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Combined experimental and computational investigations of rhodium- and ruthenium-catalyzed C−H functionalization of pyrazoles with alkynes

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journal contribution
posted on 22.10.2014, 10:32 by Andrés G. Algarra, Warren B. Cross, David L. Davies, Qudsia Khamker, Stuart A. Macgregor, Claire L. McMullin, Kuldip Singh
Detailed experimental and computational studies are reported on the mechanism of the coupling of alkynes with 3-arylpyrazoles at [Rh(MeCN)[subscript 3]Cp*][PF[subscript 6]][subscript 2] and [RuCl[subscript 2](p-cymene)][subscript 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 κ²−κ¹ 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[subscript H]/k[subscript 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 κ²−κ¹ displacement of acetate and so is still consistent with the lack of a significant experimental isotope effect (k[subscript H]/k[subscript D] = 1.1 ± 0.2).

History

Citation

Journal of Organic Chemistry, 2014, 79 (5), pp. 1954-1970

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Chemistry

Version

AM (Accepted Manuscript)

Published in

Journal of Organic Chemistry

Publisher

American Chemical Society

issn

0022-3263

eissn

1520-6904

Copyright date

2014

Available date

24/02/2015

Publisher version

http://pubs.acs.org/doi/abs/10.1021/jo402592z

Notes

The file associated with this record is embargoed until 12 months after the date of publication. The final published version may be available through the links above.

Language

en