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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Ni(II) complexes containing (S)-o-[N-(N-benzylprolyl)amino]benzophenone as an auxiliary chiral moiety in the form of a Schiff base with α-amino acids (α-amino acid = glycine, alanine, dehydroalanine; Gly-Ni, Ala-Ni, and Δ-Ala-Ni) were subjected to various types of electrochemical activation (oxidation, reduction, and a treatment with electrogenerated base), affording regio- and diastereoselective synthesis of novel types of binuclear Ni(II) complexes via C–C coupling. New compounds were fully characterized by HRMS, MALDI-TOF, CD, and 1H and 13C NMR (including two-dimensional techniques) spectroscopy; two complexes were characterized by X-ray diffraction analysis. The structures of the novel complexes obtained via electrosynthesis completely match the predictions (made from preliminary voltammetric investigations of the starting complexes as well as from DFT estimations of the energy and symmetry of their frontier molecular orbitals) about the nature of chemical transformations which may follow the electron transfer steps. Electrochemical oxidation of Gly-Ni and Ala-Ni allows access to new dimeric complexes linked via benzophenone moieties in the Ni(II) coordination environment. These new binuclear Ni(II) complexes are of interest as chiral redox mediators for both oxidative and reductive transformations, since they exhibit quasi-reversible electrochemical behavior (their reduced and oxidized forms are stable, at least on the time scale of cyclic voltammetry). Three other binuclear Ni(II) complexes which were obtained via reductive dimerization of the Δ-Ala-Ni complex, via nucleophilic addition of electrochemically deprotonated Gly-Ni to Δ-Ala-Ni, and via oxidative electrochemical dimerization of deprotonated Gly-Ni are of interest as convenient precursors for the stereoselective preparation of diamino dicarboxylic acids HO(O)CCH(NH2)(CH2) n (NH2)CHC(O)OH (n = 2–0), since the obtained binuclear Ni(II)–Schiff base complexes can be easily disassembled using aqueous HCl in methanol.

A Ni(II) glycine/Schiff base complex containing (S)-o-[N-(N-benzylprolyl)amino]benzophenone as an auxiliary chiral moiety was deprotonated using electrochemically generated azobenzene radical anion and used in nucleophilic addition to Michael acceptors, terminal 2,2-and 1,2-disubstituted alkenes ((2E)-1,3-diphenylprop-2-en-1-one, (E)-2-nitroethenylbenzene, 2-methylprop-2-enenitrile, Ni(II) dehydroalanine complex), creating a preparatively convenient path for asymmetric functionalization of the α-glycine carbon in the Ni(II) coordination environment, yielding new chiral Ni(II) complexes. The main advantage of the application of electrochemical techniques is the possibility of precise control of the concentration of a base and its in situ reaction with the complex. This opens up the possibility to carry out further functionalization of the anionic adduct formed in Michael addition via a successive one-pot reaction with the other electrophile. A one-pot cascade reaction of electrochemically deprotonated Ni(II) glycinate with (E)-2-nitroethenylbenzene and the successive interaction with benzyl chloride or dimethyl sulfate allowed a new oxime-containing Ni(II) complex to be obtained, which might be considered as an important synthon. All complexes were reliably characterized using HRMS and 1 H and 13 C NMR (including 2D techniques); an adduct with (2E)-1,3-diphenylprop-2en-1-one was also characterized by X-ray diffraction studies and CD spectrum. The manner of stereocontrol in the Michael addition of electrochemically deprotonated Ni(II) glycinate was shown to be different for terminal 2,2-and for 1,2-disubstituted alkenes. In the case of the 1,2-disubstituted alkene both stereocenters are already formed in the first reaction step, which is reversible and thermodynamically controlled. The second step (protonation of the anion) is fast and irreversible, and it does not influence the stereochemical result of the reaction. In contrast to the previous case, only one stereocenter is formed in the first thermodynamically controlled step for terminal alkenes, whereas the configuration of the second stereocenter is determined by a kinetically controlled protonation step.

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