Radicals ambiphilic

Table 6 -CFbCChBu - An Ambiphilic Radical R %, +. CH2C02Bu --------- R/ c°2But...

There are several examples of the addition reactions of caibonyl-substituted radicals to alkenes by the tin hydride method. The first reaction cited in Scheme 32 is a clear-cut example of reversed electronic requirement an electrophilic radical pairing with a nucleophilic alkene.60 Because enol ethers are not easily hydrostannylated, the use of a chloride precursor (which is activated by the esters) is possible. Indeed, the use of a bromomalonate results in a completely different product (Section 4.1.6.1.4). The second example is more intriguing (especially in light of die recent proposals on the existence of ambiphilic radicals) because it appears to go against conventional wisdom in the pairing of radicals and acceptors.118,119... 

Data is also available for addition reactions of electrophilic radicals, CH(CN)2, and so-called ambiphilic radicals, CH(C02Et)2 and CH2C02-/-Bu, which derive their electrophilic character from n-delocalization of the carbon-centered radical onto electron-attracting substituents [108-111], and for which the enthalpy of the addition process rather than the polar nature of the radicals may be the primary rate-determining factor [112,113]. 

Beranek I, Fischer H (1989) Polar Effects on Radical Addition Reactions An Ambiphilic Radical. In Minisci F (ed) Free Radicals in Synthesis and Biology. Kluwer, Amsterdam, p 303... 

The source of H-atoms has not been identified, but the position a to the phenolic ether would be reactive toward electrophilic or ambiphilic radicals. 

The intermolecular addition of carbon-centered radicals to unactivated alkenes followed by azidation (a formal carboazidation of alkenes) has been reported. A one-pot procedure similar to the one used for intramolecular reactions gives good results (Scheme 8.28). Slow addition of benzenesulfonyl azide is not necessary because this electrophilic reagent does not react with the initial electrophilic or ambiphilic radicals. Excellent results are obtained with a-iodo and -xanthate esters. a-Bromoacetates give also satisfactory results. The carboazidation process allows to prepare pyrroUdinone derivatives in a straightforward manner (Scheme 8.28, bottom example). A tin-free version of this reaction using triethylborane instead of hexabutylditin has also been reported. ... 

However, consideration of polar factors in the traditional sense does not provide a ready explanation for the regiospecificity shown by the r butoxy radicals (which arc electrophilic, Tabic 1.3) in their reactions with the tluoro-olcfins (Tabic 1.2).22,23 Apparent ambiphilicity has been reported21 for other not very electrophilic radicals in their reactions with olefins and has been attributed to the polarizability of die radical. 

This scheme can be extended by using mixtures of dienes with electron-deficient alkenes such as acrylonitrile. Due to its nucleophilic nature, addition of radical 68 to acrylonitrile is faster than addition to butadiene. The resulting ambiphilic adduct radical then adds to butadiene to form a relatively unreactive allyl radical. Oxidation and trapping of the allyl cation by methanol lead, as before, to products such as 72 and 73, which are composed of four components the radical precursor 67, acrylonitrile, butadiene and methanol (equation 30)17,94. 

The ambiphilic reactivity of aromatic cation radicals, as described in Schemes 12 and 13, is particularly subtle in the charge-transfer nitration of toluene and anisole, which afford uniformly high (>95%) yields of only isomeric nitrotoluenes and nitroanisoles, respectively, without the admixture of other types of aromatic byproducts. Accordingly, let us consider how the variations in the isomeric (ortho meta para) product distributions with... 

Radicals are often classified according to their rates of reactions with alkenes. Those radicals that react more rapidly with electron poor alkenes than with electron rich are termed nucleophilic radicals. Conversely, those that react more rapidly with electron rich alkenes than electron poor are termed electrophilic radicals. Recently, it has been found that this simple division does not suffice because certain radicals react more rapidly with both electron rich and electron poor alkenes than they do with alkenes of intermediate electron density. These radicals are termed ambiphilic. The appropriate pairing of a radical and an acceptor is important for the success of an addition reaction. 

From the Fischer rate study, it appears that primaiy ester-substituted radicals are not electrophilic but ambiphilic and the borderline between ambiphilic and electrophilic radicals is not at all clear. Consider our results68 (Scheme 16) on the atom transfer additions of ester-substituted radicals to alkynes (with the caution that it may be dangerous to compare yields in place of rate constants). The primary ester-substituted radical adds more efficiently to 1-heptyne but the tertiary ester-substituted radical prefers ethyl propiolate. 

The ionic stability of phenyl selenides can also be advantageous in the choice of addition reagents. Reagents serving as precursors to heteroatom-stabilized radicals are more accessible because of the poorer leaving group ability of the phenylseleno substituent, as shown in Scheme 18. Phenylseleno precursors to captodative radicals have been shown to be ambiphilic in nature, with successful additions to electron-rich as well as electron-deficient olefins [54], The stable precursor to a highly nucleophilic radical, 2-phenylseleno-l,3-dithiane has been shown to add to electron-deficient olefins [55]. 

Apparent ambiphilicity has been reported" for other not very electrophilic radicals in their reactions with olefins and has been attributed to the polarizability of the radical. 

Sigma bond metathesis is another important pathway that could be followed for the C-H bond activation of (hetero)arenes. Recently, the term a-bond metathesis has undergone a radical change and has been rather appropriately coined Concerted Metallation Deprotonation (CMD) pathway," sometime also known as internal electrophihc substitution or ambiphilic metal-ligand activation. ... 

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