Titanium Alkoxide-Based Method for Stereoselective Synthesis of Functionalized Conjugated Dienes

Treatment of an internal acetylene such as 1-silyl-1-octyne (3) with a low-valent titanium reagent, (η²-propene)Ti(O-i-Pr)₂ (1) readily prepared from Ti(O-i-Pr)₄ and i-PrMgCl in situ, generates an acetylene−titanium complex. This complex was allowed to react with a terminal acetylene, 4-(benzyloxy)-1-butyne (5), to give the intermediate titanacyclopentadiene (6) which, upon hydrolysis, deuteriolysis, or iodinolysis, gave diene 8, or its bis-deuterated (>99% d₂) and bis-iodinated counterparts (9 and 10), in good yields and with high selectivities. This reaction is applicable to the cross-coupling reaction of functionalized acetylenes such as 2-alkynoates and 2-alkynamides 12−18 and a variety of terminal acetylenes 24−28 to give dienes 36−50 in good yields. A terminal acetylene 28 carrying an olefinic bond at the other terminus reacted with a silylpropiolate to afford the expected diene 42 without any complication.