Vinylic Organoboranes. 5. An Improved, Convenient Synthesis of Unsymmetrical Alkynes via Iodination of Lithium Alkynyl "Ate" Complexes of Thexylalkylborinates

Article abstract of DOI:10.1021/jo00374a006

The transfer reaction induced by iodination of lithium alkynyl "ate" complexes of organoboranes represents a novel route to unsymmetrical alkynes.Several potential "blocking" groups were examined in order to achieve the selective migration of one primary alkyl group and thereby increase the efficiency of this process.Best results were obtained with the combined use of the thexyl and methoxy moieties as "blocking" groups in this reaction.The required thexylalkylborinate intermediates were conveniently prepared in high yield from thexylchloroborane via hydroboration and methanolysis.Subsequent complexation with an appropriate lithium alkyne, followed by iodination, produced the desired unsymmetrical alkyne in high yield.Minimum amounts of the product resulting from competitive migration of the thexyl group were observed.Under these conditions, an efficient utilization of a primary alkyl group in this transfer reaction is achieved.Furthermore, the high tolerance of thexylchloroborane toward many functional groups and its high regioselectivity in terminal alkene hydroboration produces intermediates that are particularly useful for the synthesis of insect pheromones and not readily accessible via conventional organometallic procedures.