Sensitizers 9,10-dicyanonaphthalene

The cyclization was followed by hydrolysis and oxidation. The transformation is a photochemical analogue of the Mannich or the Pictet-Spengler reaction. A photochemicaUy sensitized cyclization was applied to the synthesis of the tetrahydroisoquinoline derivative 107. The reaction starts with an electron transfer from 106 to the photochemical excited sensitizer dicyanonaphthalene (DCN). The resulting radical cation W undergoes cyclization followed by the release of a trialkylsilyl cation. Intermediate X then yields the final product 107 by hydrogen loss. 

A series of aluminum(III) naphthalocyanines of potential value as PDT sensitizers has been synthesized. Thus, treatment of 2,3-dicyanonaphthalene with aluminum(III) chloride in refluxing quinoline for 2h gave 48% of ClAl(NPc).353,354 Hydrolysis gave HOAl(NPc), from which tri-alkylsiloxy derivatives could be made.354 Although nonenveloped viruses were resistant, various enveloped viruses were inactivated with aluminum(III) complexes of benzonaphthoporphyrazine sulphonic acids as sensitizers.355... 

The presence of hetero-atoms within the system, remote from the alkene double bonds, does not have an adverse influence on the SET processes that occur. Thus irradiation of the diene 33 in benzene solution with 1,4-dicyanonaphthalene as the electron-transfer sensitizer affords the cyclobutane 34 in 78% yield. Various examples of the reaction were described giving cyclobutane derivatives in 54-69% yield. Benzene, or an arene solvent, is vital for the success of the reaction. When acetonitrile is used, allylation of the sensitizer (akin to the photo-NOCAS reaction) results in the formation of the three products 35-3718. (2 + 2)-Cyclization of this type described for 33 is also seen with the dialkenyl ether 38. When 38 is irradiated using X > 350 nm or X > 450 nm in acetonitrile... 

Hirano et al. reported on the stereoselective cyclization to give tetralin derivatives using the phenanthrene-p-dicyanobenzene sensitizer system. Pandey independently reported the intramolecular photocyclization of methoxybenzene derivatives bearing silyl enol ether chromophore via their heterodimer radical cations in the presence of 1,4-dicyanonaphthalene gave benzo-annulated cyclic ketones in 70% yields [490] (Scheme 133). 

Diels-Alder reactionsBoth 1,4-dicyanonaphthalene (DCN) and 2,6,9,10-te-tracyanoanthracene (TCA) have been used as sensitizers to effect photochemical [4 + 2]cycloadditions of electron-rich dienes and electron-rich dienophiles, which do not normally undergo thermal cycloadditions. These cycloadditions are known as triplex Diels-Alder reactions because they are postulated to involve as an intermediate a three-membered complex of sensitizer, dienophile, and diene. This reaction is useful for synthesis of bicyclo[2.2.2]octenes from some silyl enol ethers, alkenes, or arylalkynes. 

A similar study has been reported on the 1,4-dicyanonaphthalene (DCN)-sensitized light-induced desilylation (PET process) of iV-alkenyl-substituted RSMA to form pyrrolidine and piperidine derivatives in high yields, involving a delocalized a-silyl-methylamine cation as the key intermediate in these cyclization reactions.279,340,341... 

Fig. 7. Cyclodimerization of l-acetoxy-l,3-cyclohexadiene (17b), sensitized by A = 1,4-dicyanonaphthalene (21) in the absence and presence of Q = 1,2,4-trimethoxybenzene (23) (full symbols absence of Q open symbols presence of Q)...

Fig. 8. Pressure dependence of the endo-exo radio (In relative endo-exo ratio) in the determination of 1,3-cydohexadiene (17 a), sensitized by 1,4-dicyanonaphthalene (21) in aceto-nitrille and benzene [64]...

In benzene solvent, Calhoun and Schuster [110] proposed this intermediate with 1,3-cyclohexadiene and 1,4-dicyanonaphthalene as sensitizer. The same group expanded this mechanism to other substrates in benzene solution, e.g., 1,3-cyclohexadiene-indene [111], cyclopentadiene-indene [65], 1,3-cyclohexadiene-enol... 

With high indene concentration, high yields of 1 1 photoadducts of indene and 1,3-cyclohexadiene are formed in benzene solution. This concentration dependence was established with 1,4-dicyanonaphthalene as sensitizer, whereas with 9,10-di-cyanoanthracene no such observation was made. 

A [2 + 2] photocycloaddition with two alkenes can also be induced by photochemical electron transfer [16,17]. In such cases, sensitizers are frequently used and the reactions therefore occur under photocatalysis [18]. Under photochemical electron transfer (PET) conditions, the diene 10 yielded in an intramolecular reaction the cyclobutane 11 (Scheme 5.2) [19], such that in this reaction a 12-membered cyclic polyether is built up. The reaction starts with excitation of the sensitizer 1,4-dicyanonaphthalene (DCN) only 0.1 equivalents of the sensitizer are added to the reaction mixture. Electron transfer occurs from the substrate 10 to the excited sensitizer, leading to the radical cation I. This intermediate then undergoes cycli-zation to the radical cation of the cyclobutane (II). Electron transfer from the radical anion of the sensitizer to the intermediate II leads to the final product 11, and regenerates the sensitizer. In some cases, for example the cydodimerization of N-vinylcarbazole, the effidency is particularly high because a chain mechanism is involved [20]. 

