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^‡

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 photoelectron spectrum of the butterfly compound exhibits three isolated bands at low energy. The ground state of the radical cation (X) over tilde((2)A(g)) at 8.15 eV and the first exited state (A) over tilde((2)A(g)) at 8.7 eV arises from pi-electron ionization at the benzene molecule center. The calculated values with 8.35 and 8.82 eV are in agreement with the experimental vertical ionization energies.