Atom and group transfer

The above model has been further explored to account for reaction efficiencies in terms of a scheme where nucleophilicities and leaving group abilities can be rationalized by a structure-reactivity pattern. Pellerite and Brau-man (1980, 1983) have proposed that the central energy barrier for an exothermic reaction (see Fig. 3) can be analysed in terms of a thermodynamic driving force, due to the exothermicity of the reaction, and an intrinsic energy barrier. The separation between these two components has been carried out by extending to SN2 reactions the theory developed by Marcus for electron transfer reactions in solutions (Marcus, 1964). While the validity of the Marcus theory to atom and group transfer is open to criticism, the basic assumption of the proposed model is that the intrinsic barrier of reaction (38)... 

Most of the predictions were confirmed experimentally (apart from some anomalous cases), but one prediction, the inverted effect , (the decrease of rate constant with increasingly favorable (increasingly negative) AG°, when AG° is very large) was counterintuitive, but clearly evident in equation (1.4) and took 25 years before it was confirmed [26]. This story has been told many times and so I will not describe it here. However, the inverted effect is believed to have implications for efficiency of solar energy conversion in photosynthetic systems, as discussed elsewhere. Also, the ET theory had spin-offs for ion, atom and group transfer reactions considered in the next section. 

The single-electron shift mechanism appears to be general and applicable for electron, proton, atom, and group transfer reactions. The assumptions for this proposition include ... 

Allylations of -alkoxy esters were demonstrated under atom and group transfer conditions using allylsilanes and chelating Lewis acid conditions [28]. Equation (13.18) illustrates the basic mechanism for this reaction. Once again, selectivities greater than 100 1 favoring the anti product and yields of 87% are reported. Results from this study suggest that inclusion of Lewis acids may help facilitate the atom transfer step. 

Asymmetric reactions that can exhibit this type of behavior include atom and group transfer reactions, such as the asymmetric oxidation of sulfides, some asymmetric epoxidations of olefins, " asymmetric aziridination of olefins, - and as)rmmetric cyclo-propanation of olefins. In the asymmetric oxidation of sulfides, a non-racemic, cliiral, low-valent metal complex is oxidized, in this case by iodosobenzene, to generate a highly reactive 0x0 intermediate. The 0x0 is then transferred directly to the sulfur to form the sulfoxide in the enantioselectivity-determining step. A representative example is illustrated in Equation 14.12 that involves a chiral salen-based catalyst. ... 

Keywords Atom and group transfer Catalysis Hypervalent iodine Structure Transition metal complexes... 

Scheme 2 (a) Atom and group transfer radical addition reaction and... 

Guindon Y, Guerin B, Chabot C, Ogilvie W. Stereoselective radical carbon-carbon bond forming reactions of 3-alkoxy esters atom and group transfer aUylations under bidentate chelation controlled conditions. J/Im Chem Soc. 1996 118 12528-12535. 

---