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

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(almost equal)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.