2012 Volume 85 Issue 3 Pages 316-334
The mechanism of two enantioselective reactions, direct amination of α-cyanoacetates 3 with azodicarboxylates 4 and C–C bond formation reaction of α-cyanoacetates with acetylenic esters 6, catalyzed by chiral bifunctional Ir and Ru complexes, Cp*Ir[(S,S)-N-sulfonated dpen] 1 and Ru[(S,S)-N-sulfonated dpen](η6-arene) 2 (DPEN: 1,2-diphenylethylenediamine) was studied by NMR spectroscopic analysis combined with DFT analysis. Notably, these two reactions using the same chiral amide catalysts 1, 2 and pronucleophile, α-cyanoacetates 3 gave quantitatively the conjugate adducts bearing quaternary chiral carbon centers in excellent enantiomeric excess albeit with the opposite absolute configuration depending on the acceptor molecules 4 and 6. NMR investigation of the reactions between Ir complexes 1a–1c with α-cyanoacetates 3 showed that a stereoselective deprotonation reaction takes place to give an equilibrium mixture of N-bound amine complexes 8 and 9, the former with intramolecular hydrogen bonding and the latter without it, respectively. Computational study revealed the full details of the mechanism of the asymmetric C–N and C–C bond forming reactions catalyzed by the chiral Ir catalyst 1b. In the C–N bond forming reaction, the dimethyl azodicarboxylate 4a undergoes productive bifunctional activation by a non-hydrogen-bonded N-bound complex 9b(re) resulting in the formation of the R-product through the energetically favorable transition state. On the other hand, the linear geometry of the acetylenic ester molecule 6 allows its bifunctional activation with both types of the N-bound complexes: 8b and 9b with and without the intramolecular hydrogen bond respectively. The hydrogen-bond stabilized transition state for the C–C bond formation leading to the S-enantiomer is significantly lower in energy than the corresponding non-hydrogen-bonded transition state leading to the R-enantiomer. Thus, chiral induction of these two reactions is determined by the structures of the acceptor molecules.
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