Photoinduced reactions, aryl halides

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

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. 

The reductive photodehalogenation of aryl halides has been actively investigated in recent years. Special attention has been given to (poly)halobenzenes and (poly)halo-biphenyls. The reactions are of interest in view of their mechanisms, and because of the importance of chlorinated aromatic hydrocarbons as environmental pollutants and the possibility of their photoinduced degradation. The photochemistry of aryl halides and related compounds in general14 and the photochemistry of polyhaloarenes in particular18 have been reviewed. 

C. Intramolecular Photoinduced Reaction between Aryl Halides and Enamides... 

Sodium benzenetellurolate is arylated in liquid ammonia via photoinduced reactions with aryl halides Aryl iodides reacted with sodium benzenetellurolate in the presence of copper(I) iodide in hexamethylphosphoric triamide (but not in DMF or DMSO) to produce aryl phenyl tellurium compounds. However, lithium benzenetellurate reacted with 1,2-bromoiodobenzene and lithium methane tellurolate with 1,2-dibromobenzene in tetrahydrofuran at 20° to yield l,2-bis[organotelluro]benzenes . 

One way of carrying out nucleophilic aromatic substitution reactions under mild conditions is the Ar RNl process, which is initiated by (usually, but not necessarily, photoinduced) electron transfer to an aryl halide, e.g., from an enolate Cleavage of the resulting aryl radical anion with loss of a halide anion gives an aryl radical that combines with the enolate, thus forming the desired aryl-carbon bond. 

In the field of (hetero)aromatic photochemistry substitution reactions are also quite useful. The two most useful classes are the SrnI reaction [16] and SnI reaction [17], involving respectively the aromatic radical anion and the aryl cation as the key intermediates. In the former case, (generally photoinduced) electron transfer generates the radical anion of an aryl halide. With less strongly bonded derivatives (usually iodides) the intermediate cleaves to an aryl radical that gives the new product via a chain process (see Scheme 2.6). 

These reactions are relevant to this review in that they may be photoinduced, but as radical chain processes many of them can be initiated in other ways. Radical chain dehalogenation of aryl halides has been reviewed previously.The chain propagation sequence for the deiodination of aryl iodides with CH5O-/CH3OH is shown in Scheme 1. The species (Arl) is very short lived and deiodinates efficiently. Reactivity diminishes in the order Arl > ArBr > ArCl and is enhanced in cases where there is relief of steric strain. Photochemical dehalogenations of aryl halides with AIH4 and BH4 have been proposed to follow similar radical chain mechanisms. Evidence for electron transfer from BH4 to ArCl has been present by Freeman and Ramnath. ... 

The reaction took place either via the ArSsl or via the AtSn2 mechanism (see Scheme 14.1, paths h,c), and occurred successfully via the first mechanism when using aryl hahdes or esters with electron-donating substituents. Among halides, both aryl chlorides and fluorides underwent photosubstitution when irradiated in an aqueous MeCN solution of KCN (Scheme 14.10, left part) [59]. It should be noted that, in transition metal-catalyzed reactions, the substitution of a chloro- by a cyano-group occurs only under relatively harsh reaction conditions, whereas such a process does not take place at all with aryl fluorides [57]. In the case of aryl esters the photoinduced cyanation occurred conveniently. As esters are easily... 

In some cases, fluorination was photoaccelerated in the reaction of aryl diazonium salts in the presence of etherated boron trifluoride [221], Photodecomposition of diazonium salts in the presence of other halide anions (bromide or chloride) was in practice not explored [222,223]. Although aryl iodides can be obtained thermally from diazonium and iodonium precursors, some photoinduced syntheses were also reported [224,225]. The reaction seemed to involve aryl radicals [225] in the former case and aryl cations in the latter one [224],... 

---