Structure, spectra, photochemistry, and thermal reactions of the 4a,4b-dihydrophenanthrenes

Abstract

Ultraviolet irradiation of solutions of stilbene and its derivatives gives mixtures of their respective cis and trans isomers, together with, in many cases, 4a,4b-dihydrophenanthrene, or its corresponding derivatives. The extent of the photocyclisation depends strongly on the wavelength of the light used for irradiation. In the photostationary state in stilbene, the maximum photocyclisation (21%) is attained by irradiation at 280 mµ. In diphenylcyclopentene, 68% of the starting compound undergoes cyclisation into cyclopentenodihydrophenanthrene. The absorption spectra of the cyclic products are all similar with three main bands, at approximately 460, 310, and 240 mµ. The number, position, and intensity of these bands were correctly predicted by Simpson's exiton method. The n.m.r. spectrum and the molecular weight of hexamethyldihydrophenanthrene, formed from hexamethylstilbene, give unambiguous proof for the structure suggested for 4a,4b-dihydrophenanthrene and its derivatives. The photocyclisation is a completely reversible reaction, since the parent cis-stilbene, or its respective derivative, are reformed either thermally or by irradiation with light in the visible region. The photochemical ring-formation and ring-fission, as well as the thermal ring-fission, were investigated in detail, in particular in the system diphenylcyclopentene ⇌ cyclopentenodihydrophenanthrene, which is not complicated by cis–trans interconversion. The quantum yields of photochemical ring-fission were found to be independent of the temperature, and retained their high value (about 0·6) even in a hydrocarbon glass at –180°. The yield of photocyclisation [0·4 for (diphenylcyclopentene → cyclopentenodihydrophenanthrene) at –20°] was found to decrease sharply on cooling, with a concurrent increase in the quantum yield of the fluorescence of diphenylcyclopentene. The results can be explained satisfactorily by assuming that the only temperature-dependent reaction of the first excited singlet state of diphenylcyclopentene is the photocyclisation, with an activation energy of about 3 kcal./mole. No photosensitisation was observed for either ring-formation, or ring-fission. The kinetic parameters of the first-order thermal ring-opening reaction of 4a,4b-dihydrophenanthrene, cyclopentenodihydrophenanthrene, and hexamethyldihydrophenanthrene were determined. 4a,4b-Dihydrophenanthrene and cyclopentenodihydrophenanthrene are oxidised by oxygen in the dark at rates which increase with increases in the concentration of oxygen and the compound being oxidised. The products are hydrogen peroxide and phenanthrene, or its analogue. Oxidation is inhibited by aniline and by 2,6-di-t-butylcresol. It is suggested that the radical formed by hydrogen abstraction from dihydrophenanthrene, and the HOO· radical, are intermediates in this oxidation. At very low oxygen concentrations a photo-oxidation of either cyclopentenodihydrophenanthrene or, directly of diphenylcyclopentene, to give the corresponding phenanthrene derivative, was also observed. The system diphenylcyclopentene ⇌ cyclopentenodihydrophenanthrene represents the simplest photochromic system described hitherto.