Synthesis of a tert-butyl substituted bis(silirane) and comparison with its methyl and phenyl analogs

The detailed syntheses of a series of substituted bis(silirane)s (Me(4)C(2)SiR)(2) {R - Me (1a), Ph (1b), tert-butyl (1c)} are reported. The tert-butyl derivative (1c) shows steric congestion which results in conformational rigidity in solution with respect to Si-Si bond rotation. This contrasts with the methyl (1a) and phenyl (1b) analogs. The rotational barriers of all three bis(silirane) s were examined by Density Functional Theory (DFT) at the B3LYP/6-31G* level. The calculated rotational barriers were 4.1 and 4.7 kcal/mol for 1a and 1b, respectively, but significantly higher for 1c, 29.9 kcal/mol. The thermolysis of 1c at 145 degrees C in the presence of bis(trimethylsilyl) acetylene gave 1,4-di-tert-butyl-2,2,3,3-tetramethyl-5,6-bis(trimethylsilyl)-1,4-disilabicyclo[2.2.0]hex-5-ene (A) resulting from the [2 + 2] cycloaddition of bis(trimethylsilyl)acetylene with a 1,2-disilacyclobut-1-ene intermediate (B). The 1,2-disilacyclobut-1-ene (B) presumably originates from the rearrangement of a transient siliranylsilylene produced by loss of tetramethylethylene from 1c. Purification of the 1,4-disilabicyclo[2.2.0] hex-5-ene (A) by column chromatography resulted in its reaction with air to afford a stable 1,4-di-tert-butyl-2,2,3,3-tetramethyl-5,6-bis (trimethylsilyl)-7-oxa-1,4-disilabicyclo[2.2.1] hept-5-ene (C). (C) 2010 Elsevier B.V. All rights reserved.