V. I. Bakhmutov

General method for the asymmetric synthesis of α-amino acids via alkylation of the chiral nickel(II) Schiff base complexes of glycine and alanine

Nickel(II) complexes of Schiff bases derived from (S)-o-[(N-benzylprolyl)amino] benzaldehyde and alanine (3), or (S)-O-[(N-benzylpropyl)amino]benzophenone and alanine (4), or glycine (5) have been used for the asymmetric synthesis of α-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 allyl bromide. Large selectivity (80%) is observed for the alkylation of complex (4). Optically pure (R)- and (S)-α-methyl-α-amino acids [(S)-α-methylphenylalanine, (S)-α-allylalanine and (S)-O-benzyl-α-methyltyrosine] were obtained (70–90%) after the alkylated diastereoisomeric complexes had been separated on SiO2 and hydrolysed with aqueous HCl. 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 3,4dimethoxyphenylalanine with optical yields of 70–92%. The optically pure α-amino acids were obtained after the separation of the alkylated diastereoisomeric complexes on SiO2. The stereochemical mechanism of the alkylation reaction is discussed.

Crown Compounds for Anions: Sandwich and Half-Sandwich Complexes of Cyclic Trimeric Perfluoro-o-phenylenemercury with Polyhedralcloso-[B10H10]2− andcloso-[B12H12]2− Anions

It has been shown by IR and NMR spectroscopy that cyclic trimeric perfluoro-o-phenylenemercury (o-C6F4-Hg)3 (1) is capable of binding closo-[B10H10]2- and closo-[B12H12]2- anions to form complexes [[(o-C6F4Hg)3](B10-H10)]2- (2), [[(o-C6F4Hg)3]2(B10H10)]2-(3), [[(o-C6F4Hg)3](B12H12)]2- (4), and [[(o-C6F4Hg)3]2(B12H12)]2- (5). According to IR data, the bonding of the [B10H10]2- and [B12H12]2- ions to the macrocycle in these complexes is accomplished through the formation of B-H-Hg bridges. Complexes 2, 3, and 5 have been isolated in analytically pure form and have been characterized by spectroscopic means. X-ray diffraction studies of 3 and 5 have revealed that these compounds have unusual sandwich structures, in which the polyhedral di-anion is located between the planes of two molecules of 1 and is bonded to each of them through two types of B-H-Hg bridges. One type is the simultaneous coordination of a B-H group to all three Hg atoms of the macrocycle. The other type is the coordination of a B-H group to a single Hg atom of the cycle. According to X-ray diffraction data, complex 2 has an analogous but half-sandwich structure. The obtained complexes 2-5 are quite stable; their stability constants in THF/acetone (1:1) at 20 degrees C have been determined as 1.0 x 10(2)Lmol(-1), 2.6 x 10(3)L(2)mol(2), 0.7 x 10(2)Lmol(-1), and 0.98 x 10(3)L(2)mol(-2), respectively.

In situ IR and NMR study of the interactions between proton donors and the Re(I) hydride complex [{MeC(CH2PPh2)3}Re (CO)2H]. ReH…H bonding and proton-transfer pathways

Abstract The reactions of various proton donors (phenol, hexafluoro-2-propanol, perfluoro-2-methyl-2-propanol, monochloroacetic acid, and tetrafluoroboric acid) with the rhenium (I) hydride complex [(triphos)Re(CO)2H] (1) have been studied in dichloromethane solution by in situ IR and NMR spectroscopy. The proton donors from [(triphos)Re(CO)2H…HOR] adducts exhibiting rather strong H…H interactions. The enthalpy variations associated with the formation of the H-bonds (−ΔH = 4.4–6.0 kcal mol−1) have been determined by IR spectroscopy, while the H…H distance in the adduct [(triphos)Re(CO)2H…HOC(CF3)3] (1.83 A) has been calculated by NMR spectroscopy through the determination of the T1min relaxation time of the ReH proton. It has been shown that the [(triphos)Re(CO)2H…HOR] adducts are in equilibrium with the dihydrogen complex [(triphos)Re(CO)2(η2-H2)]+, which is thermodynamically more stable than any H-bond adduct.

