Hydride-ion transfer

It has been suggested that the Sommelet reaction proceeds by a hydride ion transfer, the acceptor being the conjugate acid of a Schiff base ... 

Several mechanisms have been postulated, all of which propose a hydride ion transfer as a key step. On the basis of the following results, postulate one or more mechanisms that are consistent with all the data provided. Indicate the significance of each observation with respect to the mechanism(s) you postulate. 

These mechanistic interpretations can also be applied to the hydrogenation of cyclohexanones. In acid, the carbonium ion (19) is formed and adsorbed on the catalyst from the least hindered side. Hydride ion transfer from the catalyst gives the axial alcohol (20). " In base, the enolate anion (21) is also adsorbed from the least hindered side. Hydride ion transfer from the catalyst followed by protonation from the solution gives the equatorial alcohol (22). 

Laali and Lattimer (1989 see also Laali, 1990) observed arenediazonium ion/crown ether complexes in the gas phase by field desorption (FD) and by fast atom bombardment (FAB) mass spectrometry. The FAB-MS spectrum of benzenediazonium ion/18-crown-6 shows a 1 1 complex. In the FD spectrum, apart from the 1 1 complex, a one-cation/two-crown complex is also detected. Dicyclo-hexano-24-crown-6 appears to complex readily in the gas phase, whereas in solution this crown ether is rather poor for complexation (see earlier in this section) the presence of one or three methyl groups in the 2- or 2,4,6-positions respectively has little effect on the gas-phase complexation. With 4-nitrobenzenediazonium ion, 18-crown-6 even forms a 1 3 complex. The authors assume charge-transfer complexes such as 11.13 for all these species. There is also evidence for hydride ion transfer from the crown host within the 1 1 complex, and for either the arenediazonium ion or the aryl cation formed from it under the reaction conditions in the gas phase in tandem mass spectrometry (Laali, 1990). 

Tn a double mass spectrometer several types of ion-molecule reactions - can be observed (a) charge exchange, A++B- A + B+, often followed by dissociation of B+ (b) transfer of part of A+ or B (e.g., proton transfer or hydride ion transfer) during the collisions (c) reactions at increased pressure in the collision chamber. 

The major problem in method (a) is that in ion-molecule interchange, considerable momentum in the direction of travel of the incident ion is imparted to both final products. Hence, in a perpendicular type apparatus only transfer of low weight particles can be observed at all and only at very low velocities of the incident ions (1, 9, 10, 11, 12, 13, 19, 20, 23, 27). Cross-sections cannot be measured. The value of these investigations is that some ion-molecule reactions—e.g., proton transfer and hydride ion transfer—can be identified. The energetics and the competition between charge exchange and ion-molecule reactions can be discussed, and by using partially deuterated compounds, one can obtain a detailed picture of the reaction. 

Stripping (Proton or hydride ion transfer) Complex formation... 

The pulsed source method, despite several limitations, appears to be a very useful technique for studying ion-molecule reactions at thermal energies. Although the studies to have date been limited primarily to simple hydrogen transfer reactions, the technique should also prove useful for studying charge transfer and hydride ion transfer reactions at thermal energies. 

Thus the reactant ions for chemical ionization formed in the methane plasma consists of approximately equal amounts of a strong gaseous Bronsted acid (CH5+) and ions which can act either as Lewis acids or Bronsted acids (C2H5+ + C3H5+). These reactant ions will effect the chemical ionization with an added substance by proton transfer or hydride ion transfer, both of which may be accompanied by fragmentation of the ion initially formed. 

Isocyanine, hydride ion transfer in, 43 Isoindoles, from 5-hydroxy-THlSs, 9 Isothiocyanates, adducts with thiazolium salts, 35... 

The addition of monomer to the latter anion leads to the polymer, In this procedure the reaction of sec-BuLi with the C = 0 group is prevented, and due to the very low concentration of the reactive CH2C(CH3) (CO OCH3), Li+ their disproportionation, presumably involving hydride ion transfer, is minimized also. All these factors contribute to the cleanness of polymerization. 

In the case of Tl(III) the overall rate coefficient has been resolved into a product kK for the two steps The large positive AS is due almost entirely to the initial association, which was also studied spectroscopically. An alternative rate determining step in the Pd(II) oxidation is hydride ion transfer to Pd(II) . 

An unusual reaction was been observed in the reaction of old yellow enzyme with a,(3-unsat-urated ketones. A dismutation took place under aerobic or anaerobic conditions, with the formation from cyclohex-l-keto-2-ene of the corresponding phenol and cyclohexanone, and an analogous reaction from representative cyclodec-3-keto-4-enes—putatively by hydride-ion transfer (Vaz et al. 1995). Reduction of the double bond in a,p-unsaturated ketones has been observed, and the enone reductases from Saccharomyces cerevisiae have been purified and characterized. They are able to carry out reduction of the C=C bonds in aliphatic aldehydes and ketones, and ring double bonds in cyclohexenones (Wanner and Tressel 1998). Reductions of steroid l,4-diene-3-ones can be mediated by the related old yellow enzyme and pentaerythritol tetranitrate reductase, for example, androsta-A -3,17-dione to androsta-A -3,17-dione (Vaz etal. 1995) and prednisone to pregna-A -17a, 20-diol-3,ll,20-trione (Barna et al. 2001) respectively. 

The reduction of 7,8-dihydrofolate (H2F) to 5,6,7,8-tetrahydrofolate (H4F) has been analyzed extensively14 26-30 and a kinetic scheme for E. Coli DHFR was proposed in which the steady-state kinetic parameters as well as the full time course kinetics under a variety of substrate concentrations and pHs were determined. From these studies, the pKa of Asp27 is 6.5 in the ternary complex between the enzyme, the cofactor NADPH and the substrate dihydrofolate. The second observation is that, contrary to earlier results,27 the rate determining step involves dissociation of the product from the enzyme, rather than hydride ion transfer from the cofactor to the substrate. 

Figure 4. NADPH structure (A) and the hydride ion transfer pathway (B-D).

This potential was developed to ensure that the molecules inside the sphere never escape and maintain a fully solvated system during molecular dynamics. Here, es, Rs, ew and Rw are the van der Waals constants for the solvent and the wall and rj is the distance between the molecule i and the center of the water sphere, Ro is the radius of the sphere. The quantities A, B and Rb are determined by imposing the condition that W and dW/dr, vanish at r, = Ro. The restraining potential W is set to zero for r, < R0. The van der Waals parameters Es, ew, Rs and Rw can also be specifically defined for different solvents. The constants Awaii and Cwan are computed using a well depth of es = ew = 0.1 kcal and the radius of Rs = Rw = 1.25 A. For the other set of simulations, especially for the hydride ion transfer, we applied periodic boundary conditions by using a spherical boundary shell of 10.0 A of TIP3P40 water to cover the edges of the protein. 

Figure 9. Plot of energy profiles during hydride ion transfer as a function of the H41N...

The free energy associated with the hydride ion transfer from C4 of the nicotinamide ring to C6 of the pteridine ring was computed in a manner... 

Figure 10. The ternary complex of the enzyme dihydrofolate reductase, the substrate and the cofactor during the transition state of the hydride ion transfer. The enzyme backbone atoms are shown alone for clarity and are colored blue. The substrate is shown in yellow and the cofactor is in red. The bond colored in light blue indicates the hydride ion being shared by both the cofactor and the substrate before the transfer to the substrate. Water molecules around the residue pteridine of the substrate and the nicotinamide ring of the cofactor alone are shown and colored in light blue. The yellow spheres represent the sodium ions and the pink spheres the chloride ions.

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