Frustrated Lewis Pairs DOI: 10.1002/ange.201002119 Exploring the Reactivity of Carbon(0)/Borane-Based Frustrated Lewis Pairs** Manuel Alcarazo,* Catherine Gomez, Sigrid Holle, and Richard Goddard Dedicated to Professor Rosario Fernµndez Since the unveiling of the concept of frustrated Lewis pairs (FLPs) by Stephan et al. , [1] the chemistry of these systems has flourished. Arguably their most attractive application has been the heterolytic activation of H 2 , [2] and the subsequent development of metal-free hydrogenation catalysis directly employing dihydrogen [3] rather than a surrogate. [4] Although several other bonds such as C ÀO, [5] C ÀH, [6] B ÀH, [7] S ÀS, [8] and C ÀC, [9] have also been cleaved by using FLPs, these systems largely rely on P- or N-based Lewis bases combined with a polyfluorinated borane. [10] The sole exceptions are the steri- cally crowded carbene 1,3-di-tert-butyl-1,3-imidazol-2-ylidene (ItBu) in combination with B(C 6 F 5 ) 3 , a pair that contains a C- derived base, [11] and the use of Al(C 6 F 5 ) 3 instead of a borane as Lewis acid. [12] Clearly, extension of the FLP concept to include currently unexplored partners is desirable, as it may lead to the discovery of a range of interesting new applica- tions. In our research to broaden the range of bases that can be used in FLP chemistry, we were inspired by the computational investigations of Tonner and Frenking on the nature of carbodiphosphoranes. [13, 14] They proposed that these com- pounds should be considered to comprise two phosphine ligands coordinated to a central zero-valent carbon atom that retains its four valence electrons. This view has been subsequently confirmed experimentally by the work of Bertrand et al., Fürstner et al., and others. [15] The available information suggests that C 0 compounds must be exceptionally good nucleophiles. In fact, the calcu- lated proton affinity for carbodiphosphoranes surpasses the values reported for amines, phosphines, and even N-hetero- cyclic carbenes. It can be envisaged therefore that, if sufficiently sterically hindered, C 0 compounds should be qualified to function as bases in the framework of FLP chemistry. Herein, in an attempt to address this issue, the pair hexaphenylcarbodiphosphorane (1)/B(C 6 F 5 ) 3 is studied and its reactivity towards several small molecules evaluated. [17] Initially, we carried out the reaction of 1 with B(C 6 F 5 ) 3 in toluene at room temperature. The NMR spectroscopic data of the obtained product suggested formation of 2 by nucleophilic attack at the para position of the pentafluorophenyl ring and trapping of the generated fluoride anion by the boron atom. X-ray crystallographic analysis later confirmed the structure of 2 (see Supporting Information). [18] However, when the same reagents were mixed at À78 8C, NMR spectroscopy indicated no interaction between the partners, that is, “frustration”. Purging this stoichiometric mixture with H 2 resulted in formation of a white precipitate which could be isolated in 91 % yield. The NMR data support the formulation for this product as [(Ph 3 P) 2 CH][HB(C 6 F 5 ) 3 ](3 ; Scheme 1). The cation exhibits a 1 H signal at d = 1.73 ppm with 2 J( 1 H, 31 P) of 5.4 Hz and a 31 P{ 1 H} resonance at d = 21.3 ppm, while the anion gives the expected 11 B signal of a borohydride at d = À25.5 ppm with a 1 J( 1 H, 11 B) of 100 Hz. Single crystals were obtained by slow diffusion of pentane into a solution of 3 in CH 2 Cl 2 , and X-ray structure analysis confirmed not only the proposed structure (Figure 1), but also the ability of 1/ B(C 6 F 5 ) 3 to function as an FLP. As expected for an FLP, 1/B(C 6 F 5 ) 3 in solution in THF resulted in ring opening of the ether to produce phosphonio borate 4, also confirmed by X-ray analysis (see Supporting Information). This reactivity was extended to ethylene carbonate and the non-enolizable ester 3,3-dimethyl-g-butyr- olactone to produce zwitterionic species 5 and 6, respectively. Moreover, addition of one equivalent of 1-fluoropentane to a suspension of 1/B(C 6 F 5 ) 3 at À78 8C generates [(Ph 3 P) 2 C- Scheme 1. Some reactions of FLP 1/B(C 6 F 5 ) 3 . Reagents and conditions (yields): a) toluene, À78 8C !RT (74 %); b) H 2 1 atm, toluene, À78 8C !RT (91%); c) THF, À78 8C !RT (76%); d) nC 5 H 11 F, toluene, À78 8C, quant.; e) ethylene carbonate, toluene, À78 8C !RT (84%); f) 2,2-dimethyl-g-butyrolactone, toluene, 78 8C !RT (71 %). [*] Dr. M. Alcarazo,Dr. C. Gomez, S. Holle, Dr. R. Goddard Max Planck Institut für Kohlenforschung 45470 Mülheim and der Ruhr (Germany) Fax: (+ 49) 208-306-2994 E-mail: alcarazo@mpi-muelheim.mpg.de [**] We thank Prof. A. Fürstner for generous support and constant encouragement. The NMR spectroscopy and X-ray crystallography departments of our institute are also gratefully acknowledged for excellent support, as well as H. Bruns and R. Schinzel for the preparation of starting materials. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201002119. Zuschriften 5924  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2010, 122, 5924 –5927