Nitrenes matrix isolation

Morawietz, J. Sander, W. Photochemistry of fluorinated phenyl nitrenes matrix isolation of fluorinated azirines, J. Org. Chem. 1996, 61,4351-4354. 

Matrix isolation photolytic studies on tetrazolo[l,5- ]pyridazine 25 have been reported by Hill and Platz <2003PCP1051> (Scheme 5) and formation of the l-cyano-3-diazopropene 27, triazacycloheptatetraene 28, and cyano-cyclopropene 29 was detected. Upon the absence of electron spin resonance absorptions at 7 K, the authors concluded that triplet nitrene was not formed but, instead, the resulting singlet nitrene rapidly underwent further ring openings. 

For species of higher than singlet multiplicity, matrix isolation coupled with ESR spectroscopy is often used for detection and characterization purposes. This is how the first examples of exotic organic compounds with quintet ground states were obtained. Since then, this kind of spectroscopy has been closely linked to the developments in the area of carbenes and nitrenes. 

Azido-4,6-dichloro-133-triazine 35, matrix isolated in argon at 10 K, yields triplet nitrene 36 and the strained cyclic carbodiimide 37 upon photolysis <99CC2113>. 

Matrix isolated NH reacts with NO, an excellent nitrene trap, to form trans-HNNO, which was characterized by infrared (IR) spectroscopy. Triplet imidogen also reacts with oxygen in matrices. 

No transient absorption >350 nm is detected upon LFP of 1-naphthylazide. A band with absorption maxima at 370 nm is formed with a time constant of 2.8 ps after the laser pulse. The carrier of the 370-nm absorption reacts over >100 ps to form azonaphthalene. The carrier of the 370-nm absorption is identified as triplet 1-naphthylnitrene that has previously been characterized as a persistent species at 77 K by UV-vis (A,nmx = 367 nm) and EPR spectroscopy. Azirine 43, detected by TRIR spectroscopy must not absorb significantly >350 nm, a fact that was established later by the matrix isolation studies of Wentrup s and Rally s groups. Assuming a rapidly equilibrating mixture of azirine and nitrene, and given that kisc = 1 X 10 s (determined by Tsao by LFP at 77 K and assumed to have the same value at 298 then K = [singlet nitrene]/[azirine 43] = 0.038 at 298 K. 

The great majority of matrix isolation studies of carbenes and nitrenes have employed their formal adducts with molecular nitrogen, that is, diazo compounds or diazirines in the case of carbenes, azides in the case of nitrenes, as precursors for their in situ generation. Usually, these compounds will readily release N2 on irradiation with a low-pressure mercury lamp (254 nm), and this fragment has the advantage that it will usually not react with or perturb the targeted reactive intermediate (see Scheme 17.2). 

Azides are virmally the only nitrene precursors that have been used in matrix isolation studies. They are usually easily accessible, but should only be made and handled in very small quantities because certain azides can be violently explosive. 

The thermal decomposition of 2-azidoacetic acid (N3CH2CO2H) in the vapour phase has been shown, by photoelectron and matrix isolation infrared spectroscopy, to involve simultaneous formation of CO2 and methanimine (CH2NH) with concerted ejection of N2.52 No evidence was found for formation of intermediate nitrene (NCH2CO2H) or the imine (HNCHCO2H) to which it could be converted by 1,2-hydrogen shift. 

Figure 6. Absorption spectra of 1,4-diazidobenzene (I) and its photoproducts matrix isolated at 77°K. la, diazide Ij, azido-nitrene dinitrene Ij, dini-trene cation.

The photolysis of benzoyl azide (PhCON3) has been investigated by means of matrix isolation, time-resolved IR spectroscopy, and computations at various levels of theory.In argon at 12 K, formation of two species was observed. One of these was phenyl isocyanate (PhCNO), while the other had an IR spectrum consistent with the predicted spectrum of the reactive singlet species PhCON, which appears to have a structure intermediate between that of a carbonyl nitrene and an oxazirene. Analogous observations were also made for 4-acetyl-benzoyl azide. Time-resolved IR spectroscopy on the nanosecond time scale provided additional evidence for the singlet ground state of benzoylnitrene. 

