Relations Between Structure and Reactivity in Free‐Radical Chemistry
TL;DR: In this article, the relations between structure and reactivity in free-radical chemistry are interpreted in terms of bond dissociation energies, as well as polar and steric effects, and it is shown that the usual interpretation of reactivities by means of the stability of the radicals involved is greatly simplified and often incorrect.
Abstract: An analysis of the relations between structure and reactivity in free-radical chemistry has shown that the usual interpretation of reactivities by means of the stability of the radicals involved is greatly simplified and often incorrect. The CX bond energies of the alkanes and simple alkyl derivatives can be explained qualitatively by strain effects in the ground state on the basis of the VSEPR theory and nonbonding interactions. To be able to explain reactivities in free-radical chemistry, it is necessary to deduce information about the geometry of the transition states during free-radical formation from experimental measurements. The relations between structure and reactivity in free-radical chemistry are interpreted in terms of bond dissociation energies, as well as polar and steric effects.
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TL;DR: In this article, the rate of addition of alkyl radicals to alkenes is controlled by steric and polar effects, and the stabilities of the educts and products are of only limited importance, since the transition states for these exothermic reactions occur very early on the reaction coordinate.
Abstract: There are many reactions in which CC bonds are formed by addition of free radicals to alkenes. Information about the mechanism is important for the synthesis of specific target molecules. The rate of addition of alkyl radicals to alkenes is controlled by steric and polar effects. The stabilities of the educts and products are of only limited importance, since the transition states for these exothermic reactions occur very early on the reaction coordinate. Variations in reactivity and selectivity can be described using frontier orbital theory: for nucleophilic radicals the dominant interactions are those between SOMO's and LUMO's, and for electrophilic radicals those between SOMO's and HOMO's. The large differences in the steric effects of α - and β- substituents of alkenes can be explained by postulating an unsymmetrical transition state— the radical approaches one of the C atoms preferentially. Regioand stereoselectivities can be predicted and are determined, in general, by steric effects.
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TL;DR: In this paper, the relative importance of bond strength, steric effects, and polarity in determining the rate and orientation of free radical subsitution and free radical addition reaction is considered.
Abstract: The relative importance of bond strength, steric effects, and polarity in determining the rate and orientation of free radical subsitution and free radical addition reaction is considered. The factors which control substitution reaction (radical transfer reaction) are gathered together as five “rules”, and a similar five “rules” are proposed for addition rections. These “rules” are shown to be special cases of two “laws” which govern all free radical reactions.
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324 citations
TL;DR: In this article, the stereoselectivity of cyclic radicals with olefins has been investigated and it has been shown that under certain conditions such systems can also react in a stereoelective manner, such as temperature, solvent, and the nature of the radical scavenger used.
Abstract: The regio- and chemoselectivities of free radical reactions are often high and largely predictable; systematic studies have now shown that the stereoselectivity of free radical reactions can also be directed. Examples involving five- and six-membered cyclic radicals will be used to show how steric and stereoelectronic effects influence the diastereoselectivity of reactions of cyclic radicals with olefins. The temperature, the solvent, and the nature of the radical scavenger used also play a role, so that, if the correct reaction conditions are used, the stereoselectivity of reactions for cyclic reactants can be very high. Lower stereoselectivities are often observed for reactions between acyclic radicals and acyclic alkenes. However, preliminary experiments have indicated that under certain conditions such systems can also react in a stereoselective manner.
295 citations
TL;DR: This Perspective offers a historical view of why it was thought that the differences between aliphatic C-H bonds of the same bond type were not large enough to distinguish them preparatively with small-molecule catalysis in the absence of directing groups or molecular recognition elements.
Abstract: The atomistic change of C(sp3)–H to C(sp3)–O can have a profound impact on the physical and biological properties of small molecules. Traditionally, chemical synthesis has relied on pre-existing functionality to install new functionality, and directed approaches to C–H oxidation are an extension of this logic. The impact of developing undirected C–H oxidation reactions with controlled site-selectivity is that scientists gain the ability to diversify complex structures at sites remote from existing functionality, without having to carry out individual de novo syntheses. This Perspective offers a historical view of why, as recently as 2007, it was thought that the differences between aliphatic C–H bonds of the same bond type (for example, 2° aliphatic) were not large enough to distinguish them preparatively with small-molecule catalysis in the absence of directing groups or molecular recognition elements. We give an account of the discovery of Fe(PDP)-catalyzed non-directed aliphatic C–H hydroxylations and ...
263 citations