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

Abstract

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. Copyright © 1984 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim