The solid-state reaction of isotope exchange of L-alanine (L-AIa) with spillover-hydrogen activated on a Rh(Pd)-supt~orted catalyst was studied. The reactivity of the carbon atoms and the activation energies of isotope exchange of the hydrogen at the C(2~ and C(3) atoms of the t.-Ala molecule were determined using tritium NMR. The ab initio calculations of the activation energy of a model reaction between the alanine molecule and a hydroxonium cation were carried out. The mechanism and plausible structures of the transition states of this reaction were proposed.Key words: isotope exchange, quantum-chemical calculations, activation energy, hydrogen spillover.The interaction of gaseous hydrogen with metals of the platinum group results in molecular sorption, atomic chemisorption, and dissolution of hydrogen in the metal. In the case of supported catalysts such as Pd/BaSO 4 and Pd/A1203, the H atoms bound to the surface metal atoms can migrate to the support (spillover hydrogen, SH). t This activated hydrogen, located on the support surface, can enter the reactions characteristic of hydrogen on the surface of metals of the platinum group. The hydrogenation of unsaturated compounds, reduction of WO 3 to tungsten bronze, and isotope exchange in the hydroxyl groups of the inorganic supports, proceeding with participation of SH are known. Stereoselective hydrogenation of the aromatic group with the formation of an optically active compound 2 proceeds in a solid "heterogeneous catalyst--asymmetric crystals of a substituted phenol" mixture under the action of SH at room temperature. The nature of SH has not been established unambiguously up to the present despite the fact that processes using hydrogen spillover have been known for many years. According to several existing hypotheses, it is a solvated proton, 3 a proton-electron pair, 4 or an atomized hydrogen, s No theoretical studies using a quantum-chemical simulation of the reactions with participation of SH have beet~ published yet.Reaction based on applying high-temperature solidstate catalytic isotope exchange (HSCIE) 6,7 appears to be particularly efficient for the synthesis of biologically active compounds labeled by hydrogen isotopes. Intense isotope exchange between the H atoms of a solid organic compound and the activated hydrogen occurs in the solid mixture formed by a metal of the platinum group, a solid organic compound, and an inorganic support. As a rule, HSCIE proceeds to a large extent with retention of configuration of the asymmetric carbon atoms. H SCI E was the first reaction that made it possible to obtain organic compounds uniformly labeled with tritium; such compounds were inaccessible previously. The present work is dedicated to a theoretical and experimental study of the mechanism of HSCIE with tritium in alanine. ExperimentalSolid-state isotope exchange between L-alanine and tritium. A solid mixture containing 0.5 mg of L-alanine (L-Ala) {"Sigma") was placed into an ampule (10 mL). For this purpose, 25 mg of activated carbon (Norit A, "Serva") c...
The simplest quantum-chemical models of hydrogen spillover over a graphite-like surface as a proton or radical have been considered. The condensed planar C24H12 molecule was tzsed as a model surface. The ab initio calctdations of the interaction of hydrogen with the model surface were carried ot:t by the restricted Hartree--gock (tIF) method in the STO-3G and 6-31G" basis sets. The radical hydrogen can not bind to such a surface, whereas tile proton binds to it with an energy release of 186 kcal rod[ -1. The activation energy of the transfer of the proton between two neighboring carbon atoms (10 kcal tool -1) has been determined. The simplest model of the hydrogen migration as a proton over tile model surface can be used for describing the spillover of hydrogen over the graphite surface.
Ab initio calculations of molecular and electronic structures of neutral molecules and protonated forms of methionine and its derivatives in the gaseous phase were carried out by the Hartree--Fock method using the 6-31G* basis set with full geometry optimization. Proton affinities of methioninc (I), methionine sulfoxide (2), and methionine sulfone (3) were calculated for different modes of coordination of the proton. The results of calculations demonstrated that in protonated forms of 1 and 3, bonding between the proton and the N atom is most favorable, while in protonated form of 9_ bonding between the proton and the O atom of the SO group is most favorable. The proton aff• of the amino acids are as follows: 223.2 (1), 241.2 (2), and 22t.5 (3) kcal tool -1, Le., methionine sulfoxide Z exhibits the highest proton affinity in the series of the amino acids under consideration.
The solid-phase catalytic hydrogenation of (R)-4-tert-butoxy-~XLpyrroline-2-carboxylic acid under the action of hydrogen spillover was studied. The reaction proceeds stereoselectively with the predominant formation of the L-amino acid. The configuration of the asymmetric center formed is determined by that of the asymmetric C(4) atom. The major portion of the isotope label is incorporated into the allylic C(3) and C(5) positions, and the ~-[-t atoms are more mobile. Using quantum-chemical calculations, the geometric structure of the L-hydroxyproline molecule was calculated, and the spin-spin coupling constants for this tritium-!abeled amino acid were determined.Key words: solid-phase hydrogenation; quantum-chemical calculation; stereochemist~ of addition; 3H NMR; hydrogen spillover.Tritium labeling of amino acids is widely used in medical biological studies. A multiple tritium label, which decreases the limits of determination of labeled compounds, is especially valuable. Liquid-phase catalytic reduction of unsaturated compounds-precursors is the main method for the incorporation of tritium into aliphatic amino acids) If this reaction is carried out in a solid mixture of a finely dispersed platinum group metal, an inorganic support, and an unsaturated amino acid derivative, the degree of incorporation of tritium into the amino acid increases. 2,3 Solid-phase catalytic hydrogenation (SCH) can be performed in the absence of a direct contact between the catalyst and the substrate. The transfer of hydrogen activated on the catalyst to the reaction zone is called hydrogen spillover. 4 Asymmetric cyclohexanols were obtained by the reaction of hydrogen spillover (HS) with asymmetric crystals of substituted phenol. 5 In this work, the stereochemistry of the solid-phase hydrogenation of the diastereotropic multiple bond in (R)-4-tert-butoxy-M-pyrroline-2-carboxylic acid under the action of HS was studied. ExperimentalThe studied transformations of L-hydroxyproline derivatives are presented in Scheme 1. Synthesis of (R)-4-tert-butoxy-ALpyrrolin-2-carboxytie acid(2). (R)-4-tert-Butoxy-L-proline 1 (0.56 g, Bachem) was added to a 0.3 M solution of CuSO 4 (5 mL). 7"lien 5 M KOH
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