The reaction pathways for the thermal additions of disilacyclobutenes and acetylene are discussed from B3LYP density-functional-theory computations. Butadiene is more stable in energy than cyclobutene, the corresponding ring compound, whereas disilabutadiene is less stable than disilacyclobutene. From detailed analyses of the potential energy surfaces, disilabutadienes formed by thermal ring opening of disilacyclobutenes are confirmed to play a central role in the addition reactions as intermediates in a manner similar to the Diels-Alder reaction. The activation energies for the symmetry-allowed conrotatory ring opening of 1,2-disilacyclobut-3-ene, 1,1,2,2-tetramethyl-1,2-disilacyclobut-3-ene, and 3,4-benzo-1,1,2,2-tetramethyl-1,2-disilacyclobutene are 41.5, 46.7, and 61.9 kcal/mol, respectively, and the activation energies for the Diels-Alder coupling reactions with acetylene are 1-4 kcal/mol when measured from the disilabutadiene intermediates at the B3LYP/6-31G** level of theory. Therefore the ring opening should be the rate-determining step in these reactions; once the four-membered ring of disilacyclobutene is opened by heat treatment, the addition reactions should readily take place, leading to six-membered ring products. The transition state for the addition of 1,4-disila-1,3-butadiene and acetylene is symmetrical with respect to the Si-C bonds being formed, whereas those of 1,1,4,4-tetramethyl-1,4-disila-1,3-butadiene and acetylene and of 1,2-bis(dimethylsilylene)cyclohexa-3,5-diene and acetylene are not symmetrical. There are good reasons that such asymmetrical transition states occur in these Diels-Alder reactions; one is the asymmetrical frontier orbitals in the methyl-substituted disilabutadienes, and the other is just a steric effect of the methyl groups.
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