Carbene complex chromium copper

Narasaka and Sakurai found that chromium carbene complexes, when exposed to a copper(II) reagent, generate acyl radicals by a one-electron oxidation, and these then undergo addition to electron-deficient alkenes (Scheme 4-27) [50J. The resulting copper(I) species reduces the resulting radical to an anion, and subsequent protonation leads to the addition product. This redox type acyl radical transfer reaction works particularly well for aromatic acyl radical systems, for which decarbonylation is not a problem. Related work has also recently appeared [51]. 

Finally, when chromium (-)-menthyloxycarbene 108 was exposed to 50 mol-% [Cu(MeCN)4][PFg], a copper bis-carbene complex was formed along with acetonitrile(pentacarbonyl)chromium. Using dichloromethane and diethyl ether as solvents, a colour change occurred and crystals of copper(I) carbene complex 109 were deposited (Scheme 47). [98] So far, there has been only a single report on the synthesis of a stable copper(I) carbene complex. [99]... 

Under the eonditions of erystallization of eopper carbene complex 109, the carbene ligand tends to be retransferred to chromium while the vacant coordination site at copper is occupied by diethyl ether. [98] The X-ray structure of 109 provides a rare example of a tricoordinated metal center. Both copper containing species, the bis-carbene and the mono-carbene 109, undergo a carbene dimerization in the presence of tributylphosphine to yield (l , 3 , 5 )-l,3,5-trienes in a process analogous to the formation of 107. [98]... 

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