A study on chiral organocalcium complexes: Attempts in enantioselective catalytic hydrosilylation and intramolecular hydroamination of alkenes

The chiral beta-diketimine ligand [(S)-Ph(Me)CH-N=C(Me)]CH2 was prepared by condensation of acetylacetone with the commercially available chiral building block (S)-Ph(Me)CH-NH2. Reaction of bis(o-Me2N-alpha-Me3Si-benzyl)calcium with this beta-diketimine led to double deprotonation. Reaction of bis(o-Me2N-alpha-Me3Si-benzyl)calcium with the commercially available chiral bis-oxazoline (S)-Ph-BOX gave diastereopure [(S)-Ph-BOX](o-Me2N-alpha-Me3Si-benzyl)calcium which in solution slowly decomposed with formation of o-Me2N-alpha-Me3Si-toluene. The corresponding amide complex [(S)-Ph-BOX]CaN(SiMe3)(2)center dot(THF)(2) is stable and the crystal structure has been determined. In solution, this heteroleptic amide is in Schlenk equilibrium with the homoleptic species [(S)-Ph-BOX](2)Ca and Ca[N(SiMe3)(2)](2)center dot(THF)(2). This Schlenk equilibrium can be steered to the heteroleptic side. Use of the enantiopure calcium amide catalyst for the hydrosilylation of styrene with PhSiH3 or in the intramolecular hydroamination of aminoalkenes gave good product yields, but only small ee-values were observed (5-10%). From stoichiometric reactions of the catalyst with the substrates it is concluded that the "true" catalytically active species is mainly present as a homoleptic calcium complex, which explains the poor enantioselectivities.