Ketenimine 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. 

To add to the confusion, various groups reported that gas-phase photolysis of phenyl azide produced the absorption and emission spectra of triplet phenylni-trene. " These observations were reconciled by the work of Leyva et al. who discovered that the photochemistry of phenyl azide in the presence of diethylamine was very sensitive to temperature. Above 200 K, azepine 30 is formed, but <160 K, azobenzene, the product of triplet nitrene dimerization, is produced. The ketenimine can react with itself or with phenyl azide to produce a polymer, which can be converted into an electrically conducting material. Gritsan and Pritchina pointed out that at high-dilution ketenimine 30 can interconvert with singlet phenylnitrene which eventually relaxes to the lower energy triplet that subsequently dimerizes to form azobenzene. 

In 1997, two groups simultaneously reported that LFP of phenyl azide 32 or compounds 34 and 35 produces a previously undetected transient with Lmax = 350 nm and a lifetime of 1 ns at ambient temperature.The transient decays at the same rate that cyclic ketenimine 30 is formed, implying that the newly detected transient is singlet phenylnitrene 33s. 

Figure 11.5. The magnitude of ko s decreases with decreasing temperature until 170 K, whereupon it reaches a value of 3.2 x 10 s. Below this temperature, koBs remains constant. " The breakpoint in the Arrhenius plot is 180-200 K, which is in exactly the same temperature range in which the solution phase chemistry changes from the trapping of ketenimine 30 with diethylamine to the dimerization of 33t. Thus, the low-temperature data of Figure 11.5 were associated with k]sc, the rate constant for intersystem crossing of singlet to triplet phenylnitrene, and the high temperature data with k., the rate constant for rearrangement of 33t.

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 . 

Reiser and co-workers published an important series of papers beginning in 1965. They were the first to observe the low-temperature UV-Vis spectrum of triplet phenylnitrene. Later studies in low-temperature glassy matrices by Leyva et alP would reveal an additional long-wavelength band in the spectrum of PN and that the spectrum of PN, originally reported by Reiser et al. was contaminated by the presence of ketenimine K. The difficulty is that PN is extremely light sensitive and, upon excitation at 77 K, rapidly isomerizes to the isomeric ketenimine. ... 

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. 

The currently accepted spectroscopic assignments were obtained by a combination of multiple techniques. Leyva et applied matrix absorption and emission spectroscopy along with flash photolysis techniques. Chapman and LeRoux obtained the matrix IR spectrum of cyclic ketenimine K and Hayes and Sheridan obtained the matrix IR and UV-Vis spectrum of triplet phenylnitrene and cyclic ketenimine K. Schuster and co-workers applied time resolved IR and UV-Vis spectroscopy and demonstrated the formation of cyclic ketenimine K in solution, the species that absorbs strongly at 340 nm. 

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... 

The transient decays at the same rate as cyclic ketenimine K is formed," implying that the newly detected transient is singlet phenylnitrene. The assignment was secured with the aid of computational chemistry" and by studying the temperature dependence of the kinetics. " In 1986 we guessed that the ISC rate constant of singlet phenylnitrene would resemble the same rate constants as those of aryl carbenes, which were known at that... 

Singlet phenylnitrene thermally ring expands in the inner phase of a hemicarcerand to the cyclic ketenimine (54), whose polymerization is prevented by the surrounding host.104 This allowed the activation parameters for the ring contraction of (54) to be measured and for the NMR spectroscopic characterization of (53). 

By 1992, Schuster and Platz could write Scheme 5.1, which economically explained much of the condensed-phase photochemistry of 46. UV photolysis of 46 produces singlet phenylnitrene and molecular nitrogen. In the liquid phase, 49 isomerizes over a small barrier to form cyclic ketenimine... 

Li et also performed calculations on azacyclohep-tatrienylidene (55), the planar carbene isomer of ketenimine 51. On the basis of their MNDO results, they proposed that the experimentally observed thermal reversion of 51 to triplet phenylnitrene ( 49) occurs not via singlet 49 but rather via a triplet state of 55. 

Formation of triplet phenylnitrene ( 52) was detected by EPR spectroscopy after photolysis of 47 in glassy matrices at 77 K. The tanperature dependence of the EPR signal demonstrated that the triplet state is the ground state of phenylnitrene. Shortly thereafter, Reiser s group recorded the LFV-Vis spectrum of 52 in a glassy matrix. Later it was found that 52 is extremely light sensitive and that upon photoexcitation at 77 K, it rapidly isomerizes to the isomeric ketenimine 51. Figure 11.5 shows the spectrum of 52 recorded in ERA at 77 K. 

The singlet phenylnitrene 52, which is formed upon photodissociation of 47, has a considerable excess of vibrational energy and can easily overcome the potential energy barrier for isomerization. Indeed, studies utilizing femtosecond time-resolved IR spectroscopy have demonstrated that a portion of the total yield of ketenimine 51 is formed on a picosecond time scale. The ketenimine 51 was also formed in the vibrationally hot state. The formation of ketenimine 51 and its vibrational cooling proceed within 10-50ps. Unfortunately, attempts to separate these two processes and determine the characteristic time of the ketenimine formation failed. " ... 

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. 

Along with the substituent effect on the reactivity of singlet phenylnitrenes, the influence of substituents on the reactions of ketenimines with nucleophiles was also studied in detail. As in the case of unsubstituted ketenimine 51, its simple derivatives could be trapped by nucleophiles in solution. The primary products, corresponding IH-azepines, undergo subsequent isomerization to final products. Reaction of ketenimines with primary and secondary amines is the most studied of the reactions with nucleophiles. Rate constants of this reaction with DEA (Table 11.4) were measured for a series of substituted ketenimines using TRIR spectroscopy, as well as conventional LFP techniques. ... 

When phenyl azide (74) was photolyzed in Ar matrices, phenylnitrene (75) was not detected in the resulting IR spectrum, but a product with an intense absorption at 1895 cm was formed. It was concluded that the only plausible candidate for the observed product was the strained cyclic ketenimine 76 (Scheme 9), with the 1895 cm IR band corresponding to the v(C=C=N)js vibration. The identity of 76 received further support when it was found that the N-isotopomer had this IR band shifted to lower frequency by about 15 cmAt the same time, it was found that a series of 3- and 4-substituted phenyl azides gave the same type of ketenimine products when photolyzed in Ar and Nj matrices, aU with IR bands in the 1910-1880 cm region. " ... 

As with phenylcarbene (35) (c/. Scheme 5), the ring expansion of phenylnitrene (75) could occur directly on the singlet potential energy surface or via a bicycHc intermediate, in this case azirine 92. Such azirines were first detected in the matrix photolysis of 1- and 2-azidonaphthalene. For example, photolysis of 93 generated a photoproduct with an IR absorption at 1730 cm, which in turn gave a cyclic ketenimine [v(C=C=N) at 1926 cm ] on further photolysis. It was proposed that the intermediate with the 1730 cm IRband was the tricyclic azirine 94, which subsequently rearranged to 95. At the time of these experiments, however, reliable computations of IR transitions were not available, so a definitive identification of the azirines was scarcely possible. Nevertheless, a clear parallel was found here with the behavior of aryl carbenes, which give detectable cyclopropenes in the naphthalene series (c/. 44 and 45) but not with the monocyclic carbenes (see the brief discussion of this point in Section 14.4). 

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