Hydrogen abstraction thermodynamics

If hydrogen transfer is under thermodynamic control, then the vitamin will experience cleavage of the weakest CH (or OH) bond. Compare energies ofradicals derived from hydrogen abstraction at different positions from a model a-tocopherol (R = CH3). Which radical is most stable Are there alternative radicals of similar stabihty ... 

Hydrogen abstraction by fluorine is thermodynamically advantageous, since C—H bond strengths are about 410kJ/mol compared to 560kJ/mol for H—F and 450-500 kJ/mol for C—F bonding. 

In a photosensitized reaction, radical induced isomerization can occur if the sensitizer undergoes either homolytic decomposition or hydrogen abstraction, or if the system contains impurities which give radicals on irradiation. The result may be to shift the measured photostationary state in the direction of thermodynamic equilibrium and to give anomalously high values of the quantum yields for cis-trans isomerization. 

Prior to the work described below (50), hydrogen transfer to superoxometal complexes has been proposed by some (124 126) and questioned by others (127) who introduced plausible alternative mechanistic pathways. The work with rhodium hydrides (50) sought to establish whether hydrogen abstraction by superoxo complexes is a feasible and reasonable mechanism for thermodynamically favorable cases. 

Hydrogen abstraction by Craq002+ from C-H, OH, and Rh-H bonds is kinetically facile provided the thermodynamics are favorable (143). Much work remains to be done with other LMOO compounds to sort out various influences on the strength of the LMOO H bonds, and thus, by implication, the tendency of LMOO to abstract hydrogen. 

The Hammond Postulate applies only if both forward reactions are fast. Obtain energies for the transition states leading to 1 -propyl and 2-propyl radicals (propane+F end andpropane+F center). Draw an energy diagram for each hydrogen abstraction reaction (place the diagrams on the same axes). Do these diagrams indicate that use of the Hammond Postulate is justified Calculate the barrier for each reaction, and calculate the relative concentrations of 1-propyl and 2-propyl radicals that would form at 298 K if each reaction were irreversible. Use equation (2). How does this (kinetic) ratio compare to the equilibrium (thermodynamic) ratio of these radicals ... 

Our earlier studies of the bimolecular alkene addition reactivity of a,a-difluoro alkyl radicals indicated that they exhibited little philicity , reacting with styrene and pentafluorostyrene (IPs 8.43 and 9.20, respectively) at virtually the same rate [70]. Their significantly greater reactivity in bimolecular additions, hydrogen-abstraction reactions and unimolecular cyclizations can therefore be largely attributed to their pyramidal nature, with some possible thermodynamic con-... 

In the case of the epimeric pairs [146], [147], and [34], the experiments are open to the objection that the thermodynamically most stable epimer is the slowest to react, as would be expected anyway. This is not the case with epimeric pair [148], where abstraction of axial hydrogen by (CH3)3CO is still preferred despite the epimer with the axial methyl group being the less stable (Beckwith and Easton, 1981). However, since the electron —13C coupling constants indicate that dialkoxy radicals are not completely tetrahedral, it follows that there will be some least motion effects favouring axial hydrogen abstraction, as there will with the epimeric pairs [146] and [147]. 

Previtali and Scaiano have attempted to correlate rates of hydrogen abstraction with thermodynamic parameters 69> along the lines of Polanyi 70>. In this approach (AH proportional to AH) the exothermicity of hydrogen abstraction varies with the triplet excitation energy and the carbonyl 51-bond energy, at constant C—H and O—H bond energies. 

Whether these radicals are rapidly inverting a radicals or n delocalized radicals cannot be ascertained but the almost exclusive product in both these cases is the thermodynamically less stable isomer 75 and 79. It is in the hydrogen abstraction step that the overall stereochemistry is controlled and not by the a-substituent (see below). 

In a series of papers, Fuchs has reported allylation [104], alkenylation [105, 106] and alkynylation [107-110] of C-H bonds. All these processes are based on the radical -fragmentation of triflones liberating a trifluroromethyl radical suitable for hydrogen atom abstraction (Scheme 24). The regioselectivity of the hydrogen abstraction has been examined and is governed by polar and thermodynamic effects as shown in Scheme 24 [107]. 

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