Anion, reaction with aryl halides

The anion of DMSO undergoes a phenylation reaction with aryl halides under sunlight stimulation38. The presence of benzhydryl methyl sulfoxide (maximum yield 5%) in all runs, the sunlight activation, the order of reactivity of halobenzenes (I > Br > Cl), the inhibition of the reaction with oxygen, all hint at the SRN139-44 mechanism (Scheme 3). 

Dimsyl anion fails to undergo SrnI reactions with aryl halides,134,143 and the occurrence of fragmentation processes similar to those in equations (11) and (45) have been used to explain this failure,143 correcting the earlier, falacious claim that dimsyl anion and halobenzenes react to give benzyl methyl sulfoxide in high yield.144... 

There has been a review of the effects of high pressure on the substitution reactions of amines with haloaromatic compounds, including polyhalobenzenes.17 Nucleophilic substitutions by amines often proceed readily in dimethyl sulfoxide (DMSO). The pAa values, in DMSO, have been reported for some ammonium ions derived from amines widely used as nucleophiles in, S Ar reactions.18 Correlations have been established19 between the oxidation potentials and the basicities of some arylamine and diarylamine anions and the rate constants for their reactions with aryl halides in DMSO. 

This method has also been quite useful for the synthesis of triorganoarsenic compounds. Arsenide anions, which can be prepared by several methods (vide infra), react with alkyl and aryl halides. Several examples are shown in equations 18 , 19 , 20 °, 21 , 22 23 , 24 and 25 . In the reaction with alkyl halides inversion of configuration at the sp carbon atom takes place and in the reaction of vinyl halides retention of configuration has been observed (equations 18 and 21). Photostimulated reaction with aryl halides has been reported to proceed via the Sjj l mechanism thus, the ketone group is compatible with the reaction conditions as shown in equation 23 . 

Nickel.— Phosphine and phosphite complexes of nickel(0) react with strong acids to produce complexes [NiHL4]+. Reaction with weak acids may proceed further, with attack of the anion at the nickel. Thus the complex Ni(LL)a, where LL = l,4-bis(diphenylphosphino)butane, reacts with hydrogen cyanide to form firstly a hydride, which reacts quickly with cyanide to give the bimolecular intermediate Ni2(CN)2(LL)a. The ultimate products are the nickel(n) monomer Ni(CN)2(LL)2 and dimer [Ni(CN)2-(LL)]2. Ni(PPh3)4 undergoes normal oxidative elimination reactions with aryl halides to produce the new nickel(n) complexes Ni(aryl)(X)-(PPh3)2. ... 

The photoinduced reactions of aryl halides with the thiourea anion afford arene thiolate ions in DMSO. These species can be used without isolation for... 

The Heck reaction is a C-C coupling reaction where an unsaturated hydrocarbon or arene halide/triflate/sulfonate reacts with an alkene in presence of a base and Pd(0) catalyst so as to form a substituted alkene. Kaufmann et al. showed that the Heck reaction carried out in presence of ILs such as tetra-alkyl ammonium and phosphonium salts without the phosphine ligands, resulted in high yields of product. They attributed the activity to the stabilizing effect of ammonium and phosphonium salts on Pd(0) species. Carmichael et al. used ionic liquids containing either A,A -dialkylimidazolium and A-alkylpyridinium cations with anions such as halide, hexafluorophosphate or tetrafiuoroborate to carry out reactions of aryl halide and benzoic anhydride with ethyl and butyl acrylates in presence of Pd catalyst. An example of iodobenzene reacting with ethyl acrylate to give trans-et vy cinnamate is shown in Scheme 14. 

A variety of substituted aromatic compounds have been prepared through addition of anionic nucleophiles to arynes generated from readily accessible precursors.1 Most of the laboratory preparations start with aryl halides. The coupling yields are usually good to modest (equations 13-15) but can be poor (equation 16).83 Sometimes, a dramatic improvement in reaction efficiency can be achieved by the change of the base/solvent pair or other reaction conditions. For instance, in arylation of phenoxides and benzenethiolates, a switch over to DMSO as the solvent boosted the yield considerably (equation 17).86 Another example, illustrative of this point, is the reaction of N-methylpyrrolidone with aryl halides where an acceptable yield could not be obtained under a variety of conditions except with LICA in THF (equation 18).71... 

Argiiello, J.E., Schmidt, L.C. and Penenory, A.B. (2003) One pot two-step synthesis of aryl sulfur compounds by photoinduced reactions of thiourea anion with aryl halides. Organic Letters, 5, 4133—4136. 

When aryl halides were applied in catalytic coupling reactions, the mechanistic evidence points to initial SET reduction by low-valent nickel phosphine species (selected investigations in [23, 24]). The competition of cage collapse to ArNi(PR3)2X vs. dissociation of the aryl halide radical anion to a free radical and Ni(I) complexes determines the cross-coupling manifolds. Thus, Ni(0)-Ni(II) and Ni(I)-Ni(III) catalytic cycles can occur interwoven with each other and a distinction may be difficult. Common to both is that the coupling process with aryl halides is likely to occur by a two-electron oxidative addition/reductive elimination pathway. 

The carbanions derived from A,A-disubstituted amides and lactams react with certain aromatic halides in liquid ammonia under photostimulation [85,86] to form the expected a-arylated compounds in good yields. Unsymmetrical a, a-diaryl amides can be formed by reaction of aryl halides with the anion of the oc-aryl-A,7V-dimethyl acetamides [85]. 

On the other hand, difficulty has been found in detecting the reaction intermediates using ESR spectroscopy. Initial studies39 with aryl halides using this technique at low temperature in solid matrices identified the n and a radical-anions of Phi. Dissociation of the radical anion (Phi)- to phenyl radical and iodide anion was also observed. These studies therefore provided evidence for the first two steps in the aromatic S l mechanism. 

The photostimulated reaction of aryl halides with carbazole230, phenothiazine 231 and benzimidazole231 anions in DMSO proceeds by the S l mechanism, giving N-arylated products, although in very low yields. 

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