- 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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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. (Chemical Equation Presented).
- McKay, David,Riddlestone, Ian M.,Macgregor, Stuart A.,Mahon, Mary F.,Whittlesey, Michael K.
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p. 776 - 787
(2015/03/04)
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- FLUORINATIONS WITH COMPLEX METAL FLUORIDES. PART 6. FLUORINATION OF PYRIDINE AND RELATED COMPOUNDS WITH CAESIUM TETRAFLUOROCOBALTATE(III)
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Pyridine has been fluorinated over caesium tetrafluorocobaltate(III) (CsCo(III)F4) at 300-400 deg C to give a mixture of undecafluoro-N-methyl-pyrrolidine, bis(trifluoromethyl)amine, pentafluoropyridine and several polyfluoropyridines; the product composition depend to some extent on the geometry of the reactor.The fluorinations of pentafluoropyridine, piperidine and undecafluoropiperidine were also investigated.
- Plevey, Raymond G.,Rendell, Richard W.,Tatlow, John Colin
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p. 159 - 170
(2007/10/02)
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- HIGHLY FLUORINATED HETEROCYCLES PART XV . THE METAL CATALYSED DEFLUORINATIVE REARRANGEMENT OF POLYFLUORINATED PYRROLIDINES TO POLYFLUOROPYRIDINES
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Pyrolysis of a number of polyfluoro-1-alkyl pyrrolidines over iron gauze at 500-525o affords a small conversion to polyfluoropyridines.
- Coe, P. L.,Sleigh, J. H.
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p. 403 - 408
(2007/10/02)
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