Photocyanation, aromatics

Beneficial Micro Reactor Properties for the Photocyanation of Aromatic Hydrocarbons... 

Photocyanation of Aromatic Hydrocarbons Investigated in Micro Reactors Oiganic synthesis 43 [OS 43] Photocyanation of pyrene... 

Nitrobenzenes react with potassium cyanide in the presence of cetyltrimethylammo-nium bromide to yield benzonitriles [71], The reaction also requires the presence of chloro substituents on the ring and at least two nitro groups (Table 2.9). Diazosulphides, ArN=NSPh, are converted into the benzonitriles, ArCN, by a photochemically induced SRN1 reaction with tetra-n-butylammonium cyanide [72, 73], Yields vary from <20% to >70%. Photocyanation of aromatic hydrocarbons has been achieved using tetra-n-butylammonium cyanide in acetonitrile or dichloromethane [74, 75]. 

In many of the cases studied a nitro-group is present as a substituent in the aromatic reactant and one gets the impression that this is favourable to the reaction. On the other hand, quite a few examples are known where no nitro-group plays a role, e.g., in the reactions of anisoles (Bcirltrop et al., 1967 Nilsson, 1971 Lok and Havinga, 1973), in the photocyanation of aromatic hydrocarbons (Vink et al., 1972a), and in the photosubstitution of aromatic ketones (Letsinger and Colb, 1972). 

The effect of entibititic residues m dairy products Photocyanation of aromatic compounds Hydrocarbon-water emulsions as fuels... 

Photonucleophilic substitution of fluoro- and chloro-anisoles has been the subject of three reports within the year. Cornelisse and co-workers have studied the photocyanation and photohydrolysis of 4-fluoro- and chloro-anisoles by laser spectroscopy and report that the initial step of the reaction involves formation of a triplet state transient complex composed of a ground state and an excited state aromatic molecule. Only in the presence of water does the complex yield radical ions and it is this process which determines the product quantum yield. The radical cation then reacts with the nucleophile to give a neutral radical which yields the substituted arene in a single step. Liu and Weiss report on anomalous effects during photonucleophilic aromatic substitution of 2- and 4-fluoroanisoles and also on the photo-... 

Nucleophilic photosubstitutions are typical reactions of aromatic halides. The variability of the process in terms of mechanism is presented in detail, along with many examples, in Section 6.2.3 therefore only one example of the reaction is mentioned here. Multiple photocyanation of hexachlorobenzene (492) takes place in aqueous acetonitrile containing a cyanide ion to give pentacyanophenolate (493) (Scheme 6.240).1347 The reaction involves the triplet state apparently via an SN2Ar (Scheme 6.93) pathway. 

Beugelmans, R.,A. Ginsburg,A. Lecas,M.T. LeGoff,andG. Roussi, Use of Phase Transfer Agents for Photocyanation of Aromatic Hydrocarbons, Tetr. Lea., 3271 (1978). 

Photocyanation of aromatic compounds is dealt with in several papers this year. In the presence of an electron acceptor such as p-dicyanobenzene, aromatic hydrocarbons such as naphthalene (81), substituted naphthalenes, phenanthrene, or anthracene give mixtures of products on irradiation with sodium cyanide. The major products involve substitution of hydrogen by cyanide or addition of hydrogen cyanide to the aromatic hydrocarbon. When oxygen is present, the product mixture is less complex, and a good yield of cyano-substituted compound is obtained. It is proposed that the aromatic radical cation is involved in the... 

The PET-generated arene radical cations also undergo nucleophilic substitution via the a-complex. Photocyanation of arenes may be cited in this context as a very early example [139], where hydrogen served as the group undergoing displacement. This concept is further extended [140] for the direct amination of polynuclear aromatic hydrocarbons with ammonia or primary amines via the arene radical cation produced by irradiating arenes in die presence of DCNB. Another potentially useful application of this methodology is... 

Photosubstitutions. Selective control of reactivity at different positions of the aromatic ring included in the CD cavity has been obtained in the nucleophilic photohydroxylation and photocyanation of fluoroanisoles (FA) by Liu and Weiss [308,309]. The reaction (see Scheme 27 for 4-FA) involves the FA" " radical cation intermediate formed by electron transfer between a triplet and a ground state molecule. Tables 19 and 20 list the relative quantum yields of the two photoreactions in mixtures of bound and free 4-FA and 2-FA. Both reactions appear to be almost totally inhibited in the... 

