Thermodynamics of the bonding and cleavage

Mislow and co-workers (258) and Hammond (259) have shown that optically active diaryl sulfoxides, which are configurationally stable in the dark at 200°C, lose their optical activity after 1 hr at room temperature on irradiation with ultraviolet light. Similarly, an easy conversion of the trans isomer of thianthrene-5,10-oxide 206a into the thermodynamically more stable cis isomer takes place upon irradiation in dioxane for 2 hr. However, the behavior of a-naphthylethyl p-tolyl sulfoxide under comparable irradiation conditions is different, namely, it is completely decomposed after 4 min. These differences are not surprising because the photochemical racemization of diaryl sulfoxides occurs by way of the pyramidal inversion mechanism whereas decomposition of the latter sulfoxide occurs via a radical mechanism with the cleavage of the sulfur-carbon bond. It is interesting to note that photoracemization may be a zero-order process in which the rate depends only on the intensity of the radiation and on the quantum yield. 

Less attention has been paid, however, to C02 organometallic chemistry during the past decade. Whilst many reduction or coupling reactions are known to proceed in the presence of stoichiometric or catalytic amounts of transition metal complexes, very few examples remain where the formation of a metal-C02 complex has led to an effective, catalytic reduction reaction of C02. Carbon dioxide complex photoactivation also represents an attractive route to CO bond cleavage, coupled with O-atom transfer. However, progress in the area of C02 utilization requires a better understanding of the reaction mechanisms, of the thermodynamics of reaction intermediates, and of structure-reactivity relationships. 

The term alkene (olefin) metathesis refers to the equilibrium reaction shown in equation (1) in which the alkylidene groups of a pair of alkenes are exchanged with one another in the presence of a transition metal-containing catalyst. The reaction involves the net cleavage of the bonds of the substrate(s) and formation of the new carbon-carbon double bonds of the prodncts. Once equilibrium has been established, the resultant prodnct mixture has a distribution of alkenes (including isomers) that is determined solely by the relative thermodynamic stabilities of the prodncts. 

Proper kinetic and thermodynamic meshing of the reactants is necessary for hydride transfer from a donor to an acceptor. There are at least three obviously different mechanisms by which hydride equivalents can be transferred. These are (i) concerted transfer of the proton and two electrons (equation 5) (ii) homolytic cleavage of the carbon-hydrogen bond followed by subsequent transfer of an electron (equation 6) and (iii) initial loss of a proton followed by transfer of two electrons, either together or stepwise (equation 7). Fusion of pathways is imaginable, (tependent on the structures of the participating molecules and possibilities for catalysis. 

This approach has been used to study the mechanism of a bond-breaking reaction following electron transfer (a dissociative electron transfer). Consider, for example, the case where species Z is an aryl halide, ArX, that becomes reduced by the electrogenerated re-ductant to yield the ultimate products Ar and X . This result can occur either by a concerted path, where bond cleavage occurs simultaneously with electron transfer, or by a stepwise path, where the radical anion, ArX", is an intermediate. Investigations of such reactions have been carried out by redox catalysis, and theoretical analysis of the structural and thermodynamic factors that affect the reaction path have been described (14, 41, 42). Similar considerations apply to oxidation reactions, such as of C20l to form two molecules of CO2. 

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