Hydrogen transfer reactions, table

I 5 Catalytic Activity of Cp Iridium Complexes in Hydrogen Transfer Reactions Table 5.3 Transfer hydrogenation of ketones and imines catalyzed by ll. "... 

The results in Table IV suggest that the condensation reactions cannot be described adequately by the ion-induced dipole model. In this regard the results agree with conventional studies which have frequently found a higher power inverse dependence of the cross-section on the field strength E for condensation reactions than for hydrogen transfer reactions. 

TABLE 9. The primary hydrogen-deuterium kinetic isotope effects for the hydrogen transfer reactions between alkyl radicals and tributyltin hydride (deuteride)... 

Hydrogen transfer reactions are highly selective and usually no side products are formed. However, a major problem is that such reactions are in redox equilibrium and high TOFs can often only be reached when the equilibria involved are shifted towards the product side. As stated above, this can be achieved by adding an excess of the hydrogen donor. (For a comparison, see Table 20.2, entry 8 and Table 20.7, entry 3, in which a 10-fold increase in TOF, from 6 to 60, can be observed for the reaction catalyzed by neodymium isopropoxide upon changing the amount of hydrogen donor from an equimolar amount to a solvent. Removal of the oxidation product by distillation also increases the reaction rate. When formic acid (49) is employed, the reduction is a truly irreversible reaction [82]. This acid is mainly used for the reduction of C-C double bonds. As the proton and the hydride are removed from the acid, carbon dioxide is formed, which leaves the reaction mixture. Typically, the reaction is performed in an azeotropic mixture of formic acid and triethylamine in the molar ratio 5 2 [83],... 

Table 7 Reaction enthalpies (in kj/mol) for hydrogen-transfer reactions (Eqs. 10-12)... 

The aromaticity of azines is reduced relative to benzene, as is evidenced by the RCI values (83JOC1344) (Table IV) as well as by the RE values calculated from the energies of hydrogen-transfer reactions (89JA4178) (Table VII). For example, in the case of pyridine the MP3/6-31G calculated energy of the homodesmotic reaction (50) equals -1.8 kcal/mol. Since the RE of benzene determined from the hydrogenation enthalpies is 36 kcal/mol, pyridine s RE will, accordingly, be 34.2 kcal/mol. 

The chain fragments formed by the recombination of free radicals can be reconverted into radicals by a variety of reinitiation processes, some of which are listed in Table 1. Such reactions can occur in the gas phase via electron collision and on the polymer surface by impact of charged particles or photon absorption. Reinitiation may also be induced in both the gas phase and on the polymer surface by hydrogen transfer reactions. These last processes are similar to the chain transfer processes which occur during homogeneous polymerization. Expressions for the rates of reinitiation are given by Eqns. 20 through 23. 

In some cases when detailed ab initio calculations of potential surfaces have become available, they have confirmed the major qualitative features of the surfaces deduced from experimental data and from preliminary data on correlation diagrams and the asymptotic properties of reactant, product, and intermediate states. One such case is the C+-H2 system,451 which has aroused considerable interst. Angular and energy distributions were determined experimentally for the CH+ + H products from the reactions of the C+(2P) ground state and the C+(4P) excited state with H2 (Table I, Jones et al.9b and references cited therein, and Jones et al.326). Chemiluminescence was also observed from this hydrogen-atom transfer reaction (Table IV).442,443 In addition, the reaction... 

Table 13.1 Hydrogen transfer reactions carried out in 2-propyl alcohol as solvent and inert atmosphere.

Table III shows the relationship between polymerization activity and the evolution of methane as a function of reaction time. Nearly 15 min after mixing of the metallocene and MAO, the catalyst attains its maximum activity the production of methane is low. After 2 h at 10°C, the equilibrium mentioned previously is reached. This corresponding activity is then nearly constant for 17 h. This result shows that another important function of MAO is the reactivation of inactive complexes formed by hydrogen-transfer reactions.

The rate of the hydrogen transfer reaction depends on the structure of the alcohol, amine or thiol as well as on the structure of the cycloalkyne. Cyclooctyne (14) dehydrogenates ethanol, but the reaction is 102—103 times slower than with (31)213). The half-lives of some dehydrogenation reactions are given in Table 12 208). 

The experimental studies reported here in Tables VII-XI indicate that os the propane-to-propylene ratio in alkylation feed was increased from 0 to 3.6/1, alkylate yield and isobutane consumption decreased significantly. The main effect on alkylate composition was an increase in the isoheptanes at the expense of the trimethylpentanes. Since, with propylene feed, trimethylpentane formation is a result mainly of hydrogen transfer reactions, the synthetic propane would be expected to decrease. This decrease was observed. A similar result occurred when normal butane was used as a diluent in alkylation feed. 

Ru(II), Ru(I) and Ir(I) in an aqueous medium (90). Data for the different complexes are shown in Table II. The highest conversion is observed with the ruthenium(II) complexes, which correlates with the facility of Ru complexes to catalyze other hydrogen transfer reactions. The complex PdCl2(TPPMS)2 has been used as a two-phase aqueous-organic catalyst for the carbonylation of allylic chlorides (Eq. 30) 91). The reaction pro-... 

It is clear that cobalt catalysts 10—44 are much less active than cobaloximes, generally by 2 orders of magnitude. It is concluded that the hydrogen transfer reaction is not diffusion controlled in their case. This difference in reactivity also suggests that some of the trends found for cobaloximes may not work for other cobalt chelates. Unfortunately, there have been few studies to this end. Most of the values of Cc in Table 4 were calculated having only one or two points on the Mayo dependence. For cases when Cc < 50, it is usually necessary to carry out ad-... 

---