Stilbenes, photochemical phenanthrenes

The photochemical synthesis of helicenes by irradiation of 1,2-diarylethylenes in adilute solution and in the presence of an oxidizing agent is based on the well known photocyclodehydrogenation of stilbene into phenanthrene. There is an overwhelming amount of literature on this type of photoreaction. Details about scope and limitation can be found in previous reviews 7,8). Therefore, only a short survey will be given of the mechanism of the reaction. 

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

Photochemical conversion of stilbenes to phenanthrenes via a six 7t-elec-tron conrotatory cyclization according to an electrocyclic mechanism to the dihydrophenanthrenes and subsequent dehydrogenation is a very famous and useful synthetic reaction (103). 

Photochemical cycUzation of heterocyclic stilbenes. The photochemical oxidative cyclization of stilbenes to phenanthrenes proceeds in low yield when applied to heterocyclic analogs. Cava et alJ have examined a number of oxidants other than O2 for in situ oxidation of the dihydroheterocycle formed initially and find that 5% Pd on carbon is the reagent of choice. In addition, a hydrogen acceptor, p-nitrobenzoic acid, can improve yields. 

The photochemical isomerization of E-stilbenes has been applied in the preparation of phenanthrenes, as Z-stilbenes undergo electrocyclie ring closure (cf. chapter 3.1.3) to dihydrophenanthrenes which in turn are easily oxidized to phenanthrenes (3.1) 305). This sequence has also been employed in the synthesis of benzoquinolines 306) or of benzoquinolizines (3.2) 307). 

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). ... 

In the presence of an oxidizing species (oxygen, iodine, tetracyanoethylene and others) DHP is dehydrogenated into phenanthrene. This oxidation can also be thermal or photochemical. In general, meta substituted stilbenes give rise to two isomeric substituted phenanthrenes ortho-substituted stilbenes can lose the substituent on cyclization. 

The stilbenes have played a crucial role in the development of modern photochemistry. Direct or triplet sensitized irradiation of trans-stilbene (t-1) in dilute solution results in isomerization to cis-stilbene (c-1) as the exclusive uni-molecular photochemical reaction (1-3). Direct irradiation of c-1 results in isomerization to both t-1 and trans-4a,4b-di-hydrophenanthrene (2), which revert to c-1 both thermally and photochemically and can be trapped by oxidants such as iodine or oxygen to yield phenanthrene (3) (4-6). Triplet sensitized irradiation of c-1 yields only t-1. These unimolecular isomerization pathways are summarized in eq. 1. 

The synthesis of 9-phenylphenanthrene (Expt 6.14) illustrates the formation of the phenanthrene system by the cyclisation of a 1,2-diphenylethylene (stilbene). The process involves an allowed photochemical cyclisation which gives initially a dihydrophenanthrene. This is readily dehydrogenated in situ by molecular oxygen in the presence of iodine. 

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]. 

Photocyclization of 1-benzylideneisoquinolines to the Aibexao[d,e,g -quinolines is a well-studied example of the above stilbene-phenanthrene cyclization and therefore has been extended to the development of new photochemical methods for the synthesis of aporphine alkaloids (11-14). In this section, photocyclization of 2-acyl-1 -benzylideneisoquinolines to dehy-droaporphines and their subsequent conversion to aporphines and oxo-aporphines are summarized. 

The photochemistry of linear conjugated trienes is closely connected to the isomeric 1,3-cyclohexadienes which are both their photochemical precursors and products. Such systems have been dealt with under cyclic 1,3-dienes in the previous section. The photochemical cyclization of m-stilbene to dihydrophenanthrene and ultimately to phenanthrene and hydrogen is formally analogous to reaction (13). This reaction (17) has been observed in a large class of aromatic compounds under a... 

Physical details relating to the isomerization of the stilbenes (12) have been determined.A study of the photophysical properties of the styrylstiIbenes (13) has shown that quantum yields for trans-cis isomerism are low from the singlet but high from the triplet state. The photochemical isomerization of the alkene (14) in an ethanol glass affords the trans-isomer with high efficiency even at liquid helium temperatures. Photochemical cis-trans-isomerization of cis-1,2-di-l-naphthylethylene (14) has also been studied in the crystalline phase. A study of the photochemical isomerization of a series of styryl phenanthrenes has been reported. The mechanism of the reaction involved was discussed. ... 

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). 

Based on the experience obtained from the synthesis of the [4]helicene 37b, we initially sought to prepare 4-(9-triptycyl)phenanthrene (36) also using a stilbene photocyclization strategy. Unfortunately, various photochemical approaches failed in the final photocyclization step. Given these results, we... 

Z)-Diarylethenes, such as stilbene, also undergo a 671-electrocyclization reaction upon irradiation from the singlet n,n state, but not upon triplet sensitization.487 529,599 622 The photochemical orbital symmetry-allowed conrotatory closure leads to the trans-dihydro derivative 66 (Scheme 6.23), which can subsequently be oxidized by oxygen or iodine to the corresponding arene (phenanthrene) in nearly quantitative yield.561... 

Scheme 12. Different photochemical reaction pathways of czs-stilbene (1) (i) to phenanthrene (2) by conrotatory [n6] electrocyclization, and (ii) to tetraphenylcyclobutane (33) by a [2+2] photocycloaddition [67a-d, 68]...

The degradation of 2,2-dimethylisoquinolinium iodides 101 was carried out for the synthesis of phenanthrene alkaloids. Hoffmann elimination of the quaternary ammonium salt gave a stilbene intermediate 102 (Scheme 45), which was used as substrate for a photochemical electrocyclization to afford phenanthrene derivatives 103 in moderate to good yields <04TL4171>. 

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). 

A common type of six electron electrocycUc reaction occurs in the photochemical reaction of 1,2-diaryl alkenes. ° The parent substrate, stilbene can be converted to phenanthrene, a process that involves conrotatory electrocyclization under photochemical conditions and subsequent oxidation of the product to the polycyclic aromatic structure (3.218). 

Stilbenes exhibit a diverse photochemical behavior in solution such as reversible cis/transisomerization, cyclizationofcis-stilbene 1 todihydrophenanthrene (DHP) and further oxidation to phenanthrene 3, and dimerization of traws-stilbene to yield tetraphenylcyclobutane products 4 and 5 [1] (Figure 4.1). 

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