RADICAL ANION ARYLATION

RADICAL ANION ARYLATION, 58, 134 Raney nickle, 57, 19, 58, 114, 116 Raney nickel,W-2, 56, 16, 74 Reduction, carboxyl groups, 56,83 Reductive alkylation, 56, 52 Resolution of amines, 55, 80, 83 Rexyn 201, 55,4 Rhodium(III) oxide, 57,1 Ring contraction, 56. 107... 

Nitrogen heterocycles continue to be valuable reagents and provide new synthetic approaches such as NITRONES FOR INTRAMOLECULAR -1,3 - DIPOLAR CYCLOADDITIONS HEXAHYDRO-1,3,3,6-TETRAMETHYL-2,l-BENZISOX AZOLINE. Substituting on a pyrrolidine can be accomplished by using NUCLEOPHILIC a - sec - AM IN O ALKYL ATION 2-(DI-PHENYLHYDROXYMETHYL)PYRROLIDINE. Arene oxides have considerable importance for cancer studies, and the example ARENE OXIDE SYNTHESIS PHENANTHRENE 9,10-OXIDE has been included. An aromatic reaction illustrates RADICAL ANION ARYLATION DIETHYL PHENYLPHOSPHONATE. 

Bunnett, J.F., and Weiss, R.H., Radical anion arylation. Diethyl phenylphosphonate (phosphonic acid, phenyl-diethyl ester), Org. Synth., 58, 134, 1978 Org. Synth. Coll. Vol. VI, 451, 1988. 

The mechanism is by no means completely clear, but the photodehalogenation reaction is postulated to occur through the formation of a pair of radical ions from an exciplex resulting in the excited state (Grimshaw and de Silva, 1981). The precursor of the reduction product (Aryl-H) is suggested to be a radical anion (Aryl-Ck-), while a radical cation (Aryl-Cl+ ) is postulated as the precursor of the substitution product (Aryl-OR). In a less polar solvent, e.g., iso-propanol, direct homolysis of the C-Cl bond occurring from the triplet state has been suggested,... 

The distinguishing feature of this mechanism is the second step, in which an electron is transferred from the organometallic reagent to the carbonyl compound to give the radical anion of the carbonyl compound. Subsequent collapse of the ion pair gives the same product as is formed in the normal mechanism. The electron transfer mechanism would be expected to be favored by structural features that stabilize the radical anion. Aryl ketones and diones fulfill this requirement, and much evidence for the electron transfer mechanism has been accumulated for such ketones. In several cases, it is possible to observe the intermediate radical anion by EPR spectroscopy. ... 

Another way to prepare fluorinated sulfides is the photochemical alkylation of sulfides or disulfides by perfluoroalkyl iodides [69, 70, 71] (equations 62-64). Reaction of trifluoromethyl bromide with alkyl or aryl disulfides in the presence of a sulfur dioxide radical anion precursor, such as sodium hydroxymethanesulfi-nate, affords trifluoromethyl sulfides [72] (equation 65). 

Packer et al. (1981) found that y-irradiation reduces arenediazonium tetrafluoro-borates to aryl radicals. Packer and Taylor (1985) investigated the y-irradiation of 4-chlorobenzenediazonium tetrafluoroborate by a 60Co source in the presence of 1 alone or I- +13 . The major product in the presence of iodide was 4,4 -dichloroazo-benzene. With I- + 1 ", however, it was 4-chloroiodobenzene. Two other investigations of the reactivity of aryl radicals with iodine-containing species are important for the understanding of the chain process of iodo-de-diazoniation that starts after formation of the aryl radical. Kryger et al. (1977) showed that, in the thermal decomposition of phenylazotriphenylmethane, the rate of iodine abstraction from I2 is extremely fast (see also Ando, 1978, p. 341). Furthermore, evidence for the formation of the radical anion V2 was reported by Beckwith and Meijs (1987) and by Abey-wickrema and Beckwith (1987) (see Sec. 10.11). 

