Nucleophile olefin combination, aromatic

The photochemical nucleophile-olefin combination aromatic substitution (photo-NOCAS) reaction received considerable attention from many groups not only because of its synthetic value because the yields of nucleophile-olefm-arene (1 1 1) adducts can be high but also because of interesting mechanistic details (Scheme 48). 

The photochemical nucleophile olefin combination, aromatic substitution (photo-NOCAS) reaction, formulated below for 2,3-dimethylbutene-methanol-p-dicyano-benzene, has some synthetic utility. The final step, loss of cyanide ion, is not shown. 

There has been a study of photo-induced intramolecular cyclization of some o-haloarylheterylamines which may lead to pyridof 1,2-a]benzimidazole derivatives.51 Several studies have been reported of photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reactions with fluoride,52 cyanide,53 or acetonitrile54 acting as the nucleophile, hi the example illustrated in Scheme 3,... 

Previously, Ohashi and his co-workers reported the photosubstitution of 1,2,4,5-tetracyanobenzene (TCNB) with toluene via the excitation of the charge-transfer complex between TCNB and toluene [409], The formation of substitution product is explained by the proton transfer from the radical cation of toluene to the radical anion of TCNB followed by the radical coupling and the dehydrocyanation. This type of photosubstitution has been well investigated and a variety of examples are reported. Arnold reported the photoreaction of p-dicyanobenzene (p-DCB) with 2,3-dimethyl-2-butene in the presence of phenanthrene in acetonitrile to give l-(4-cyanophenyl)-2,3-dimethyl-2-butene and 3-(4-cyanophenyl)-2,3-dimethyl-l-butene [410,411], The addition of methanol into this reaction system affords a methanol-incorporated product. This photoreaction was named the photo-NO-CAS reaction (photochemical nucleophile-olefin combination, aromatic substitution) by Arnold. However, a large number of nucleophile-incorporated photoreactions have been reported as three-component addition reactions via photoinduced electron transfer [19,40,113,114,201,410-425], Some examples are shown in Scheme 120. 

In some cases the nucleophilic capture of a radical cation is followed by coupling with the radical anion (or possibly with the neutral acceptor), resulting ultimately in an aromatic substitution reaction. Thus, irradiation of 1,4-dicyanobenzene in acetonitrile-methanol (3 1) solution containing 2,3-dimethylbutene or several other olefins leads to capture of the olefin radical cation by methanol, followed by coupling of the resulting radical with the sensitizer radical anion. Loss of cyanide ion completes the net substitution reaction [144]. This photochemical nucleophile olefin combination, aromatic substitution (photo-NOCAS) reaction has shown synthetic utility (in spite of its awkward acronym). 

The photochemical Nucleophile-Olefin Combination, Aromatic Substitution (Photo-NOCAS) reaction, in Handbook of Organic Photochemistry and Photobiology, 2nd edn (eds W. Horspool and F. Lenci), CRC Press, Boca Raton, pp. 40-1-40-17. 

In the examples above, the attacking radical arises from the fragmentation of the radical cation, but, as already noted, in the presence of a nucleophile, radicals are also formed by addition to olefinic (Scheme 4) [45,78-79], aromatic [80] or heterocyclic [81] radical cations, and react in the same way. For the reactions involving alkenes the acronym Nucleophile-Olefin Combination Aromatic Substitution (NOCAS process) has been proposed in this process, the radical cation of strained alkenes may rearrange before addition (Scheme 20) [79]. 

Arnold, D. R., Chan, M. S. W., McManus, K. A., Photochemical Nucleophile Olefin Combination, Aromatic Substitution (Photo NOCAS) Reaction. 12. Factors Controlling the Regiochemistry of the Reaction with Alcohol as the Nucleophile, Can. J. Chem. 1996, 74, 2143 2166. 

