Radical anion substitution reactions

At room temperature under photostimulation a-nitrosulfones react with a variety of nucleophiles via radical anion chain reactions interestingly, in none of the cases where the PhSOj group is involved in SrnI type of substitution does the O end of the ambident anion " play a role. This strong regioselectivity is reminiscent of the one reported for other ambident anions involved in these radical chain substitutions. ... 

Komblum, in his investigations of the factors that control the course of ambident anion alkylation reactions [64, 65], found a new type of radical chain substitution reaction that involves anion-radicals and free radical intermediates. When... 

A new and general method of substitution at aromatic carbon has been uncovered. Reactions involve electron transfer steps and the formation of radical anion and radical intermediates. Many of the transformations are unprecedented in aromatic chemistry they represent major, new sequences which will have considerable value in syntheses. The radical anion method of substitution at aromatic carbon was first reported by Kim and Bunnett in 1970.96,97) The method is related to radical-anion substitution at aliphatic carbon.98, ")... 

Nucleophilic substitution by S and Sg (generated electrochemically) at nitrohaloaromatics have been studied, and the mechanism suggested involves Sl and Sg as the active species, and not radical anions. The reaction of sulfur with thiolate ions RS in dimethylacetamide has been studied electrochemically. The reaction leads to the formation of RS, which is then oxidized to RSSR and polysulfide ions. Rate constants for the inter-molecular thiolate/disulfide exchanges, shown in Eq. (33), are much faster ... 

The neat resin preparation for PPS is quite compHcated, despite the fact that the overall polymerization reaction appears to be simple. Several commercial PPS polymerization processes that feature some steps in common have been described (1,2). At least three different mechanisms have been pubUshed in an attempt to describe the basic reaction of a sodium sulfide equivalent and -dichlorobenzene these are S Ar (13,16,19), radical cation (20,21), and Buimett s (22) Sj l radical anion (23—25) mechanisms. The benzyne mechanism was ruled out (16) based on the observation that the para-substitution pattern of the monomer, -dichlorobenzene, is retained in the repeating unit of the polymer. Demonstration that the step-growth polymerization of sodium sulfide and /)-dichlorohenzene proceeds via the S Ar mechanism is fairly recent (1991) (26). Eurther complexity in the polymerization is the incorporation of comonomers that alter the polymer stmcture, thereby modifying the properties of the polymer. Additionally, post-polymerization treatments can be utilized, which modify the properties of the polymer. Preparation of the neat resin is an area of significant latitude and extreme importance for the end user. 

Kattenberg and coworkers54 studied the chlorination of a-lithiated sulfones with hexachloroethane. These compounds may react as nucleophiles in a nucleophilic substitution on halogen (path a, Scheme 5) or in an electron transfer reaction (path b, Scheme 5) leading to the radical anions. The absence of proof for radical intermediates (in particular, no sulfone dimers detected) is interpreted by these authors in favour of a SN substitution on X. 

Novi and coworkers124 have shown that the reaction of 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene with sodium benzenethiolate in dimethyl sulfoxide yields a mixture of substitution, cyclization and reduction products when subjected at room temperature to photostimulation by a sunlamp. These authors proposed a double chain mechanism (Scheme 17) to explain the observed products. This mechanism is supported by a set of carefully designed experiments125. The addition of PhSH, a good hydrogen atom donor, increases the percent of reduction products. When the substitution process can effectively compete with the two other processes, the increase in the relative yield of substitution (e.g., with five molar equivalents of benzenethiolate) parallels the decrease in those of both cyclization and reduction products. This suggests a common intermediate leading to the three different products. This intermediate could either be the radical anion formed by electron transfer to 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene or the a radical formed... 

In certain reactions where nucleophilic substitutions would seem obviously indicated, there is evidence that radicals and/or radical ions are actually involved. The first step in such a process is transfer of an electron from the nucleophile to the substrate to form a radical anion ... 

Some of the reactions in this chapter operate by still other mechanisms, among them an addition-elimination mechanism (see 13-15). A new mechanism has been reported in aromatic chemistry, a reductively activated polar nucleophilic aromatic substitution. The reaction of phenoxide with p-dinitrobenzene in DMF shows radical features that cannot be attributed to a radical anion, and it is not Srn2. The new designation was proposed to account for these results. 

As the final example in this section, a Li-mediated carboaddition/carbocycliza-tion process will be described. Thus, Cohen and coworkers observed a 5-e%o-trig-cy-clization by reaction of the lithium compound 2-349 and a-methyl styrene 2-350 to give 2-352 via 2-351 (Scheme 2.82). Quenching of 2-352 with methanol then led to the final product 2-353 [189]. In this process, 2-349 is obtained by a reductive lithia-tion of the corresponding phenyl thioether 2-348 with the radical anion lithium 1-(dimethylamino)naphthalenide (LDMAN) (2-354). Instead of the homoallylic substance 2-348, bishomoallylthioesters can also be used to provide substituted six-membered ring compounds. 

Such nucleophilic displacements are likely to be addition-elimination reactions, whether or not radical anions are also interposed as intermediates. The addition of methoxide ion to 2-nitrofuran in methanol or dimethyl sulfoxide affords a deep red salt of the anion 69 PMR shows the 5-proton has the greatest upfield shift, the 3- and 4-protons remaining vinylic in type.18 7 The similar additions in the thiophene series are less complete, presumably because oxygen is relatively electronegative and the furan aromaticity relatively low. Additional electronegative substituents increase the rate of addition and a second nitro group makes it necessary to use stopped flow techniques of rate measurement.141 In contrast, one acyl group (benzoyl or carboxy) does not stabilize an addition product and seldom promotes nucleophilic substitution by weaker nucleophiles such as ammonia. Whereas... 

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