Silicon−Carbon Unsaturated Compounds. 75. Thermal Isomerization of 2-Alkyl- and 2-Aryl-2-trimethylsiloxy-1,1-bis(trimethylsilyl)-1-silacyclohex-4-enes

Abstract

The thermolysis of 2-tert-butyl-, 2-adamantyl-, and 2-phenyl-4,5-dimethyl-2-(trimethylsiloxy)-1,1bis(trimethylsilyl)-1-silacyclohex-4-ene (1-3), synthesized by the reactions of acylpolysilanes with 2,3dimethylbutadiene, in a sealed glass tube at 250 °C for 24 h gave the products, arising from dyotropic rearrangement, cis-2-tert-butyl-, cis-2-adamantyl-, and cis-2-phenyl-4,5-dimethyl-1-(trimethylsiloxy)-1,2bis(trimethylsilyl)-1-silacyclohex-4-ene (5-7), respectively, in high yields. The theoretical calculations using of 2-methyl-2-siloxy-1,1-bis(silyl)-1-silacyclohex-4-ene as a starting compound and cis-2-methyl-1-siloxy-1,2-bis(silyl)-1-silacyclohex-4-ene as a product indicated that a pentacoordinate silicon species plays an important role for the present dyotropic rearrangement. The thermolysis of the 4,5-diphenyl-1-silacyclohex-4-ene derivative 8, prepared by the reaction of pivaloyltris(trimethylsilyl)silane with 2,3diphenylbutadiene, also afforded cis-2-tert-butyl-4,5-diphenyl-1-(trimethylsiloxy)-1,2-bis(trimethylsilyl)-1-silacyclohex-4-ene (9), whose structure was confirmed by X-ray crystallographic analysis. The thermolysis of 4,5-dimethyl-2-mesityl-2-(trimethylsiloxy)-1,1-bis(trimethylsilyl)-1-silacyclohex-4-ene (4) at 250 °C for 24 h, however, produced a ring-contracted compound, 3,4-dimethyl-1-mesityl-1-[trimethylsiloxybis(trimethylsilyl)silyl]cyclopent-3-ene (10).

Dyotropic Reactions: Mechanisms and Synthetic Applications

“…Heating of silacyclohexenes 64 in a degassed sealed tube at 250 °C for 24 h results in a clean reaction to give the cis -silacyclohexenes 65 in excellent yields (Scheme ) . The corresponding trans -isomer is not detected in these reactions, thus confirming the stereoselectivity of the transformation.…”

“…The computed activation barrier for this process was ca. 5 kcal/mol higher than that associated with transition structure 216 …”

“…5 kcal/mol higher than that associated with transition structure 216. 50 Klingebiel et al An interesting example of a type I dyotropic rearrangement involving a concerted (albeit highly asynchronous) hydrogen and carbon migration on a static C-C scaffold has been proposed for the biosynthesis of pentalenene 227 from farnesyl diphosphate 223 (Scheme 81). 134 Carbocation 224 transforms stereospecifically into the more stable intermediate 226 via transition structure 225 with an activation barrier of ca.…”

Carbon, silicon, germanium, tin and lead

“…Thermolysis gives the rearranged trans-4-adamantyl-2-tert-butyl-and trans-2-adamantyl-4-tert-butyl-1-(triemthylsiloxy)-1,2-bis(trimethylsilyl)-1-silacyclobut-3-ene 66 where 2-adamantyl-4,5-dimethyl-2-(triuemthylsiloxy)-1,1-bis(triemthylsilyl)-1-silacyclohex-4-ene rearranges to cis-2-adamantyl-4,5-dimethyl-2-(triemthylsiloxy)-1,1-bis(trimethylsilyl)-1silacyclohex-4-ene in a process that exchanges the trimethylsiloxy group with a trimethylsilyl. 67 Sticking with the theme of rearrangements, a study of 1,2-and 1,4-retro-Brook rearrangements of a,g-disilyloxy organolithium compounds shows that a larger siloxyl derivative at the g-relative to the size of that at the a-position results in higher 1,4-selectivity 68 where photochemical and thermal 1,3-and 1,5-migrations of silyl groups in polysilylenones gives silenes. 69 The irradiation of 1,1-diphenyl-, 1,1-bis-(4-trifluormethylphenyl)-or 1,1-bis(3,5-bis(trifluoromethyl)phenyl)germacyclobutane in the presence of methanol as a trapping reagent results in the extrusion of the diarylgermene and ethylene and the diarylgermylene and cyclopropane with differing selectivity, where the corresponding silicon compounds typically form silenes and ethylene only.…”