Organometallic compounds of group 13, 55 - Novel organoaluminum Lewis acids via selective aluminum-tin exchange processes - Electrophilic initiation by the aluminum halide and ensuing complexation by the resulting tin halide

With the goals of preparing novel carbaluminating reagents and mono- and bi dentate organoaluminum Lewis acids. the scope and Limitations of synthesizi ng the requisite organoalanes by the aluminum-tin exchange between an alumi num halide and the appropriate organostannane have been examined in detail. The interactions of such tin precursors as 1,2-bis(trimethylstannyl)ethyne , allyltri-n-butyltin, benzyltrimethyltin, and 1,2-bis(trimethylstannyl)ben zene and various aluminum chlorides of the type, RnAlCl3-n (R = Me, Et), ga ve selective aluminum-tin exchange at the sp- or sp(2)-hybridized carbon-ti n bond and produced such organoalanes as allyl(methyl)aluminum chloride, be nzylaluminum dichloride, 1,2-bis(diethylalumino)ethyne, 1,2-bis(dimethylalu mino)benzene, 1,2-bis[chloro(methyl)alumino]benzene, and 1,2-bis(dichloroal umino)-benzene in high yield. A complicating factor was the tendency of the R3SnCl by-product to complex with the resulting organoalane, In some cases , exemplified by allyl(methyl)aluminum chloride, such complexation did not interfere with the carbaluminating action of the reagent; in other cases, e xemplified by 1,2-bis[chloro(methyl)-alumino]benzene, the R3SnCl could be r emoved by means of pi-bases and reduced pressures; and in still other struc tures, as with 1,2-bis(dichloroalumino)benzene, the tin chloride could not be dislodged at moderate temperatures. The structure and bonding in such ti n halide-aluminum halide complexes in solution were investigated with the 1 :I adducts of AlCl3 with Me3SnCl and with nBu(3)SnCl, respectively, by mean s of multinuclear NMR spectroscopy. Furthermore, an XRD of the solid comple x, Me3SnCl . AlCl3, was carried out. Such complexes were shown to consist o f 1:I. ion pairs of the type [R3Sn+][AlCl4-] in dilute solution and of a ch iral polymeric helix in the solid state wherein planar Me3Sn+ units are lin ked to each other via bridging tetrachloroaluminate anions, Cl-(AlCl2)-Cl-. Treatment of such complexes with either benzyltriethylammonium chloride (o ne or two equivalents) or tetramethylphosphonium chloride leads to the disp lacement of Me3SnCl and the formation of the expected ionic complexes. Fina lly, the importance of such novel reagents and chelating Lewis acids to org anic synthesis and olefin polymerization is discussed and elucidated.