4a,4b-Dihydrophenanthrene

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

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

Table 5. 4a,4b-Dihydrophenanthrenes. Systems with five conjugated rings derived from 1,2-dinaphthylethylenes

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

Table 9. 4a,4b-Dihydrophenanthrenes with 7 conjugated rings derived from dixanthylidene and its dithio analog

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.  

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.

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. 

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 

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

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

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

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. 

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

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. 

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

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], 

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