Q5(c). Photoenolisation of 2-methylbenzophenone

Introduction. Photoenolisation is an intramolecular hydrogen abstraction by an excited carbonyl group from an ortho-alkyl group (a Norrish type II-like 1,5-H shift), giving a dienol (an o-xylylenol) instead of a cleaved or cyclised product.

Why it is rapid

• On excitation (n→π^*) benzophenone undergoes very fast intersystem crossing (about 10¹¹ s⁻¹) to the n,π^* triplet, whose half-filled oxygen n orbital behaves like an alkoxy radical.
• The ortho CH₃ hydrogens are held close to this oxygen, in a six-membered transition state (1,5-H shift). Abstraction is intramolecular, needs no diffusion, and is faster than any competing reaction.
• The 1,4-biradical formed is a hydroxy-benzyl / benzyl radical pair; it relaxes at once to the ground-state dienol (photoenol), an o-quinodimethane.

Why it is reversible

The photoenol has lost the aromatic sextet of one ring, so it is high in energy. It returns to 2-methylbenzophenone by a thermal [1,5]-sigmatropic H shift from OH to the =CH₂ carbon (or by tautomerisation through the solvent). The net result is no permanent chemical change, unless a dienophile traps the enol.

Scheme: The two photoenols differ in the geometry of the exocyclic C=C(OH)Ph bond.

Stereoisomerism of the photoenol

The new exocyclic C=C bond carries OH and Ph on one carbon, so the photoenol exists as two geometrical isomers:

• (Z)-enol: OH on the same side as the =CH₂ group. The OH hydrogen is close to the methylene carbon, so it returns to the ketone by a fast [1,5]-H shift (lifetime of microseconds to milliseconds).
• (E)-enol: OH turned away from =CH₂; the [1,5] path is blocked, so it lives much longer (seconds in non-polar solvents) and reverts only by slower intermolecular routes.

Both are formed because the triplet biradical can rotate before it closes to the enol. The long-lived (E)-enol is the one trapped by dienophiles (for example maleic anhydride) to give tetrahydronaphthol adducts; deuterium exchange of the ortho CH₃ in CH₃OD under light shows the enol is formed.

Conclusion. 2-Methylbenzophenone photoenolises rapidly because its triplet n,π^* oxygen abstracts an ortho methyl H through a six-membered transition state, and reversibly because the dienol re-forms the aromatic ketone by a [1,5]-H shift. The photoenol exists as short-lived (Z) and long-lived (E) isomers.

Application. Photoenols are used as light-generated dienes in Diels–Alder synthesis and in photo-click ligation of proteins; photoenolisation also makes ortho-alkyl benzophenones useful as UV-stable photoinitiators.