Ti(II)-mediated allenyne cyclization as a new tool for generation of chiral organotitanium compounds. Asymmetric generation of cyclic allyltitanium reagents with no chiral auxiliary

Treatment of a variety of 1,2-dien-7-ynes with a slight excess of (eta(2)-propene)Ti(O-i-Pr)(2) (1), prepared in situ from Ti(O-i-Pr)(4) and 2 equiv of i-PrMgCl in ether, afforded 1-alkenyl-2-alkylidenecyclopentanes in moderate to good yields after aqueous workup. The intermediate titanabicycle such as 7 was identified by deuterolysis, which gave d(2)-8 with exclusive deuterium incorporation. The use of an optically active allene such as 19 (less than or equal to 83% ee) realized a highly efficient axial to centered chirality transfer to give 21 (80-83% ee). Analogously, the cyclization of 22 (less than or equal to 86% ee) with 1 followed by carbonylation (under ca. 1 atm of CO) afforded the optically active bicyclic ketone 25 (86% ee). When a homologous 1,2-dien-6-yne 27 was subjected to the cyclization as above, a different type of titanabicycle 28 was generated, which was identified by hydrolysis giving 29 or the following carbonyl addition. Upon reaction with nonanal, 28 afforded the alcohol 32 with very high regio-(with respect to the allylic system), stereo-(the diene moiety), and diastereoselectivities (the hydroxy group). Various 1,2-dien-6-ynes underwent the cyclization followed by the addition to carbonyl compounds in comparable selectivities as above. Chiral 1,2-dien-6-ynes 46 (less than or equal to 83% ee) achieved a nearly complete chirality transfer to generate a chiral titanabicycle 48, which, in turn, reacted with nonanal, acetone, or N-benzylidenepropylamine to afford the alcohols 51 and 52 (both in 80% ee) or the amine 53 (81% ee) in good yields with a very small loss of the enantiopurity.