Chloro compounds diazirines

Halogen exchange of F is usually with chloro compounds however, replacement of bromine has enabled fluorodiazirines to be obtained (83JA6513 86TL419). Diazirine (2) was previously obtained by a difficult route involving F2. The relative ease of access to (2) enables a carbene whose reactivity is intermediate between that of electrophilic ( CF2) and nucleophilic ( C(OMe)2) carbenes to be studied. 

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). 

Diazirine was used as the carbene precursor in an experiment to make diazomethane from methylene and nitrogen in the gas phase. As shown by means of labeled nitrogen, there was 4% diazomethane formation. During the past 10 years thermal as well as photochemical decomposition of 3-chloro- and 3-bromodiazirine has been investigated in detail. Compounds 175 and 176 as well as analogous compounds are easily obtained by reaction of amidines with hypohalites according to Graham. ... 

Three-membered rings with two heteroatoms are usually encountered only as reagents. Diazirines are nsefnl carbene precnrsors" - they are generally more stable than the equivalent isomeric diazo compounds, thongh they are sometimes explosive in the pure state. They can be prepared by oxidation of diaziridines that, in tnm, are available via the condensation of a ketone or aldehyde with ammonia and chloramine. Chloro-diazirines, from the reaction of amidines with hypochlorite, wiU undergo Sn2 or Sn2 displacement reactions."" ... 

A kinetic study of the thermal decomposition of 3-chloro-3-methoxydiazirine showed that the reaction was unimolecular both in the gas phase and in solu-tion. A direct two-bond cleavage to carbene was suggested for this reaction, without the formation of a diazo-compound. The authors suggest, however, that the decomposition of diazirine involves both pathways as competitive processes in the general case. 

Full details have been given for the preparation of chloro(phenyl)carbene (3), from the corresponding diazirine, and for its subsequent trapping by /3-methylstyrene, leading to the cyclopropene (4) after dehydrochlorination. An excellent new catalyst for the synthesis of cyclopropanecarboxylates (5), from olefins and diazoesters, is the rhodium cluster compound, Rh6(CO)i6. In general, good yields of (5) are only obtained if a large excess of olefin is used, but other work... 

Due to the complication of alternate pathways involved in the photochemistry of diazo compounds or diazirines, new strategies for carbene generation from non-nitrogenous precursors have been studied. Phenyliodonium ylides" (27) andmethyl-8-chloro-3a,7a-methanoindan-8-carboxylates" (28) are alternative precursors to dicarboethoxycarbene and carbomethoxychlorocarbene, respectively. Photoreactions from these precursors presumably will not be plagued by the excited-state chemistry observed for the diazo compounds and diazirines. 

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