Arenes, cyanation

Within the year a wide range of photoreactions in which an aromatic residue undergoes change in substitution has been published. As previously, the diversity of the various processes makes any classification of the reactions unrealistic, and so their order of presentation here is somewhat arbitrary. Aromatic photosubstitution reactions have been reviewed by Parkanyi although the treatment is not extensive, the processes of free radical, electrophilic, and nucleophilic photoinduced substitutions of arenes are well covered.Arene photoreactions initiated by electron transfer with electron donors or acceptors are the subject of a review by Pac and Sakurai. The requirements for the efficient photogeneration of the ion radicals are considered and the synthetic utility of the photoreactions, which include reduction, cyanation, and amination, is discussed. 

There have been several recent investigations into the mechanism of photo-cyanation of aromatic hydrocarbons. The process with naphthalene, biphenyl, and phenanthrene has been subjected to a kinetic analysis the reactions in dry or aqueous methyl cyanide are shown to involve two transient species, the first of which is an ionic complex formed from a triplet excimer of the arene, or, in the presence of an electron acceptor, from a triplet exciplex. Reaction of the transient complex with the cyanide ion yields the radical ArHCN, and in aqueous methyl cyanide this second transient reacts with itself to produce dihydrocyano- and cyano-compounds. In dry methyl cyanide the radical species is oxidized to the cyano product. 

Transition metal organometallic complexes like dicarbonyl cyclopentadienyl iron [128], tricarbonyl cyclopentadienyl manganese [129] and iron-arene complexes [130,131] have also been reported as photoinitiators for photochemical crosslinking of cyanate esters. Photosubstitution of carbonyl groups by -OCN during irradiation initiates the reaction in the former case whereas photochemical dissociation of arene triggers it in the latter system. 

Recyclable [bis(trifluoroacetoxy)iodo]arenes 66 and 70 are excellent reagents for oxidative coupling of thiophenes or pyrroles [79] and for direct oxidative cyanation of heteroaromatic compounds using Me3SiCN as source of cyanide anion [80,81]. 

The cyanide anion has a strong tendency to attack non-substituted positions in aromatic rings [6, 7]. Therefore, the use of electrochemical methods described in Sect. 2.2 seems to be an appropriate approach for the Sn cyanation of arenes [22]. 

In general, electrochemical C-H functionalization of arenes and heteroarenes has been developed, as a new synthetic route to structurally modified aromatics. A number of transformations, such as cyanation, amination, C-arylation of ketones, alkylation, and phosphorylation have been performed by reacting arenes or heteroarenes with the cyanide ion, amines, ketones, RM and tetraalkylborate ions, and phosphorous compounds as nucleophiles, respectively. 

Cyanation of aryl hahdes, boronates, boronic acids, arene carboxylic acids, and (hetero)arene C—H bonds using nonmetaUic cyano-group sources 12AG(E)11948. 

Tetracyano-/ -quinodimethanes from / -bis(cyanomethyl)arenes H - CN One-pot cyanation-dehydrogenation... 

Other isocyanate syntheses that have recently been reported include several well-known reactions. One area which has attracted considerable attention is that of the direct production of isocyanates by the carbonylation of nitro-arenes. Both mono- and di-isocyanates are claimed to have been produced using various catalysts palladium, rhodium, and iron compounds often being cited. Other preparative reactions for isocyanates which have appeared in the literature include the acid catalysed hydrolysis of isocyanide dihalides and the reaction between alkyl halides and alkali-metal cyanates, although the latter has been given a modern flavour by the use of a polymer-supported reagent. ... 

Scheme 20 Cyanation of aryl halides and arenes with nitriles catalyzed or mediated by copper...

Hong, X. Wang, H. Qian, G. Xu, B. Rhodium-catalyzed direct C—H bond cyanation of arenes with isocyanide. J. Org. Chem. 2014,79, 3228-3237. 

Kim S, Choi J, Shin K, Chang S (2012) Copper-mediated sequential cyanation of aryl C-B and arene C-H bonds using anrmonium iodide and DMF. J Am Chem Soc 134 (5) 2528- 2531... 

More recently, Jin et al. discovered an efficient copper-catalyzed cyanation of arenes 119 nsing benzyl nitrile as a cyanide anion surrogate, which fiunishes aromatic nitriles 120 (Scheme 5.84) [85]. The process involves at least three steps, where (1) benzyl nitrile undergoes a copper-catalyzed oxidation with air to generate A and B, accompanied by the generation of Cu(II) species (2) cyanide anion is formed throngh a retro-cyanohydrination of A or/and hydrolysis of B and (3) the in situ-generated cyanide anion participates in the copper(II)-catalyzed aerobic oxidative C—H fnnctionalization to afford the cyanation product. 

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