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

Abstract

The {2 + 2} cycloaddition of alkyne across the Ti=N bond of [(H(3)SiO)(2)Ti(=NSiH(3))] 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[triple bond]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(pi)-p(pi) bonding orbital largely consists of the p(pi) orbital of the N and moderately of the d(pi) 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(pi) orbital of the N, into which the pi orbital of 2-butyne mixes in an antibonding way with the p(pi) orbital of N. As a result, the C[triple bond]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)[triple bond]C(beta)OMe) 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[triple bond]C bond. In the reverse regioselective {2 + 2} 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 + 2} 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.