Radicals, reduction substitution

Of some relevance in this connection is a study216 on the structure of the anion radicals formed when diaryl sulphones react with n-butyllithium in hexane-HMPA solution under an argon atmosphere. Apparently, a dehydrogenative cyclization and a further one-electron reduction occurs to produce the anion radicals of substituted dibenzothiophene-S, S-dioxides. These anion radicals were studied by ESR spectroscopy. 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

In 1969, Elschenbroich and Cais reported the ESR spectra of several ferrocenyl anion radicals, including benzoyl, p-tolyl, p-carbomethoxy-benzoyl, p-nitrophenyl, p-cyanophenyl, and nitroferrocene, prepared by electrolytic reduction in either acetonitrile or DMF (5S). In general, the ferrocenyl group destabilizes the anion radicals compared to a phenyl substituent. When both groups are present, delocalization of the unpaired electron into the phenyl substituent is more extensive, and the ESR spectra resemble, for the most part, anion radicals of substituted aromatics. There is small spin density in the ferrocenyl moiety, which appears as small hyperfine couplings for the cyclopentadienyl protons ortho to the point of substitution (38). 

The usually considered monomolecular mechanism of substitution implies that one-electron reduction activates a substrate sufficiently so that it could dissociate with no further assistance from a nucleophile. The next steps of the reaction consist of transformations of the resultant radical. However, in substrates having sp3 carbon as a reaction center, the influence of the leaving group has been fixed (Russell Mudryk 1982a, 1982b). This led to the formulation of the SRN2 bimolecular mechanism of radical-nucleophilic substitution. In this mechanism, the initial products of single-electron transfer are combined to form the... 

In comparison, the stereoselectivity of the radical reduction slightly decreases if the anomeric center is substituted by an electron withdrawing substituent (CN) using AIBN as radical initiator [17 b]. The corresponding radical is supposed to be more planar than pyramidal to explain the lower degree of stereoselectivity for the reduction process of 8 b into 9. 

Simple radicals derived from aldoses have been considered exclusively in the preceding sections, from the point of view of structure and transformation. Application of radical reduction to tertiary anomeric radicals, produced from ketoses, higher sugars and other substituted analogues, and having generally alkoxy and... 

Lastly, Gualtieri reported the use of binaphthyl-substituted germanes (8, 9) in enan-tioselective radical chemistry26. For example, an enantioselectivity of 59% was reported for the reaction of 126 with 8 at —60° (equation 127). To the best of our knowledge, this represents the first account of the use of a chiral germanium hydride in free-radical reduction chemistry. 

Malacria s group recently developed a new method for the synthesis of allenes based on the radical reduction of bromides 7181. Equation 32 shows the general reaction for the terminally substituted allenes. Eight examples were reported with yields in the range of 30-80%. As pointed out by the authors, the use of large excesses of initiator suggests that these transformations do not involve straightforward chain reactions. 

Not only radical scavengers, radical reduction and/or radical dimerization products but also radical probes were used in order to prove the presence of radicals as intermediates along the S l propagation cycle. Thus the formation of cyclized and uncyclized substitution products was taken as an indication of radical intermediates in the reaction of neopentyl-type halides containing a cyclizable probe of the 5-hexenyl type 2. These reactions were performed with PhS and Ph2P ions as nucleophiles (equation 13)52. 

Again, no reaction was observed in the absence of mercaptoethanol. The mechanistic steps for the transformation were elucidated by the Chatgilialoglu et ah and are represented in Scheme 4.13, in analogy with the pathway reported for the radical reduction of aromatic azides with triethylsilane in toluene, with the addition of silyl radicals to the azide function, liberation of nitrogen and formation of silyl-substituted aminyl radical. The thiol is the hydrogen atom donor to this intermediate and it can be regenerated by its interaction with the silane, thus propagating the chain. The hydrolysis of the silylamine occurred... 

Work carried out in our laboratories over the past three years and in conjunction with the research group of Daktemieks at Deakin University has been directed toward the development of novel enantiomerically pure stannanes for use in free-radical reduction chemistry. To that end Dunn prepared a series of menthyl-substituted stannanes 18 -20 and some others derived from aromatic amines (eg. 21, 22).Perchyonok tested these reagents against a series of substrates while Henry modelled the reactions in question through the use of ab initio molecular orbital theory. 

The radical reduction of ester-substituted alkyl iodides bearing a chiral tetrahydrofuranyl substituent proceeds with high selectivities1522. A bulky group in the 4-position of the tetra-hydrofuran ring has little influence on the diastereomeric excess, whereas an alkoxy substituent in the 3-position increases the synjanti ratio. 

Scheme 1. Chelation-controlled radical reduction of substituted a-bromo-/ -alkoxy esters [3]...

A-Azinylpyridinium A-aminides 73 have been shown to be convenient precursors for the regioselective synthesis of 3-fluoro-2-aminopyridines 76" " (Scheme 6.25). First, compound 73 was treated with xenon difluoride, which resulted in the regioselective introduction of fluorine in 3 of substituted pyridine ring leading to compounds 74, which were further alkylated to give compounds 75 and finally, the radical reduction was used to form 3-fluoropyridines 76. 

H transfer to organic radicals is a common reaction of transition metal hydride complexes. The reaction in Equation 3.128, which is parallel to RjSnH reductions in organic chemistry, is frequently used for the preparation of stable derivatives and for quantifying the amount of hydride present (by measuring the amount of CHClj formed). The mechanism is related to that of the radical chain substitution reactions discussed in Chapter 4. 

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