Radical reactions 3 cleavage

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

The broadest classification of reactions is into the categories of heterolytic and homolytic reactions. In homolytic (free radical) reactions, bond cleavage occurs with one electron remaining with each atom, as in... 

Alkyl halides can be reduced to alkanes by a radical reaction with tributyltin hydride, (C4H9)3SnH, in the presence of light (hv). Propose a radical chain mechanism by which the reaction might occur. The initiation step is the light-induced homolytic cleavage of the Sn— H bond to yield a tributyltin radical. 

The cleavage of phenylmercuric iodide by iodine in the presence of excess iodide ion (to suppress free-radical reactions) at 25 °C in aqueous dioxan was reported to be first-order in both aromatic and tri-iodide ion, and faster than the reaction of alkylmercuric iodides724. A further study, together with bromodemercuration, both reactions being generally represented by... 

The radical-induced cleavage of a,/ -aziridino alcohols proceeds in analogy to that of a,/ -epoxyalcohols, leading regiospecifically to allyl amines (a). By successive reactions with double bond systems, pyrrolidines are formed (b) [68]... 

As can be seen from Figure 3, the ratio of the isopropyl ether to isobutyrate is about 1 1. It is clear that after a-cleavage of the ketone the two radicals primarily formed are captured directly by nitroxide. This takes place without decarbonylation of the acyl radical (reaction (7)) ... 

Because reductive cleavage of aliphatic nitro compounds with Bu3SnH proceeds via alkyl radicals, nitro compounds are also used as precursors to alkyl radicals. Reactions using nitro compounds may have some advantages over other ones, since aliphatic nitro compounds are available from various sources. For example, the sequence of the Michael additions of nitro compounds provides an excellent method for the construction of quaternary carbon compounds (Eq. 7.79).126 Newkome has used this strategy for the construction of dendritic polymers (Eq. 7.80).127... 

Kharasch has pointed out that there are two very similar and not easily distinguished reactions of oxygen radicals, /9-cleavage and rearrangement.111 The difficulty is that they can often lead to the same final product. 

Optically active iV-unprotected-2-pyrrolidinones 194 were obtained from selenocarboxylate or allylamine via radical cyclization and subsequent one-step cleavage of the C-O and C-N bond of the inseparable mixture of the two bicyclic oxyoxazolidinones 192 and 193 with -Bu4NF. The initial radical reaction is highly stereoselective. Products were obtained with ee up to 90%. The mandelic acid 195, which served as the chiral auxiliary in this method, was recovered with no loss of optical activity (Equation 33) <2003T6291>. 

CLEAVAGE OF ION RADICALS. REACTION OF RADICALS WITH NUCLEOPHILES... 

Reaction 13.22 is the net result of two steps, the first being the photochemical cleavage of the 0-0 bond in di-terf-butylperoxide (reaction 13.23), followed by the abstraction of the hydroxylic hydrogen in phenol by the ferAbutoxyl radical (reaction 13.24). 

The bottom part of Scheme 18.5 is intended to show another fate for the phenyl radicals produced other than those already discussed in Schemes 18.1 and 2. Simple hydrogen abstraction yields a phenyl radical as discussed above in conjunction with Scheme 18.1. Reaction with other chains yields a branch point as also discussed above. However, if after hydrogen abstraction by the phenyl radical the cleavage reaction that produced the phenyl radical originally were to be applied to the remaining ester group on the ring, then one could account for the report of benzene [25] as well. 

Cobaloxime(I), electrochemically regenerated from chloro(pyridine)-cobaloxime (III) (232), has been employed as a mediator in the reductive cleavage of the C—Br bond of 2-bromoalkyl 2-alkynyl ethers (253), giving (254) through radical trapping ofthe internal olefin (Scheme 95) [390]. An interesting feature of the radical cyclization (253) (254) is the reaction in methanol, unlike the trialkyltin hydride-promoted radical reactions that need an aprotic nonpolar solvent. An improved procedure for the electroreductive radical cyclization of (253) has been attained by the combined use of cobaloxime(III) (232) and a zinc plate as a sacrificial anode in an undivided cell [391]. The procedure is advantageous in terms of the turnover of the catalyst and the convenience of the operation. 

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