Diazirin

Prepared by the latter route, decomposition is - 10% in 24 h. This reaction involves direct Cl transfer to NH2CI via the intermediate NH CE NH2CI hydrolysis playing Htde or no role. The presence of NH" 4 improves stabiUty by reacting with HOCl which tends to increase decomposition (39). Trichloramine also increases decomposition, whereas NH2CI has Httle effect. Dichloramine is useful for preparation of diazirine (40). 

It is interesting to note that the existence of an isomer of diazomethane with a diazirine structure was never discussed, so the report on its synthesis 61TL612) simultaneously started and ended the discussion on its existence. 

Among unsaturated C—N—N three-membered rings only the 3H-diazirines (3) are known. IH-Diazirines with a C=N double bond were never obtained. Diazirines with one or two alkyl groups at carbon were prepared in many cases, aryldiazirines only in some cases. An important role is played by difluorodiazirine as well as by diazirines containing chlorine or bromine (9). 

Whereas oxaziridine and diaziridine were partial subjects of comprehensive theoretical studies on cyclic compounds (73MI50800), diazirine and some of its simple derivatives were the special target of quantum chemical investigations. Since diazirine, the lowest molecular weight heterocycle, has only five atoms and is of high symmetry, there was a chance for ab initio calculations, which followed some semiempirical studies. 

Later there was an attempt by ab initio calculation to fit the electron structure of diazirine into the Walsh model of cyclopropane (69MI50800). According to these SCF-LCAO-MO calculations three MOs add to the description of the lone electron pairs, all of which also contribute to some extent to ring bonding. As to strain, 7r-character and conjugative effect, the term pseudo-rr-character was used. 

There was less agreement between calculated and experimental energy values. The use of 6-3IG, the best procedure in energy calculations of three-membered rings, yielded a value too low by more than 40 kJ moF in the case of diazirine bond separation energy was calculated as -45 kJ moF the experimental value is +0.4 kJ moF . Vibrational correction and extrapolation to 0 K would reduce this difference by several kJ moF . 

Even more recent calculations using STO-3G and 6-31G basis sets could not safely predict diazomethane as the more stable compound in comparison with diazirine, although there is an experimental energy difference of 125 kJ moP 79JST(52)275). 

Some theoretical investigations have attempted to follow reactions of diazirines, among them reactions of 3-chloro- and 3-fluoro-diazirines. 

Structural data of a diaziridine come from gas phase electron diffraction measurements (74CC397). The N—N bond of 3-methyldiaziridine (24) is longer than in hydrazine (1.449 A) the C—N bond distances in (24) and in diazirine are nearly equal (1.479 versus 1.482 A),... 

The molecular geometry of diazirine (3 R = H) was analyzed by microwave spectroscopy (62JA2651). Rotatory spectra of diazirine, of ( C)diazirine and of ( N- N)diazirine yielded the bond lengths and bond angles shown. The dipole moment of diazirine is 1.59 D. 

According to measurements on steroidal diazirines (49) there is also a high field shift of equatorial methyl groups, although their protons are more distant by one position from the diazirine ring. Reported values lie between 0.1 and 0.2 p.p.m., about 0.8 p.p.m. higher than in alkanes (65JA2665). 

In the IR spectrum of diazirine (44), complete assignment of vibrations could be performed when deuterated derivatives were used (Table 1) (64JCP(40)1693). 

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). 

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). 

Diaziridines (156) unsubstituted on both nitrogens decompose at 125 °C by a redox reaction yielding one mole of a diazirine (157) together with two moles of ammonia and one mole of ketone from two moles of (156). The reaction proceeds below 60 °C when copper salts are present (64AG(E)229). 

It was not their reactivity but their chemical inertness that was the true surprise when diazirines were discovered in 1960. Thus they are in marked contrast to the known linear diazo compounds which are characterized by the multiplicity of their reactions. For example, cycloadditions were never observed with the diazirines. Especially surprising is the inertness of diazirines towards electrophiles. Strong oxidants used in their synthesis like dichromate, bromine, chlorine or hypochlorite are without action on diazirines. Diazirine formation may even proceed by oxidative dealkylation of a diaziridine nitrogen in (186) without destruction of the diazirine ring (75ZOR2221). The diazirine ring is inert towards ozone simple diazirines are decomposed only by more than 80% sulfuric acid (B-67MI50800). 

The chemical inertness of the three-membered ring permitted many conversions of functional groups in diazirines. Esterifications, cleavage of esters and acetals, synthesis of acid chlorides, oxidation of hydroxy groups to carboxyl groups as well as Hofmann alkenation all left the three-membered ring intact (79AHC(24)63). 

Reductions of the N —N double bond yield diaziridines and were carried out for proof of structure, using for example sodium amalgam or catalytic hydrogenation. They are unimportant beyond that, because most diazirine syntheses start with diaziridines. 

Diazirines (3) smoothly add Grignard compounds to the N—N double bond, giving 1-alkyldiaziridines. Reported yields are between 60 and 95% without optimization (B-67MI50800). The reaction is easily carried out on a preparative scale without isolation of the hazardous diazirines and may serve as an easy access to alkylhydrazines. The reaction was also used routinely to detect diazirines in mixtures. The diaziridines formed are easily detected by their reaction with iodide. Phenyllithium or ethylzinc iodide also add to (3) with diaziridine formation. 

Metal carbonyls react with diazirines with complex formation at one or both N atoms (75JOM(94)75). The 1 2 complex (187) is converted to (188) by N —N cleavage in acidic media. 

Photolytic transformation of diazirines to diazoalkanes was observed in some cases. The parent compound (44) on irradiation in the gas phase with light (A = 3200 A) yields diazomethane. The quantum yield is 0.2 (64JA292). 

Formation of diazomethane from diazirine was also observed in a solid nitrogen matrix on irradiation (64JCP(41)3504). Labeling experiments demonstrated that elimination and uptake of nitrogen occurred. 

With some special diazirines interconversion with diazoalkanes was observed on illumination (79AHC(24)63), e.g. with diazirinecarboxylic acid piperidide (194), spirodiazirine (195) and the tetracyclic ketodiazirine (196) (78CC442). 

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). 

Methylvinyldiazirine (199) rearranges at room temperature in the course of some days. Formation of the linear isomer is followed by electrocyclic ring closure to give 3-methyl-pyrazole. The linear diazo compound could be trapped by its reaction with acids to form esters, while the starting diazirine (199) is inert towards acids (B-71MI50801). 

A formal ring enlargement of diazirines to five-membered rings is also observed with some hydrazones of ketodiazirines (65CB2509). On attempted preparation of hydrazones (201) from ketodiazirine (200) at 0 C the diazo compounds (202) are plausible intermediates since their transformation to aminotriazoles (203) is known. 

Formation of azines on thermolysis of diazirines was reported repeatedly (B-67MI50800), e.g. with perfluorodimethyldiazirine (204), with chloromethoxydiazirine and with chloro-phenyldiazirine. 

There are some reports on reactions involving complete N—N cleavage in diazirine reactions such as formation of amidine (205) from chlorophenyldiazirine, or on formation of products containing only one nitrogen atom. Betaine (206) was described as a product from difluorodiazirine and triphenylphosphine. Compound (207) is formed from decomposing (204) and cyclohexane (79AHC(24)63). 

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