Carbon radicals homolytic addition reactions

Additions. Homolytic bimolecular addition reactions of carbon-centered radicals to unsaturated groups have been studied in detail because these are the reactions of synthesis and polymerization. Within this group, radical additions to substituted alkenes are by far the best understood. An excellent compilation of rate constants for carbon radical additions to alkenes is recommended for many specihc kinetic values. ... 

Whereas additions of carbon radicals to alkene moieties are the best characterized homolytic additions, carbon radicals are known to add to a wide range of unsaturated systems. These include polyenes, alkynes, arenes, heteroarenes, carbon monoxide,isonitriles, °° ° nitriles, ° imines and derivatives, ° ° aldehydes,nitrones, and thiones. ° Many of these reactions, such as addition of an alkyl radical to a carbonyl group, ° are thermodynamically unfavorable and readily reversible, and they form the basis of composite group-transfer reactions discussed below. 

Another important homolytic reaction of tin hydrides is the reduction of carbon-halogen bonds the reaction is promoted by initiators and retarded by radical traps (Scheme 13). Reactivity decreases in the sequence X = I > Br > Cl and BusSnH > Bu2SnH2 Ph3SiiH > BuSnH3. Other groups X in RX that can be reduced by tin hydrides include -OC(S)R, SR, SePh, TePh, NC, and NO2. The intermediate radical, R in Scheme 13, can be trapped by additional substances, e.g. alkenes, or may undergo... 

More recently, a radical-mediated variation of this addition-fragmentation has been explored. The reaction, summarized in Scheme 77 for a one-carbon expansion, involves the generation of a radical at the terminus of a chain by homolytic cleavage of a carbon-heteroatom bond. Addition of the radical to the carbonyl produces a bicyclic intermediate, which on cleavage of the alternate bond regenerates the ketone carbonyl group with formation of a new radical. The sequence is terminated by the reduction of the radical with the tributyltin hydride reagent. The near neutral conditions of the reaction avoid the reclo-... 

In this chapter, we discuss reactions that either add adjacent (vicinal) groups to a carbon-carbon double bond (addition) or remove two adjacent groups to form a new double bond (elimination). The discussion focuses on addition reactions that proceed by electrophilic polar (heterolytic) mechanisms. In subsequent chapters we discuss addition reactions that proceed by radical (homolytic), nucleophilic, and concerted mechanisms. The electrophiles discussed include protic acids, halogens, sulfenyl and selenenyl reagents, epoxidation reagents, and mercuric and related metal cations, as well as diborane and alkylboranes. We emphasize the relationship between the regio-and stereoselectivity of addition reactions and the reaction mechanism. 

Homolytic cleavage of the carbon-boron bond of a trialkylborane can be promoted by oxygen. The so-formed alkyl radical can be used in synthesis (for radical reactions, see Section 4.1). Indeed, triethylborane in air can be used to generate radicals from precursors such as alkyl iodides or selenides. Hydroboration followed by addition of an a,(3-unsaturated aldehyde or ketone leads to transfer of an alkyl group from the boron atom via an alkyl radical intermediate. The reaction takes place by addition of the alkyl radical to the conjugated system to form an enol borinate, hydrolysis of which gives the aldehyde or ketone product (5.37). 

The most notable addition reaction is that of ground state oxygen with carbon centered radicals. This leads to peroxy radicals that can abstract hydrogens from other molecules regenerating carbon-centered radicals. Additionally, the peroxide so formed can then undergo homolysis to yield alkoxy and hydroxy radicals. Consequently, this homolytic initiation leads to additional radicals that propagate and accentuate the autocatalytic nature of the reaction. 

Radical addition to alkenes (Sections 10.9 and 10.10) A process by which an atom with an unshared electron, such as a bromine atom, adds to an alkene with homolytic cleavage of the IT-bond and formation of a cr-bond from the radical to the carbon the resulting carbon radical then continues the chain reaction to product the final product plus another species with an unshared electron. 

The free-radical mechanism involves several reaction types (1) Initiation - the introduction of free radicals into the reaction system. In pyrolysis, the reaction is initiated thermally with a probable homolytic cleavage of a carbon-carbon bond. (2) Propagation - a series of reactions that converts reactants to products while leaving the radical concentrations unchanged (radical decomposition and isomerization, hydrogen transfer and radical addition). (3) Termination - the combination or disproportionation of radicals to give stable products (Rebick, 1983). 

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