Hydrogenation by Frustrated Lewis Pairs: Main Group Alternatives to

Review pubs.acs.org/OPRD

Hydrogenation by Frustrated Lewis Pairs: Main Group Alternatives to Transition Metal Catalysts? Lindsay J. Hounjet and Douglas W. Stephan* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6 ABSTRACT: Since the discovery of “frustrated Lewis pairs” in 2006, these metal-free systems have been exploited to activate a variety of small molecules, with H2 being perhaps the most significant among them. This finding has since allowed for the development of metal-free strategies to hydrogenation catalysis. In this review, progress toward the development of these new catalysts for reductions of polar organic substrates, olefins, alkynes, and aromatic systems, is described.

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INTRODUCTION Hydrogenation reactions rank among the most important chemical processes employed for the production of commodity chemicals, finding widespread application within petrochemical, pharmaceutical, materials, and food industries. Today, hydrogenations of unsaturated organic compounds, including olefins, alkynes, arenes, imines, aldehydes, and ketones, are predominantly carried out using transition metal-based catalysts. Heterogeneous catalysts, in which a metal is deposited on a support, are most frequently employed. However, homogeneous precious metal catalysts derived from Rh, Ir, Pd, Pt, or Ru, provide greater selectivity, can be adapted for enantioselective syntheses, and are generally operative under milder conditions with higher turnovers. Thus, homogeneous catalysts are also employed in many catalytic hydrogenation processes on an industrial scale. For example, Wilkinson’s rhodium catalyst, [RhCl(PPh3)3], discovered in the 1960s, continues to find use on a large scale. Its high activity and stability, together with robust recovery technologies, make it the catalyst of choice for many commodity applications of hydrogenation (Scheme 1A).1 In the pharmaceutical realm, a variety of transition metal catalysts are also used. One example is the ruthenium-based technology

developed by Noyori et al. This catalyst enables highly effective enantioselective reductions of polar unsaturates, including aldehydes, ketones, and imines (Scheme 1B).2,3 A third example of a precious metal catalyst is Lindlar’s Pd catalyst (Scheme 1C),4 which is used for stereoselective partial hydrogenations of alkynes to cis-alkenes. While homogeneous precious metal catalysts are known for their high product selectivities, the high cost and low natural abundance of the constituent metals necessitated the development of protocols for the cost-efficient recovery from products. Catalyst removal is particularly important for those products intended for human consumption, such as pharmaceuticals and food ingredients, since metallic contaminants pose health hazards. In such cases, the catalyst and the engineering required to recover it, can have a significant impact on total production cost. There are other strategies to the reduction of unsaturated organic compounds. For example, hydride transfer from Hantzsch esters,5 Birch reduction of arenes with sodium in ammonia,6 and Meerwein−Ponndorf−Verley (MPV) transfer hydrogenation catalysis using a sacrificial alcohol in the presence of aluminum alkoxide7 are all useful reduction methods for academic purposes. However, in contrast to the use of hydrogen, these procedures are stoichiometric in a reducing agent which generates an equivalent of waste. Thus, such methods are generally more costly for production processes. As recently as 2005, the idea of an alternative reduction strategy in which a metal-free system activates H2 seemed at best fanciful, and at worst contrary to the almost 100 years of synthetic chemistry dogma. This situation changed in 2006 when a molecule that incorporated sterically encumbered phosphonium and hydridoborate sites, Mes2PH(C6F4)BH(C6F5)2, 1, was reported.8 Not unexpectedly, this species liberated hydrogen on heating. However, in a startling finding, the resulting phosphine−borane species was also shown to react with H2 to reform the phosphonium borohydride (Scheme 2). This concept has since proved to be much more general as it is not limited to the rather eclectic molecule used

Scheme 1. Transition metal-catalyzed hydrogenation

Received: November 4, 2013 Published: February 4, 2014 © 2014 American Chemical Society

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dx.doi.org/10.1021/op400315m | Org. Process Res. Dev. 2014, 18, 385−391

Organic Process Research & Development

Review

Scheme 2. Metal-free, reversible H2 activation

Scheme 3. FLP hydrogenation of polar substrates

in the original paper. Indeed, simple combinations of Lewis acids and bases, which are sterically frustrated from forming classical Lewis acid−base adducts, are capable of cleaving dihydrogen in a heterolytic fashion. For example, while combining B(C6F5)3 and tBu3P in solution results in no reaction at room temperature, addition of H2 rapidly affords the salt [tBu3PH][HB(C6F5)3].9 Such combinations of Lewis acids and bases have been coined “frustrated Lewis pairs” (FLPs) (Scheme 1).10 A relatively small range of acids and bases have been shown to generate FLPs, and their reactivity with a variety of small molecules have been examined in detail. Nonetheless, it is the original discovery of reversible H2 activation that prompted the development of metal-free hydrogenation catalysts that has drawn the most attention. While this development has not been commercialized, nor is it ready to be, FLP-catalyzed hydrogenation is a rapidly developing area of main group chemistry that may find important applications in the future. It is the advancement of these systems’ applicability toward hydrogenation catalysis that is the focus of this short review.