On the basis of all these results and his own investigations on chloro- and bromo-de-diazoniations (Galli, 1981), Galli proposed in 1988 that iodo-de-diazoniation, after formation of the aryl radical in the initiation reaction (Scheme 10-22) follows three coupled iodination chain reactions based on the formation of the I2 molecule and the If anion in the step shown in Scheme 10-23, namely iodine atom (I ) addition (Scheme 10-24), and iodine abstraction from I2 and If in Schemes 10-25 and 10-26 respectively. Aryl radicals and iodine molecules are regenerated as indicated in Scheme 10-27. The addition of iodide ion to aryl radicals forming the radical anion [Arl] -, as in Scheme 10-28, is considered an unlikely pathway, as that reaction has been found to be reversible (Lawless and Hawley, 1969 Andrieux et al. 1979). 

The cleavage of alkyl aryl sulfones by sodium amalgam and alcohols65 probably proceeds also through the intermediacy of a radical anion, followed by splitting to the arylsulfinate anion and an alkyl radical. Both the sulfinate anion and the disproportionation products of the radical have been observed. 

Three possible mechanisms may be envisioned for this reaction. The first two i.e. 1) Michael addition of R M to the acetylenic sulfone followed by a-elimination of LiOjSPh to yield a vinyl carbene which undergoes a 1,2 aryl shift and 2) carbometallation of the acetylenic sulfone by R M followed by a straightforward -elimination, where discarded by the authors. The third mechanism in which the organometallic reagent acts as an electron donor and the central intermediates is the radical anion ... 

Normally, the radical anion could be the starting point for a competitive process leading also to the same arylated nucleophile but the presence of radical scavengers, such as dinitrobenzene or ditertiobutylphenol would inhibit this secondary way of arylation, increasing the overall yield of arylation. 

In a reaction similar to 12-50, azoxy compounds can be prepared by the condensation of a nitroso compound with a hydroxylamine. The position of the oxygen in the final product is determined by the nature of the R groups, not by which R groups came from which starting compound. Both R and R can be alkyl or aryl, but when two different aryl groups are involved, mixtures of azoxy compounds (ArNONAr, ArNONAr, and Ar NONAr ) are obtained and the unsymmetrical product (ArNONAr ) is likely to be formed in the smallest amount. This behavior is probably caused by an equilibration between the starting compounds prior to the actual reaction (ArNO -I- Ar NHOH Ar NO - - ArNHOH). The mechanism has been investigated in the presence of base. Under these conditions both reactants are converted to radical anions, which couple ... 

Chemiluminescence also occurs during electrolysis of mixtures of DPACI2 99 and rubrene or perylene In the case of rubrene the chemiluminescence matches the fluorescence of the latter at the reduction potential of rubrene radical anion formation ( — 1.4 V) at —1.9 V, the reduction potential of DPA radical anion, a mixed emission is observed consisting of rubrene and DPA fluorescence. Similar results were obtained with the dibromide 100 and DPA and/or rubrene. An energy-transfer mechanism from excited DPA to rubrene could not be detected under the reaction conditions (see also 154>). There seems to be no explanation yet as to why, in mixtures of halides like DPACI2 and aromatic hydrocarbons, electrogenerated chemiluminescence always stems from that hydrocarbon which is most easily reduced. A great number of aryl and alkyl halides is reported to exhibit this type of rather efficient chemiluminescence 155>. 

The electrochemical reduction of aryllead triacetates was smdied by Chobert and Devaud82, as a re-examination of some previous work83 to detect the role of intermediates such as [ArPb(OAc)2]V The reductions were carried out by polarography in acetic acid or acidic alcohol solutions and show three diffusion controlled waves. The first step involves a single electron transfer to produce a radical anion which dimerizes, arylates the electrode or hydrolyzes to phenol ... 

Simple Group 14 tetraaryl complexes (Ar4E) appear to exhibit no particular photochemical behaviour. In the majority of aryl-substituted Group 14 complexes, the principal photochemical step is the loss of a ligand other than an aryl group. The initial photoproducts are either the radical A E-, the radical anion A E-- or A E . 

---