β-Silylated Homopropargylic Amines via the Asymmetric Allenylboration of Aldimines

The asymmetric synthesis of α-trimethylsilylpropargylic carbamines (7) through the addition of allenylboranes 4 to N-H aldimines is reported. The insertion of TMSCHN2 into enantiomerically pure B-alkynyl-10-TMS-9-borabicyclo[3.3.2]decanes 3 followed by a sterically driven 1,3-suprafacial borotropic shift proceeds with complete stereospecificity to produce 4 in diastereomerically and enantiomerically pure form. These reagents give 7 (51−85%, syn/anti >99%, 92−9% ee) permitting the recovery of 8 (53−63%). Allenylboranes 4 also provide a convenient route to optically pure allenylsilanes 13 (55−94%) through their protonolysis.