Photochemical cyanation

Photochemical cyanation of nitroimidazoles (106) to give cyanoimidazoles (107) has been reported as a synthetically useful procedure. 

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). 

Similar photochemical cyanation reactions were investigated for 0- and p-dlmethoxybenzenes (5a and 5b) with KCN in an aprotic solvent, in the presence/absence of 2 when PEG is used as the cosolvent (Scheme 7). The results are shown in Table 3. 

Autoxidation of secondary acetonitriles under phase-transfer catalytic conditions [2] avoids the use of hazardous and/or expensive materials required for the classical conversion of the nitriles into ketones. In the course of C-alkylation of secondary acetonitriles (see Chapter 6), it had been noted that oxidative cleavage of the nitrile group frequently occurred (Scheme 10.7) [3]. In both cases, oxidation of the anionic intermediate presumably proceeds via the peroxy derivative with the extrusion of the cyanate ion [2], Advantage of the direct oxidation reaction has been made in the synthesis of aryl ketones [3], particularly of benzoylheteroarenes. The cyanomethylheteroarenes, obtained by a photochemically induced reaction of halo-heteroarenes with phenylacetonitrile, are oxidized by air under the basic conditions. Oxidative coupling of bromoacetonitriles under basic catalytic conditions has been also observed (see Chapter 6). 

Again, a hnear relationship of and [CNS] i shows a bimolecular reaction between the excited triplet state of 1 and the nucleophile to take place. The triplet lifetime of la is 4.7 X 10 s in water and 1.2xl0 8s in aqueous 10 2M solutions of potassium cyanide as determined from quenching studies The nitro group in 7 a is likewise replaced photochemically by methoxide and cyanate ions. 

In contrast to the deactivation observed in the metal-catalyzed process, the photochemical reaction tolerates the use of an excess of KCN, a relatively inexpensive cyanating agent, to generally improve the yield. 

Additional photochemical studies on a-arylvinyl bromides were undertaken in order to study their synthetic utility [66,67]. The synthetic use of vinyl halides had been limited to conditions which required a high temperature and a protic, polar solvent. The advent of photolytically generated vinyl cations has allowed the use of arylvinyl halides to construct heterocycles in the presence of azide anion [66] and isoquinolines in the presence of cyanate ion [67] (Scheme 23 and 24). 

All the crystalline metal fulminates and the heavy metal azides explode and detonate on the application of thermal shock and in some cases in the presence of an electric field, while the heavy metal cyanates and thiocyanates only deflagrate at high temperatures. According to Bowden and Williams (83) the stability to thermal shock increases in the order fulminate heavy metal salts are more thermally unstable. During the last decade Yoffe and co-workers have tried to explain the thermal and photochemical behaviour of these meterials in terms of their electronic structure. This work has been extensively reviewed by Yoffe (2) and will not be considered here. We will however explore briefly a few ideas which qualitatively correlate the geometry of the electronic and atomic configurations with the kinetic stability of these materials. 

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. 

In addition to RB, eosin Y could be used for similar transformations of amines. Konig et al. utilized eosin Y and green LEDs to oxidize THIQ derivatives. The transient iminium ion was then trapped with a variety of nucleophiles including nitroalkanes, malonates, and diall lphosphonates (Scheme 10.31)." The addition of malonitrile, however, led to a-cyanation rather than formation of the (3-malonitrile, a result which has also been observed under non-photochemical oxidative conditions. ... 

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