The [2 + 2) cycloaddition of alkyne across the Ti=N bond of [(H3SiO)(2)Ti(=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 equivalent to CMe) easily occurs with moderate activation barrier (7.6 kcal/mol) and considerably large exothert- nicity (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(pi)-p(pi) bonding orbital largely consists of the p, orbital of the N and moderately of the d, orbital of the Ti, the pi* orbital of 2-butyne interacts with the d(pi)-p(pi) bonding orbital so as to form a bonding overlap with the p, orbital of the N, into which the pi orbital of 2-butyne mixes in an antibonding way with the p, orbital of N. As a result, the C equivalent to 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 alpha equivalent to(COMe)-O-beta) occurs with smaller activation barrier (3.2 kcal/mol) than that of 2-butyne, when the C-alpha and C-beta approach the Ti and N, respectively. The higher reactivity of this alkyne is interpreted in terms of its polarized C equivalent to C bond. In the reverse regioselective 12 + 21 cycloaddition in which the C-alpha and C-beta approach the N and Ti, respectively, the activation barrier becomes larger. From these results, it is concluded that the regioselective (2 + 21 cycloaddition can be performed by introducing such pi-electron donating group as methoxy on one C atom of alkyne. The major product contains the Ti-C-alpha and N-C-beta bonds, where the methoxy group is introduced on the C-beta. The ratio of the major to minor products is theoretically estimated to be very large.