For oxidative purposes, electron-deficient arenes e.g., dicyanoanthra-cene (DCA), dicyanonaphthalene (DCN), or triphenylpyrylium salts are frequently used. For reductive PET reactions (not shown in Sch. 2) electron donor substituted arenas or amines are generally applied. The latter substrates are consumed during the reaction and are thus sacrificial co-substrates rather than sensitizers. Nevertheless, the strategy of a sacrificial electron transfer provides an effective way to avoid back electron transfer. 

Toluene, durene, hexamethylbenzene, 1- and 2-methylnaphthalenes are oxidized to the corresponding benzaldehydes by irradiation in oxygen-equilibrated acetonitrile sensitized by 1,4-dicyanonaphthalene, 9-cyano-, 9,10-dicyano-, and 3,7,9,10-tetracyanoanthracene. The reaction involves proton transfer from the radical cation of the donor to the sensitizer radical anion or the superoxide anion, to yield the benzyl radical which is trapped by oxygen. In the case of durene, some tetramethylphthalide is also formed with this hydrocarbon it is noteworthy that the same photosensitization, when carried out in an nonpolar medium, yields the well-known singlet oxygen adduct, not the aldehyde [227,228] (Sch. 20). 

Nitroaromatics also sensitize the oxidation of methylarenes and it has been found that silica-grafted 2,4,6-trinitrobenzene is a convenient heterogeneous sensitizer, giving the aldehydes in 79-90% yields with 100% selectivity [229]. Bibenzyls, pinacols and pinacol ethers are likewise oxidized to ketones or respectively esters through carbon-carbon bond fragmentation upon dicyanonaphthalene sensitization [230]. A good method for benzylic oxidation is based on titanium dioxide photocatalysis [231-233]. 

There are a number of other mechanisms by which alkenes can undergo photochemical (2 + 2) cycloaddition, one of which works well for electron-rich alkenes and electron-acceptor sensitizers. The pathway is through the radical cation of the alkene, which attacks a second, ground-state alkene molecule and then cyclizes and accepts an electron to give the product cydobutane. Typical of this group of reactions is the formation of 1,2-dialkoxycyclobutanes from alkoxy-ethylenes with dicyanonaphthalene as sensitizer (2.78). 

A characteristic feature of some of these reactions is the dependence of their efficiency on the basicity of the radical anion [108], The differences are especially manifested in non-polar solvents, where the CIP are expected to dominate. Some of these cleavage processes are more efficient than expected, based on the thermodynamic evaluations of the unassisted fragmentation (Sect. 3.2). Also a stereochemical preference for cleavage is observed for erythro isomers as compared to the threo isomers (Scheme 6). In benzene erythro/threo selectivity is high, being highest for the relatively basic radical anion of dicyanonaphthalene and lowest for the relatively nonbasic radical anion of thioindigo. The stereochemical preference disappears in acetonitrile if biphenyl is used as a co-sensitizer [108]. 

The present radical formation was then applied to intramolecular cyclization. 3-Substituted cyclohexenone derivative 64 which has a (tributylstannyl)methylthio group in the side chain was irradiated (hv > 300 nm) in methanol. The desired spiro compound 65 was, however, obtained in low yield (23%) along with methyl sulfide 66 (60%) and dienone 67 (S%). In contrast, when 64 was irradiated in the presence of a 0.3 molar amount of 1,4-dicyanonaphthalene (DCN) as sensitizer in acetonitrile with 5 molar amounts of water, spiro ketone 65 was produced in 84% yield without a detectable amount of 66 and 67. Thus, by employing the DCN-mediated one-electron oxidation, the intramolecular coupling reaction proceeds smoothly (Scheme 27). 

Scheme 61) [167-169]. In these photoreactions, dicyanonaphthalene acts as an effective sensitizer for generation of the radical cations of the heterocyclic compounds. 

The 1,4-dicyanonaphthalene-sensitized photocyclodimerization of the vinyl ethers (192), yielding (193) has been described. 9,10-Dicyanoanthracene-sensitized dimerization of cyclohexa-1,3-diene affords the two [4+2] adducts (194) and (195) in a total yield of 60% and in a ratio of 4 1. This is to be contrasted with the previous report of cyclohexadiene dimerization where [2 + 2] dimers were also obtained." A detailed study of the photodimerization of some acenaphthylenes (196) has been reported. " The structures and configurations of the products have been elucidated by spectroscopic methods." ... 

