Stilbene photocyclization, oxidative

Stilbenes, photocyclization of, 30, 1 StiUe reaction, 50, 1 Stobbe condensation, 6, 1 Substitution reactions using organocopper reagents, 22, 2 41, 2 Sugars, synthesis by glycosylation with sulfoxides and sulfinates, 64, 2 Sulfide reduction of nitroarenes, 20, 4 Sulfonation of aromatic hydrocarbons and aryl halides, 3, 4 Swem oxidation, 39, 3 53, 1... 

Further extensions of the stilbene photocyclization are seen in analogous reactions of compounds containing the imine chro-mophore (e.g. 3,71) or an amide group (3.72). The amide reaction can be considered formally as giving a zwitterion intermediate, which undergoes proton shifts and oxidation to form the observed product. Non-oxidative cyclizations that start with either N-vinyl aromatic carboxamides (C=C— N— CO—Ar) or N-aryl a,p-unsaturated carboxamides (Ar— N—CO—C=C) have been extensively used to make quinoline or isoquinoline alkaloids and their derivatives a fairly simple example is given in (3.73). 

The photochemical reactions of some a-arylvinyl bromides (31), in acetic acid in the presence of sodium acetate and tetraethylammonium bromide (labeled with 8zBr) give nucleophilic substitution, reductive debromination, cis-stilbene photocyclization, and oxidation as the primary pathways to product... 

The photocyclization of c/s-stilbene gives dihydrophenanthrene (5), which may be thermally or photochemically converted back to c/.s-stilbene or oxidized to phenanthrene (Moore et al., 1963 Muszkat, 1980). 

V-Phenylbenzylimines undergo stilbene-type photocyclizations (equation 83) to yield heterocyclic compounds143. The reactions usually take place via the iminium salt and need oxidants like oxygen or iodine. Six-electron electrocyclic reactions have been observed for 1-aza-l,3-dienes144 (equation 84). 

The l,3,5-triene-l,3-cyclohexadiene interconversion is a six-electron electrocycli-zation that requires a cis central double bond to occur.245 An important application of this rearrangement is the photocyclization of cis-stilbene to dihydrophenanthrene [Eq. (4.45)], which is usually further oxidized to phenanthrene 249... 

A number of l-aryl-2-thienylethylenes have been photocyclized in the presence of an oxidizing agent (usually iodine) to polycyclic aromatic compounds. Representative examples are given in Table 1. The mechanism, as with the conversion of stilbene to phenanthrene, probably involves conversion of the trans-alkene to the c/s-form, cyclization to the dihydro isomer, and oxidation of the latter to the fully aromatic compound. The yield of the cyclized product seems to decrease when the ethylene is attached to the /3-position of the thiophene. 

Numerous examples of stilbene-to-dihydrophenanthrene photocyclization incorporating oxygen and sulfur heterocycles have been reported. Oxidation to the phenanthrene is usually effected by added iodine or by oxygen. Thus, irradiation of 2,3-diphenylchromone (37) results in the formation of phenanthro[9,10 -2,3]chromone (38)29 analogous photocyclizations have... 

Oxidative photocyclization of stilbene to phenanthrene, which was discovered in 1960, was first applied to the synthesis of [7]helicene by Martin and coworkers in 1967 [26, 44, 45]. This approach was extended to the syntheses of long [njhelicenes (n < 14) and [n]thiahelicenes (n < 15), using both mono- and diannelations. Numerous [nfhelicenes (n < 13) and [n]thiahelicenes (n < 13) were obtained in non-racemic form via the following methods (1) seeded crystallization of conglomerate (e.g. [7]-, [8]- and [9]helicene) [46, 47], (2) resolution by chromatography (e.g. [13]thiahelicene) [48] and (3) photocyclization from a resolved precursor (e.g. [13]helicene from hexahelicene-2-carboxylic acid) [49]. The oxidative photocyclization of stilbenes is still the method of choice for the preparation of selected [n]helicenes and their heteroatom analogs [50-58]. 

Syntheses of [n]helicenes (and [n]thiahelicenes) via oxidative photocyclizations of stilbenes have serious constraints. They must be carried out in dilute solutions ( 1 mM) and the functional groups that significantly affect relaxation of the singlet excited states of stilbenes such as bromo, iodo, keto, amino and nitro, are typically not compatible for longer helicenes, problems with regioselectivity of photocyclization may be encountered [61]. These limitations have spurred the recent developments of nonphotochemical syntheses for efficient preparation of highly functionalized, nonracemic helicenes. 

