Dirhodium(II) Tetrakis[N-benzene-fused Phthaloyl-(S)-piperidinonate] as a Chiral Lewis Acid: Catalytic Enantioselective Aldol Reactions of Acetate-derived Silylketene Acetals and Aldehydes

Abstract

A first example of chiral dirhodium(II) complex-catalyzed enantioselective Mukaiyama aldol reactions is described. The aldol addition reaction of methyl acetate-derived trimethylsilylketene acetal with specific aldehydes such as benzyloxyacetaldehyde and electron-poor aromatic aldehydes is effectively catalyzed by dirhodium(II) tetrakis[N-benzene-fused phthaloyl-(S)piperidinonate] (1a), providing silylated aldol adducts in up to 94% ee. The Lewis acid-catalyzed addition of silyl enol ethers and silylketene acetals to carbonyl compounds, popularly known as the Mukaiyama aldol reaction, is one of the most powerful methods available for achieving carbon-carbon bond formation. 1 Over the past fifteen years, remarkable progress has been achieved in the development of enantioselective variants through catalysis by chiral Lewis acid complexes derived from Sn(II), Ti(IV), B, Al, Cu(II), Ag(I), and Zr(IV) with excellent levels of enantioselection. 2-10 Aside from their effectiveness in diazo decomposition, 11 dirhodium(II) complexes are now becoming recognized as a new class of chiral Lewis acid catalysts. Doyle and co-workers demonstrated that chiral dirhodium(II) carboxamidates function as effective catalysts for enantioselective hetero-Diels-Alder (HDA) reactions and [2+2] cycloaddition reactions of trimethylsilylketene with ethyl glyoxylate. We also have disclosed that Rh 2 (S-BPTPI) 4 (1a), a dirhodium(II) carboxamidate complex Dedicated to the memory of the late