Photolysis of 3,5‐diphenylisoxazole in argon matrices

Abstract

Broadband irradiation of 3,5‐diphenylisoxazole 1 in an argon matrix results in formation of azirine 3. Further irradiation of the matrix reduces the amount of azirine 3 with concurrent formation of ylide 4. Thus, it is theorized that the conversion of isoxazole 1 to azirine 3 goes through a triplet vinylnitrene 2 that does not intersystem cross to ketenimine 6. Hence, the reactivity of triplet vinylnitrene 2 is different from similar vinylnitrene intermediates with α‐methyl substituents that intersystem cross to form corresponding ketenimines. Density functional theory calculations support the notion that the conjugation of the α‐phenyl group to the vinylnitrene moiety in vinylnitrene 2 renders it more flexible than vinylnitrenes with α‐methyl substituents, and therefore, vinylnitrene 2 intersystem crosses to azirine 3, rather than ketenimine 6. Copyright © 2016 John Wiley & Sons, Ltd.

On the Carbenic Nature of Nitrile Ylides: Experimental and Computational Characterization of Hydroxy and Amino Nitrile Ylides

“…Species 2 a and 2 b (primary photoproducts of 1 a and 1 b , respectively) [32] are readily identified as 3‐hydroxy‐2‐formyl‐2 H ‐azirine and 3‐amino‐2‐formyl‐2 H‐ azirine, respectively [characteristic IR bands (argon matrix): 1818 [ν(C=N)] and 1733 [ν(C=O)] cm −1 for 2 a ; 1827 [ν(C=N)] and 1727 [ν(C=O)] cm −1 for 2 b ]. This assignment is supported by the previous IR spectral characterization reported for these molecules, [33] and is also consistent with the reported formation of 2 H ‐azirines as primary photoproducts in studies with other matrix‐isolated isoxazoles [13,34,35] . Subsequent irradiations were performed using longer wavelenghts where the isoxazole precursors 1 a and 1 b do not absorb.…”

“…This assignment is supported by the previous IR spectral characterization reported for these molecules, [33] and is also consistent with the reported formation of 2H-azirines as primary photoproducts in studies with other matrix-isolated isoxazoles. [13,34,35] Subsequent irradiations were performed using longer wavelenghts where the isoxazole precursors 1 a and 1 b do not absorb. It was observed that species 3 a and 3 b can be selectively converted to 4 a and 4 b, respectively, facilitating their unambiguous identification.…”

Mechanism and cis/trans Selectivity of Vinylogous Nazarov-type [6π] Photocyclizations

“…The reaction proceeds via (i) a photochemical rearrangement of isoxazoles 1 to azirines 2 , (ii) a cobalt­(II)-catalyzed condensation reaction forming pyrroles 3 , and (iii) a [6π] photocyclization yielding the cycloheptadienones 4 . Curiously, only the trans product was observed for the phenyl derivative 4a , whereas cis / trans mixtures were obtained for furanyl and thienyl compounds 4b,c , an experimental finding that could not be rationalized at that time.…”

Preface for the special issue of ISRIUM‐2015 in Honolulu, Hawaii, USA

“…Professor Anna Gudmundsdottir and coworkers find the photochemical transformation of 3,5‐diphenylisoxazole to an azirine in argon. Further irradiation was found to produce the corresponding ylide in the low‐temperature matrix conditions. The formation of the azirine from the isoxazole is proposed to occur via a triplet nitrene intermediate, which is confirmed by theoretical calculations.…”