Synthesis of enantio- and diastereo-isomerically pure β- and γ-substituted glutamic acids via glycine condensation with activated olefins

The glycine fragment in the nickel(II) complex formed from the Schiff base of glycine and (S)-o[(N-benzylprolyl)amino]benzophenone undergoes base-catalysed Michael addition in methanol in the presence of MeONa to the activated olefins methyl acrylate, acrylonitrile, methyl methacrylate, acrolein, and methyl trans-cinnamate. Complexes of substituted (S)-glutamic acid or its derivatives were formed in good chemical yields with almost complete diastereoselection at the α-carbon atom of the amino acid moiety. Diastereoselection at the β- and γ-atoms was not significant, but the isomeric complexes could be easily separated chromatographically. Cleavage of the pure diastereo-isomers with aqueous HCl gave, in good yields, optically pure glutamic acids and regenerated the original chiral reagent. The configurations of the amino acid β- and γ-carbon atoms were determined by 1H n.m.r. spectroscopy and crystal structure X-ray analysis of the corresponding original complexes. The addition to acrolein, catalysed by triethylamine in methanol, leads to the 1,4-adduct exclusively. The amino acid thus obtained could be converted into (S)-proline by reduction with NaBH4.

Dihydrogen bonding of decahydro-closo-decaborate(2−) and dodecahydro-closo-dodecaborate(2−) anions with proton donors: experimental and theoretical investigation

Abstract The interactions of [Bu 4 N] 2 [B 10 H 10 ] and [Bu 4 N] 2 [B 12 H 12 ] with various proton donors (MeOH, EtOH, Pr i OH, PhOH, 4-FC 6 H 4 OH, 4-NO 2 C 6 H 4 OH, CF 3 CH 2 OH, (CF 3 ) 2 CHOH, (CF 3 ) 3 COH) in low polarity media were investigated. The site of coordination for [B 10 H 10 ] 2− and [B 12 H 12 ] 2− was found to be hydride hydrogen. Spectral (IR, NMR) evidences for the BH⋯HO hydrogen bonding between the boron hydrides and the OH proton donors in solution are presented. Spectral (Δ ν , Δ ν 1/2 , ΔA) and thermodynamic (Δ H °, Δ S °) characteristics of the H-complexes were determined. The BH⋯HO bonding strength increases from [B 12 H 12 ] 2− to [B 10 H 10 ] 2− . The geometry, energy, as well as electron distribution in the [B 10 H 10 ] 2− ·HOCH 3 , [B 10 H 10 ] 2− ·HOCF 3 , [B 10 H 10 ] 2− ·HCN, and [B 12 H 12 ] 2− ·HOCH 3 complexes were studied using ab initio HF/6-31G approximation. It was shown that increase of the proton donor ability of acids leads to formation of bifurcate H-bonds.

General method for the asymmetric synthesis of anti-diastereoisomers of β-substituted L-2-aminobutanoic acids via chiral nickel(II) Schiff's base complexes of dehydroaminobutanoic acid. X-Ray crystal and molecular structure of the nickel(II) complex of the Schiff's base from [(benzylprolyl)amino]benzophenone and dehydroaminobutanoic acid

An efficient approach to the asymmetric synthesis of (L)-allo-isomers of β-substituted α-aminobutanoic acid is described. The chiral NiII complex of a Schiffs base derived from (S)-o-[N-(N-benzylprolyl)amino]benzophenone (BBP) and glycine was treated with acetaldehyde in MeOH. The addition proceeds with high diastereoselectivity to give, if catalysed by MeONa, the corresponding complex of (R)-threonine, and, if catalysed by Et3N, the corresponding complex of (S)-allo-threonine. The (R)-threonine complex was converted into the chiral NiII complex of dehydroaminobutanoic acid, and a X-ray diffraction structural study of its major isomer showed that the dehydroaminobutanoic acid moiety was in the E-configuration. The complex, in turn, entered into Michael addition reactions with nucleophiles, including MeOH, EtOH, PhSH, and PhCH2SH. The reaction proceeded with high diastereoselectivity, producing predominantly complexes of the allo-threonine derivatives (d.e. > 90%). Diastereoisomerically and enantiomerically pure α-amino acids were obtained after chromatographic purification, decomposition of the complexes, and recovery of the initial chiral auxiliary, BBP. The thiol addition reaction is accompanied by a side reaction leading to the formation of sizeable amounts of the vinylglycine complex. An approach to the synthesis of optically active vinylglycine starting with racemic methionine is described.

