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, (η 2 -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% d 2 ) 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.