Organosilicon compounds. Part LIII. Base cleavage of substituted fluoren-9-yltrimethylsilanes and related compounds

Abstract

The rates of cleavage of substituted fluoren-9-yltrimethylsilanes by methanol-aqueous base (5 : 1 v/v) relative to that of the unsubstituted compound expressed as 1000 are : (a) at 50°, 9-phenyl-, 140; 9-methyl-, 29; 9-ethyl-10; 9-trimethylsilyl-, 3·5; 2-trimethylsilyl-9-methyl-, 30; 9-m-chlorophenyl-, 560; 2-bromo-9-methyl-, 190; (b) at 30°, 9-chloro-, 44,000; 9-bromo-, 36,000; 2-bromo-, 5800; 2,7-dibromo-, 32,000. The factors at 30° for 1-trimethylsilylindene and 1,1-bis(trimethylsilyl)indene are 10,000 and 18, respectively. For fluoren-9-yl-trimethylsilane and the 2-bromo- and 2,7-dibromo-fluoren-9-yl and inden-1-yl compounds, the reactivities are in accord with the acidities of the corresponding carbon acids, but for the other compounds, each of which carries an additional α-substituent, steric hindrance appears to reduce the reactivity. The reactivity of phenylethynyltrimethylsilane, which is cleaved three times as rapidly as fluoren-9-yltrimethylsilane at 30°, is much lower than would be expected from the kinetic acidity of phenylacetylene.