Carbenes, from diazo compounds

Durr, H. Triplet-Intermediates from Diazo-Compounds (Carbenes). 55, 87-135 (1975). 

Triplet-Intermediates from Diazo-Compounds (Carbenes)... 

Throughout this chapter, we have almost always ignored the role of the carbene precursor. Carbenes are generally made from diazo compounds, or from a variety of surrogate diazo compounds including diazirines, tosylhydrazone salts, and aziridyl imines, all of which probably decompose through nonisolable diazo compounds. Not surprisingly, it turns out that diazo compounds have a rich chemistry of their own, especially when irradiated. Moreover, that chemistry often closely resembles the reactions of carbenes. Much of intramolecular carbene chemistry is, in fact, diazo compound chemistry. 

Complexes were also considered for other carbenes and even suggested for reactions in alkane solution( ) A counterproposal, backed by the inability of theory to find support for such stable complexes, held that the second source was not the carbene-alkene complex but instead was the diazo compound, formed from isomerization of excited diazirine. " Other LFP studies reinforced the need for two intermediates, but could not finally resolve the question of carbene complexes ver-sus diazo compound. However, the question is now settled in this case,... 

Like electrophilic addition to diazo compounds [7] from which diazonium ions and, subsequently, carbocations are generated, transition-metal compounds that can act as Lewis acids are potentially effective catalysts for metal carbene transformations. These compounds possess an open coordination site that allows the formation of a diazo carbon-metal bond with a diazo compound and, after loss of dinitrogen, affords a metal carbene (Scheme 5.2). 

Diirr, H., Triplet-intermediates from diazo-compounds (carbenes), 1975, volume 55, 85-135, Springer, Berlin. 

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