Photoreactions of biacetyl with electron‐rich olefins. An extended mechanism

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Abstract
The photoreactions of biacetyl (1) with various electron‐rich olefins have been investigated. Oxetanes and allyl alcohols are formed upon irradiation with λ=400–480 nm (nπ* excitation of biacetyl) depending on the nature of the olefin. An unusual relationship between the rate constants of luminescence quenching of biacetyl by olefins (log kq) and the free enthalpies of electron transfer (ΔG2) indicate formation of an exciplex first. A reasonable good fit between experimental results of log kq and ΔG2 has been obtained on the basis of a mechanistic model which includes the exciplex intermediate as well as an ionic photodissociation pathway and some empirical adjustment of the parameters. Complete electron transfer as a consecutive process has been proven for some strong electron‐donating olefins by means of ESR spectroscopy and scavenging of the radical cations. The solvent dependence of the product quantum yields further confirms the competition between ionic photodissociation and product formation. The preferred formation of allyl alcohols (reduction products) with strong electron‐donating olefins and the highly regioselective oxetane formation with ketene acetal are discussed in terms of exciplexes of strong CT character or even contact ion pairs and dipolar intermediates.