4a,4b-Dihydrophenanthrene

Z-Stilbene also undergoes photocyclization to 4a,4b-dihydrophenanthrene via an electrocyclization. ... 

Fig. 13.11. A schematic drawing of the potential energy surfaces for the photochemical reactions of stilbene. Approximate branching ratios and quantum yields for the important processes are indicated. In this figure, the ground- and excited-state barrier heights are drawn to scale representing the best available values, as are the relative energies of the ground states of Z- and E -stilbene 4a,4b-dihydrophenanthrene (DHP). [Reproduced from R. J. Sension, S. T. Repinec, A. Z. Szarka, and R. M. Hochstrasser, J. Chem. Phys. 98 6291 (1993) by permission of the American Institute of Physics.]...

When stilboestrol (diethylstilbestrol, 62) was photolysed in aqueous methanol at 254 nm it gave the stable 4a,4b-dihydrophenanthrene dione (63). The mechanism required a photo trans-cis isomerization, photocyclization and spontaneous enol-keto tautomerism [53,54]. Previous workers had carried out a similar irradiation in dilute acetic acid and obtained the expected aromatic product (64) [55],... 

Thus 4a,4b-dihydrophenanthrenes (f), the parent molecule, is formed from optically excited cis-stilbene,... 

As in many other instances, 4a,4b-dihydrophenanthrenes were purposedly studied only in recent years even though their formation was unknowingly observed many years ago. Thus, Lewis, Magel and Lipkin noted already in their pioneering 1940 paper Hhat... 

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. 

Tables 1 -9 list all the 4a,4b-dihydrophenanthrenes and their analogs that have been observed (to the best of our knowledge) up to the time of the writing of this review. Several of the compounds described in Tables 1-9 (e.g. 6,15-19, 21, and 60) were studied on various occasions in Rehovot but were not included in previous publications. The data on several other systems listed in these Tables are published for the first time though the compounds themselves were mentioned in previous reports.

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 1. 4a,4b-Dihydrophenanthrenes. Systems derived from Stilbenes...

Table 2. 4a,4b-Dihydrophenanthrenes bridged at position 4 and 5, derived from 12.2)metacyclophanes...

Table 3. 4a,4b-Dihydrophenanthrenes. Heterocyclic analogs with three conjugated rings...

Table 8. 4a,4b-Dihydrophenanthrenes with 7 conjugated rings, derived from bianthrones...

The majority of the 4a,4b-dihydrophenanthrenes listed in Tables 1-9 are definitely thermally unstable at room temperature or below and undergo rapid dehydrogenation by molecular oxygen according to the overall process D. ... 

Table 10. Chemical Shifts of 4a,4b-Dihydrophenanthrenes. Atoms are numbered as in Tables 1 —9...

The basic chromophore in 4a,4b-dihydrophenanthrenes is a highly folded fully conjugated hexa-ene bridged by the central two atom unit of carbon atoms 4a and 4b (7 a). This bridge acts both as a poly alkyl substituent and also as a skeletal constraint. 

The simplest theoretical description of the electronic absorption spectra of 4a,4b-dihydrophenanthrenes seems to be provided by Simpson s exciton theory of the spectra of polymers. Compared with equally applicable but more complicated MO treatments (e.g. see Ref. and for tr-electron SCF MO analyses of 1, 44 and of 45), Simpson s model offers (at least for 1) some advantages such as numerical simplicity and sufficient transparency without losing too much of physical meaning. In the case of 1 Simpson s exciton model predicts the correct number of transitions and gives estimates of their energies and of their relative intensities. 

Fig. 3. Bond transition moment diagrams (Exciton model) for first three electronic transitions of 4a,4b-dihydrophenanthrene and of the iinear hexaene...

Table 13. Excited state vibrational spacings (in cm ) of 4a,4b-dihydrophenanthrenes ...

Table 14. Ring opening and fluorescence quantum yields of iJ-naphthylethylene- derived 4a,4b-dihydrophenanthrenes 2+-26)...

As stated previously 4a,4b-dihydrophenanthrenes cannot be isolated pure but can be obtained only in reaction mixtures containing both cis- and trans-isomers of the parent diarylethylene. Thus studies of 4a,4b-dihydrophenanthrenes always depend on the prior development of conditions providing considerable conversion of diaryl-ethylenes into their DHP photoisomers. One requirement for obtaining maximum conversion consists in minimizing the rates of decomposition processes such as photochemical ring cleavage,... 

---