- Photoreactions of Biacetyl with Electron-rich Olefins. An Extended Mechanism
-
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 (n?* 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.
- Mattay, Jochen,Gersdorf, Joachim,Buchkremer, Karl
-
p. 307 - 318
(2007/10/02)
-
- Additions of 1,1-Diethoxyethene to 1,2-Diketones
-
The ketene acetal 2 undergoes thermally noncatalyzed additions to various 1,2-diketones under formation of - and -adducts.Only with biacetyl 1a could an oxetane, 3, be isolated whereas mainly substituted 6-oxo-2,4-hexadienoic acid ethyl esters 6 and 7 are formed with aromatic 1,2-diketones.The regioselectivities and the product ratios are discussed in terms of a mechanism via zwitterionic intermediates.
- Mattay, Jochen,Runsink, Jan
-
p. 2815 - 2818
(2007/10/02)
-
- THERMAL AND PHOTOCHEMICAL REACTIONS OF BIACETYL WITH 1,1-DIETHOXYETHENE. "UMPOLUNG" OF THE REACTIVITY OF BIACETYL BY PHOTOCHEMICAL INDUCED ELECTRON TRANSFER.
-
The thermal and photochemical reactions of biacetyl 1 and 1,1-diethoxyethene 2 are totally complementary: 1 and 2 exclusively form the 2,2-diethoxyoxetane 4 in polar solvents at room temperature.Contrary to the literature this thermal cycloaddition proceeds uncatalyzed. 1 and 2 react photochemically under exclusive formation of the regioisomer oxetane 3 - preferably in nonpolar solvents.The inversion ("Umpolung") of the reactivity of 1 is caused by the photoinduced electron transfer.
- Mattay, Jochen,Gersdorf, Joachim,Freudenberg, Ulrich
-
p. 817 - 820
(2007/10/02)
-