The aim of this study was to demonstrate the absolute necessity of control experiments for a correct interpretation of mercury drop test results when applied to mechanistic studies of palladacycle-catalyzed reactions. It was shown that the interaction of diverse azapalladacycles with metallic mercury leads to the formation of organomercuric chlorides during the redox-transmetalation process. The structure of these organomercurials was confirmed by elemental analysis, 1H, 13C­{1H}, and 199Hg­{1H} NMR spectra, X-ray diffraction analysis, and DFT calculations. The behavior and properties of C,N-mercuracycles bearing the weak and labile N···Hg bond are discussed on the basis of the temperature dependence of the NMR spectra and calculated thermodynamic parameters of the dechelation process.

Nickel(i1) complexes of Schiff bases derived from (S) -0 - [ (N-benzylprolyl)amino] benzaldehyde and alanine (3), or (S) -0-[ (N-benzylpropyl)amino] benzophenone and alanine (4), or glycine (5) have been used for the asymmetric synthesis of a-amino acids under a variety of conditions. The method of choice consists of the reaction of the corresponding complex with the appropriate alkyl halide in DMF at 25 "C using solid NaOH as a catalyst. Low diastereoselective excess (d.e.) is observed for the alkylation of complex (3) with benzyl bromide and ally1 bromide. Large selectivity (80%) is observed for the alkylation of complex (4). Optically pure (R) -and (S) -a-methyl-a-amino acids [ (S) -a-methylphenylalanine, (S) -a-allylalanine and (S)-0-benzyl-a-methyltyrosine] were obtained (70-90%) after the alkylated diastereoisomeric complexes had been separated on SiO, and hydrolysed with aqueous HCI. The initial chiral reagents were recovered (80-92%). The alkylation of complex (5) gave (S)-alanine, (S) -valine, (S) -phenylalanine, (S) -tryptophan, (S) -isoleucine, (S) -2-aminohexanoic acid, and 3,4-dimethoxyphenylalanine with optical yields of 70-92%. The optically pure a-amino acids were obtained after the separation of the alkylated diastereoisomeric complexes on Si O,. The stereochemical mechanism of the alkylation reaction is discussed.a-Amino acids and their derivatives have numerous biological uses. Non-proteinogenic amino acids are important both because of their pharmaceutical properties l b and their ability to serve as building blocks for physiologically active peptides. In recent years the application of a-amino acids in organic synthesis has grown.2 In all these applications the enantiomerically pure amino acid is needed, and this is the underlying reason for recent progress in the field of asymmetric synthesis of a-amino acids.3 The most significant results were achieved by Schollkopf and his group who developed a general method for the efficient asymmetric synthesis of a-amino acids via alkylation of chiral bis-lactim ethers of dioxopipera~ines.~~ Unfortunately, the method has drawbacks including use of expensive reagents, multi-stage syntheses and, probably, difficulties in scale-up.We believed that the important feature of the successful asymmetric synthesis via the bislactim ethers, e.g. rigid mutual arrangement of the chiral-inducing centre and the prochiral groups, could be realized in chiral a-amino acid complexes with transition metals. The advantages of such a system could be ready formation, the easy recovery of a-amino acids, and also, probably, greater CH acidity of the a-amino acid fragment allowing use of mild alkylation reaction conditions. The application of simple chiral complexes of a-amino acids for the asymmetric synthesis of t h r e~n i n e ,~ asymmetric decarb~xylation,~ and asymmetric transformation of a-amino acids in cobalt(ii1) complexes are well documented. Unfortunately the reaction with alkyl halides gave only very low yields of the described amino acids,7 probably, because the a-...

Achiral, diamagnetic Ni(II) complexes 1 and 3 have been synthesized from Ni(II) salts and the Schiff bases, generated from glycine and PBP (7) and PBA (11), respectively, in MeONa/MeOH solutions. The requisite carbonyl-derivatizing agents pyridine-2-carboxylic acid(2-benzoyl-phenyl)-amide 7 (PBP) and pyridine-2-carboxylic acid(2-formyl-phenyl)-amide 11 (PBA) were readily prepared from picolinic acid and o-aminobenzophenone or picolinic acid and methyl o-anthranilate, respectively. The structure of 1 was established by X-ray crystallography. Complexes 1 and 3 were found to undergo C-alkylation with alkyl halides under PTC conditions in the presence of β-naphthol or benzyltriethylammonium bromide as catalysts to give mono-and bis-alkylated products, respectively. Decomposition of the complexes with aqueous HCl under mild conditions gave the required amino acids, and PBP and PBA were recovered. Alkylation of 1 with highly reactive alkyl halides, carried out under the PTC conditions in the presence of 10% mol of (S)or (R)-2-hydroxy-2′-amino-1,1′-binaphthyl 31a (NOBIN) and/or its N-acyl derivatives and by (S)-or (R)-2hydroxy-8′-amino-1,1′-binaphthyl 32a (iso-NOBIN) and its N-acyl derivatives, respectively, gave rise to R-amino acids with high enantioselectivities (90-98.5% ee) in good-to-excellent chemical yields at room temperature within several minutes. An unusually large positive nonlinear effect was observed in these reactions. The Michael addition of acrylic derivatives 37 to 1 was conducted under similar conditions with up to 96% ee. The 1 H NMR and IR spectra of a mixture of the sodium salt of NOBIN and 1 indicated formation of a complex between the two components. Implications of the association and self-association of NOBIN for the observed sense of asymmetric induction and nonlinear effects are discussed. † A. N. Nesmeyanov Institute.

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