Radical Additions and Eliminations

Addition reactions such as addition of enolates to carbonyl compounds, photochemical cydoaddition, radical addition and elimination reactions such as dehydration can be carried out in microflow reactors. Addition-elimination reactions such as Wittig reaction, Homer-Wadsworth-Emmons reaction, Baylis-Hillman reaction. 

The first three chapters discuss fundamental bonding theory, stereochemistry, and conformation, respectively. Chapter 4 discusses the means of study and description of reaction mechanisms. Chapter 9 focuses on aromaticity and aromatic stabilization and can be used at an earlier stage of a course if an instructor desires to do so. The other chapters discuss specific mechanistic types, including nucleophilic substitution, polar additions and eliminations, carbon acids and enolates, carbonyl chemistry, aromatic substitution, concerted reactions, free-radical reactions, and photochemistry. 

The stereospecificity of the methylene transfer provides compelling support for a concerted mechanism and this conclusion has rarely been disputed. It is instructive, however, to review the experimental evidence that allowed for the elimination of the alternative mechanistic proposals, namely, a radical addition and a carbome-tallation (Scheme 3.4). 

In a similar manner, many additions of heteroatom radicals to unsaturated positions have been studied. In many cases, addition reactions of heteroatom radicals to alkenes are reversible and thermodynamically disfavored, but their occurrence is apparent. For example, the rapid addition and elimination of thiyl radicals to unsaturated fatty acid methyl esters results in isomerization reactions from which kinetic parameters can be obtained. Additions of group 14 (IV A) metal-centered... 

At present, bimolecular allylation is limited to unsubstituted and 2-substituted allylstannanes (which are symmetrical so that addition and elimination of a tin radical are degenerate). 1-Substituted allylstannanes (for example, crotylstannane) are not useful reagents because the substituent decelerates the rate of the radical addition step below the useful limit.139 The usefulness of 3-substituted allylstannanes has been limited by their relatively facile isomerization to the inert 1-substituted isomers under typical reaction conditions.140... 

Roder M, Wojnarovits L, Foldiak G, Emmi SS, Beggiaro G, D Angelantonio M (1999) Addition and elimination kinetics in OH radical induced oxidation of phenol and cresolsin acidic and alkaline solutions. Radiat Phys Chem 54 475-479... 

Another group of reagents that can distinguish between Fe O2 + and HO" radicals are the sulfoxides, R2SO, which react with Fe 02+ by O-atom transfer to yield the sulfones, and with HO" radicals by addition and elimination of CH3" radicals ... 

Addition and elimination reactions of RS radicals derived from cysteamine and 2-mercaptoethanol involving C5-C6 double bond in various pyrimidines (thymine, uracil, and cytosine) were studied by the pulse radiolysis technique in aqueous solutions.For this purpose the kinetic parameters, i.e. the rate constants of the addition and elimination reactions, were determined using two chemical-monitoring systems. 

The rate constants for addition and elimination of RS radicals were determined by applying the modified version of ACUCHEM for simulations of the experimentally observed kinetic traces in (RS.. SR) and LA systems (Table 5), at the specified wavelengths ... 

Jenn, T., and Heissler, I ),. Synthesis of tricyclopolyprenols via a radical addition and a stereoselective elimination. Part 1. Methodology, Tetrahedron, 54, 97, 1998. 

The flavin enzymes contain the riboflavin derivatives flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) (D 10.4.3) as prosthetic groups. They act as the carrier of hydrogen or electrons and participate in a large number of oxido-reductions. Flavin enzymes may be oxidized or reduced in a one-step (Fig. 13, pathway A) or a two-step reaction (Fig. 13, pathway B). In the case of the one-step mechanism the addition and elimination of a hydride ion plays an important role. In the two-step transformation a flavin radical appears as an intermediate. 

This chapter focused upon reactions at unsaturated centers alkenes, alkynes, arenes, and carbonyls. A large variety of additions and eliminations was examined. The complexity in the examples showed how diverse the possibilities are for additions and eliminations to such centers. One can have electrophilic, nucleophilic, and radical pathways for almost all reaction classes, and the dominant product depends upon the structures of the reactants and the experimental conditions. You should now be able to write the electron pushing for the standard reaction mechanisms (El, E2, SeAr, etc.), as well as be able to predict and examine experimentally the variations that are possible. Given this background on unsaturated centers, we turn our attention to reactions that occur at saturated centers, which is the first major topic of the next chapter. 

Miyata, O., Ozawa, Y., Ninomiya, L, and Naito, T. (1997) An enantioselective synthesis of (—)-a-kainic acid via thiyl radical addition-cyclization-elimination reaction. Synlett, 275-276. 

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