Phenanthrene from stilbene

The valence tautomerism 251 257 has been proposed to account for the formation of iV-methylthiobenzamide by irradiation (2537 A) in methyl cyanide solution. The valence tautomer 257, R = Me, = Ph, has been detected spectroscopically (p , 2060 cm —N=C=S) either during photolysis at 25° or by heating at 160°. Photochemical oxidative cyclization of 4,5-diaryl derivatives (251, R = R = Ar) analogous to the formation of phenanthrene from stilbene has been reported. Thus, irradiation of the 4,5-diphenyl derivative 251, R = R = Ph, yields the tetracyclic meso-ionic compound (258). ... 

Iodine irradiati on air Cyclodehydrogenation Phenanthrenes from stilbenes... 

A very large number of substituted phenanthrenes have been made from stilbenes by this photocydization method, as have more complex polycyclic aromatic compounds by related reactions involving a single cydizatlon (e.g. 3.64 for chrysenes) or two, or more, successive cydizations le.g. 3.65). The reaction can be nicely adapted to provide a route from 1-benzylidenetetra-hydroisoquinolines to alkaloids of the aporphine family (e.g. 3.66). 

Synthesis from Stilbene and Di-tolyl.— Two similar syntheses indicate the constitution of phenanthrene. When stilbene, (p. 762),... 

If an ortho substituent is present in a stilbene it can direct the regiochemistry of the photocyclisation towards or away from the position of the substituent. In addition, it can act as a potential leaving group, obviating a need for an oxidant to generate a phenanthrene from the dihydrophenanthrene Intermediate. Thus, for... 

Since the first photochemical synthesis of phenanthrene from (E)stilbene was realized [80] photochemical methods became a convenient way of aryl-aryl bond formation (Scheme 47). 

The reaction of cinnoline 2-oxide with phenylmagnesium bromide gives phenanthrene, trans- and cfs-stilbene, 2,3-diphenyl-l,2-dihydrocinnoline and 2-styrylazobenzene in yields of 1-15%. Analogous results are also obtained from 4-methylcinnoline 2-oxide. 

Cohare and co-workers reported that aristolactam BU (22) was prepared, following Kupchen s method, by Perkin condensation of 6-bromo-3,4-di-methoxy phenyl acetic acid (119) and o-nitrobenzaldehyde (120) (Scheme 14). The 2-bromo-4,5-dimethoxy-2 -nitro-ds-stilbene-a-carboxylic acid (121) was obtained. The nitro group of 121 was reduced with ferrous sulfate and ammonium hydroxide, and the resulting 2-bromo-4,5-dimethoxy-2 -amino-cw-stilbene-a-carboxylic acid (122) without purification was submitted to the Pschorr phenanthrene synthesis to yield l-bromo-3,4-dimethoxyphen-anthrene-lO-carboxylic acid (123). The phenanthrylamine 124 was prepared from 123 via a Schmidt reaction, and, by treatment with n-butyllithium and CO2, 124, afforded 22 (42). 

Fig.6 The distance dependence of electron-transfer rates in DNA hairpins [51]. The acceptor is a photoexcited derivatized stilbene (SA) or phenanthrene (PA) the electron donor is guanine (G), deazaguanine (Z), or inosine (I). The decay is much more rapid in the Z-PA couple compared to the G-SA couple because the tunneling energy is further from the bridge states in the case of Z-PA...

Stilbenes and associated molecules provide very good examples of the formation of intermediate unstable isomers which give a chemical route for internal conversion. Upon irradiation, stilbenes undergo a cis-trans isomerization as the predominant reaction. However, under oxidative conditions phenanthrene is also formed.12 It was shown that the phenanthrene came only from c/s-stilbene (13),61 and that an intermediate unstable isomer, nms-dihydrophenanthrene (14), was the precursor of the phenanthrene.62-64 The dihydrophenarithrene was in its ground state, but vibrationally excited, and was formed by a process calculated to be endothermic by 33 10 kcal/mole-1.02 Oxygen or other oxidants converted it to phenanthrene (15), but in the absence of oxidants it was either collisionally stabilized or reverted to m-stilbene. 

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]. 

Cis-stilbenes, which are easily obtained from their trans-isomers by trans, cis-photo-isomerization, can give rise to dihydrophenanthrene derivatives upon direct irradiation (Scheme 9.17). The latter compounds are easily converted into phenanthrenes under oxidizing conditions (oxygen, I2, or rc-electron acceptors such as tetracyano-ethene) [24]. 

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. 

Photochemical reactions of substituted phenylethylenes, frans-stilbene and 1,1-diphenylethylene, on silica gel, has been reported by Sigman et al. [40]. Irradiation of fraus-slilbcnc adsorbed on silica gel produced two dimers, along with ds-stilbene, phenanthrene, and a small amount of ben-zaldehyde which arose from a type II oxidation mechanism (Scheme 9). The formation of photodimers was claimed to be promoted by inhomogeneous surface loading and slow diffusion of fraus-slilbcnc on silica [40]. 

Epoxidation. In the presence of catalytic amounts of 18-crown-6 and an acyl chloride, KO, can effect epoxidation of alkenes and of some polycyclic arenes derived from phenanthrene. The most effective acyl chlorides are phosgene and benzoyl chloride. The highest yields of epoxides are formed from fran,v-stilbene (89%) and cyclohexene (80%). The 9,10-epoxide of phenanthrene is obtained in 38% yield. 

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... 

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