Exothermic hydride transfer reactions

For these aromatic compounds, the major competing reaction channel Is charge transfer, whl h accounts for 8 and 15% of the reactive encounters between CF3 and benzene or toluene, respectively. In toluene, the highly exothermic hydride transfer reaction ... 

That is, hydrocarbon carbonium ions will nearly always undergo exothermic hydride transfer reactions with alkanes having a larger number of carbon atoms, but this is apparently not the case In fluorocarbon systems. An examination of the thermochemistry of these reactions explains these di fering trends.Table X contrasts the nergy gained in going from R to RH for, and sec-... 

On the basis of the kinetic and thermodynamic data, a plausible mechanism for the Tishchenko reaction is presented in Scheme 15. In the first step of the reaction, the precatalyst 1 reacts with two equivalents of the aldehyde to give exothermically the alkoxo complex 42 (Step i in Scheme 15 AHcaic = —68 kcal/mol). A second insertion of an aldehyde into the thorium-alkoxide bond yields complex 43 (step ii in Scheme 15). The concomitant hydride transfer from complex 43 to an additional aldehyde releases the ester 44 and produces the active catalytic species 45 (step iii in Scheme 15). The insertion of an aldehyde into complex 45 (step iv, AHcaic = —25 kcal/mol) gives complex 46, and its hydride transfer reaction (step v, rate determining step, AHcaic = —22 kcal/mol) with an additional aldehyde via a plausible six-centered chair-like transition state (47) produces the ester 38 and regenerates the active complex 45. 

Thus, these simple QM-cluster calculations gave deep insight into Nature s design of the three families of mono-nuclear Mo oxo-transfer enzymes. We have found that the Mo ligands are not selected to make the oxo-transfer reactions as favourable as possible, but rather exothermic by a minimal amount of energy. This is important, because the active-site must be re-reduced or re-oxidized after the oxo-transfer reaction and the reaction must also be favourable. The reduction of DMSO is trivial and can be performed by all three families. The oxidation of sulfite is also rather simple, provided that the repulsion between the active site and the substrate can be overcome. However, the hydroxylation of xanthine is much more complicated and seems to require a unique MoOS active site, in which the S ligand makes the formal hydride-transfer reaction possible. 

As already mentioned, the hydride-transfer reaction is highly exothermic for alkyl ions. This is true for both primary and secondary structures of the propyl ion product. It was therefore of interest to establish the positional probabilities for H transfer from primary and secondary carbons of the neutral molecule. For the reactions of ethyl ions with the molecule CH3CD2CH3, the relative probability H"/D transfer was found to be 1.85, while the ratio D /H transfer from CD3CH2CD3 to ethyl ions was found to be 3.0 0.1. Thus there is an isotope effect slightly favoring transfer of H over D and no substantial preference for transfer from the secondary position of the neutral molecule. It is plausible to attribute this lack of positional discrimination to the substantial exothermicity of reaction for both primary (AH — 6 kcal mole" and secondary (AH — 25 kcal mole H" transfer. These values are based on recent estimates of heat of formation of ground-state alkyl ions. It is expected that ethyl... 

The hydride-transfer reactions of the sec-C H-j ion are only 0.16 eV (3.6 kcal) more exothermic than the corresponding reactions of the sec-C4H9 ion. Indeed, the results given in Table III show that the ion reacts only slightly faster (1.2-1.5 times) than the sec-C Hg ion, and that generally the rates of reaction with different molecules follow the same trends. (The collision rate constant for the propyl ion is about 3-9% greater than that for the butyl ion.)... 

In Table I, we also estimate the probabilities that the collisions between these reactant pairs lead to reaction. Except for the reactions of CF2, we see that the F or F transfer reactions observed in perfluorinated alkanes are extremely inefficient. This low reaction probability is especially striking when compared with the efficiencies of the corresponding reactions in hydrocarbon systems ( ). The methyl ion reaction with ethane corresponding to reaction 4 occurs at every collision between the reactant pair the hydride transfer reactions from propane to vinyl ions or ethyl ions, corresponding to reactions 6 and 7, occur with efficiencies of about 0.5. These differences between the efficiencies of ion-molecule reactions in fluoroalkanes and alkanes can be explained in terms of the thermochemistry of these systems, remembering that endothermic or thermoneutral ion-molecule reactions are quite inefficient, and often can not even be seen on the time scale of ion collection in a mass spectrometer. The highly efficient hydride transfer reactions observed in hydrocarbon systems are all exothermic. In the fluorocarbon systems, o the other j hand, the F transfer reactions listed for 2 3 2 5 actually slightly endothermic, as... 

Solutions of aluminium bromide in dichloromethane used as a catalyst in hydride-transfer equilibrium experiments should be kept cold, as a potentially dangerous exothermic halide exchange reaction occurs on warming. 

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 deuterium KIE values are generally in the range expected for linear three-center hydrogen transfer reactions,44107 and they track nicely with the rate constants for the reactions with the faster, more exothermic reactions displaying smaller KIEs. The large KIE value for reaction of the benzyl radical is noteworthy in that it exceeds the theoretical maximum for the classical model in a manner apparently similar to that seen with tin hydride (see below). 

The formaldehyde disproportionation has been examined by semi-empirical MO methods (Rzepa and Miller, 1985). With the MNDO procedure, transfer of hydride from hydrate mono-anion to formaldehyde is exothermic by 109 kJ mol-1, and the transition structure [29], corresponding to near symmetrical transfer of hydride, lies 72 kJ mol -1 above the separated reactants. Inclusion of two water molecules, to model solvation effects, stabilizes reactants and transition structures equally. Hydride transfer from the hydrate dianion was found to have a less symmetrical transition structure [30] not unexpected for a more exothermic reaction, but the calculated activation energy, 213 kJ mol-1, is unexpectedly high. Semi-classical primary kinetic isotope effects, kH/kD = 2.864 and 3.941 respectively, have been calculated. Pathways involving electron or atom transfers have also been examined, and these are predicted to be competitive with concerted hydride transfers in reactions of aromatic aldehydes. Experimental evidence for these alternatives is discussed later. 

Despite the different nature of the protonating agents, the different exothermicity of their reactions, and the large differences in the composition and the pressure of the gaseous system, the mass-spectrometric results of Field and Munson appear in excellent agreement with those obtained from the study of the reactions the HeT+ ion, as illustrated in Table 22 for the case of c-CeHjg. The formation of protonated cyclohexane, and the occurrence of hydride-ion transfer reactions from c-CgHig to propyl and pentenyl ions (equations 77 and 78 of Fig. 9) were observed by Abramson and Futrell (1967) in their study of the protonation of cyclohexane with CHO+ ions. It should be mentioned, however,... 

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