Abstract
The ring-opening reaction of metallacyclobutenes derived from the addition of alkyne (HC⋮CR, R = H, Me) to molybdenum alkylidene Mo(NH)(OR‘)2(CHR‘ ‘) (R‘ = Me, CF3, R‘ ‘ = H, Me) has been studied using quantum mechanics methods. Two ring-opening paths, NH outward and NH inward ring-openings, have been found for metallacyclobutene intermediates Mo(NH)(OR‘)2(CH2)(HCCH) (R‘ = Me, CF3) and α- and β-addition metallacyclobutene intermediates Mo(NH)(OMe)2(CH2)(C2HMe). The NH outward ring-opening is found to be favored over the NH inward ring-opening. The electron-withdrawing group (OCF3) in molybdenum alkylidene and α-methyl group further enhance the preference for the NH outward ring-opening pattern. Four ring-opening paths have been found for Mo(NH)(OMe)2(CHMe)(C2H2), an intermediate in the addition of acetylene to Mo(NH)(OMe)2(CHMe). The conformation of the product is found to be dependent upon the conformation of the molybdenum alkylidene. That is, when trans-molybdenum alkylidene is used as the catalyst, an anti-polymer is generated, while using cis-molybdenum alkylidene may lead to the polymer in different conformations. The anti/syn-isomerization of the electron-withdrawing group substituted molybdalkylidene is calculated to be much slower than the polymerization steps; therefore, an all-trans-polymer can be obtained.