The influence of copper halides on the course of photolysis of α,α′-Disubstituted stilbenes

Abstract

Photolysis of 4,4?-dimethoxy-α,α?-dimethylstilbene (Id) in the presence of air, in cyclohexane containing iodine (Method A), gave an abnormally low yield (11%) of 3,6-dimethoxy-9,10-dimethylphenanthrene (IId), together with 9% of p-methoxy-acetophenone formed by photooxidative fission. Irradiation of (Id) in the presence of air in absolute ethanol containing iodine and cupric chloride (or bromide) (Method B), afforded a significantly higher yield (54%) of (IId), while a similar reaction in which cupric chloride was omitted produced only 0.6% of (IId). The yields of phenanthrenes obtained by photolysis by Methods A and B of various α,α?-disubstituted, α-monosubstituted, and α-unsubstituted stilbenes are compared. The nature of the photooxidative cleavage of (Id) (using Method A), and the possible role of cupric and/or cuprous ions in facilitating the photoconversion of (Id) into (IId), are discussed. Demethylation of (IId), (IIe), and (IIf) afforded the phenanthrene diols (IIa), (IIb), and (IIc), respectively.