Tertiary amine and N-heterocyclic carbene coordinated haloalanes--synthesis, structure, and application.

The preparation of several tertiary amine and N-heterocyclic carbene coordinated chloro- and bromoalanes has been studied and routes to their gram-scale synthesis optimized. This provides a catalogue of well-characterized, thermally stable haloalanes for future application. All complexes have been investigated by spectroscopy (IR, NMR) and, where possible, single-crystal X-ray diffraction structure determination. A particular focus of this article is the relative thermal stabilities of the complexes, which provides a useful handle for the aerobic stability of Group 13 hydride complexes. These thermal data have been elucidated in full and rationalized relative to one another on the basis of Lewis base donation, steric shielding, and relative inductive halide strengthening of the aluminum hydride bonds by halides. All of the four-coordinate complexes reported exist as distorted tetrahedra in the solid state with aluminum to N/C-donor bonds that shorten with the increasing Lewis acidity of the aluminum Lewis acid. The five-coordinate complexes [AlBrH(2)(Quin)(2)] and [AlBr(2)H(Quin)(2)] (Quin = quinuclidine) exist in a trigonal-bipyramidal geometry in the solid state with the amine donors situated in the apical positions. Five chloroalanes; [AlClH(2)(Quin)], [AlClH(2)(Quin)(2)], [AlCl(2)H(Quin)(2)], [AlClH(2)(IMes)], and [AlCl(2)H(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), the latter two of which are aerobically stable, have been applied to the hydroalumination of carbonyl and heterocycle substrates and their chemo-, regio-, and stereoselectivities compared to those of Group 13 hydride reagents cited in the literature. Overall, the reactivities of these species are comparable to non-halogenated alane complexes with the additional benefit of aerobic stability, non-pyrophoricity, and enhanced regioselectivity borne out of greater Lewis acidity.