In the presence of methanol as solvent and 1,4-dicyanobenzene as acceptor, photoinduced electron transfer from 1,4-bis(methylene)cyclohexane gives 4-(methoxymethyl)-1 -methylenecyclohexane and 4-(4-cyanophenyl)-4-(methoxy-methyl)-l-methylenecyclohexane which arise by nucleophilic attack of the solvent on the radical cations, followed either by reduction and protonation, or by combination with the radical anion of the electron acceptor.These observations are in accordance with the proposed mechanism of the nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction. The same group has also investigated the use of cyanide ion as nucleophile and report that irradiation of a mixture of 1,4-dicyanobenzene in the presence of biphenyl as donor, KCN, and 18-crown-6 gives a mixture of (79) and (80). These workers have also extended the scope of NOCAS to fluoride ion. In particular, use of 2,3-dimethylbut-2-ene and 2-methylbut-2-ene gives 4-cyanophenyl substituted... 

The scope of the photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction has been extended to include cyanide. 

When the radical cation DX does not undergo fragmentation, as is typical when arising from an olefin, an alternative is found in the attack by a nucleophile Nu". The resulting radical X-D-Nu couples with the aryl radical anion ArZ " and regains aromaticity, yielding again a three-components adduct X-ArD-Nu [the NOCAS (Nucleophile Olefin Combination Aromatic Substitution) process, as shown in Scheme 14.2, path g [14, 15]. In Scheme 14.2, the aromatic compound... 

NOCAS nucleophile olefin combination aromatic substitution... 

As an example, this apply to enols or tautomeric enols such as maleic acid derivatives. While with a chemical reagent (cerium ammonium nitrate) the only process occurring is oxidative dimerization, when aromatic nitriles are used as the photochemical oxidant, selective trapping of the radicals by an electrophilic alkenes or by the nitrile itself occurs. Under these conditions, both the alkylation of alkenes and the oxidative alkylation/dimerization of dienes have been smoothly obtained (see Scheme 8) and side processes such as double alkylation or polymerization often occurring with other methods have been avoided. A three-component (Nucleophile-Olefin Combination, Aromatic Substitution) process is also possible. ... 

Some interesting photo-NOCAS-type reaction (photochemical nucleophile-olefin combination, aromatic substitution) have been reported by three groups. Arnold has developed the photo-NOCAS reaction as three components photoaddition.Xu et al., reported the intramolecular photocyclization of A-(co-hydroxyalkyl)-tetrachlorophthalimide (138, 141) with alkenes to give medium- and large-ring heterocycles (140,143). These photoreactions proceeded via 1, n-biradicals generated from the nucleophilic attack of alcohols to alkenes between the radical anions of phthalimides and the radical cations of alkenes. 

Another three components synthesis is involved in the extensively investigated NOCAS process (nucleophile olefin combination aromatic substitution, path h) [89,90], In this case, a nucleophile adds to an alkene radical cation and again the interaction between the resulting radical and the radical anion of the aromatic nitrile may follow two paths. The first is electron transfer, which results in sensitized anti-Markovnikov addition onto the alkene, and is favored with stabilized, reducible radicals such as the benzyl radicals obtained from aryl olefins. The latter one is... 

Arnold, D.R. and Snow, M.S., The photochemical nucleophile-olefin combination, aromatic substitution reaction. Part 2. Cyclic olefins, methanol, 1,4-dicyanobenzene, Can. J. Chem., 66, 3012, 1988. 

The photoinduced charge-transfer reactions between N or Q and 1,4-dicyanobenzene (DB) in aceto-nitrile/methanol lead to adducts 17 and 18, photoinduced nucleophile-olefin-combination-aromatic-substitution (NOCAS) products 19-22 and two acetonitrile adducts, 23 and 24, which are formed only from N. ... 

Mangion, D. and Arnold, D.R., Photochemical nucleophile-olefin combination, aromatic substitution reaction. Its synthetic development and mechanistic exploration, Acc. Chem. Res., 35, 297, 2002. 

Mangion, D., Arnold, D.R., Cameron, T.S., and Robertson, K.N., The electron transfer photochemistry of allenes with cyanoarenes. Photochemical nucleophile—olefin combination, aromatic substitution (photo-NOCAS) and related reactions, /. Chem. Soc., Perkin Trans. 2, 48, 2001. 

The Photochemical Nucleophile-Olefin Combination, Aromatic Substitution (Photo-NOCAS) Reaction... 

---