Silicon−Carbon Unsaturated Compounds. 76. Photochemical and Thermal Behavior of 1-Silacyclobut-3-enes Generated from the Reaction of Pivaloyltris(trimethylsilyl)silane with tert-Butylacetylene

Abstract

Irradiation of trans-1-silacyclobut-3-ene 2, obtained by thermal isomerization of [2 þ 2] cycloadduct 1 arising from the co-thermoysis of pivaloyltris(trimethylsilyl)silane with tert-butylacetylene, with a low-pressure mercury lamp afforded an equilibrium mixture consisting of 2 and its isomer, cis-1-silacyclobut-3-ene 3, in a ratio of 1:1. Similar irradiation of the cis isomer 3 gave the mixture involving equal amounts of 2 and 3. The photolysis of 2 in the presence of tert-butyl alcohol gave two isomers of the cyclopropane derivative, arising from the reaction of silabicyclobutane, A and A 0 , with alcohol. The thermolysis of 3 at 250 °C produced 2 in quantitative yield. Similar thermolysis of 3 in the presence of tert-butyl alcohol, however, afforded no alcohol adducts, but 2 was obtained as the sole product. The DFT calculations with the use of cis-2,4-dimethyl-1-siloxy-1,2-bis(silyl)-1-silacyclobut-3-ene as a starting compound and trans-2,4-dimethyl-1-siloxy-1,2-bis(silyl)-1-silacyclobut-3ene as a product indicated that a pentacoordinate silicon intermediate plays an important role in the thermal isomerization of 3 to 2.

Synthesis and thermal behavior of cis- and trans-1-tert-butyl-4,5-dimethyl-2-phenyl-2-(trimethylsiloxy)-1-(trimethylsilyl)-1-silacyclohex-4-ene

“…The solvent ether was evaporated and the residue was chromatographed on silica gel eluting with hexane:ethyl acetate/1:1, to give 3.095 g (56% yield) of 1: Anal. Calcd for C 17 H 32 OSi 3 : C,60.64;H,9.58. Found: C,60.33;H,9.44 …”

“…Calcd for C 23 H 42 OSi 3 : C, 65.96; H, 10.11. Found: C,66.01;H,9.98 123.4, 124.9, 125.8, 127.4, 127.8, 145.9 (phenyl ring and olefinic carbons); 29 Si NMR d (CDCl 3 ) À1.8 (ring Si), 3.5 (Me 3 SiC), 7.8 (OSiMe 3 ).…”

Substituent Effects in Thermal Reactions of a Silene with Silyl-Substituted Alkynes: A Theoretical Study

“…In 1979, Brook and co-workers reported the photolysis and thermolysis of pivaloyltris­(trimethylsilyl)­silane to afford a silene with a moderate lifetime and confirmed the presence of the silene by trapping experiments (Scheme ) . Nowadays it is well known that photochemical and thermal reactions as well as Peterson-type reactions of the acylpolysilanes offer a convenient route for the preparation of silenes. − Silenes having a highly reactive SiC bond are utilized as key reaction intermediates for the synthesis of various organosilicon compounds. − On the other hand, the silenes have also been used as silicon-based reagents for organic synthesis …”

“…Ishikawa, Naka, and co-workers have been investigating the unique reactivity of silenes in the co-thermolysis of acylpolysilanes and silyl-substituted alkynes. , Silenes derived from the acylpolysilanes generally react with alkynes to afford [2 + 2] cycloadducts, 1-silacyclobut-3-enes, which are isolable in many cases. For example, the [2 + 2] cycloadditions of silene 1 to bis­(trimethylsilyl)­butadiyne and tert -butyldimethylsilylacetylene at 120 °C gave the corresponding silacyclobutene derivatives 2a and 2b in high yields, respectively (Scheme ).…”

Theoretical Study on the Formation of Silacyclopropene from Acylsilane and Acetylene via Silene-to-Silylene Rearrangement

“…Unsaturated silicon compounds such as silene (SiC), − disilene (SiSi), , and silylene (divalent Si) are known to be highly reactive and used as reagents and key reaction intermediates in the synthesis of various organosilicon compounds . Although a great deal of experimental and theoretical efforts have been devoted to the chemistry of unsaturated silicon compounds, a systematic understanding is still lacking, particularly on mechanistic aspects of their diverse chemical reactivities.…”