Carbenes, coupling direct insertion

Xanthylidene does not react measurably with cyclohexane at room temperature (Table 5). Thermolysis of DAX at high temperature does, however, give some of the expected coupling product 9-cyclohexylxanthene. The crossover experiment (1 1 C6H12, C6D12) reveals that this product is not formed by the abstraction-recombination sequence. This observation is consistent with the direct insertion characteristic of a singlet carbene. 

The established activity of ethereal a-C-H bonds toward carbene and nitrene insertion has evoked new applications for sulfamate oxidation [76-78] In principle, a C-H center to which an alkoxy group is attached should be a preferred site for amination irrespec-hve of the addihonal functionality on the sulfamate ester backbone (Scheme 17.20). Such a group can thus be used to control the regiochemistry of product formation. The N,0-acetal products generated are iminium ion surrogates, which may be coupled to nucleophiles under Lewis acid-promoted conditions [79]. This strategy makes available substituted oxathiazinanes that are otherwise difficult to prepare in acceptable yields through direct C-H amination methods [80]. 

A(-Iminopyridinium yhdes (62) undergo direct C-H bond alkylation by cross-coupling with A(-tosylhydrazones, using unhganded copper(I) iodide and hthium t-butoxide. DFT calculations suggest a Cu carbene migratory insertion. Direct Cu carbene C-H insertion was ruled out by a diphenyldiazomethane control reaction which only gave (63) if the requisite base was present (the direct carbene process does not need base). 

C(OMe)C6H4-o-C=CPh (CO)j leads directly to the formation of a chrysene derivative via the formal dimerization of the carbene ligand. A plausible explanation for the formation of the final product involves a doubly alkyne-bridged dinuclear complex, alkyne insertions into metal-carbene bonds, and coupling of the carbene carbons. 

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