Organoboranes. 36. A Simple, General Synthesis of Alkynyl Ketones 1 and [E]‐1,3‐Alkenynes via Thexylchloroborane‐Dimethyl Sulfide

Thexylchloroborane‐dimethyl sulfide ( 1 ) hydroborates representative 1‐bromo‐1‐alkynes in the presence of 10% boron tribromide as a catalyst to afford thexyl‐( cis ‐1‐bromo‐1‐alkenyl)‐chloroboranes ( 2 ). Treatment of these intermediates with isopropyl alcohol provides the corresponding isopropyl thexyl( cis ‐1‐bromo‐1‐alkenyl)borinates ( 3B ). Subsequent treatment with alkynyllithium gives the corresponding “ate” complexes ( 4 ) which undergo intramolecular migration of the alkynyl group from boron to the adjacent carbon, displacing the bromine, producing the corresponding cis ‐vinylboranes ( 5 ). These intermediates ( 5 ) provide the corresponding alkynyl ketones ( 6 ) on oxidation and the corresponding stereodefined conjugated enynes ( 7 ) of high isomeric purity on protonolysis. Yields in the range of 60–65% were realized.