Stereochemistry of Disilanylene-Containing Cyclic Compounds. Thermal Reactions of cis- and trans-3,4-Benzo-1,2-diisopropyl-1,2-dimethyl-1,2-disilacyclobut-3-ene

The thermal reaction of cis-3,4-benzo-1,2-diisopropyl-1,2-dimethyl-1,2-disilacyclobut-3-ene (1a) with tert-butyl alcohol at 300 °C for 24 h proceeded with high stereospecificity to give erythro-1-(tert-butoxyisopropyl­methylsilyl)-2-(isopropylmethylsilyl)benzene (2a) in 71% yield, as a single stereoisomer. A similar reaction of trans-benzodisilacyclobutene (1b) with tert-butyl alcohol produced the threo isomer 2b in 81% yield, as the sole product. The photolysis of 1a,b in the presence of tert-butyl alcohol afforded a mixture of two adducts formed by the reaction of 1-[2-(isopropylmethylsilyl)phenyl]-1-isopropylsilene with tert-butyl alcohol, respectively. The reaction of 1a with ethylene in an autoclave at 300 °C for 24 h gave 3,4,9,10-dibenzo(r-1)-trans-2,trans-5,cis-8-tetraisopropyl-1,2,5,8-tetrasilacyclododeca-3,9-diene (10a), as a single stereoisomer in 71% yield. The reaction of 1b with ethylene under the same conditions, however, produced poly[1,2-bis(isopropylmethylsilylene)phenylene–ethylene], whose molecular weight was determined to be 61000 (M w/M n = 2.32), relative to polystyrene standards. The reaction of 1a with 1-hexyne, tert-butylacetylene, phenylacetylene, and (trimethylsilyl)acetylene in benzene in a sealed degassed tube at 150 or 200 °C proceeded stereospecifically to give the respective cis-5,6-benzo-1,4-diisopropyl-1,4-dimethyl-1,4-disilacyclohexa-2,5-diene derivatives (11a–14a) in high yields as the sole products. The reaction of 1b with the alkynes under the same conditions afforded the respective trans-benzodisilacyclohexadienes (11b–14b), as the sole stereoisomers in high yields. Theoretical treatment for the reaction of 3,4-benzo-1,1,2,2-tetramethyl-1,2-disilacyclobut-3-ene (1c) with tert-butyl alcohol and acetylene indicated that the substrates react directly with 1c to give the respective adducts. For the reaction of 1c with acetylene, it has been shown that the acetylene molecule inserts directly into a silicon–silicon bond in 1c to give the benzodisilacyclohexadiene system. Such direct reactions of the substrates with 1c were shown to be energetically more advantageous than the reaction of o-quinodisilane, 1,2-bis(dimethylsilylene)cyclohexa-3,5-diene, arising from a conrotatory ring-opening reaction of 1c with substrates.