The first double helicenes, consisting of head-to-tail annelated [6]helicenes, were prepared by oxidative photocyclization of stilbenes to helicenes [21]. Recently, double helicene 48, in which two benzene rings are shared by [5]- and [7]helicene, was prepared via nonphotochemical Pd-catalyzed cydotrimerization of 3,4-didehydrophenanthrene derived from 47 (Fig. 15.17) [88]. The H NMR spectrum for racemic 48 showed better agreement with the calculated spectrum for the diastereomer with homochiral versus heterochiral helicenes. The characterization of double helicenes did not indude X-ray crystallography [21, 88],... 

Phenanthrenequinone from benzoin. The photocyclization of stilbene to phenanthrene requires a trans to cis isomerization. This step can be circumvented by reaction of benzoin with phenylboric acid to form the 1,3-dioxoborole 2. This adduct can be converted to phenanthrenequinone by photocyclization in the presence of diphenyl diselenide as the oxidant. 

A study has been reported on the photocyclization of several ortho-halogenated stilbene derivatives (190) under both oxidative conditions (I2, cyclohexane) and basic conditions (MeONa, MeOH)573. The major products were those anticipated from photodehydrogenation, i.e. 191, and photodehydrohalogenation (192), respectively (equation 141). 

There are a large number of photochemical cyclizations of aromatic compounds that lead initially to polycyclic, non-aromatic products, allhough subsequent rearrangement, elimination or oxidation occurs in many instances to form aromatic secondary products. The archety pe for one major class of photocyclization is the conversion of stilbene to phenanthrene by way of a dihydrophenanthrene (3.60). 

Diaryl amines, ethers or sulphides, or their aryl vinyl analogues, provide another 6-electron system related to stilbene, but for these a pair of electrons is provided by the single heteroatom (ArXAr or ArXC=C). With diaryl compounds the initial photocyclized product is a zwitterion that undergoes a proton shift to give, for example, /V-methy -4a,4b-dihydrocarbazo e from methyldiphenylamine, with subsequent oxidation to N-methylcarbazole (3.74). With the ary vinyl analogues the product after the proton shift can normally be isolated (3.75). An especially useful variation of this reaction employs... 

The photocyclization of stilbenes (211) (including its in situ oxidation) to phenanthrenes (213) and that of conjugated arylalkenes to polycyclic aromatics constitute one of the most studied and widely used applications of organic photochemistry. Its potential synthetic utility is amplified by the existence of a number of natural products (mainly alkaloids) that contain a phenanthrene subunit in their structure. In view of the plethora of examples contained in several excellent reviews, only selected examples will be presented here with focus on the selectivity of the process. 

By contrast to the thermal process, the photochemistry of 2-vinylbiphenyl (249) and 4-vinylphenan-threne (250) has been widely used in connection with syntheses of polycyclic aromatic compounds, complementing the most commonly used stilbene to phenanthrene transformation/ For example, the product obtained from the ctnirotatory photocyclization of 2-(a-styryl)biphenyl (252) can be trapped by an oxidant to give the phenanthrene (254) or suffers a 1,5-H shift to give the dihydro d)enandiieiie... 

The analogous photocyclization of N-methyIdiphenylamine has been studied in detail (Forster et al., 1973 Grellmann et al., 1981) and utilized synthetically (Schultz, 1983). In contrast to stilbene, the reaction proceeds from the triplet state of the amine by an adiabatic conrotatory ring closure to give a dihydrocarbazole (93), in accordance with the Woodward-Hoff-mann rules. After return to the ground state, the initial product is oxidized to a carbazole (Scheme 31) ... 

Esters (21), readily formed from 2-hydroxy-l,2,2-triphenylethanone and carboxylic acids, undergo efficient photocyclization to the benzofuran (22), which then yields the benzophenanthro[9,10-d]furan (23) by the usual 6ji-oxidative closure process of the cis stilbene moiety (Ashraf et al. Chapter 4). The overall reaction is so convenient that the authors propose the triphenylethanone as a new photolabile protecting group for carboxylic acids. 

Pheuanthrene synthesis. Stilbene derivatives are oxidatively cyclized to phenanthrenes by vanadyl trifluoride in TFA-CH2CI2 at 0°. An example is the conversion of 1 into 2. This reaction had been conducted by photocyclization. 

As both symmetrical and unsymmetrical stilbenes can be readily synthesized by employing Wittig [126], Heck [127], or McMurry [128] reactions, various graphene-type molecules have been prepared via photolytic cyclodehydrogenation, some of which will be discussed at this point A typical example of photocyclization is the irradiation of the [2.2.2]paracydo-phane 96 in the presence of an oxidant, such as iodine, leading to the corresponding coronene derivatives 97 in high yield. This offers a versatile method for the preparation of new coronene derivatives with different functionality and substitutional symmetry (Scheme 13.26) [129]. 

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