Mechanistic Consideration of Asymmetric C-N and C-C Bond Formations with Bifunctional Chiral Ir and Ru Catalysts

作者：Hasegawa Yasuharu; Gridnev Ilya D; Ikariya Takao^*

来源：Bulletin of the Chemical Society of Japan, 2012, 85(3): 316-334.

DOI：10.1246/bcsj.20110307

The mechanism of two enantioselective reactions, direct amination of alpha-cyanoacetates 3 with azodicarboxylates 4 and C-C bond formation reaction of alpha-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](eta(6)-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, a-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 Jr complexes 1a-1c with alpha-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 1r 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.

• 出版日期2012-3-15