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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

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journal contribution
posted on 17.12.2015, 07:14 by David McKay, Ian M. Riddlestone, Stuart A. Macgregor, Mary F. Mahon, Michael K. Whittlesey
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.