Novel Organoaluminum Lewis Acids via Selective Aluminum-Tin Exchange Processes – Electrophilic Initiation by the Aluminum Halide and Ensuing Complexation by the Resulting Tin Halide

Abstract

With the goals of preparing novel carbaluminating reagents and mono- and bidentate organoaluminum Lewis acids, the scope and limitations of synthesizing the requisite organoalanes by the aluminum-tin exchange between an aluminum 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)benzene and various aluminum chlorides of the type, RnAlCl3–n (R = Me, Et), gave selective aluminum-tin exchange at the sp- or sp2-hybridized carbon–tin bond and produced such organoalanes as allyl(methyl)aluminum chloride, benzylaluminum dichloride, 1,2-bis(diethylalumino)ethyne, 1,2-bis(dimethylalumino)benzene, 1,2-bis[chloro(methyl)alumino]benzene, and 1,2-bis(dichloroalumino)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, exemplified by 1,2-bis[chloro(methyl)alumino]benzene, the R3SnCl could be removed by means of π-bases and reduced pressures; and in still other structures, as with 1,2-bis(dichloroalumino)benzene, the tin chloride could not be dislodged at moderate temperatures. The structure and bonding in such tin halide–aluminum halide complexes in solution were investigated with the 1:1 adducts of AlCl3 with Me3SnCl and with nBu3SnCl, respectively, by means of multinuclear NMR spectroscopy. Furthermore, an XRD of the solid complex, Me3SnCl · AlCl3, was carried out. Such complexes were shown to consist of 1:1 ion pairs of the type [R3Sn+][AlCl4−] in dilute solution and of a chiral polymeric helix in the solid state wherein planar Me3Sn+ units are linked to each other via bridging tetrachloroaluminate anions, Cl–(AlCl2)–Cl−. Treatment of such complexes with either benzyltriethylammonium chloride (one or two equivalents) or tetramethylphosphonium chloride leads to the displacement of Me3SnCl and the formation of the expected ionic complexes. Finally, the importance of such novel reagents and chelating Lewis acids to organic synthesis and olefin polymerization is discussed and elucidated.