Free radical reactions overview

The use of free-radical reactions in organic synthesis started with the reduction of functional groups. The purpose of this chapter is to give an overview of the relevance of silanes as efficient and effective sources for facile hydrogen atom transfer by radical chain processes. A number of reviews [1-7] have described some specific areas in detail. Reaction (4.1) represents the reduction of a functional group by silicon hydride which, in order to be a radical chain process, has to be associated with initiation, propagation and termination steps of the radical species. Scheme 4.1 illustrates the insertion of Reaction (4.1) in a radical chain process. 

The use of free radical chemistry at the anomeric center to produce carhon-carhon bonds, especially in an intramolecular fashion, remains a popular method for C-glycoside synthesis. A review entitled C-Glycosidation Technology with Free Radical Reactions appeared in early 1998, and the reader is referred to that somce [2] as well as to the more traditional ones for complete overviews on the subject. Again, the coverage here focuses on the most recent developments. 

In this chapter we have attempted to provide an overview of the application of computational chemistry to free-radical reactions of synthetic utility. We hope that the reader will be encouraged to add various modeling techniques to their chemical... 

III. Overview and Comments on Free Radical Reactions in Reactive Plasmas. 233... 

In this section, therefore, an overview of and comments on free radical reactions are given from a physicochemical viewpoint. 

This discussion is not intended to be an exhaustive review of the wood-polymer literature, but rather an overview of the processing procedures used today. In general, the free radicals used for the polymerization reaction come from two sources, temperature sensitive catalysts and Cobalt-60 gamma radiation. In each case a free radical is generated by the process, but from that point the vinyl polymerization mechanism is the same. Each... 

The thermal pyrolysis of hydrocarbons proceeds by free radical chain reaction processes. These processes are exceedingly complex and this overview concentrates on the details as it impacts on the technology and economics of olefin production. 

Manganese(III)-mediated radical reactions have become a valuable method for the formation of carbon-carbon bonds over the past thirty years since the oxidative addition of acetic acid (1) to alkenes to give y-butyrolactones 6 (Scheme 1) was first reported by Heiba and Dessau [1] and Bush and Finkbeiner [2] in 1968. This method differs from most radical reactions in that it is carried out under oxidative, rather than reductive, conditions leading to more highly functionalized products from simple precursors. Mn(III)-based oxidative free-radical cyclizations have been extensively developed since they were first reported in 1984-1985 [3-5] and extended to tandem, triple and quadruple cyclizations. Since these additions and cyclizations have been exhaustively reviewed recently [6-11], this chapter will present an overview with an emphasis on the recent literature. 

This chapter has attempted to present a thorough overview of alkaloid syntheses in which free-radical cyclizations have played a pivotal role. It is not meant to be a comprehensive review, but focusses on syntheses in which nitrogen plays a clear role in the cyclization process, either as an attenuator of radical reactivity (Sections 4,1.2 and 4.1.3), a tether (Section 4.1.4), or a radical acceptor (Section 4.1.5). Several other notable alkaloids syntheses have been reported in which carbocyclizations play the pivotal role and introduction of nitrogen is secondary, for example Sha s syntheses of (-)-dendrobine [76] and (-t-)-paniculatine [77], and Clive s synthesis of (+)-fredericamycin [78]. Syntheses in which nitrogen-centered radicals play a critical role are also known, such as the Zard synthesis of (—)-dendrobine [79]. My apologies to these authors for not elaborating on their fine contributions, to authors who have nicely used intermolecular radical addition reactions in alkaloid synthesis, and to others whose contributions may have escaped my attention. 

Microflow reactors serve as powerful tools for accomplishing gas-Uquid-phase reactions in addition to liquid- and liquid-liquid-phase reactions. This chapter provides an overview of electrophilic and free-radical substitution under gas-liquid-phase conditions using microfiow reactors. 

The book starts with an introductory overview, which is intended to refresh the reader s memory on key aspects of free radicals and reactions thereof. Subsequent sections cover free radicals and food chemistry, natural antioxidants, and nutritional biochemistry and health. In the food chemistry section, topics range from analysis of free radicals within food matrices to Maillard reactions, emulsions, dairy, and meat products. In the antioxidant section, results are presented on the efficacy of antioxidants from tea, seeds, and selected naturally occurring compounds. Finally, in the nutritional biochemistry and health section, free radical inhibition is discussed in relationship to biochemical paths and cancerous cell cultures. 

An overview of die free radicals and reactions thereof is presented. Free radicals are atoms or groups having an unpaired electron and hence are paramagnetic. Electron paramagnetic resonance spectroscopy (EPR) and trapping methods are used to analyze radicals. In lipids, radical reactions lead to autoxidation and hence flavor reversion. Reactive oxygen species are key components involved in such reactions. Finally, descriptions for phenolic, sequesterant and enzymatic antioxidants and their mode of action are provided. 

The driving force for a HAT reaction, AG°xh/y =- 7 ln. Kxh/y. is best determined by direct equilibrium measurements in the solvent of interest. However, this is typically limited to reactions where IAG°xh/yI is small, less than about 5 kcal mol . Also, this is only possible for reactions in which all of the species are fairly stable, which is unusual for organic radical reactions. The AG° for a HAT reaction is typically more easily derived as the difference in bond dissociation free energies (BDFEs) of X-H and Y-H in the solvent of interest. We have recently reviewed BDFEs of common organic and biochemical species and how they are obtained, so only an overview is given here. 

Cerium(IV) ions are widely used as initiators for radical polymerizations of vinyl monomers (acrylamide, acrylonitrile, methyl methacrylate, vinyl acetate,. ..). In order to act as an initiator, a reductant has to be added to the solutions containing the monomer and a cerium(IV) salt. Free radicals are produced by the oxidation of the reductant by cerium(IV) and these free radicals can initiate the polymerization reaction. Table 3 gives an overview of the different... 

In some cases where a reaction involving a radical species occurred within cobalt porphyrin complexes, it has been possible to trap transient cobalt porphyrin hydride species. This was indeed observed during the synthesis of organocobalt porphyrin that resulted from the reaction of cobalt(n) porphyrin and dialkylcyanomethylradicals with alkenes, alkynes, alkyl halides, and epoxide. A transient hydride porphyrin complex was also involved in the cobalt porphyrin-catalyzed chain transfer in the free-radical polymerization of methacrylate. The catalytic chain transfer in free-radical polymerizations using cobalt porphyrin systems has been extensively investigated and will not be treated in this section. Gridnev and Ittel have published a comprehensive overview of the catalytic chain transfer in free-radical polymerizations. ... 

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