Diazirines stability

The diaziridine ring exhibits a surprising stability towards strong oxidizing agents. Diaziridines unsubstituted at both N atoms can be transformed into diazirines by dichromate in acidic solution or by chlorine (Section 5.08.4.3). Radical attack by decomposing peroxide converts (136) to the diaziridinyl radical (137), as evidenced by ESR spectroscopy (76TL4205). 

The stabilization reactions of alkylcarbenes were used preparatively in some cases. The diazirine derived from adamantanone gave the dehydroadamantane (2l7) thermally in 96% yield 73ZOR430). Alkene formation was reported for a steroid with its C-3 atom part of a diazirine ring. At 140 °C a A-2-unsaturated steroid was formed 65JA2665). 

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 all cases the formation of carbenes was accompanied by partial photoisomerization of the precursor diazirines [53] into the corresponding diazo compounds [55] which possess a higher photochemical stability than the diazirines. 

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. 

Aryl diazomethanes vary in stability. They are formed when diazirines are irradiated and will be be discussed below. 

Depending on the mode of generation, a carbene may be initially formed in either the singlet or triplet state, irrespective of its stability. Common methods used for the generation of carbenes include photolytic, thermal, or metal catalyzed decomposition of diazocompounds, elimination of halogenfrom gem-dihalides, elimination of Hx from CHX3, decomposition of ketenes, thermolysis of a-halo-mercury compounds and cycloelimination of shelf stable substrates such as cyclopropanes, epoxides, aziridines and diazirines. 

Some aldehyde-derived diaziridines are unstable and hamper the Schmitz synthesis of the corresponding diazirines <1975J(P2)686>. A one-pot derivative of the standard Schmitz reaction has been developed to circumvent such problems, in which an intermediate diaziridine is stabilized by a trimethylsilyl group <2000TL795>. Treatment of the aldehydes 68a-c with LiHMDS, then HOSA, gave the silicon-stabilized diazirines 69a-c. In situ oxidation with f-butyl hypochlorite generated the diazirines 70a-c in moderate yields (Scheme 24). 

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