Asymmetric Synthesis of Isomerically Pure Allenyl Boranes from Alkynyl Boranes through a 1,2‐Insertion–1,3‐Borotropic Rearrangement

Razzle-dazzle…︁ and allenyl boranes 1 are obtained in enantiomerically pure form from 2 through a novel CHTMS insertion followed by a sterically driven suprafacial 1,3-borotropic rearrangement. Asymmetric allenylboration with 1 gives syn β-trimethylsilyl (TMS) homopropargylic alcohols 3 in 80–96 % yield with d.r. 99:1 and 94–99 % ee. When R=isopropenyl, allylboration occurs to provide the 1,2,3-trienyl alcohol 4 with >99 % ee.