Diphenylcarbene, from decomposition

Japanese chemists2 have also reported cases in which the addition of the copper chelate modifies the reactions of carbenes. Thus diphenylcarbene (thermal decomposition of diphenyldiazomethane, 1) is converted mainly into benzophenone azine (2) when generated in an aprotic medium and into 1,1,2,2-tetraphenylethane (3) when generated in a protic medium. The second reaction is considered to proceed by abstraction of hydrogen from the solvent to form benzhydryl radicals which then... 

The electrophilic carbene carbon atom of Fischer carbene complexes is usually stabilised through 7i-donation of an alkoxy or amino substituent. This type of electronic stabilisation renders carbene complexes thermostable nevertheless, they have to be stored and handled under inert gas in order to avoid oxidative decomposition. In a typical benzannulation protocol, the carbene complex is reacted with a 10% excess of the alkyne at a temperature between 45 and 60 °C in an ethereal solvent. On the other hand, the non-stabilised and highly electrophilic diphenylcarbene pentacarbonylchromium complex needs to be stored and handled at temperatures below -20 °C, which allows one to carry out benzannulation reactions at room temperature [34]. Recently, the first syntheses of tricyclic carbene complexes derived from diazo precursors have been performed and applied to benzannulation [35a,b]. The reaction of the non-planar dibenzocycloheptenylidene complex 28 with 1-hexyne afforded the Cr(CO)3-coordinated tetracyclic benzannulation product 29 in a completely regio- and diastereoselective way [35c] (Scheme 18). 

In the absence of acid, diphenyldiazomethane, or DDM as it is conveniently abbreviated, decomposes unimolecularly with the loss of nitrogen, either on heating or on irradiation. In alcoholic solutions in the presence of oxygen the products of both thermal and photochemical decompositions have heen well established as the benzhydryl ether of the solvent, diphenylketazine, tetraphenylethane and benzophenone (Bethell et al., 1965 Kirmse, 1963 Bartlett and Traylor, 1962). All of these products can be visualized as coming from a diphenylcarbene intermediate. 

If phenylsodium is used in the reaction along with phenyllithium, the metallated ylid would be less stable, and the products would be those expected from the reaction or decomposition of the free ylid and not of its lithio derivative. When this reaction was carried out by Tschesche 148> in the presence of an excess of cyclohexene, the benzhydryl ylid was rapidly formed as indicated by the red color of the solution. However, the color was quickly discharged to give 28% 32, 27% 31, 12% 29, 32% 30. To account for these results, it was assumed 148> that the benzhydryl ylid is initially formed in 70% yield whereas the methyl ylid is formed in a 30% yield. Half of the benzhydryl ylid decomposes to give diphenylcarbene whereas the other half undergoes a Stevens rearrangement to give 31. 

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