Hydrodefluorination

Hydrodefluorination (HDF) is a type of organic reaction in which in a substrate of a carbon–fluorine bond is replaced by a carbon–hydrogen bond.[1][2] The topic is of some interest to scientific research. In one general strategy for the synthesis of fluorinated compounds with a specific substitution pattern, the substrate is a cheaply available perfluorinated hydrocarbon. An example is the conversion of hexafluorobenzene (C6F6) to pentafluorobenzene (C6F5H) by certain zirconocene hydrido complexes. In this type of reaction the thermodynamic driving force is the formation of a metal-fluorine bond that can offset the cleavage of the very stable C-F bond. Other substrates that have been investigated are fluorinated alkenes. Another reaction type is oxidative addition of a metal into a C-F bond [3] followed by a reductive elimination step in presence of a hydrogen source. For example, perfluorinated pyridine reacts with bis(cyclooctadiene)nickel(0) and triethylphosphine to the oxidative addition product and then with HCl to the ortho-hydrodefluorinated product. In reductive hydrodefluorination the fluorocarbon is reduced in a series of single electron transfer steps through the radical anion, the radical and the anion with ultimate loss of a fluorine anion. An example is the conversion of pentafluorobenzoic acid to 3,4,5-tetrafluorobenzoic acid in a reaction of zinc dust in aqueous ammonia.

Specific systems that have been reported for fluoroalkyl group HDF are triethylsilane / carborane acid,[4][5] and NiCl2(PCy3)2 / (LiAl(O-t-Bu)3H) [6]

References
  1. Kuehnel, M. F., Lentz, D. and Braun, T. (2013), Synthesis of Fluorinated Building Blocks by Transition-Metal-Mediated Hydrodefluorination Reactions. Angew. Chem. Int. Ed., 52: 3328–3348. doi:10.1002/anie.201205260
  2. C−F and C−H Bond Activation of Fluorobenzenes and Fluoropyridines at Transition Metal Centers: How Fluorine Tips the Scales Eric Clot, Odile Eisenstein, Naseralla Jasim, Stuart A. Macgregor, John E. McGrady, and Robin N. Perutz Accounts of Chemical Research 2011 44 (5), 333-348 doi:10.1021/ar100136x
  3. Activation of Carbon-Fluorine Bonds by Metal Complexes Jaqueline L. Kiplinger, Thomas G. Richmond, and Carolyn E. Osterberg Chemical Reviews 1994 94 (2), 373-431 doi:10.1021/cr00026a005
  4. Meier, G. and Braun, T. (2009), Catalytic C[BOND]F Activation and Hydrodefluorination of Fluoroalkyl Groups. Angew. Chem. Int. Ed., 48: 1546–1548. doi:10.1002/anie.200805237
  5. Hydrodefluorination and Other Hydrodehalogenation of Aliphatic Carbon−Halogen Bonds Using Silylium Catalysis Christos Douvris, C. M. Nagaraja, Chun-Hsing Chen, Bruce M. Foxman, and Oleg V. Ozerov Journal of the American Chemical Society 2010 132 (13), 4946-4953 doi:10.1021/ja100605m
  6. Juan Xiao, Jingjing Wu, Wenwen Zhao, Song Cao and NiCl2(PCy3)2-catalyzed hydrodefluorination of fluoroarenes with LiAl(O-t-Bu)3H, Journal of Fluorine Chemistry, Volume 146, February 2013, Pages 76-79, ISSN 0022-1139, doi:10.1016/j.jfluchem.2012.12.002.
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