Arylsilanes and aromatic reactivity.

A review of literature on cleavages of aryl-silicon bonds by electrophilic reagents is presented. Trimethylsilyl derivatives of furan, benzothiophene, dibenzothiophene, dibenzofuran, N-ethylcarbazole, diphenyl ether, diphenyl sulphide, diphenylmethane, fluorene, pyrene, anthracene, phenanthrene and dihydrophenanthrene have been prepared, including the following new compounds (R = trimethylsilyl):- 2- and 3-R-benzothiophene, 1-, 2-, 3-, and 4-R-dibenzo- furan, 2- and 4-R-diphenyl ether, 2- and 4-R-diphenyl sulphide, 4-R-diphenylmethane, 9-R-9,10-dihydroanthracene, 9,10-di-R- 9,10-dihydroanthracene, 9-R-anthracene, 2-R-fluorene, 2-R-9-methylfluorene and 2-R-bis-9-dimethylfluorene. Points noted during these preparations include the following:- (i) Bromination of 9,10-dihydrophenanthrene leads to the formation of some 2-bromophenanthrene, in addition to 2-bromo-9.10-dihydrophenanthrene. (ii) The 9-R-anthracene compound cannot be made by conventional methods, and the novel route devised involved treatment of 9,10-di-R-9,10-dihydroanthracene with sulphur. The resulting reaction is the first example of cleavage of a carbon-silicon bond with sulphur. (iii) A modified "entrainment" method for the preparation of Grignard reagents, which was recently described80, is very useful when working with these compounds of high molecular weight. The rates of cleavage of the aryl-silicon bonds by aqueous methanolic perchloric acid have been measured spectrophotometrically, and compared with the rate of cleavage of the aryl-silicon bond in phenyltrimethylsilane. In this way a measure has been obtained of the reactivity of the various positions of the aromatic structures examined, and in several cases the results provide the first quantitative data on these reactivities. The results are compared with the relevant qualitative and quantitative data for the more familiar aromatic substitutions such as bromination and nitration; some anomalies are noted. For the structures it is shown that the observed rates can be roughly interpreted by considering the structures as substituted phenyltrimethyl-silanes, The behavior of the fluorene system, which appears anomalous, is analysed.