Thc photorcactions of biacctyl (I) with various clcctron-rich olcfins have been investigated. Oxetanes and allyl alcohols are formed upon irradiation with )c = 400-480 nm (nx* excitation of biacetyl) depending on the nature of the olefin. An unusual relationship between the rate constants of luminescence quenching of biacctyl by olefins (log kq) and the free enthalpies of electron transfcr ( A G ) indicatc formation of an cxciplcx first. A rcasonablc good fit bctwccn cxperimcntal results of log k, and AGz 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 clectron-donating olefins by means of ESR spectroscopy and scavcnging of the radical cations. The solvent dcpcndcnce 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 cxciplcxcs of strong CT character or even contact ion pairs and dipolar intermediates.
Derivatives of 2-C-branched-chain erythrono-1 ,Clactones of the type 1 have been synthesized under reductive conditions using DIBAL from the oxetane 2, which is easily accessible from diethyl mesoxalate (4) and 2,2-diisopropyl-1,3-dioxole (3) in high yield by means of a catalytic cycloaddition. Similarly, 2 rearranges in presence of A1Me3 under formation of an erythronolactone with an additional branching at C(4). The corresponding oxetanes 7-9 from ethyl pyruvate (5) and ethyl trimethylpyruvate (6), respectively, have been studied with regard to their reductions yielding building blocks of branched-chain sugars.
Derivatives of 2‐C‐branched‐chain erythrono‐1,4‐lactones of the types 1 have been synthesized under reductive conditions using DIBAL from the oxetane 2, which is easily accessible from diethyl mesoxalate (4) and 2,2‐diisopropyl‐1,3‐dioxole (3) in high yield by means of a catalytic cycloaddition. Similarly, 2 rearranges in presences of AlMe3 under formation of an erythronolactone with an additional branching at C(4). The corresponding oxetanes 7–9 from ethyl pyruvate (5) and ethyl trimethylpyruvate (6), respectively, have been studied with regard to their reductions yielding building blocks of branched‐chain sugars.
077ChemInform Abstract Products of cycloaddition and reduction are obtained by photolysis of (I) and (II) with quantum yields which increase with solvent polarity. The formation of reduction products connected with the presence of allylic hydrogen is enhanced with increasing oxidation potential. The photochemical and thermal reactions of (IX) with (II) are totally complementary, representing an example of a photochemically induced "Umpolung" of the reactivity of a 1,2-diketone. While (Id) and (II) react in a "clean" S1 reaction, the conversion with (IX) is a "clean" T1 process. The formation of (XI) and (XII) in addition to (X) in the photolysis in a polar solvent (MeCN) indicates photoionization (ESR of the radical cation derived from (Id)). It is concluded that an exciplex is formed reversibly which can be deactivated by electron transfer (ionic photodissociation), by product formation, and by decay to the ground state.
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