Mattay and coworkers reported that direct irradiation (2 > 300 nm) of 2-cyclohexene-l-one (251) in the presence of A, A -dimethylallylamine (252) leads to 2,4 y -dimethyl-ds-decahydro-S-isoquinolineone (253) in yields up to 20 % along with trace amounts of 254 (Scheme 60), depending on the polarity of the medium (Table 12) [307]. In polar solvents like acetonitrile containing 10 % water the PET promoted addition-cyclization process competes efficiently with the [2 + 2] cycloaddition pathway leading to dimers (Table 12). Irradiation in the presence of PET sensitizer such as dicyanonaphthalene (DCN) led to only trace amounts of products. 

Zimmerman and Hofacker have studied the photochemically induced SET reactivity of the 1,4-dienes (74). The sensitizers used were dicyanoanthracene and dicyanonaphthalene. The radical cations of the 1,4-dienes undergo regioselective cyclization to the cyclic radical cations (75) which ultimately afford the final products (76). The SET-induced photochemistry of other non-conjugated dienes such as geraniol (77) has been studied. The results demonstrate that with DCA as the sensitizer in methylene chloride a contact radical-ion pair is involved and this yields the cyclopentane derivatives (78) and (79) in the yields shown. The cyclization is the result of a five-centre cyclization. With the more powerful oxidant dicyanobenzene as the sensitizer and in acetonitrile as solvent, separated radical-ion pairs are involved and this leads to the formation of the bicyclic ethers (80) and (81). DCA-sensitized reactions of the dienes (82) and E,E-(S3) and the bicyclohexane (84) have been studied. At low conversion the irradiation of (84) under these conditions affords a mixture of the dienes (82) and , -(83) in ratios that are independent of temperature. 

Photoelectron transfer oxidation of phenols, 3,5-dunethyl and 2,6-dunethylphenol, takes place using 2-nitrofluorene as the electron-accepting sensitizer in both acetonitrile and cyclohexane solution. In acetonitrile the anion radical of 2,6-dimethylphenol is observed as the final product . Other phenols such as the 2,4,6-trunethyl derivative also undergo electron transfer reactions with 1,1 -, 1,2 - and 2,2 -dinaphthyl ketones. Other sensitizers such as 1,4-dicyanonaphthalene with biphenyl as a co-sensitizer in acetonitrile have also been used. The resultant phenol radical cations (4a-h) have absorption maxima in the 410-460 nm region with the exception of 4i that absorbs at 580 nm. When the reactions are carried out in the presence of a trace of water, the radical cations are not observed. Instead, phenoxyl radicals are detected. This presumably is due to the reaction shown in equation 2. 

G. Pandey. Upon electron transfer sensitization of the donorsubstituted arene by means of dicyanonaphthalene oxidation occurs to form a radical cation which is intramolecularly trapped by the acid group to form the coumarin. 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

SENSITIZERS AP = acetophenone BP = benzophenone A = acetone B = methyl 3,5-bis(trifluoromethyl)benzoate C = (-)-tetrabomyl 1,2,4,5-benzene tetracarboxylate D = (-)-hexakis(l-methylheptyl) benzenehexacarboxylate MB = methyl benzoate P = phenan-threne CNN = 1-cyanonaphthalene DCA = 9,10-dicyanoanthracene DCB = 1,4-dicyano-benzene DCN = 1,4-dicyanonaphthalene CuOTf = copper trifluoromethanesulfonate benzene complex TPT = tetraphenylpyrylium tetrafluoroborat. 

Similarly bis (3,5-dimethyl phenyl) disulfides yielded the corresponding polymer 209b in the absence of acid. In both cases it is suggested that ArSS+(Ar)SAr, formed in the reaction, effects electrophilic aromatic substitution by acting as an ArS+ donor. This same polymerization process can also be achieved by photooxidation of bis (3,5-dimethylphenyl) disulfide in the presence of 2,3-dicyanonaphthalene as sensitizer in acetonitrile containing trifluoro-acetic acid [486]. 

Many examples of cyclobutane formation involving cyano-substituted alkenes or enone components have been reported over the years. Some of these reactions arise as a result of SET to an electron-accepting sensitizer such as 1,4-dicyanonaphthalene. One such reaction among the many reported involves the photoreaction of the diene 244 in benzene solution. Under these conditions the (2 + 2)-cycloadduct 245 is formed in reasonable yields ca 70%). In acetonitrile, however, a different reaction occurs leading to the allylation of the sensitizer affording 246, which undergoes (2 + 2)-cycloaddition to yield the cyclobutane 247 ". Other intramolecular cycloadditions have also been reported, such as the formation of the cage compound 248 from irradiation of the pentaene 249 " ". ... 

Stereomutation of 1,2-diary Icy clopropane (20) takes place under DCA, 1,4-dicyanonaphthalene (DCN), or 3,3, 4,4 -benzophe-nonetetracarboxylic dianhydride (BTDA) SET-sensitized conditions as well as when the EDA complex of 20 and tetracyanoethylene (TCNE) is directly irradiated. Interestingly, the aminium salt catalyzed reaction of cis-21 also involves stereomutation with double epimerization, affording trans-21a and trans-2th. ... 

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