{2 + 2} Cycloaddition of Alkyne with Titanium−Imido Complex: Theoretical Study of Determining Factor of Reactivity and Regioselectivity

The {2 + 2} cycloaddition of alkyne across the TiN bond of [(H₃SiO)₂Ti(NSiH₃)] 1 was theoretically investigated. Though this cycloaddition is symmetry forbidden in a formal sense by the Woodward−Hoffmann rule, the cycloaddition of 2-butyne (MeCCMe) easily occurs with moderate activation barrier (7.6 kcal/mol) and considerably large exothermicity (41.0 kcal/mol), where the CCSD(T)-calculated energies are presented hereafter. The moderate activation barrier is interpreted in terms of the considerably polarized TiN bond; Because the d_π-p_π bonding orbital largely consists of the p_π orbital of the N and moderately of the d_π orbital of the Ti, the π* orbital of 2-butyne interacts with the d_π-p_π bonding orbital so as to form a bonding overlap with the p_π orbital of the N, into which the π orbital of 2-butyne mixes in an antibonding way with the p_π orbital of N. As a result, the CC bond of 2-butyne is polarized in the transition state and the symmetry forbidden character becomes very weak, which is the reason of the moderate activation barrier. The {2 + 2} cycloaddition of 1-methoxy-1-propyne (MeC^αC^βOMe) occurs with smaller activation barrier (3.2 kcal/mol) than that of 2-butyne, when the C^α and C^β approach the Ti and N, respectively. The higher reactivity of this alkyne is interpreted in terms of its polarized CC bond. In the reverse regioselective {2 + 2} cycloaddition in which the C^α and C^β approach the N and Ti, respectively, the activation barrier becomes larger. From these results, it is concluded that the regioselective {2 + 2} cycloaddition can be performed by introducing such π-electron donating group as methoxy on one C atom of alkyne. The major product contains the Ti−C^α and N−C^β bonds, where the methoxy group is introduced on the C^β. The ratio of the major to minor products is theoretically estimated to be very large.