Aryl radicals specificity

A number of methods are available for generating aryl radicals. They have been reviewed recently as has the evidence that the processes result in the generation of free aryl radicals, Those methods which have been used for the arylation of heterocyclic compounds are described here, and their applications to the arylation of specific heterocycles are discussed and tabulated in Section II,C,D and E. 

Aryl radicals are produced in the decomposition of alkylazobenzenes and diazonium salts, and by f)-scission of aroyloxy radicals (Scheme 3.73). Aryl radicals have been reported to react by aromatic subsitution (e.g. of Sh) or abstract hydrogen (e.g. from MMA10) in competition with adding to a monomer double bond. However, these processes typically account for <1% of the total. The degree of specificity for tail vs head addition is also very high. Significant head addition has been observed only where tail addition is retarded by sleric factors e.g. methyl crotonate10 and -substituted methyl vinyl ketones 79, 84). 

The plan for a tandem cyclization as the key step in the synthesis of morphine alkaloids has been successfully performed [95]. Three examples are given in Scheme 7.10. Specifically, the initially formed aryl radical, generated by bromine abstraction from compound 74, underwent a tandem cyclization to construct the desired carbocyclic skeleton. Depending on the nature of the Z substituent to the double bond, the product radicals either abstracted hydrogen from silane (Z = C02Me or CN) or eliminated thiyl radical (Z = SPh). 

Free radicals produced by the thermolysis of typical radical Initiators as AIBN, benzoplnacole and phenylazotrlphenylmethane reduce the dlaryllodonlum salt generating aryl radicals and solvent derived radicals which In a chain reaction Induce the decomposition of more dlaryllodonlum salt. Through the selection of particular radical Initiators with specific decomposition rates, It Is possible to adjust the Initiation temperature of the cationic polymerization with considerable latitude. 

A more recent determination of the rate constant for reaction of an aryl radical with Bu3SnH at ambient temperature is available, viz. ki = 7.8 x 10 M s [42], If one assumes that this kinetic value can be used for any aryl radical reacting with tin hydride, then the rate constants for cyclization of the aryl radical clocks shown below can be calculated from the reported relative Arrhenius functions [29]. Specifically, radicals 13 and 14 cyclize with rate constants of 5 x 10 s and... 

Homolysis of photoexcited aryl iodides and bromides has been exploited as a means of producing aryl radicals in some cases, these reactions have synthetic potential for the formation of biaryls, including the use of intramolecular arylation in order to construct polycyclic systems. Reductive dehalogenation has been suggested as a synthetic route to specifically deuterated aromatic compounds by photolysis of aryl halides in solvents containing abstractable deuterium atoms. 

Diacyl or diaroy] peroxides (36, R- alkyl or aryl respectively) are given specific coverage in reviews by Fujimori,141 Bouillion et c//.,14 and Hiatt.14j They are sources of acyloxy radicals which in turn are sources of aryl or alkyl radicals. Commercially available peroxides of this type include dibenzoyl peroxide (BPO), didodecanoyl or dilauroyl peroxide (LPO), didecanoyl peroxide (42) and succinic acid peroxide (43). 

The distinction between electrophilic and electron-transfer mechanisms of addition reactions to vinyl double bonds of ArX—CH=CH2 (X = S, O, Se) has been achieved by studying substituent effects. Specifically, the effects of meta and para substituents on the rates of electrophilic additions correlated with Hammett radical cations correlates with statistical tests. The ofclcctrophilicj/o-1 (FT) dichotomy is in accord with the conventional paradigm for cr/cr+ correlations and further support has been found by ah initio calculations. Interestingly, the application of this criterion to the reactions of aryl vinyl sulfides and ethers with tetracyanoethylene indicates that cyclobutanes are formed via direct electrophilic addition to the electron-rich alkene rather than via an electron-transfer mechanism.12... 

AIBN, the radical 15 can attack the aryl ring to give an intermediate cyclohexadienyl radical 16 this is then converted to the fully aromatic product 17. The feature of this reaction that is particularly noteworthy is that the conversion of 16 to 17 is formally an oxidation reaction, yet it takes place in the presence of BusSnH, a reducing agent various mechanistic possibilities could explain this anomaly. The approaches applied to investigate this reaction further illustrate some of the methods used to elucidate radical reactions generally and are applicable to other specific examples. 

---