Is RuH4(PPh3)3 in solution indeed a non-classical hydride?

Abstract A novel systematic investigation of temperature dependences of 1 H, 2 H, 31 P NMR spectra and 1 H T 1 , T 2 and 2 H T 1 relaxation times has been carried out for solutions of RuH 4 (PPh 3 ) 3 ( 5 ) and its isotopomers in toluene. The linewidth of the H-ligands of 5 in the 180-310 K temperature range is governed by T 2 and J (HP), the chemical shift does not depend on the temperature and the character of the T 1 change is the same for all protons of the complex. It has been shown that the 1 H T 1 ( T ) dependences are not consistent with the theoretical data, but the T 1 min value can be used to calculate the distances between H-ligands. The calculated distances under the assumption of the classical and non-classical models of 5 , the found quadrupole coupling constant of D-ligands (68 ± 3 kHz) and J (HD) ( c . 2.7 Hz) have revealed an inadequacy of those models by a whole set of experimental data. A dynamic structure involving fast pairwise approachment-detachment of the H-ligands is suggested and discussed.

Crown compounds for anions. Spectroscopic and theoretical studies of complexation of borohydride anions with cyclic trimeric perfluoro-o-phenylenemercury

It has been shown by IR and NMR spectroscopy that cyclic trimeric perfluoro-o-phenylenemercury (o-C6F4Hg)3 (1) is capable of binding borohydride anions in THF and 1,2-dichloroethane solutions to form complexes {[(o-C6F4Hg)3](BH4)2}2− (2) and {[(o-C6F4Hg)3]2(BH4)}− (3). According to the IR data, complex 2 contains both terminal and coordinated B–H bonds while all four B–H bonds of the BH4− ion are coordinated with the mercury atoms in complex 3. The use of NMR spectroscopy provided the identification of one more complex of 1 with BH4−, which can be formulated as {[(o-C6F4Hg)3](BH4)}− (4). The stability constants of complexes 2 and 3 have been determined by IR spectroscopy. Quantum-chemical calculations of 2–4 by the AM1 method suggest that the complexes have the unusual bipyramidal, sandwich and half-sandwich structures, respectively. In contrast to 1, the interaction of (C6F5)2Hg with BH4− anions in THF gives a single complex {[(C6F5)2Hg](BH4)}− (5) independently of the reagent ratio. This complex is much less stable than 2 and 3.

Synthesis of a chiral nickel(II) complex of an electrophilic glycinate, and its use for asymmetric preparation of α-amino acids

A chiral NiII complex of a Schiffs base derived from (S)-o-[(benzylpropyl)amino]benzophenone and α-bromoglycine has been obtained and its stereoselective reaction with nucleophiles studied; the synthesis of aspartic acid with 80% optical purity is described.

Enantioselectivity of nickel(II) and copper(II) complexes of Schiff bases derived from amino acids and (S)-o-[(N-benzylprolyl)amino]-acetophenone or (S)-o-[(N-benzylprolyl)amino]benzaldehyde. Crystal and molecular structures of [Ni{(S)-bap-(S)-Val}] and [Cu{(S)-bap-(S)-Val}]

Copper(II) and nickel(II) complexes of Schiff bases derived from (S)-o-[(N-benzylprolyl)amino]-acetophenone [(S)-bap] and the amino acids (aa) : glycine, (R)- and (S)- and (S)-valine (val), (R)- and (S)-adamant-1-ylalanine, and (R)- and (S)-adamant-1-ylglycine have been synthesized. The structure of the complexes has been determined by physical and chemical methods; in addition the structures of [Cu{(S)-bap-(S)-Val}] and [Ni{(S)-bap-(S)-Val}] have been determined by X-ray crystallographic analysis. The kinetic CH-acidity and deuterium exchange of the hydrogen of an amino acid moiety under the action of bases has been studied. The deuterium exchange is accompained with epimerization which results in 80% excess of the (S)-2-[2H]amino acid. The stereoselectivity in the nickel complexes has been found to be higher than in the copper ones. The epimerization rate constant lies within the range 5 × 10–4–1.2 × 10–1 dm3 mol–1 s–1. The observed stereoselective effects are interpreted on the basis of X-ray analysis, circular dichroism, and 1H n.m.r. spectroscopic data. It is suggested that the [Ni{(S)-bap-aa}] complexes can be used for determining the absolute configuration of the amino acids.