Cyclic ketenimine

Banks and co-workers found that gas-phase pyrolysis of pentafluorophenyl azide yields a diazaheptafulvalene (Scheme 16).50 The formation of the product was viewed as dimerization of the perfluoro analogue of 4b, although it is possible that it could, instead, have been formed by dimerization of the corresponding cyclic ketenimine. 

The CASPT2/6-311G(2d,p) barrier for the process 2b — 3b is only ca. 3 kcal/mol, and this reaction is calculated to be exothermic by about 6 kcal/mol. These computational results are consistent with the failure of Schuster s time-resolved IR experiments to detect 2b.38b,c A barrier of 3 kcal/mol implies rapid conversion of 2b to 3b at room temperature, and a 6 kcal/mol difference in energy between 2b and 3b means that at 25°C the equilibrium would overwhelmingly favor 3b. In addition, azirine 2b probably absorbs less strongly than cyclic ketenimine 3b in the infrared, thus rendering detection of the azirine even more difficult. 

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. 

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 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. 

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... 

The assignment of the transient absorption spectrum of Fig. 2 to PN is supported by the similarity of its spectrum to that of the longer-lived per-fluorinated singlet arylnitrenes. " The decay of this transient absorption is accompanied by the formation of cyclic ketenimine K. Furthermore, the electronic absorption spectrum of PN in the A2 state calculated at the CASPT2 level is in good agreement with the transient spectrum (Fig. 2). 

In the absence of nucleophiles the cyclic ketenimine polymerizes. At high dilution it can slowly revert to benzazirine BZ, and to the singlet nitrene. Eventually the singlet nitrene relaxes to the lower-energy triplet nitrene (at high dilution), which subsequently dimerizes. ... 

There is rather little direct experimental evidence for the intermediacy of BZ. Cyclic ketenimine K has been detected by matrix IR spectroscopy, benzazirine BZ has not. However, fluorinated and naphthalenic derivatives of BZ have been generated as persistent species in cryogenic matrices and characterized. Parent benzazirine BZ has been intercepted with ethanethiol, and certain derivatives of BZ have been trapped with... 

Ultraviolet irradiation of mesityl azide 288 in the presence of tetracyanoethylene has resulted in the isolation of the intermediate azomethine ylide 289 (from trapping of the aryl nitrene) together with its rearrangement product, the spiroazepine 290 (Scheme 36) <1997JOC3055>. Photolysis (at 313 nm) at low temperature of 1- and 2-azido-naphthalenes in an Ar matrix provided access to the novel seven-membered cyclic ketenimines 291 and 292, respectively <2004JA237>. 

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). 

Photolysis of 5,5-dimethyl-3-azido-2-cyclohexene-l-one gave only the azepine (52).20 This product probably arises by a Curtius-type rearrangement to give a cyclic ketenimine which is hydrolyzed to the product 52. 

An elegant application of the isoxazole -azirine rearrangement has been used in the generation of the fused azirine 64 which, on further photolysis, underwent ring expansion to the cyclic ketenimine 65 as shown in Eq. (19).66... 

The products of nucleophilic trapping (47, 48), after decomposition of 46, were initially rationalized as arising from benzazirine 50. This explanation was generally accepted in subsequent studies " and supported by calculations,but in 1978, Chapman and LeRoux detected 1-aza-l,2,4,6-cycloheptatetraene (51) using matrix isolation techniques. The existence of the cyclic ketenimine 51, was confirmed by later spectroscopic studies in matrices and in solution. It was also established that ketenimine 51 is the species trapped by nucleophiles in solution to form azepines 47 and 48. ... 

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... 

The formation of the products (cyclic ketenimine 51 and triplet nitrene 49) was monitored at 380 nm. The decay of 49 and growth of the products are first order and can be analyzed to yield an observed rate constant, koBs- -An Arrhenius treatment of the koes data (open circles) is presented in Figure 5.15. 

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