Hydriding reaction

Above 40 wt % hydrogen content at room temperature, zirconium hydride is brittle, ie, has no tensile ductiHty, and it becomes more friable with increasing hydrogen content. This behavior and the reversibiHty of the hydride reaction are utilized ki preparing zirconium alloy powders for powder metallurgy purposes by the hydride—dehydride process. The mechanical and physical properties of zirconium hydride, and thek variation with hydrogen content of the hydride, are reviewed in Reference 127. 

Digitoxigenin, structure of, 1097 Digitoxin, structure of, 989 Dihalide, alkynes from, 261 Dihedral angle, 94 Diiodomelhane. Simmons-Smith reaction with, 228-229 Diisobutylaluminum hydride, reaction with esters, 812 structure of, 699 Diisopropylamine, pK.d of, 923 1,3-Diketone, pfCa of, 852 Dimethyl disulfide, bond angles in, 20 structure of, 20 Dimethyl ether, electrostatic... 

Lithium aluminum hydride, reaction with aldehydes, 610 reaction with carboxylic acids. 611-612... 

Sodium cyanoborohydride. reductive ami nation with, 931 Sodium cyclamate, LP50 of, 26 Sodium hydride, reaction with alcohols, 605... 

Tributyltin hydride, reaction with alkyl halides. 358 Tricarboxylic acid cycle, see Citric acid cycle... 

Mn02) [56], The XANES spectra at the Ni K-edge indicates that, unlike the ABS alloys, there is very little interaction between hydrogen and Ni but rather strong interactions with Ti, V, and Zr. The hydrogen is presumably located in tetrahedra that contain large fractions of these three elements, whereas the Ni-rich sites are probably empty. Thus the function of Ni in AB2 alloys may be primarily to serve as a catalyst for the electrochemical hydriding reactions. 

The reaction of thiyl radicals with silicon hydrides (Reaction 8) is the key step of the so-called polariiy-reversal catalysis in the radical chain reduction. The reaction is strongly endothermic and reversible with alkyl-substituted silanes (Reaction 8). For example, the rate constants fcsH arid fcgiH for the couple triethylsilane/ 1-adamantanethiol are 3.2 x 10 and 5.2xlO M s respectively. 

Two closely related reactions, (a) and (b), illustrated by Eq. (12) (Rj = HPhj, Etj, Phj, CI3, CljPh) and (13), of silicon hydrides with transition metal complexes generate compounds with Si—M bonds with elimination of hydrogen (a) cleavage of metal-metal bonds and (b) reaction with transition metal hydrides. Reactions discussed in this section are relevant to... 

Organotin Hydrides, Reactions with Organic Compounds, 1, 47... 

It is well recognized that protonated phosphine complexes such as [M(dppe)(H)2]+ (dppe = 2-bis(diphenylphosphine)ethane), M = Co, Ir),39 [Fe(dppe)(L)H]+,40 or [Pt(PEt3)3H]+41 catalyze proton reduction at very negative potentials, 2 V vs. SCE. In contrast, the protonated [(,/s-CsI Is)CoIII P(OMe)3 2I I]1 complex is a catalyst for hydrogen production at —1.15 V vs. SCE at a Hg-pool cathode in pH 5 aqueous buffer.42 Dihydrogen is evolved from the reduced [(r/5-Cd fdCo1"-(P(OMe)3)2H]° form of the complex, which decays to H2 or reacts in a proton-hydride reaction. 

Deuterated and tritiated tin hydrides have been used to prepare deuterated saccharides93 and tritiated steroids46 from alkyl bromides, (equations 68 and 69). It is important to note that isomerization has occurred at the chiral reaction centre in the saccharide reaction (equation 68). For the steroid, the tin hydride reaction is regiospecific, i.e. it only reacts at the more reactive bromide rather than the less reactive chloride site and does not react with the keto group, the hydroxyl group or the acetal group. 

