Stability of diazirines

While 3-H-3-phenyldiazirine was reported to be unstable on storage as a neat liquid (Smith and Knowles, 1975), its stability was improved in hexane solution at —20°C. Higher molecular weight derivatives of this compound have been stored for months in solution at —80°C without decomposition. 

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Theoretical predictions on photochemical and thermal isomerization of diazirines may explain numerous experimental values. Due to low and reversible relative stability of diazirines and diazomethanes, which depend on substituents (Figures 1 and 2), the spontaneous <63TL1405>, photochemical <64JA292>, or thermal <77JCS(P2)1214> isomerization of diazirines into diazoalkane has been observed, as well as reversible isomerization of diazocarbonyl compounds and corresponding diazirines. Interconversions of diazirines and diazo compounds have been considered in detail [Pg.352]

When first prepared in 1960, the thermal stability of diazirines was astonishing. Although they are more energy-rich than their linear isomers by ca. 30 kcal mol", they are decomposed only at temperatures above... 

The same dichotomy was observed with hexafluorodimethylcarbene (228), formed by thermolysis of diazirine (227) at 150 °C. The carbene (228) can stabilize itself either intramolecularly to perfluoropropene (229), or intermolecularly by addition to multiple bonds. Oxirane (230) is formed with hexafluoroacetone, cyclopropene (231) with 2-butyne (66MI50800). 

The stabilization of chloromethoxycarbene (234) was intensively studied. It is formed from diazirine (233) in a first order reaction with fi/2 = 34h at 20 C. It reacts either as a nucleophile, adding to electron poor alkenes like acrylonitrile with cyclopropanation, or as an electrophile, giving diphenylcyclopropenone with the electron rich diphenylacetylene. In the absence of reaction partners (234) decomposes to carbon monoxide and methyl chloride (78TL1931, 1935). 

The photolysis of chlorodiazirine was investigated in several cases. From chloromethyl-diazirine (232) vinyl chloride was formed as the stable primary product of stabilization of chloromethylcarbene, with acetylene and hydrogen chloride as secondary products. Some 1,1-dichloroethane was assumed to have been formed through a linear diazo compound by reaction with HCl. Added HBr yielded 1-bromo-l-chloroethane (76MI5Q800). 

The reaction of diazirines 2 with the kinetically stabilized cyclobutadiene 1 gives 577-1,3-di-azepines 4 via the initial cycloadducts 3.159... 

In the cyclobutylfluoro system, the excited diazirine, 21-F, is considerably less involved. A parallel analysis indicates that only about 12% of 22-F and 23-F arise from excited diazirine, while 88% of those products descend from carbene 17-F.28 The increased carbene involvement in the photolysis of 21-F presumably reflects the greater stability of fluorocarbene 17-F over its chloro analogue 17-C1 and, consequently, a more favorable partition (ki/k3) of excited diazirine 21-F to the carbene. 

Figure 1 Relative thermodynamic stability of substituted diazirines and diazomethanes (kcal mol).

Figure 2 The order of thermodynamic stability of valence isomers of diazirine (relative stability of isomers) at the MP3/6-31 //3-21G level (89CPH157) and relative energies (kcal mol-1).

Valence isomerization of the intermediate [4 + 2] cycloadducts of diazirines and kinetically stabilized cyclobutadiene was also observed. It occurs with the opening of the ring at the N—N bond to form 5//-l,3-diazepines, 2//-1,3-diazepine, and the highly stable 1,4-diazaquadricyclone (Scheme 6). 

Diazirine 192 loses nitrogen even at 75°. Cyclopropanation with added olefin must compete with intramolecular stabilization of the carbene 200 to imine 201. 

The reverse reaction is possible not only photochemically, but also thermally. This is due to the lower thermal stability of this diazirine relative to the compound without a carbonylamino group, i.e., pentamethylenediazirine (5.63, Schmitz and Ohme, 1973). This decrease in stability of 5.72 is likely to be due to the pseudoconjugation of the three-membered ring with the carbonyl group, decreasing the... 

The present author has the impression from the literature on the stability of diazomethane relative to diazirine that two different physico-chemical phenomena were called (thermal) stability in some of the publications, namely the thermodynamic stability, as defined by the free energy of formation AGf and the free enthalpy of formation A//f for the (hypothetical) formation of a compound from the elements in a gas phase reaction under standardized conditions (298 K, 1 mol). AGf and A//f are related to one another by the free entropy A5f in the Gibbs-Helmholtz equation AGf = A/ff-TASf. The absolute values of AGf, A/ff and ASf do not give definite information on the stability of a compound, as this word is used in the everyday language of a chemist, because it is related to an unrealistic chemical process, namely the formation from the elements. If A/ff is known, however, for a given compound and for a real product of one of its reactions, the difference in magnitude of the two free enthalpies tells us whether this reaction is likely to take place, but we cannot depict at all, at least in principle, the half-life of such a reaction. 

Returning to the problem of the relative stabilities of diazomethane and its various isomers like diazirine etc., we emphasize that all previously mentioned heats of formation are free enthalpies of formation, but that the diazoalkane alkyldiazirine isomerizations (5-17), (5-18), and (5-23) are cases of differences in inertness. 

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