Thermolyse von Arylhalogenboranen; Synthese von 1,3-Dibora- und 1,3-Borasilaindanen

The thermolysis of the 2-silylated or borylated arylboranes (3a-g, or 9a,b) has been investigated. The synthesis of the unknown starting boranes succeeds upon treatment of the (o-(dichloroboryl)phenyl)trimethylsilane (3b) with trimethylalane (giving 3e), tetramethylstannane (giving 3f) or diisopropylamine (giving 3g) and upon trimethylation of the 1,2-bis(dichloroboryl)benzene (9a) with tetramethylstannane (giving 9b). In solution 3b,c, and 9a,b “dimerize” readily in a temperature range between 45 and 135°C with the liberation of halotrimethylsilanes or chloroboranes yielding the 9,10-dibora-9,10-dihydroanthracenes. The reaction mechanism is discussed. To force intramolecular reactions exclusively, flash vacuum pyrolyses (FVP) were performed at about 650°C. When pyrolyzing 3b elimination of hydrogen chloride occurs yielding 1,1-dimethyl-3-chloro-1-sila-3-boraindane (11). After pyrolysis of 11 at 750°C intramolecular, mutual migration of a chloro and a methyl group takes place. Possible rearrangement mechanisms are discussed and have been experimentally in investigated. Hydrogen chloride is the best leaving molecule in these pyrolyses. Extrusion of methanol (from (o-(dimethoxyboryl)phenyl)trimethylsilane (3d)) or methane (from 3e) gives the corresponding borasilaindanes only at 750°C and in lower yields. The synthesis of the very sensitive 1,3-dimethyl-1,3-diboraindane also succeeds by FVP (550°C) starting with 9b. The high-field shifts of the ²⁹Si and the ¹¹B NMR signals of the borasilaindanes compared to the chemical shifts of the corresponding parent bora- and silaindane systems are considered as evidence for a through-bond interaction between both hetero atoms.