Photolysis of diazirines

The absorption properties of diazirines are characteristic. All possess an absorption band in the near ultraviolet (kmax 350 to 380 nm) which is resolved into a series of sharp peaks in non-polar solvents such as hexane. Extinction coefficients are modest, usually around 300 M-1 cm-1, but 

Photolysis of dlazirines to nitrogen and carbenes is a general reaction and plays a greater role in carbene chemistry than photolysis of linear diazo compounds. Whereas the latter are often obtained only under the conditions of their thermal decomposition from suitable precursors, diazirines are obtainable in a pure state in most cases. Photolysis has the further advantage to permit nitrogen extrusion at atmospheric pressure, even with low-boiling materials. 

Photolysis of the parent compound (44) yields singlet methylene, as evidenced by its stereospecific addition to ( )-butene. The cyclopropane is formed together with the characteristic insertion products of methylene (62MI50800, 64PAC(9)527) 

Methylene from diazirine has higher energy of vibration than the product from photolysis of ketene, but it is more discriminating in insertion reactions into primary and secondary C—H bonds. 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

Dimethyl- Propene Ethane, ethylene, propane, 1- butene, 2,3-dimethylbutane, 2- methyl-4-pentene, isobutane, 1,5-hexadiene 

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During photolysis of [29] in an argon matrix doped with 4% CO, bands belonging to the ketene [31] were observed along with those of the carbene [30], Upon further warming to 40-45 K the carbene bands disappeared and were replaced with bands of the ketene [31], indicating a direct interaction of carbene (CF3)2C with CO. Photolysis of diazirine [29] in a matrix doped with 17% CO resulted in the appearance of only ketene [31] bands the carbene bands were not detected in this experiment. 

Figure 4.1. TRIR difference spectra averaged over the timescales indicated following 355 nm laser photolysis of diazirine 8 (15.7mM) in C02-satnrated dichloromethane. Reprinted with permission from B. M. Showalter and J. P. Toscano, J. Phys. Org. Chem. 2004, 14, 743. Copyright 2004, John Wiley Sons Limited.

Photolysis of diazirine 75 in N2 at 9 K produced carbene 76, whose IR and UV/Vis spectra fit predictions by DFT calculations. The IR spectra were most consistent with a carbene conformation with Cl aligned 90° to the adjacent C-CH bond. The carbene was found, by IR spectroscopy, to rearrange to chloroadamantene 77 slowly at 9 K in the dark. The rate of rearrangement was somewhat faster in Ar matrices at 9 K or at higher temperatures in N2. 

As shown in Table 1, however, the product distribution depends on the method of carbene generation. Whereas thermolysis of either tosylhydrazone salt (7) or methylethyldiazirine (8) affords essentially the same product distribution (in which 95% of the products are the 2-butenes), photolysis of diazirine 8 is quite different. 

Photolysis of diazirine 14 gave >90% of vinyl chloride, but, in accord with the observations of Tomioka, Liu, and Bonneau,17-20 photolyses in the presence of increasing quantities of TME led to a strongly curved correlation of addn/rearr vs. [TME]. And, carbene 15 was trapped by TME with, at most, 66% efficiency, so that about a third of the diazirine was converted to vinyl chloride by a 1,2-H shift that seemed to bypass the carbene. 

Photolysis of diazirines 21-C1 or 21-F in the presence of varying concentrations of TME or 2-methyl-1-butene gave the 1,2-H and 1,2-C products 22... 

The Y-intercept ( 3 /k Eq. 13) of the reciprocal correlation for the photolysis of diazirine 21-C1 in TME was 2.18, which translated into a 68% incursion of diazirine excited state in the genesis of the rearrangement products, 22-C1 and 23-C1 carbene 17-C1 only accounted for 32% of these products.28 A similar conclusion followed from the ratio of rearr/addn (68 32) at a high concentration (6.7 M) of TME in pentane, where carbene 17-C1 was almost completely diverted to the cyclopropane, and 22 and 23 were exclusively derived from the excited diazirine.28... 

Related observations were reported for frans-r-butylcyclopropylcarbene, 20.46 Photolysis of diazirine 29 in freon-113 gave f-butylcyclobutene, 30 (46-49%), as well as 20% of f-butylethene (and ethene) fragmentation products Scheme 5. 

The absolute rate constants for oxygen and sulfur transfer to a range of carbenes (dialkyl, cycloalkylidene, alkylchloro, diaryl, arylchloro, arylalkoxy, and dialkoxy), generated by laser flash photolysis of diazirine or oxadiazoline precursors, were determined. No evidence was seen for ylide formation and a concerted mechanism via an ylide-like transition state was proposed. 

Diazirine, as well as diazomethane and ketene, has been used as a convcm. m source of CH2 radicals. The photolysis of diazirine in the vacuum ultravo i. i leads to... 

Carbene generation from photolysis of diazirine compounds leads to efficient insertion into C—H or N—H bonds and also causes addition reactions with points of unsaturation within target molecules. Diazirine-containing photoaffinity probes have... 

Photolysis of diazirine in the presence of a large excess of propane yielded n- and isobutane and in the presence of n-butane yielded n- and isopentane. From the relative rates of attack on the primaiy and secondary carbon-hydrogen bonds in these compounds, it was concluded that methylene derived from diazirine showed approximately the same discrimination as methylene formed by the photolysis of ketene. The results obtained, using methylene derived from the photolysis of diazomethane, gave a product ratio closer to the simple statistical ratio of the number of carbon-hydrogen bonds without correction factors for the type involved and indicated almost no differentiation between the types. 

Amrich and Bell have reported the photolysis of diazirine at pressures from 5 to 30 mm. in the presence of nitrogen at pressures from 0 to 600 mm. They employed monochromatic light of about 320 m,u. They detected the formation of diazomethane in the photolysis (monitored by ultraviolet absorption at 229.5, 217.5, and 213.5 m u), and suggested the following as part of the mechanism ... 

The photolysis of diazirine °° at 3130 A yields ethylene and nitrogen methylene is probably formed in the primary process. The long wavelength absorption ( max = 3200 A) had been identified as the allowed (a, n ) transition and semi-empirical Huckel calculations indicate that the methylene produced must be (Ai) or (Bi). There is evidence that methylene produced from the photolysis of diazirine is more selective than methylene from diazomethane, owing to decreased excess translational energy. At low pressures 5-30 torr, diazomethane was identified as an intermediate by its absorption spectrum and the question arises, is methylene formed directly, or does it arise from decomposition of diazomethane The quantum yield of disappearance of diazirine is 2.0+0.5 and the quantum yield of diazomethane formation is about 0.2. The intermediate diazomethane is... 

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