Radical reactions enthalpic effects

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

The mechanism of free-radical addition follows the pattern discussed in Chapter 14 (pp. 894-895). The method of principal component analysis has been used to analyze polar and enthalpic effect in radical addition reactions. A radical is generated by... 

Because the enthalpic effect was considered the sole driving force in free-radical processes, it was long erroneously believed that high reactivity had always to be associated with low selectivity. This is one of the reasons why radical chemistry was thought to be useless for selective synthesis of complex molecules, until Barton, in 1960 [1], showed the synthetic potentiality of free-radical reactions. 

Minisci, F., Fontana, F., Vismara, E. Polar and enthalpic effects in free-radical reactions. Free-radical diazo coupling and reactivity of carbohydrate radicals. Gazz. Chim. Ital. 1993, 123, 9-18. 

Free radical chemistry continues to be of importance in the field and a survey has been published on polar and enthalpic effects in free radical reactions. A further short review dealt with free radicals, carbenes and nitrenes at the anomeric centre of carbohydrates. A useful survey has appeared of the application of carbohydrate based chiral auxilliaries in stereoselective syntheses which covers a range of reactions, for example cycloaddition processes, reductions and Strecker reactions. ... 

F. Antonietti, A. Mele, F. Minisci, C. Punta, F. Recupero, F. Fontana, Enthalpic and polar effects in the reactions of perfluoroalkyl radicals. New selective synthetic developments with alkenes and heteroaromatic bases, J. Huorine Chem. 125 (2004) 205-211. 

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