Photochemistry Photocyanation of aromatic hydrocarbons PTC carbonylation of aryl and vinyl halides under UV irradiation Reduction of nitrobenzenes to the corresponding oximes or quinones using viologens Photohydrogenation of acetylenic groups with viologen, Pt or Pd, and a photosensitizer Photochemically induced polymerization of methyl methacrylate Beugelmans et al. (1978) Brunet et al. (1983) Tomioka et al. (1986) Maidan and Willner (1986) Shimada et al. (1989, 1990)... 

Another important class of reactions involves the introduction of a cyano group by substitution in an Ar-Z precursor. In fact, novel pathways leading to aromatic nitriles-for example, photosubstitution reactions-are desirable in view of the many applications of aryl cyanides as agrochemicals and pharmaceuticals. Today, the classical copper(l)-mediated Rosenmund-von Braun and Sandmeyer reactions, from aryl halides and aryldiazonium salts respectively, have been supplanted by reactions which employ palladium- or copper-catalysis [57]. The rather common use of excess cyanide anion may lead to a deactivation of the catalyst, and affect to a remarkable extent each of the key steps of the catalytic cycle [58aj. Although the use of complex iron cyanide has been shown to offer an effective solution to this limitation [58b,c], photocyanation provides an equally useful alternative [10],... 

The photocyanation of pyrene in a microchaimel through an oil-water interface was investigated by Ueno et al. [13]. The microchips employed are made of polystyrene by embossing with a silicone template. The phase transfer reaction proceeds in four steps as depicted in Figure 16.2. In the first step, a photoinduced electron transfer in the oil phase (polycarbonate) occurs from the aromatic hydrocarbon pyrene (DH) to the electron acceptor 1,4-dicyanobenzene (A). The cationic DH " radical is subsequently the target of the nucleophilic attack of the cyanide anion at the oil-water interface. The cyanated product DCN is insoluble in water and goes back into the oil phase. Experiments without a cyanide source (NaCN) in the aqueous phase show no reaction. Hence it can be excluded that the nudeophilic-substituted cyanide originates from the electron acceptor 1,4-dicyanobenzene. 

A different kind of nucleophilic aromatic substitution reaction, namely cyanation reactions, was described by Kitamura and coworkers [49]. Thqr investigated the photocyanation of pyrene by mixing an aqueous solution of NaCN and a propylene carbonate solution of pyrene and 1,4-dicyanobenzene in Y-shaped microfluidic chips made of polymers (Scheme 4.26). Since the reaction takes place at the oil-water interface, an increase in interfadal area was a major driver for employing microreactors. 

Photochemical cyanation of aromatic hydrocarbons in acetonitrile solution is a higher yield process when the potassium cyanide complex of 18-crown-6 is the cyanide ion source [31] compared to similar reactions in mixed organic aqueous solvent systems [32] (see Eq. 7.16). A ten-fold excess of 18-crown-6/KCN over the aromatic hydrocarbon (present in 10 " M) was used. The yield improvements were attributed to increased activity of cyanide due to diminished hydration of the ion. Biphenyl, naphthalene, phenanthrene, and anthracene were photocyanated in 50%, 15%, 25% and 20% yields respectively the latter being an equimolar mixture of mono and dicyanation products [31]. 

Photochemical nucleophilic substitutions of aromatic ring systems in protic solvents have been well documented (Scheme 1). When a crown ether is present the photocyanation proceeds better in aprotic solvents than in protic solvents (Scheme 2), while addition of an electron acceptor, such as terephthalonitrile (2), improves both the yields of the photocyanation products and the specificity of substitution (Scheme 3). This report concerns the photochemical cyanation of anisole (1) and dimethoxybenzenes (5) with KCN in an aprotic solvent (CH2CI2), in the presence and absence of 2 when polyethylene glycol (PEG) is present instead of crown ether (CE) (Schemes 4 and 7). 

The photosubstitution of many polycyclic aromatic molecules demonstrate an a-effect. For example, photocyanation of naphthalene and Azulene give the 1-cyano derivatives and phenanthrene the 9-substituted compound. 

Photocyanation of deuterium-labeUed 1-nitronaphthalenes with cyanide nucleophile has been proven to involve primarily an ipso attack followed by isomerizations. The reaction of nucleophiles with halogen-ated aromatics proceeds via attack of the nucleophile on the nitroarene, generating an anionic o-complex. This is followed by departure of the leaving group and rearomatization. Cervera et al. and Huertas et al. have shown a direct coupling of carbon nucleophiles with m-dinitrobenzene via fluoride promoted nucleophilic aromatic substitution (Scheme 5). Amines, amides, ethyl acetate, acetonitrile, and acetone... 

Lazaro, R., Bouchet, R, Sole, R., Alkorta, I., and Elguero, J., Aromatic photosubstitution, IV. Photocyanation of aromatic compounds labeling experiments, ACfl-Models Chem., 136,531,1999. 

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