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 (Me4C2SiR)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 °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).