Cyclohexadienone Reactions

Cross-Conjugated Derivatives

Derivatives of 2,5-cyclohexadienone are common in nature, and their photochemical transformations posed a challenge to early researchers. Some reactions of 4,4-diphenyl-2,5-cyclohexadienone are presented in the following diagram. Although 2,3-diphenylphenol was a product from irradiation in aqueous dioxane, it is actually formed from 6,6-diphenylbicyclo[3.1.0]hex-3-ene-2-one, the initial rearrangement product. It should be noted that a similar bicyclo[3.1.0]hexane isomer was formed by irradiation of 4,4-diphenyl-2-cyclohexenones, as shown in the gray-shaded box at the bottom right.

These photochemical rearrangements occur by way of triplet excited states, which are conveniently depicted as diradicals. Mechanisms for these reactions will be displayed above . Bond reorganization may take place in the triplets, which eventually intersystem cross to singlet species. Charge separation in these states may then lead to rearrangement to a stable product.

The photo-isomerization shown above is one example of a general family of reactions known as di-π-methane rearrangements, other examples of which are illustrated in the following diagram. These transformations are often photo-sensitized, indicating they proceed by way of triplet excited states. As the name suggests, substrates exhibiting this rearrangement are comprised of two π-functions separated by a saturated (sp³-hybrid) carbon atom (designated by a red dot in the illustration). In the event, a 1,2-shift of one ene-group, followed by a bond formation between the terminal sites of the remaining π-function leads to a vinylcyclopropane product. One of the unsaturated functions may be a carbonyl or imine group, as in examples 2. and 3.

A photo-isomerization of this kind was used by H. Zimmerman (Wisconsin) to prepare the novel self-replicating diene semibullvalene from barrelene.