Carbenes, coupling diaryl

Calculations predict that the lowest state of PN has an open-shell electronic configuration." " The Salem-Rowland Rule for ISC promoted by spin-orbit coupling (SOC) predicts that singlet to triplet relaxation will have its maximum rate when the singlet state is closed-shell. This is the case with diaryl carbenes where the absolute rate constants of ISC are in the order of Michl has recently pointed out the importance of donor-... 

To conclude the details on zero-valent nickel carboxylation catalysts, some recent synthetic approaches worthy of note showed that this area of research still has a rich chemistry. For example, Louie and coworkers reported on the use of N-heterocyclic carbenes (diaryl-imidazolylidene) as new efficient ligands in the Ni-catalyzed coupling of various symmetrical di-ynes with C02 (Scheme 5.19) [60a]. 

The N-heterocychc carbene-paUa(iitim(II)-l-methyhmidazole [NHC-Pd(II)-Im] catalyst system utilized in this procedure required potassium i-butoxide as base and the inclusion of 0.5 equiv. of water for optimal activity and produced higher yields when electron-rich aryl hahdes were introduced as coupling partners. Touzani, Doucet, and coworkers estabhshed that 2,5-diarylation of 1-substituted imidazoles by aryl bromides (160 X = CH) was promoted by the use of 2 mol% Pd(OAc)2 in the presence of the base CsOAc (2014TET8316). Optimal yields were observed with 1-methyhmidazole (159) while l- -butyhmidazole, l-aryHmidazole, and 1 -benzylimidazole all displayed reduced yields. One example of diheteroar-ylation using 3-bromopyridine (160 X = N, R = H) afforded the coupled product 161 (X = N, R = H) in 72% yield. 

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