Dihydrophenanthrene, intermediate

One may well ask why the isomerization of alkenes discussed in the preceding section requires a sensitizer. Why cannot the same result be achieved by direct irradiation One reason is that a tt — tt singlet excited state (5,) produced by direct irradiation of an alkene or arene crosses over to the triplet state (Ij) inefficiently (compared to n —> it excitation of ketones). Also, the Si state leads to other reactions beside isomerization which, in the case of 1,2-diphenyl-ethene and other conjugated hydrocarbons, produce cyclic products. For example, cw-l,2-diphenylethene irradiated in the presence of oxygen gives phenanthrene by the sequence of Equation 28-8. The primary photoreaction is cyclization to a dihydrophenanthrene intermediate, 6, which, in the presence of oxygen, is converted to phenanthrene ... 

The initial photochemical step, (211) to (212), can be most simply viewed as a perturbed 6e electrocy-clic process, suggesting a trans configuration via conrotation for Ae dihydrophenanthrene intermediate (212). In support of this hypothesis, the stabilization of (216) by tautomerism to (217) in the photolysis of diethylstilbestrol (214), followed by ozonolysis, afforded only the racemic form of 1,2,3,4-buta-netetracarboxylic acid (218). The majority of dihydrophenanthrenes, however, are thermally unstable and undergo conversion to phenanthrenes (under oxidative or non-oxidadve conditions) or 1,/-H shifts to isomeric 9,10-dihydrophenanthrenes. ... 

Laarhoven has examined the fate of the dihydrophenanthrene intermediate (148) formed by photocyclisation of the styryl benzo-phenanthrenes (149). When the reaction is performed in the presence of iodine as oxidant the expected arene (150) is obtained but in the absence of the oxidant compound (148) rearranges to the more stable, isolable dlhydroarenes (151) and (152). This rearrangement is base catalysed and the proportions of (151) and (152) isolated depend upon the nature of the base and the solvent. 

As might be expected, cyclisation is inefficient when the substituents are bulky tert.-butyl groups. More interestingly, evidence is seen for two equilibrating dihydrophenanthrene intermediates (158). Presumably these are conformers which are slow to interconvert due to a barrier to passage of the closest R groups past one another. 

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... 

Dehydrogenation of the dihydrophenanthrene intermediate formed by the cyclization of (Z)stllbene presents a general route to substituted phenanthrenes [81]. Yields can be improved by the addition of hydrogen abstractors such iodine or tetracyanoethene... 

When phenanthrene (0.65 mg/L) in hydrogen-saturated deionized water was exposed to a slurry of palladium catalyst (1%) at room temperature for approximately 2 h, 1,2,3,4,5,6,7,8-octahydro-phenanthrene and l,2,3,4,4a,9,10,10a-octahydrophenanthrene formed as products via the intermediates 1,2,3,4-tetrahydrophenanthrene and 9,10-dihydrophenanthrene, respectively (Schiith and Reinhard, 1997). 

Besides their obvious role as reactive intermediates in a powerful synthetic approach the 4a,4b-dihydrophenanthrenes offer a fascinating combination of unusual chemical and physical properties. Over the past 15 years these topics were investigated at length at the Weizmann Institute in Rehovot and elsewhere, and the present review is intended to provide an up-to-date summary of the activity in this field. 

An important point we wish to stress within the present context is that the number of observed 4a,4b-dihydrophenanthrenes is far smaller than the number of systems in which the photocyclodehydrogenation process (e.g. A. followed by D.) has been reported. In many cases the reason is simply that these intermediates were not looked for so that no special efforts were made to observe them. However, in many instances in which photocyclodehydrogenation products are known to be formed no 4a,4b-dihydrophenanthrenes can be observed even under usually favorable conditions (see below). In this case either the 4a,4b-dihydrophenanthrenes are destroyed by some subsequent process or that the photostationary concentration of these species is too low. Low photostationary concentrations are due (among other causes, see below) to low cyclization quantum yields. Such is the case, e.g., with stilbenes substituted at the 4-ring position with electron attracting groups. 

