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Mechanistic study of Ru-NHC-catalyzed hydrodefluorination of fluoropyridines: The influence of the NHC on the regioselectivity of C-F activation and chemoselectivity of C-F versus C-H bond cleavage

McKay, D., Riddlestone, Ian M., Macgregor, S.A., Mahon, M.F. and Whittlesey, M.K. (2015) Mechanistic study of Ru-NHC-catalyzed hydrodefluorination of fluoropyridines: The influence of the NHC on the regioselectivity of C-F activation and chemoselectivity of C-F versus C-H bond cleavage ACS Catalysis, 5 (2). pp. 776-787.

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Abstract

We describe a combined experimental and computational study into the scope, regioselectivity, and mechanism of the catalytic hydrodefluorination (HDF) of fluoropyridines, C5F5–xHxN (x = 0–2), at two Ru(NHC)(PPh3)2(CO)H2 catalysts (NHC = IPr, 1, and IMes, 2). The regioselectivity and extent of HDF is significantly dependent on the nature of the NHC: with 1 HDF of C5F5N is favored at the ortho-position and gives 2,3,4,5-C5F4HN as the major product. This reacts on to 3,4,5-C5F3H2N and 2,3,5-C5F3H2N, and the latter can also undergo further HDF to 3,5-C5F2H3N and 2,5-C5F2H3N. para-HDF of C5F5N is also seen and gives 2,3,5,6-C5F4HN as a minor product, which is then inert to further reaction. In contrast, with 2, para-HDF of C5F5N is preferred, and moreover, the 2,3,5,6-C5F4HN regioisomer undergoes C–H bond activation to form the catalytically inactive 16e Ru-fluoropyridyl complex Ru(IMes)(PPh3)(CO)(4-C5F4N)H, 3. Density functional theory calculations rationalize the different regioselectivity of HDF of C5F5N at 1 and 2 in terms of a change in the pathway that is operating with these two catalysts. With 1, a stepwise mechanism is favored in which a N → Ru σ-interaction stabilizes the key C–F bond cleavage along the ortho-HDF pathway. With 2, a concerted pathway favoring para-HDF is more accessible. The calculations show the barriers increase for the subsequent HDF of the lower fluorinated substrates, and they also correctly identify the most reactive C–F bonds. A mechanism for the formation of 3 is also defined, but the competition between C–H bond activation and HDF of 2,3,5,6-C5F4HN at 2 (which favors C–H activation experimentally) is not reproduced. In general, the calculations appear to overestimate the HDF reactivity of 2,3,5,6-C5F4HN at both catalysts 1 and 2.

Item Type: Article
Divisions : Faculty of Engineering and Physical Sciences > Chemistry
Authors :
NameEmailORCID
McKay, D.
Riddlestone, Ian M.i.riddlestone@surrey.ac.uk
Macgregor, S.A.
Mahon, M.F.
Whittlesey, M.K.
Date : 6 February 2015
Funders : Engineering and Physical Sciences Research Council (EPSRC)
DOI : 10.1021/cs501644r
Copyright Disclaimer : Copyright © 2014 American Chemical Society. This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.
Uncontrolled Keywords : Catalysis; DFT; Hydrodefluorination; Mechanism; N-heterocyclic carbenes; Pentafluoropyridine; Ruthenium
Depositing User : Clive Harris
Date Deposited : 24 Jun 2019 10:53
Last Modified : 24 Jun 2019 10:53
URI: http://epubs.surrey.ac.uk/id/eprint/852023

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