Using frustrated Lewis pairs to explore C-F bond activation.

Our interest in C-F bond activation prompted an investigation of the reactions of PhC(O)CF3 with a superbasic proazaphosphatrane (Verkade's base) and a corresponding FLP. The reaction of PhC(O)CF3 with P(MeNCH2CH2)3N in a 2 : 1 ratio generates [FP(NMeCH2CH2)3N][PhC(O)CF2COCF3Ph]. While this salt was not isolable, the anion could be derivatized to allow the isolation of [PhC(O)CF2C(OSiMe2tBu)CF3Ph], 1. To further probe this C-F bond activation, the FLP P(MeNCH2CH2)3N/BPh3 was allowed to react with PhC(O)CF3, which afforded the salt [PhC(CF2)OP(MeNCH2CH2)3N][FBPh3], 2. Insights into the mechanism of the reaction of the proazaphosphatrane with PhC(O)CF3 have emerged from the reactivity of 2 and these have been augmented by DFT computations. Salt 2 could be readily converted to the triflate derivative 3, which was a viable synthon for further C-F bond reactivity. Sequential reaction with Na[HBEt3] afforded the salts [PhC(CF(H))OP(MeNCH2CH2)3N][O3SCF3], 4 and [PhC(CH2)OP(MeNCH2CH2)3N][O3SCF3], 5. Interestingly, the formation of 5 contrasts with the formation of [HP(MeNCH2CH2)3N][PhC(CH2)OB(C6F5)3], 6 observed following the reaction of the FLP P(MeNCH2CH2)3N/B(C6F5)3 with PhC(O)Me. Thus these differing FLP protocols provide avenues to salts containing either an enolate derived cation or anion.

[1]  D. Stephan,et al.  New Directions for Frustrated Lewis Pair Chemistry , 2019, Trends in Chemistry.

[2]  T. C. Johnstone,et al.  Frustrated Lewis Pair Oxidation Permits Synthesis of a Fluoroazaphosphatrane, [FP(MeNCH2CH2)3N]. , 2018, Inorganic chemistry.

[3]  Richa Gupta,et al.  Selective Monodefluorination and Wittig Functionalization of gem-Difluoromethyl Groups to Generate Monofluoroalkenes. , 2018, Journal of the American Chemical Society.

[4]  T. C. Johnstone,et al.  Accessing Frustrated Lewis Pair Chemistry from a Spectroscopically Stable and Classical Lewis Acid-Base Adduct. , 2018, Angewandte Chemie.

[5]  S. Kass,et al.  Asymmetric Arylation of 2,2,2-Trifluoroacetophenones Catalyzed by Chiral Electrostatically-Enhanced Phosphoric Acids. , 2018, Organic letters.

[6]  S. Grimme,et al.  C-F Bond Activation by Silylium Cation/Phosphine Frustrated Lewis Pairs: Mono-Hydrodefluorination of PhCF3 , PhCF2 H and Ph2 CF2. , 2017, Chemistry.

[7]  Frédéric-Georges Fontaine,et al.  Metal-Free Borylation of Heteroarenes Using Ambiphilic Aminoboranes: On the Importance of Sterics in Frustrated Lewis Pair C-H Bond Activation. , 2017, Journal of the American Chemical Society.

[8]  H. Eshghi,et al.  Understanding the mechanism, thermodynamic and kinetic features of the Kukhtin–Ramirez reaction in carbamate synthesis from carbon dioxide , 2017 .

[9]  D. Stephan The broadening reach of frustrated Lewis pair chemistry , 2016, Science.

[10]  Etienne Rochette,et al.  Bench-stable frustrated Lewis pair chemistry: fluoroborate salts as precatalysts for the C-H borylation of heteroarenes. , 2016, Chemical communications.

[11]  Etienne Rochette,et al.  Metal-free catalytic C-H bond activation and borylation of heteroarenes , 2015, Science.

[12]  G. Erker,et al.  Frustrated Lewis pair chemistry: development and perspectives. , 2015, Angewandte Chemie.

[13]  D. Stephan Frustrated Lewis pairs: from concept to catalysis. , 2015, Accounts of chemical research.

[14]  D. Stephan,et al.  Activation of Alkyl C–F Bonds by B(C6F5)3: Stoichiometric and Catalytic Transformations , 2012 .

[15]  Scott J. Miller,et al.  Divergent Reactivity in Amine- and Phosphine-Catalyzed C–C Bond-Forming Reactions of Allenoates with 2,2,2-Trifluoroacetophenones , 2011 .

[16]  Stefan Grimme,et al.  Effect of the damping function in dispersion corrected density functional theory , 2011, J. Comput. Chem..

[17]  Douglas W Stephan,et al.  Frustrated Lewis pairs: metal-free hydrogen activation and more. , 2010, Angewandte Chemie.

[18]  R. Schmutzler,et al.  Über die Reaktionen sterisch aufwendig substituierter Phosphine am Beispiel des 1‐Adamantylphosphins , 1994 .

[19]  J. Verkade,et al.  An improved synthesis of the strong base P(MeNCH2CH2)3N , 1993 .

[20]  P. Vierling,et al.  Relevance of the acid-base phosphorane/phosphoranide equilibrium to the tautomeric phosphorane/phosphine equilibrium in the action of an activated ketone on tetracyclic tetraaminophosphoranes , 1988 .

[21]  H. Brown,et al.  Organoboranes. 44. A convenient, highly efficient synthesis of triorganylboranes via a modified organometallic route , 1986 .

[22]  C. P. Smith,et al.  Reaction of tertiary phosphines with hexafluoroacetone and with o-quinones. Attack by phosphorus on carbonyl oxygen and isolation of 2,2,2-trialkyl-2,2-dihydro-1,3,2-dioxaphospholanes , 1968 .

[23]  C. P. Smith,et al.  Reactions of five-membered cyclic triaminophosphines with hexafluoroacetone, trifluoroacetophenone, and fluorenone. Attack by phosphorus on carbonyl oxygen and isolation of crystalline 2,2,2-triamino-1,3,2-dioxaphospholanes , 1967 .

[24]  A. Speziale,et al.  Reactions of Phosphorus Compounds. X. The Reactions of α-Halo Ketones and Nitriles with Tertiary Phosphines and Phosphites , 1965 .

[25]  D. Stephan Frustrated Lewis Pairs. , 2015, Journal of the American Chemical Society.

[26]  I. Borowitz,et al.  Organophosphorus chemistry. XI. Mechanism and chirality of enol and ketophosphonium salt formation from the reactions of .alpha.-haloketones or .alpha.,.alpha.-dihaloketones with tertiary phosphines , 1971 .

[27]  V. Mark Nucleophilic reactions of trivalent phosphorus compounds. Synthesis of stable difluoromethylenephosphorane analogs. , 1964 .