Metal‐free Catalytic Olefin Hydrogenation: Low‐Temperature H₂ Activation by Frustrated Lewis Pairs

Abstract

The hydrogenation of double bonds is one of the most fundamental transformations [1] in organic chemistry, and has numerous applications in the commodity chemical, agrochemical, pharmaceutical, polymer, and food industries. [2] Despite significant advances in the last 100 years, efforts to improve metal-based technologies for hydrogenation are still the focus of current research. [3] In parallel to these continuing efforts, metal-free strategies for effecting reductions have also been pursued. While organic reagents such as Hantschs esters [4] and silanes [5] have been used as stoichiometric reducing agents, it was not until 2006 [6] that the first metalfree systems, the so-called frustrated Lewis pairs (FLPs), [7] were shown to reversibly activate dihydrogen. This discovery allowed the development of FLP-based catalysts for the reduction of polar unsaturated bonds such as imines, [8] nitriles, [8a,c] aziridines, [8a,c] enamines, [8b] silylenolethers, [9] and aromatic reductions of anilines.[10] Herein, we report the discovery of FLP systems which, while appearing unreactive at room temperature, in fact are capable of dihydrogen activation at temperatures as low as À80 8C. This finding was then exploited for the catalytic hydrogenation of olefins at temperatures between 25 and 70 8C. Experimental and computational data support a plausible mechanism involving protonation of the olefin with subsequent hydride transfer.These FLPs represent the first metal-free hydrogenation catalysts for the reduction of olefins bearing carbocationstabilizing moieties.It is well known that the reactions of olefins with Brønsted acids in the presence of a nucleophilic halide, leads to addition products according to a protonation/addition mechanism. In considering the potential of such a mechanism for FLP hydrogenation of C=C double bonds, it was recognized that while the generated borohydride would act as the nucleophile, this pathway would require the generation of a countercation which was sufficiently acidic to effect protonation of the olefin. While the majority of FLP activations of dihydrogen have been demonstrated for phosphine/borane combinations, [7b] a variety of other donors including amines, [8a, 11] pyridines, [12] carbenes, [13] and phosphinimines [14] have been shown to be effective when paired with boron or aluminum Lewis acids. However, in all of these cases, the generated cations are only weak Brønsted acids and thus are incapable of protonation of olefinic double bonds.Seeking to enhance the Brønsted acidity of the cation generated by the FLP activation of dihydrogen, we initiated investigations employing (C 6 F 5 ) 3 B (1) in combination with the weakly basic phosphine (C 6 F 5 )Ph 2 P (2). An NMR spectroscopic examination of a 1:1 mixture of 1 and 2 at 25 8C resulted in spectra that did not differ from those of the individual components. Exposure of this FLP to hydrogen (5 bar) did not lead to significant changes in the NMR spectra at room temperature. However, the situation altered when the temperatu...

Scope and Mechanisms of Frustrated Lewis Pair Catalyzed Hydrogenation Reactions of Electron‐Deficient CC Double Bonds

“…[2] Grimme,P aradies,a nd co-workers demonstrated that hydrogenation reactions of EDG-substituted C=Cd ouble bonds proceed through initial protonation, and that substrates with nucleophilicity parameters 1 < N < 5o n the Mayr reactivity scale [3] can be hydrogenated by the use of ac ombination of tris(1-naphthyl)phosphane (C 10 H 7 ) 3 Pa nd B(C 6 F 5 ) 3 as the catalyst. [4] On the other hand, hydrogenation reactions of nitroalkenes and acrylates were not catalyzed by FLPs derived from B(C 6 F 5 ) 3 (1a), but proceeded smoothly when aL ewis pair derived from the less acidic borane B(2,6-F 2 C 6 H 3 ) 3 (1c) was employed (Scheme 2). [5] Analogously,A lcarazo and coworkers explored the catalytic hydrogenation of electronpoor alkenes with FLPs derived from differently fluorinated triaryl boranes and 1,4-diazabicyclo[2.2.2]octane (DABCO) and found that only B(2,4,6-F 3 C 6 H 2 ) 3 (1b)and B(2,6-F 2 C 6 H 3 ) 3 (1c)s howed catalytic activity.…”

Experimental Insights into the Structure and Reactivity of Frustrated Lewis Pairs

“…[55] Paradies and Grimme reported that combinations of 27, BCF, and H 2 in methylene chloride had the same NMR fingerprints of the constituents at room temperature, suggesting, at first blush, the inactivity of the system. When these mixtures were cooled to À80 8C, the formation of phosphonium borate salt [(C 6 F 5 )Ph 2 PH] [HB(C 6 F 5 ) 3 ] was observed, while raising the temperature, the salt readily releases H 2 to restore the initial mixture.…”

B(C6F5)3‐katalysierter Diwasserstofftransfer von einem ungesättigten Kohlenwasserstoff auf einen anderen

“…[3] Diese Systeme sind auf Reduktionen, d. h. übergangsmetall-freie Hydrierungen, einer Vielzahl an Substraten anwendbar. [4][5][6][7][8][9] Wirh atten jüngst gezeigt, dass dasselbe Gesamtergebnis durch Einsatz von Cyclohexa-1,4-dienen als Diwasserstoffsurrogate erreicht wird (Schema 1, oben). [10] B(C 6 F 5 ) 3 ist in der Lage,e in Hydrid von geeignet substituierten Cyclohexa-1,4-dienen zu abstrahieren.…”

“…[14,15] Die Gegenwart von Carbeniumionen in verschiedenen Schritten dieses Prozesses lçst jedoch hçchst-wahrscheinlich kationische Oligomerisierungen beider Reaktionspartner aus.E sm uss erwähnt werden, dass Stephan und Mitarbeiter vor einiger Zeit über ein Beispiel einer B-(C 6 F 5 ) 3 -katalysierten Alkenhydrierung berichteten, in der Et 2 Oa ls Lewis-basische Komponente zugesetzt werden muss. [8,9,16,17] Dasselbe Labor arbeitete auch eine durch das Lewis-acide Phosphoniumion [(C 6 F 5 ) 3 PF] + katalysierte Tr ansferhydrierung von Alkenen aus,inder das Hydrid sowie das Proton in einer vorgelagerten dehydrierenden Si-O-oder Si-N-Kupplung generiert werden. [18] Wirs tellen hier eine Tr ansferhydrierung von 1,1-disubstituierten Alkenen vor,die nur mit B(C 6 F 5 ) 3 katalysiert wird.…”

“…[18] Wirs tellen hier eine Tr ansferhydrierung von 1,1-disubstituierten Alkenen vor,die nur mit B(C 6 F 5 ) 3 katalysiert wird. [8,9] Durch Einsatz eines 1,3,5-trisubstituierten Cyclohexa-1,4-diens als Diwasserstoffquelle werden Nebenreaktionen der gebildeten Carbeniumionen unterbunden.…”