Intramolecular Rearrangements of Silenes. 3. From 2,5-Bis(1-methyl-1-silacyclobut-1-yl)-p-xylene to 1,2,4,5-Tetrahydro-1,1,4,4-tetramethyl-1,4-disiladicyclobuta- [a,d]benzene. The First Silacyclobutene Diannelated [a,d]Benzene

Abstract

Low-pressure pyrolysis of 2,5-bis(1,1-dimethyl-1-silacyclobut-1-yl)-p-xylene (1) results in the formation of 1,2,4,5-tetrahydro-1,1,4,4-tetramethyl-1,4-disiladicyclobuta[a,d]benzene (2), here named as the butterfly compound and representing the first silacyclobutene diannelated [a,d]benzene. Both compounds, 1 and 2, were characterized by NMR, IR, and MS methods; furthermore, the molecular structures were determined by single-crystal X-ray diffraction analysis. Density functional and ab initio calculations prove that the difference in the benzene carbon−carbon bond lengths in 2 results primarily from substituent effects and is not caused by ring strain effects. Forming strongly bent silicon−carbon bonds, silicon decreases the amount of strain in the model compounds, in the 1-silacyclobut-2-ene 10 by 8.3 kcal/mol and in the 1,4-disiladicyclobuta[a,d]benzene 15 by 19.0 kcal mol-1, compared to the analogous hydrocarbons. The total strain energy is additive, depending on the number of annelated rings. The phot...