NOVEL ORGANOBORANE LEWIS-ACIDS VIA SELECTIVE BORON-TIN EXCHANGE PROCESSES - STERIC CONSTRAINTS TO ELECTROPHILIC INITIATION BY THE BORON HALIDE

With the purpose of preparing novel mono-and bidentate organoboron Lew is acids, the scope and Limitations of synthesizing the requisite orga noboranes by the boron-tin exchange between a boron halide and the app ropriate organostannane have been examined in detail. The following or ganotin derivatives have been obtained either from the corresponding R MgBr or RLi reagent and MenSnC4-n or from a Barbier procedure using th e organic halide, Me3SnCl and magnesium metal: 1,2bis (trimethylstanny l) ethyne, o-, m-, and p-bis(trimethylstannyl)benzenes, alpha,o-bis(tr imethylstannyl)toluene, alpha,alpha-bis(trimethylstannyl)-o-xylene, an d 2,2-dimethyl-2-stannaindane, The individual interaction of the 1,2bi s(trimethylstannyl) ethyne and the isomeric bis(trimethylstannyl)benze nes with Et2BBr produced the corresponding bis(diethylboryl)-derivativ es. By contrast, with Et2BCl the alpha,o-bis(trimethylstannyl)toluene gave only o-diethylboryl-alpha-trimethylstannyltoluene and with BCl3 t he alpha,alpha'-bis(trimethylstannyl)-o-xylene formed only alpha,alpha '-bis(chlorodimethylstannyl)-o-xylene. Furthermore, in the attempted d ouble boron-tin exchange between obis(trimethylstannyl)benzene and BCl 3, an unprecedented rearrangement of the 1-(dichloroboryl)-2-(trimethy lstannyl)benzene intermediate into its 1-[chloro(methyl)boryl]-2-(chlo rodimethylstannyl) isomer was observed. Likewise, o-bis(trimethylstann yl)benzene with PhBCl2 produced by a similar rearrangement l(phenyl)bo ryl]-2-(chloro-dimethylstannyl)benzene. The thermolysis of such borane s led variously to definite dimers or ill-defined oligomers, Prelimina ry studies of the properties of these organoboranes have identified th e heightened Lewis acidity of 1,2bis(diethylboryl)ethyne and the pi-el ectron delocalization involving the 2p(z)-boron orbitals in the 9,10-d ihydro-9,10-diboraanthracene system. Finally, an electronic mechanism for the boron-tin exchange has been developed to account for the selec tivity of the boron halide's attack at unsaturated carbon-tin bonds.