Combined Experimental and Computational Investigations of Rhodium- and Ruthenium-Catalyzed C-H Functionalization of Pyrazoles with Alkynes
Combined Experimental and Computational Investigations of Rhodium- and Ruthenium-Catalyzed C-H Functionalization of Pyrazoles with Alkynes
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DOI：
10.1021/jo402592z
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发表时间：
2014-03-07
影响因子：
3.6
通讯作者：
Singh, Kuldip
中科院分区：
文献类型：
--
作者：
Algarra, Andres G.;Cross, Warren B.;Singh, Kuldip
Detailed experimental and computational studies are reported on the mechanism of the coupling of alkynes with 3-arylpyrazoles at [Rh(MeCN)(3)Cp*][PF6](2) and [RuCl2(p-cymene)](2) catalysts. Density functional theory (DFT) calculations indicate a mechanism involving sequential N-H and C-H bond activation, HOAc/alkyne exchange, migratory insertion, and C-N reductive coupling. For rhodium, C-H bond activation is a two-step process comprising kappa(2)-kappa(1) displacement of acetate to give an agostic intermediate which then undergoes C-H bond cleavage via proton transfer to acetate. For the reaction of 3-phenyl-5-methylpyrazole with 4-octyne k(H)/k(D) = 2.7 +/- 0.5 indicating that C-H bond cleavage is rate limiting in this case. However, H/D exchange studies, both with and without added alkyne, suggest that the migratory insertion transition state is close in energy to that for C-H bond cleavage. In order to model this result correctly, the DFT calculations must employ the full experimental system and include a treatment of dispersion effects. A significantly higher overall barrier to catalysis is computed at {Ru(p-cymene)} for which the rate-limiting process remains C-H activation. However, this is now a one-step process corresponding to the,kappa(2)-kappa(1) K1 displacement of acetate consistent with the lack of a significant experimental isotope effect (k(H)/k(D) = 1.1 +/- 0.2). and so is still