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. 

The formation of highly strained bridgehead imines was observed on the irradiation of a series of matrix-isolated bridgehead azides.The photochemistry of matrix-isolated 1-azidonorbornane (6) was studied using monochromatic irradiation IR, UV, and ESR spectroscopy and trapping with methanol and CO. The azide photochemistry was very complicated, and the formation of two types of imines (7 and 8) and triplet nitrene 9 were observed. 

Rearrangements of aromatic and heteroaromatic nitrenes can be initiated with either heat or light. The thermal reaction is typically induced by flash vacuum thermolysis, with isolation of the products at low temperatures. Photochemical experiments are conducted either under matrix isolation conditions or in solution at ambient temperature. These rearrangements are usually initiated by ring expansion of the nitrene to a seven-membered ring ketenimine or carbodiimide (i.e., an azacycloheptatetraene). 

A somewhat similar study reporting generation of mono- di- and tri-nitrene species from a suitable matrix-isolated precursor was described by Misochko et dealing with the photolysis of 1,3,5-triazido-2,4,6-trichlorobenzene in argon matrices. In turn, Costa et published the... 

The fluorocarbonyl nitrene FC(0)N has been generated in high yield by ArF excimer laser photolysis of matrix-isolated FC(0)N3. This is the first example of a triplet... 

More recent matrix isolation studies on benzoyl nitrene, however, did show conversion to the isocyanate. Calculations show that the acyl nitrene has oxazirene (108) character and that this 0-N interaction stabilizes the singlet state of the nitrene. Interestingly, a reexamination of benzoyl nitrene, trapped in an argon matrix and irradiated at 308 nm, revealed that in addition to phenyl isocyanate, a small amoimt of phenyl cyanate (110) was formed. Direct irradiation of benzoyl azide (254 nm) also generated the... 

Isoxazole-oxazole photoisomerization was studied by irradiation of matrix-isolated 3,5-dimethylisoxazole (18) at 222 nm. 2-Acetyl-3-methyl-2H-azirine (20) was obtained, likely through an acetyl vinyl nitrene intermediate 19 as primary photoproduct, while upon longer time UV irradiation, two additional photoproducts were identified as acetyl nitrile ylide 21 and 2,5-dimethyloxazole (22) (13JOC10657). Analogously, 3,5-diphenylisoxazole and 2-benzoyl-3-phenyl-2f/-azirine behaved as precursors to triplet vinyl nitrene (of type 19) through laser flash photolysis (13JOC11349). Reductive heterocycle—heterocycle transformations of (2-nitrophenyl)isoxazole precursors, such as 23 and 26, afforded 4-amino quinolines of type 24, quinolin-4(lfJ)-ones 25, and 3-acylindoles 27. Che-moselective heterocyclizations were observed from 3,4-,4,5-, and 3,4-bis(2-nitrophenyl)isoxazoles (13OL2062). 

The photochemistry of HN3 in the argon and nitrogen matrices was studied for the first time by J. Pimentel et al. and the IR spectra of triplet nitrene NH and radical NH2 were recorded. A series of studies of matrix isolated NH (in ground triplet and excited singlet A states) and its deutero-substitute analogue (ND) were later performed using UV and luminescence spectroscopy. " The spectroscopy and relaxation of the lowest excited singlet state of NH / ND ( A) were studied in detail in Ne, Ar, Kr and Xe matrices... 

Photochemical properties of azido derivatives of six-member aza-heterocycles (pyridine, pyrimidine, triazine, quinoline, acridine) are discussed. Data on the stmcture of the reaction products formed under photolysis of azides in different conditions (solvent, temperature, additives), and also data on the matrix isolation spectroscopy of heterocyclic nitrenes, including high-spin nitrenes, produced by low-temperature photolysis of the corresponding azides are shortly examined. 

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