Cleavage of cyclodimethylsiloxanes by dialkylaluminum hydrides and the nature of the siloxyaluminum products

The interaction of R2AlH with cyclic siloxanes leads td rupture of the sili con-oxygen framework and yields aluminum polysiloxides. The room-temperatur e reaction of R2AlH with (Me2SiO)(3) yields the asymmetrical siloxide compo unds R2Al(mu-OSiMe2H)(mu-OSiMe2OSiMe2H)AlR2, R = Bu-t (1), Bu-i (2). In con trast, the reaction of (Bu-t)(2)AlH with (Me2SiO)(3) or (Me2SiO)(5), in ref luxing toluene yields [(Bu-t)(2)Al(mu-OSiMe2H)](2) (4), as does the reactio n of Al(Bu-t)(3) with (Me2SiO)(3). Me2Al(mu-OSiMe2H)(mu-OSiMe2OSiMe2H)AlMe2 (3) is formed from the reaction of Me2AlH with (Me2SiO)(3) in hexane when using a reagent Si:Al ratio of 0.5; higher Si:Al ratios result in the forma tion of the siloxide-bridged compound Me2Al(mu-OSiMe2H)AlMe2(mu-OSiMe2O)Me2 Al(mu-OSiMe2H)AlMe2 (5). The asymmetrical polysiloxide compound (Bu-t)(2)Al (mu-OSiMe2H)(mu-OSiMe2OSiMe2OSiMe2H)Al(Bu-t)(2) (6) is formed from the reac tion of(Bu-t)(2)AlH with (Me2SiO)(4), while compound 5 and its isobutyl ana logue (7) are formed with Me2AlH and (Bu-t)(2)AlH, respectively. Reaction o f R2AlH with (Me2SiO)(5) yields the siloxide-bridged compounds R2Al(mu-OSiM e2H)AlR2(mu-OSiMe2OSiMe2O)R2Al(mu-OSiMe2H)AlR2, R = Bu-t (8) and Bu-i (9). If the reaction of (Bu-t)(2)AlH with (Me2SiO)(5), is carried out in the pre sence of THF, then (Bu-t)(2)Al(mu-OSiMe2H)Al(Bu-t)(2)(mu-OSiMe2OSiMe2O)Al(B u-t)(2)(THF) (10) may be is elated. The reaction of Me2AlH with (Me2SiO)(5) yields compound 5. The symmetric siloxide [(Bu-t)(2)Al(mu-OSiMe3)](2) (11) is formed from the reaction of(Bu-t)(2)AlH with (Me3Si)(2)O. The reaction of the noncyclic siloxane Me3Si(OSiMe2)(2)OSiMe3 yields a mixture of (Bu-t) (2)Al(mu-OSiMe2H)(mu-OSiMe3)Al(Bu-t)(2) (12) and (Bu-t)(2)Al(mu-OSiMe2H)(mu -OSiMe2OSiMe3)Al(Bu-t)(2) (13). A combination of H-1, C-13, Al-27, and Si-2 9 NMR allow for characterization of all compounds. In addition, the 29Si NM R spectra of compounds 1-3 and 6 exhibit unusual (3)J(Si-H) coupling (1.0 H z), and the Al2O2 core is found to exhibit a large deshielding effect on ad jacent silicon. The molecular structures of compounds 1, 4, 8, 10, 11, and 12 have been determined by X-ray crystallography. The reaction pathway for the cleavage of polysiloxanes is discussed.