The iV-aminopyrrole - benzene ring methodology has been applied to a synthesis of the 9,10-dihydrophenanthrene juncusol (218) (81TL1775). Condensation of the tetralone (213) with pyrrolidine and reaction of the enamine with ethyl 3-methoxycarbonylazo-2-butenoate gave pyrrole (214). Diels-Alder reaction of (214) with methyl propiolate produced a 3 1 mixture of (215) and its isomer in 70% yield. Pure (215) was reduced selectively with DIBAL to the alcohol, reoxidized to aldehyde, and then treated with MCPBA to generate formate (216). Saponification to the phenol followed by O-methylation and lithium aluminum hydride reduction of the hindered ester afforded (217), an intermediate which had been converted previously to juncusol (Scheme 46). 

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. 

In the vapor phase phenanthrene and hydrogen were products of the photolysis of c/.v-stilbene in addition to the expected mww-stilbene.65 The photolysis of 1,4-diphenyl-1,3-butadiene to give a-phenylnaphtha-lene is an analogous process.65 It was proposed that the intermediate dihydrophenanthrene was a cis form.66... 

We know that C6-cyclization of 1-(naphthyl-2)-butene is possible without metal catalysts. The products are dihydrophenanthrene over quartz and 1,2,3,4-tetrahydrophenanthrene plus phenanthrene over alumina (50). The latter apparently catalyzes the redistribution of hydrogen in dihydrophenanthrene. Neither anthracene nor dihydro- or tetrahydroanthracene are formed over quartz or alumina from 1-(naphthyl-2)-butene. Plate and Erivanskaya concluded from this that the 2-alkylnaphthalene - anthracene reaction does not involve naphthylbutene intermediate (27). 

Dihydrophenanthrene (3) is dehydrogenated without added metal salt. In this case the intermediate I I complex (b) is suggested. 

Fluorene has been reported to afford the 3,9a-dihydro product, but it is almost certain that this is the 2,4a-dihydro isomer (55 = 1) by analogy with biphenyl. 9,10-Dihydrophenanthrene (56) is reduced as expected to (55 n = 2), but spontaneously reverts to the starting material on standing. These systems do not require the presence of alcohol for reduction and it is consequently possible to alkylate the intermediate anions with alkyl halides, as (56) gives (57). These products are much more stable and structural analysis is simplified accordingly oxidation of the doubly allylic methylene occurs readily to afford the dienone (58 Scheme 7). Dienones of this type have potential as intermediates for the synthesis of natural products. Anthracene and phenanthrene are both readily reduced in the central ring to form the 9,10-di-hydro derivatives as might be expected, but to avoid further reduction it is necessary to have an iron salt present. Further examples are reviewed elsewhere. ... 

Pd-C, or iodine/ " The reaction is a photochemically allowed conrotatory conversion of a 1,3,5-hexatriene to a cyclohexadiene, followed by removal of two hydrogen atoms by the oxidizing agent. The intermediate dihydrophenanthrene has been isolated. The use of substrates containing heteroatoms (e.g., PhN=NPh) allows the formation of heterocyclic ring systems. The actual reacting species must be the c/i-stilbene, but frani-stilbenes can often be used, because they are isomerized to the cis isomers under the reaction conditions. The reaction can be extended to the preparation of many fused aromatic systems, for example, ... 

The stilbene-dihydrophenanthrene photocycllsatlon reaction continues to find synthetic applications. The tetra-oxygenated methyl phenanthrene skeleton (161) has been prepared by photocyclisation of the stilbene (162) aromatisation of the intermediate dihydrophenanthrene occurs by elimination of methanol. 2 nq photocyclisation was observed In the absence of the cyano group in this compound 2 although the closely related structures (163) and (164) are said to cyclise to the phenanthrenes (165) and (166). Triarylethylenes are important... 

The report on the biosynthesis of 9,10-dihydrophenanthrene is rare. A biosynthetic pathway of 9,10-dihydrophenanthrene was proposed by Preisig-Mueller et al. Bibenzyls are bicyclic intermediates and require O-methylation as a prerequisite for their conversion into dihydro-phenanthrenes [377]. 

---