Ketenimine spectroscopy

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. 

In the absence of amine, the ketenimine-azirine singlet nitrene species can equilibrate and, eventually, the singlet nitrene can cychze to form carbazole. Berry and co-workers independently monitored the growth of carbazole ( max = 289.4 nm) by this process. In cyclohexane, some carbazole was formed this way with an observed rate constant of 2.2 x 10 at 300 K over a barrier of 11.5 kcal/mol. Tsao and co-workers recently used TRIR spectroscopy to show that ketenimine decay equals the rate of carbazole formation. 

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. 

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... 

Once the spectroscopy and dynamics of parent singlet phenylnitrene were understood, we began a systematic study of the effect of substitution on the kinetics of singlet phenylnitrenes. For most of the aryl azides of interest " the rate constants of singlet nitrene decay and product formation (triplet nitrene and/or ketenimine) are the same (Fig. 9). With these nitrenes, cyclization to substituted benzazirines is the rate-limiting step of the process of nitrene isomerization to ketenimine in a manner similar to the parent phenylnitrene. The only exception, o-fluorophenylnitrene, will be examined in detail in the last section of this review. 

Decomposition of 2,6-difluorophenyl azide by LFP (266 nm) generated a singlet nitrene which was detected by time-resolved IR spectroscopy (1404 cm-1).75 The nitrene could only be detected between 243 and 283 K. At 298 K, the nitrene decay products, a ketenimine (1576 cm-1) and a triplet nitrene (1444 cm-1), were observed. The IR assignments were consistent with DFT calculations and previous UV-visible detection results. 

An eight-membered cyclic 7,./V-bis(germadiyl)bis(ketenimine) 190 was prepared by the reaction of tert-butyl-lithium with (fluorodimesitylgermyl)phenylacetonitrile 188 leading to the lithium salt 189, which then underwent an elimination of lithium halide. Compound 190, the first ring containing two ketenimine moieties, was characterized by IR and 13C NMR spectroscopy as well as X-ray structure determination (Scheme 34) <19980M1517>. 

In 1978 Chapman and Le Roux reported that photolysis of phenyl azide in argon matrices at 10 K gave l-aza-l,2,4,6-cycloheptatetraene [24]. This species was characterized by IR spectroscopy and was found to have a prominent band at 1895 cm-1 associated with the ketenimine portion of the molecule. There was no evidence for the formation of a benzazirine type of structure,... 

These studies reveal a general problem in matrix isolation spectroscopy, that different species have very different sensitivities to different spectroscopic methods. EPR spectroscopy is a very sensitive tool for the detection of triplet phenyl nitrene but is totally blind towards a dehydroazepine. The dehydroazepine has a distinctive ketenimine chromophore enabling facile IR detection but no such characteristic vibration exists for triplet phenyl nitrene. Furthermore the molar absorptivities of the molecules of interest are not known thus it is impossible to quantify accurately the yield of a given species produced in the matrix. Thus Chapman s work [24,79] clearly demonstrated the formation of triplet phenyl nitrene and of dehydroazepine and the absence of benzazirine, but it did not reveal the ratio of nitrene to dehydroazepine present in the matrix, nor did it indicate which species is initially formed in the matrix. 

To a solution of 0.8 g dimethyl A -[l-(2-azidophenyl)ethyl]dithiocarbonimidate (3 mmol) in 15 mL dry toluene, was added 3 mL 1 M trimethylphosphane in toluene (3 mmol), and the mixture was stirred at room temperature until the evolution of nitrogen ceased (15-30 min). Then, 0.40 g methyl phenyl ketene (3 mmol) was added, and the reaction mixture was stirred at room temperature until the ketenimine band 2000 cm was not observed by IR spectroscopy (3-4 h). Upon removal of the solvent under reduced pressure, the residue was purified by silica gel column chromatography using hexanes/EtOAc (4 1 v/v) as the eluent to afford 0.90 g frans-2,8-dimethyl-l,l-bis(methylthio)-2-phenyl-l,2-dihydroazeto[2,l-i ]quinazoline as colorless prisms (Et20), in a yield of 85%, m.p. 141-142°C. 

In 1961, Smolinsky reported on vapor phase pyrolysis of a-azidostyrene (52, Scheme 5.8), which furnished 3-phenyl-2//-azirine as the main product and provided the first example of the synthesis of such strained heterocycles from vinyl azides. By analyzing the IR data of the pyrolysates produced from 52, it was shown one year later that N-phenylketenimine is formed as a side product. By utilizing IR and NMR spectroscopy at low temperature, Wentrup and coworkers have smdied recently the detailed structures of another azirine-ketenimine pair, generated by thermal or photochanical decomposition of an enazide. The transformation of vinyl azides into 2//-azirines is currently the most frequently used access to these heterocycles. The manifold chemistry of azirines was reviewed several times," " and most of the aspects of their synthesis from alkenyl azides are summarized in Chapter 6 (Gilchrist Alwes). Therefore, only some additional certain points are included here. 

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. " ... 

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. ... 

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