Abstract
The silicon directing effect on the cleavage of C-C bonds in radical cations was investigated. Electron impact induced fragmentation was performed in the gas phase on six pairs of cis and trans isomers of 5-(trimethylsilyl)bicyclo[4.4.0]dec-1-enes with an angular CHO, COOEt, or CH 2 OH group at the C-6 position. For aldehydes including trans-6-formyl-5-(trimethylsilyl)bicyclo[4.4.0]dec-1-en-3-one (2), trans-6-formyl-5-(trimethylsilyl)bicyclo[4.4.0]dec-1-ene (4), trans-6-formyl-5-(trimethylsilyl)bicyclo[4.4.0]dec-1-en-3-one 3-(ethylene thioacetal) (6), and 1-butyl r-3-[c-6-formyl-t-5-(trimethylsilyl)bicyclo[4.4.0]dec-1-enyl] sulfide (8), their parent radical cations underwent fragmentation to give (M •+ - • CHO) as the main species. By the same fashion, the radical cation of the ester ethyl trans-3-oxo-5-(trimethylsilyl)bicyclo[4.4.0]dec-1-ene-6-carboxylate 3-(ethylene thioacetal) (10) gave (M •+ - • COOEt); the radical cation of the alcohol trans-6-(hydroxymethyl)-5-(trimethylsilyl)bicyclo-[4.4.0]dec-1-ene (12) gave (M •+ - • CH 2 OH). In contrast, the corresponding cis radical cations, resulting from cis-6-formyl-5-(trimethylsilyl)bicyclo[4.4.0]dec-1-en-3-one (1), cis-6-formyl-5-(trimethylsilyl)bicyclo[4.4.0]dec-1-ene (3), cis-6-formyl-5-(trimethylsilyl)bicyclo[4.4.0]dec-1-en-3-one 3-(ethylene thioacetal) (5), and 1-butyl r-3-[c-6-formyl-c-5-(trimethylsilyl)bicyclo[4.4.0]dec-1-enyl] sulfide (7) produced (M •+ - • CH 3 ) as the main species. These results from mass spectrometry indicated that the trimethylsilyl group at the β-position directed the angular C-C bond cleavage in octalin radical cations. The trimethylsilyl group, being trans to the angular substituent, provided greater driving force than the trimethylsilyl group being cis in the cleavage of the C-C bond. The fragmentation pattern was independent of the functionalities on the octalin framework and the oxidation state of the angular oxygenated methyl substituent. © 1989 American Chemical Society.