Cation-Mediated, Substituent-Controlled, C2−C7Cycloaromatization of an Enyne−Allene

Reaction of (2-bromophenylethynyl)trimethylsilane 1 with trimethyl borate yielded the alkynearylboronic acid 2, which was coupled in situ with the bromoallenes 3 to give enyne−allenes 4 . The intermediates 4 underwent a spontaneous cation-mediated C 2 −C 7 cycloaromatization with a concomitant 1,2-shift of the trimethylsilyl group to give the naphthalene derivatives 5 . Besides 5, yne−allenes 10 were isolated by chromatography of the crude reaction products. A key step in the formation of 10 is the hydrolysis of the aryl boronic acid 2, which led to phenylethynyltrimethylsilane 8 . Under the reaction conditions used, the TMS group of 8 was subsequently removed to give phenylacetylene 9, which further reacted with the bromoallenes 3 to furnish the yne−allenes 10 .