Dihydrophenanthrene ring system

The direct formation of the 9,10-dihydrophenanthrene ring system by photo-cyclization of 2-vinylbiphenyls has provided the key step in two synthetic... 

It is apparent from Table 1 that the cyclohexadiene ring in 1,2-dihydrocorannule-nes is significantly flatter than either 1,3-cyclohexadiene itself or 9,10-dihy-drophenanthrene due to the constriction imposed by the remainder of the bowl-shaped ring system. Nonetheless, the lack of planarity means that substituents at may adopt either pseudoaxial (pa) or pseudoequatorial (pe) positions. However, the surface of the corannulene system is curved, and, unlike 1,3-cyclohexadiene and 9,10-dihydrophenanthrene, neither the two pa positions nor the two pe positions are equivalent. Thus, substituents in 1 -/ -1,2-dihydrocorannulenes may be (1) endo-pseudoaxial (endo-pa), (2) e do-pseudoequatorial (e/ido-pe), (3) exo-pseudoaxial (exo-pa), and (4) exo-pseudoequatorial (exo-pe), where endo and exo are related to concave and convex orientations, respectively. 

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 partially hydrogenated ring of dihydtocorannulene constitutes a 1,3-cyclo-hexadiene ring, a system that has been well-studied with respect to its geometry and the conformational preferences of substituents. However, the curvature of the corannulene surface introduces an additional stereochemical factor that makes the conformational analysis especially interesting. 1,3-Cyclohexadiene (23) and 9,10-dihydrophenanthrene (24) serve as models they are structurally similar systems, and their stereochemistry and conformational preferences are well documented in the literature. In both cases, the reduced ring adopts a nonplanar, semi-chair conformation of symmetry. 

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. 

Table 4. 4a,4b-Dihydrophenanthrenes. Systems with four conjugated rings derived from 1-phenyl-2-naphtylethylenes...

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. ... 

---