Vinylic Organoboranes. 4. A General, One-Pot Synthesis of 6- and 7-Alkyn-1-ols via Boracyclanes. Influence of Steric Effects in the Iodination of Lithium Alkynyl "Ate" Complexes of Dialkylborinates

Article abstract of DOI:10.1021/jo00374a005

The iodination of the "ate" complexes derived from various B-alkoxyborinane derivatives and 1-alkynyllithium has been investigated.The results indicate the ate complex from B-methoxyborinane is converted into desired 6-alkyn-1-ol in a yield of only 22percent, with much larger amounts, 65percent, of the undesired 1-iodo-1-alkyne.Increases in the steric bulk of the alkoxy group on boron increase the yield of the required 6-alkyn-1-ol with the best results realized with B-(triphenylmethoxy)borinane.Treatment of B-(triphenylmethoxy)borinane with 1-alkynyllithium affords the corresponding "ate" complex.Subsequent iodination induces the migration of one end of the cycloalkyl chain from boron to the adjacent carbon, resulting in the formation of the one-carbon homologated borepane moiety.This then undergoes a rapid deiodoboronation to afford the corresponding (6-alkyn-1-yl)boronate ester.Oxidation of these esters produces the desired 6-alkyn-1-ols in excellent yields (85percent).An attempt to extend this reaction to di-n-alkylborinates to prepare the corresponding unsymmetrical alkynes did not achieve satisfactory results.Alternatively, the iodination of the "ate" complex from B-methylborinane and 1-alkynyllithium, followed by oxidation, provides the required 6-alkyn-1-ols in high yields.This procedure has been successfully extended to the seven-membered borepane moiety to provide the corresponding 7-alkyn-1-ols.Extension of this reaction to the di-n-alkylmethylboranes provides the corresponding unsymmetrical alkynes in good yields.Thus, these procedures constitute a simple, general and one-pot synthesis of the desired alkyn-1-ols, valuable synthons in organic synthesis.Insect pheromones, (Z)-7-tetradecenaland (Z)-7-hexadecenal, were readily prepared in excellent yields by utilizing this convenient procedure.