For the primary and secondary a-alkoxy radicals 24 and 29, the rate constants for reaction with Bu3SnH are about an order of magnitude smaller than those for reactions of the tin hydride with alkyl radicals, whereas for the secondary a-ester radical 30 and a-amide radicals 28 and 31, the tin hydride reaction rate constants are similar to those of alkyl radicals. Because the reductions in C-H BDE due to alkoxy, ester, and amide groups are comparable, the exothermicities of the H-atom transfer reactions will be similar for these types of radicals and cannot be the major factor resulting in the difference in rates. Alternatively, some polarization in the transition states for the H-atom transfer reactions would explain the kinetic results. The electron-rich tin hydride reacts more rapidly with the electron-deficient a-ester and a-amide radicals than with the electron-rich a-alkoxy radicals. 

The tertiary a-ester (26) and a-cyano (27) radicals react about an order of magnitude less rapidly with Bu3SnH than do tertiary alkyl radicals. On the basis of the results with secondary radicals 28-31, the kinetic effect is unlikely to be due to electronics. The radical clocks 26 and 27 also cyclize considerably less rapidly than a secondary radical counterpart (26 with R = H) or their tertiary alkyl radical analogue (i.e., 26 with R = X = CH3), and the slow cyclization rates for 26 and 27 were ascribed to an enforced planarity in ester- and cyano-substituted radicals that, in the case of tertiary species, results in a steric interaction in the transition states for cyclization.89 It is possible that a steric effect due to an enforced planar tertiary radical center also is involved in the kinetic effect on the tin hydride reaction rate constants. 

Recently proof has been reported for a heterometallic bimolecular formation of aldehyde from a manganese hydride and acylrhodium species [2], Phosphine free, rhodium carbonyl species show the same kinetics as the cobalt system, i.e. the hydrogenolysis of the acyl-metal bond is rate-determining. Addition of hydridomanganese pentacarbonyl led to an increase of the rate of the hydroformylation reaction. The second termination reaction that takes place according to the kinetics under the reaction conditions (10-60 bar, 25 °C) is reaction (3). The direct reaction with H2 takes place as well, but it is slower on a molar basis than the manganese hydride reaction. 

We can classify the methods of synthesis into two main types, namely nucleophilic substitutions and metal IV hydride reactions. 

PhSeSiRs reacts with BusSnH under free radical conditions and affords the corresponding silicon hydride (Reaction 1.8) [19,20]. This method of generating RsSi radicals has been successfully applied to hydrosilylation of carbonyl groups, which is generally a sluggish reaction (see Chapter 5). 

The reaction of thermally and photochemically generated tcrt-butoxyl radicals with silicon hydrides (Reactions 3.13 and 3.14) has been extensively used for the generation of silyl radicals in EPR studies, time-resolved optical techniques, and organic synthesis. 

S. V. Alapati, J. K. Johnson, and D. S. Sholl, Using First Principles Calculations to Identify New Destabilized Metal Hydride Reactions for Reversible Hydrogen Storage, Phys. Chem. Chem. Phys. 9 (2007), 1438. 

Lewis base adducts, 25 64, 68-69 metal exchange reactions, 25 57 NMR spectra, 25 93-95 pyrolysis, 25 107 silicide formation, 25 110 tetracarbonylsilyl hydride reaction with isoprene, 25 75 reductive elimination, 25 81 site, formation, ribonucleotide reductase, 43 372-375... 

Starting from point 1, a small amount of hydrogen goes into solution in the metal phase as the H2 pressure increases. At point 2, the hydriding reaction begins (Eq, l) and H2 is absorbed at nearly constant pressure. This pressure Pp is termed the "plateau pressure" and corresponds to a two-phase mixture of metal. Me, and metal hydride, MeHx. At point 3, the metal has been completely converted to the hydride phase. Further increases in H2 pressure (point h) result in only a small addition of hydrogen in solution in the hydride phase. In principle this curve is reversible. Extraction of H2 from the gas phase results in the dissociation of the hydride phase in an attempt to maintain the equilibrium plateau pressure. 

---