Benzazirine phenylnitrene

However, in 1978, Chapman and LeRoux discovered that photolysis of phenyl azide, matrix isolated in argon at 10 K, produces a persistent species with a strong vibrational band at 1880 10 cm . The carrier of this species was most reasonably assigned to ketenimine 30 rather than benzazitine 29 or triplet phenylnitrene. This result imphes that it is the ketenimine 30 and not benzazirine 29 that is trapped with amines to form the 37/-azepines (27) that had been isolated earher. It does, however, raise the question as to why two groups observed triplet phenylnitrene by low temperature spectroscopy while a third observed ketenimine 30. 

The temperature-independent rate constant observed at low temperature (3.2 X 10 s ) is the rate constant for ISC to the triplet ground state (feisc)- The rate constant obs is equal to + feisc. where is the absolute rate constant for rearrangement for singlet phenylnitrene to benzazirine (29). As feoBS = +... 

The cyclic ketenimine 30 is the major trappable reactive intermediate present in solution when phenyl azide (at moderate concentrations) is decomposed photolyti-cally at 298 K. The rate of decay of singlet phenylnitrene 33s is equal to the rate of formation of the cychc ketenimine. The first step, cyclization to benzazirine (29) is rate determining, and is followed by fast electrocyclic ring opening to cychc ketenimine 30. 

For most aryl azides, the rate constants of singlet nitrene decay and product formation (triplet nitrene and/or ketenimine) are the same. Thus, in all these phenyl-nitrenes cyclization to substituted benzazirines is the rate-limiting step of the process of isomerization to ketenimine, as is the case for the parent phenylnitrene. The only known exception, o-fluorophenylnitrene, will be discussed in the next section. 

For almost a hundred years, chemists have argued over the identity(ies) of the CgHsN specie(s). Candidate structures for CeHsN have been singlet ( PN) and triplet ( PN) phenylnitrene, benzazirine (BZ) and cyclic ketenimine (K), a menagerie of species described by Schrock and Schuster as wonderfully complex . 

Confusion over the matrix and gas-phase optical spectroscopy of PN spilled over to the liquid phase. Initial flash photolysis experiments involving phenyl azide gave conflicting results, with different authors favoring the presence of triplet phenylnitrene, " benzazirine BZ, or cyclic ketenimine as the carrier of the transient spectra. 

In the liquid phase, singlet phenylnitrene is rapidly relaxed by collision with solvent and cannot surmount the barrier to form cyanocyclopentadiene at ambient temperature. Under these conditions PN isomerizes over a small barrier to form cyclic ketenimine K. Later, computational work of Karney and Borden would show this to be a two-step process involving benzazirine BZ, the species trapped by ethanethiol (Scheme 2). In the liquid phase, PN prefers rearrangement to intersystem crossing (ISC) to the lower-energy triplet state at ambient temperature. Intersystem crossing is not an activated process and its rate is not expected to vary with temperature. The rate of... 

Fig. 13 Arrhenius treatment of A r(= qbs isc) for singlet pora-fluoro- 16b (1), meto,meto-difluorophenylnitrene 16c (2) in pentane and ortho,onho-Ai uoro-phenylnitrene 16d (4) in CCI4 and for singlet 2-fluorophenylnitrene 16a (3) calculated as described in the text. (5) Arrhenius treatment of the rate constant of ring opening reaction (k ) for benzazirine 17a.

Initial flash photolysis experiments involving 46 gave conflicting results, with different authors favoring the presence of either triplet phenylnitrene 349 iso.isi benzazirine... 

At that time, the primary intermediate, singlet phenylnitrene 52, had still escaped direct detection. The benzazirine 50 had not been detected either, although the formation of 49 in ethanthiol (Scheme 11.24) was an indication of its intermediacy in the phototransformation of 47. A series of reviews were published in 1992, which economically explained much of the photochemistry of phenyl azide (Scheme 11.27). 

For most orf/to-substituted phenylnitrenes, as well as for para-substituted ones, the cyclization to benzazirine is the rate-determining step of the process of nitrene isomerization to ketenimine (Scheme 11.31), similar to the case of the parent 52. The lifetimes of these singlet nitrenes and Arrhenius parameters for their rearrangement are summarized in Table 11.2. 

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