Ketenimine azirine cyclization

Curiously, the addition of a second orf/zo-fluorine substituent (i.e. in benzazirine 17d) raises the barrier to reversion to singlet nitrene 16d, relative to the mono orf/io-fluoro system (Fig. 14). This is partly due to steric hindrance by fluorine in the transition state for cyclization, but also to the stabilization of 17d by the fluorine attached directly to the azirine ring vide suprd)F The addition of the second fluorine substituent (benzazirine 17d) decreases the barrier to conversion of azirine 17d to ketenimine 18d slightly... 

Acyl sulfenes, like all sulfenes, prefer to participate as 2it components of [2 + 2] or [4 + 2] cycloadditions (Chapter 5). Nonetheless, a range of [4 + 2] cycloaddition reactions of acyl sulfenes have been described46,47 (Scheme 8-XII), including their 4n- participation in dimerization reactions46-48 and reactions with imines,49 carbodiimides,50 ketenimines,51 1-azirines,52 vinyl ethers,53 and ketenes.47 The reactions often provide mixtures of [4 + 2] and [2 + 2] cycloadducts, and the observed course of the reaction usually depends on the reaction conditions. Consequently, many of the observed [4 + 2] cycloadditions of acyl sulfenes proceed by a stepwise, polar addition-cyclization reaction. 

Argon matrix photolysis of 1- and 2-naphthyl azides 79 and 80 at 313 nm initially produced the singlet naphthyl nitrenes, 81 and 82. Relaxation to the corresponding lower energy, persistent triplet nitrenes 81 and 82 competes with cyclization to the azirines 83 and 84 which can also be formed photochemically from the triplet nitrenes (Figures 11.20 and 11.21). On prolonged irradiation, the triplet nitrenes 81 and 82 can be converted to the 7-membered cyclic ketenimines 85 and 86, respectively, as described earlier by Dunkin and Thomson. ... 

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