographically.