Reactions of diazirines with electrophiles

A direct attack on the diazirine ring was observed only by use of almost concentrated sulfuric acid or strong Lewis acids, e.g. aluminum chloride in the case of (208). Some hydrazine was found together with cyclohexanone and cyclohexanol after decomposition of the spirodiazirine (189) with 80% sulfuric acid (B-67MI50800). 

Increased sensitivity towards acid is observed when protonation occurs on a functional group outside the diazirine ring, giving rise to electron dilution at the carbon atom adjacent to the diazirine carbon. The products isolated are in accord with the proposal (79AHC(24)63) that cation formation at this carbon atom leads to nitrogen extrusion, probably with formation of a vinyl cation. Thus protonated hydroxydiazirine (209) yields acetone, and methylvinyldiazirine (199) on treatment with acids yields butanone (67CB2093). 

Acidic cleavage of the oxirane ring in (210) results in formation of the alkynic aldehyde (211) (68TL4905). 

Especially sensitive towards acid is the ketodiazirine (200). It decomposes on contact with O.IN acid, forming methylenecyclopentanone (213) and cyclopentanecarboxylic acid (214). The products may be formed from a developing vinyl cation (212) by C—C bond shift either to the double bond (213) or across the double bond (214) (B-67MI50800). 

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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). 

Whereas diazirines exist only in the 3//-form with an intracyclic N=N bond, and are widely used as precursors of electrophilic carbenes, their 3//-diphosphirene analogues bearing an intracyclic P=P bond are unstable and are postulated only as intermediates (94CB969). Actually, Niecke and co-workers <91AG(E)90> described the reaction of persilylated 1,2-diphosphapropene-2 with hexachloroethane under mild conditions giving a trans l,2,4,5-tetraphosphatricyclo[3.1.0.0]hexane (5g) (Scheme 2). Its structure was established by x-ray analysis (Table 1) and its formation can be... 

The formation of ylides by the reaction of 4-nitrophenylchlorocarbene (generated by laser flash photolysis, LFP, of the parent diazirine) with ethers has been observed by UV-vis spectroscopy and was shown to be reversible. LFP combined with UV-vis spectroscopy has also enabled determination of the equilibrium constant for dihalocar-banion formation on reaction ofphenylchlorocarbene with halides. These experiments established a very fast equilibrium between the electrophilic carbene and the nucleophilic carbanion, which can be adjusted by careful control of the halide concentration. 

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). 

Phenylbromocarbene, generated by diazirine photolysis, is apparently more selective than the corresponding carbenoid from benzal bromide and potassium -butoxide. The electrophilic behavior of the carbenoid may be obscured by steric factors in the reaction with highly substituted olefins, as discussed above. In principle, however, complexation of a singlet carbene may increase or decrease its electrophilicity, depending on whether the com-plexing agent supplies electrons to the vacant p orbital (Lewis base) or withdraws electrons from the occupied sp orbital (Lewis acid). More data on this problem is clearly warranted. 

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