Isomerization diazirines

We have examined both isomeric diazirines, 3 H-diazirine with an N=N bond and 1 H-diazirine with a C—N bond. 3 H-diazirine is known and its structure has been determined by microwave spectroscopy. 4) In the STO-3G determination we have assumed Czv symmetry, leading to the structure compared with experiment in Fig. 1. The N=N bondlength is... 

On illumination an equilibrium between the phenoldiazonium betaine 170 and the isomeric diazirine 171 is reached, containing ca. 15% of the diazirine. 

The photolysis of diazo compounds and the isomeric diazirines continues to attract a great deal of interest, ranging from fundamental theoretical studies to applications such as photoaffinity labelling. 

The energy difference between diazirine and diazomethane, interesting from the point of view of their isomerism, came from MS measurements (63JCP(39)3534). The appearance potentials of the CH2 ion, common to both compounds, yielded a difference in heats of formation of 125kJmor A strong peak in the mass spectrum of 3-chloro-3-methyl-diazirine (50) with relative mass 55 was ascribed to the methyldiazirinium ion (51). 

Thermal conversion of diazirines to linear diazo compounds was postulated occasionally and proved by indirect methods. The existence of a diazo compound isomeric to diazirine (197) was proved spectroscopically on short thermolysis in DMSO (76JA6416). An intermediate diazoalkane was trapped by reaction with acetic acid, yielding the ester (198) (77JCS(P2)1214). 

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. 

In the discussion on the structure of the aliphatic diazo compounds, the question of the existence of isomeric diazo compounds with three-membered rings was never considered. It wms therefore a surprise when the cyclic diazo compounds, i.e. the diazirines, became known their preparation wms published independently by Paulsen and by Schmitz and Ohme. ... 

The diazirines are of special interest because of their isomerism with the aliphatic diazo compounds. The diazirines show considerable differences in their properties from the aliphatic diazo compounds, except in their explosive nature. The compounds 3-methyl-3-ethyl-diazirine and 3,3-diethyldiazirine prepared by Paulsen detonated on shock and on heating. Small quantities of 3,3-pentamethylenediazirine (68) can be distilled at normal pressures (bp 109°C). On overheating, explosion followed. 3-n-Propyldiazirine exploded on attempts to distil it a little above room temperature. 3-Methyldiazirine is stable as a gas, but on attempting to condense ca. 100 mg for vapor pressure measurements, it detonated with complete destruction of the apparatus." Diazirine (67) decomposed at once when a sample which had been condensed in dry ice was taken out of the cold trap. Work with the lower molecular weight diazirines in condensed phases should therefore be avoided. 

The properties of the diazirines and the analytical results showed that a new class of isomeric diazo compounds had been discovered. The three-membered ring structure (65), which is made probable by the synthetic methods, is confirmed by the reactions of the diazirines. 

Diazirines (isomeric with diazoalkanes) also give carbenes ... 

Some diazirines, particularly the 3-trifluoromethyl-3-aryldiazirines, can rearrange upon photolysis to a linear diazo derivative, similar in structure to the photosensitive end of the crosslinker PNP-DTP (Chapter 5, Section 3.12). These isomerized products themselves can be photolyzed to the reactive carbene. 

Time resolved (LFP) infrared spectroscopy of diazirine 9a showed a low efficiency (0 0.075) isomerization to diazo compound 38, but 38 was not... 

Irradiation of the nitrogenous precmsors 37-NR, 38-NR, and 39-NR was carried out essentially under the same experimental conditions as for 33-DN [106]. At the initial stage of the irradiation, in the fluorine case (37-NR, Scheme 8a), partial isomerization of the diazirine precmsor to its diazo isomer (37-DN) was inferred from the IR spectral changes. In the cases of the chloro and bromo derivatives (38-NR and 39-NR, respectively), the formation of azidocarbenes (38-N and 39-N, respectively. Scheme 8b) is compatible with the new azido IR peaks... 

