Synthesis, structure, and reactivity of silicon based main group metal complexes

The synthesis and reactivity of two distinctly different series of silicon based main group metal complexes are reported. The first complexes reported are tripodal donor functionalized carb- and silyl-anions of the alkali metals and the second series of complexes are bulky aluminum disiloxides. Donor functionalized carb- and silyl- anions of the alkali metals have proven to be of particular interest as spectator ligands in organometallic chemistry due to their dual functionality. The interest in these ambidentate ligands spawns from the anion which exhibits a strong trans influence when covalently bound to a metal center. A series of tripodal donor functionalized potassium silanides and methanides were prepared with different donor atoms (O, N, P, and S) incorporated within the ligand backbone.

Bulky aluminum disiloxides contain a strongly Lewis acidic center which is advantageous for the mediation of MPV reduction of aldehydes and ketones. The MPV (Meerwein-Pondorf-Verley) reduction of carbonyl substrates to alcohols is a classical reduction that involves a hydride transfer from a secondary alcohol to a carbonyl compound via coordination to a Lewis acidic aluminum center.1 The reaction is highly chemoselective, operationally simple and uses isopropanol as an inexpensive and non-toxic reducing agent. However, it has not found widespread utility in synthetic chemistry, primarily due to high catalyst loadings often required to obtain satisfactory yields of the reduced product.

Here, we report on the synthesis and structure of new cyclic aluminum siloxides. It will be demonstrated that the newly developed, sterically hindered aluminum siloxides are exceedingly active catalysts in the MPV reduction of ketones and aldehydes under mild and solvent-free conditions with low catalyst loadings. The novel reactivity of this new class of aluminum complexes particularly with regard to the activation of carbonyl substrates and the formation of stable cationic aluminum siloxides will also be discussed.