Complexes were also considered for other carbenes and even suggested for reactions in alkane solution( ) A counterproposal, backed by the inability of theory to find support for such stable complexes, held that the second source was not the carbene-alkene complex but instead was the diazo compound, formed from isomerization of excited diazirine. " Other LFP studies reinforced the need for two intermediates, but could not finally resolve the question of carbene complexes ver-sus diazo compound. However, the question is now settled in this case,... 

Occasionady a diazo-compound on irradiation isomerizes to a diazirine (5.331 although this process could in principle precede the loss of nitrogen in any diazo system, there is no evidence to suggest that such isomerization occurs in most cases. 

Trifluoromethyl)-3//-diazirin-3-yl]arenes, e.g. 36 (R1 = CF3), were introduced as photophores by Brunner et alJ116 to provide a more favorable carbene than the one produced from diazoesters[117 or from aryldiazirines. 118 Both 3//-diazirin-3-ylarenes I8-1201 and the [(3-tri-fluoromethyl)-3//-diazirin-3-yl]arenes (e.g., 36, R = H, CF3, respectively) 116 are more stable in acid than most diazoesters, and photolyze at around 360 nm to generate highly reactive carbenes such as 37. However, these diazirines can photo-isomerize to some extent to the... 

All silenes generated so far on the silylcarbene route are reactive intermediates themselves, which were characterized by typical subsequent reactions35 such as isomerization and dimerization or by trapping reactions (see below). However, photolysis of (silyl)diazo compounds in inert matrices at low temperature allowed the isolation and spectroscopic (IR, UV) characterization of several silenes (Scheme 2, Table 3). Irradiation of (dia-zomethyl)silanes 7 at X > 360 nm produced both diazirine 8 and silenes 10, but at shorter wavelength (X > 305 nm) the silenes were produced cleanly from both precursors the... 

The cyclic isomers of diazoalkanes, diazirines also decompose under the influence of heat and light to give carbenes (Schmitz, 1967 Milligan et al., 1964 Mitsch, 1965 Moss, 1967). They are, however, much less sensitive to acid-catalysed decomposition than their open-chain counterparts. They should therefore represent a useful source of carbenes, since the interpretation of the results of experiments would not be complicated by the necessity of considering the involvement of carbonium ions. In some cases, however, there is evidence that thermal and photochemical transformation of diazirines into the isomeric diazoalkane takes place (Overberger and Anselme, 1963 Amrich and Bell, 1964 see also Hoffmann, 1966), though this has not been confirmed elsewhere (Moore and Pimentel, 1964 Frey and Stevens, 1965). 

The diazirine from Step 1 (9.5 mmol) was dissolved in methylacrylate (90 mmol) and 10 ml tetrachloroethylene then refluxed 20 minutes. The crude products consisted of an isomeric cyclopropane mixture with a syn/anti ratio of 3.8 1. The mixture was purified by chromatography on silica with light petroleum/EtOAc, 5 1, and the product mixture isolated 68% yield with no change in the isomer ratio. H-NMR and data supplied. 

The first preparations of the diazirines (cyclic diazo compounds) were described independently by Paulsen and Schmitz and Ohme only a few years ago. It is surprising that these compounds were not synthesized much earlier since in many ways they are more easily obtained than the corresponding diazo compounds. Many diazirines are already known in the pure state whereas the isomeric diazo compounds are only known in dilute solution. 

The quantum yield for the disappearance of the diazirine was found to be 2 0.5. The formation of the diazomethane corresponded to that of about one-tenth of the decomposition of the diazirine but leveled off during the photolysis. The authors su ested that this leveling off may have been due to secondary photolysis of the diazomethane by scattered radiation. These results set a lower limit for 0.2 for k /ki, i.e., at least 20% of the primary decomposition of the diazirine is an isomerization to diazomethane. Very surprisingly, no appreciable effect on the yield of diazomethane was found when nitrogen was added. 

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