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POLAR ORGANIC SUBSTRATES The initial report describing metal-free hydrogenation catalysis discussed the reactivity of 1 with hydrogen to effect reductions of imines, nitriles, and aziridines.11,12 In these initial trials, 5 mol % of phosphonium-borate 1 was employed as the catalyst at 80−120 °C under 1−5 atm of H2. Subsequently, it was shown that imine substrates could themselves act as the basic components of FLPs. Thus, combining imine substrates with a catalytic amount of B(C6F5)3 2 under H2 (4 atm) resulted in hydrogenation of imine to the corresponding amine (Scheme 3A). The catalyst 2 was also applied to diimines, providing the corresponding diamines in a facile manner (Scheme 3B).13 Similarly, imines that represent precursors to herbicides, antidepressants, and anticancer candidates were readily reduced to these commercial products using the metal-free strategy (Scheme 3C−E).13 Since nitriles generally form strong stable adducts with boranes, they are not hydrogenated. However, using nitrile− borane adducts as the substrates in the presence of a catalytic quantity of 1 allowed hydrogenation to proceed slowly but ultimately in good yields, affording the corresponding amine− borane adduct (Scheme 3F). Interestingly, cis-1,2,3-triphenylaziridine was catalytically hydrogenated to the ring-opened amine in the presence of either 1 or 2/PMes3 (Scheme 3G).12 The mechanism of imine hydrogenation catalyzed by FLPs involves H2 heterolysis by a donor and boron acceptor. This process either directly generates an iminium hydridoborate intermediate or involves proton transfer from a transient phosphonium cation to N. The resulting iminium cation then

abstracts hydride from the hydridoborate anion, yielding the amine and regenerating the borane catalyst (Scheme 4).11,12 Scheme 4. Imine hydrogenation catalyzed by 2

Subsequent to these initial reports, Erker and co-workers14 developed an intramolecular FLP wherein P and B were linked by an ethylene chain, Mes2PCH2CH2B(C6F5)2 (3; Scheme 5). The proximity of Lewis acidic and basic centers, combined with steric congestion and ring strain, disfavors P−B bond formation, facilitating H2 activation to enable imine hydrogenation. This species was shown to be a highly effective catalyst for reductions of imines at 25 °C under 1.5 atm of H2, although higher catalyst loadings were required (10 mol %). Catalyst 3 was also shown to reduce enamines at 25 °C and 1.5 atm of H2, using as little as 3 mol % catalyst. 386



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The Soos group has developed a clever approach to increasing functional group tolerance based on “size exclusion.”18 These researchers utilized (C6H2Me3)B(C6F5)2 (7; Scheme 5) in combination with a nitrogen base such as CH(CH2CH2)3N or N(CH2CH2)3N, as a catalyst at 20 °C under 4 atm of H2 for reductions of less sterically encumbered imines. It is also interesting that these catalyst systems allow for reductions of p-(MeO)-m-(H 2 CCHCH 2 O)C 6 H 3 CH NtBu, MeCHCHCHNtBu, and even the enone CC bond within carvone, although the latter reduction was quite slow (6 days). The Erker group demonstrated the tolerance of FLP catalysts to organometallic scaffolds, showing that the zirconocene salt, [(C 5 H 4 CH 2 NH 2 (C 6 H 3 iPr 2 )) 2 ZrCl 2 ] [HB(C 6 F 5 ) 3 ] 2 (8) (Scheme 5)19 could hydrogenate imines and silyl enol ethers. In addition, FLP hydrogenations of metallocene-based substrates could be conducted using 3.20 Another exciting development has been pioneered by Klankermayer.21−24 His group has elegantly developed several chiral borane catalysts (9, 10; Scheme 5) that permit the asymmetric reduction of prochiral ketimines. Using the catalyst 10, enantioselectivities that are in excess of 80% ee could be achieved. Such an advance augurs well the future development of highly selective metal-free asymmetric hydrogenation catalysts. Alternatively, but in a related sense, we showed that optically pure ketimines could be reduced with high diastereoselectivity using 2.25 In a more recent effort, we sought catalysts that were not based on highly electrophilic boranes to avoid the use of C6F5 substituents. To this end, we prepared the NHC−borane adduct IiPr(9-BBN). This species is readily converted to the corresponding borenium salt 11 (Scheme 6),26 which activates

Scheme 5. FLP catalysts

Expanding upon the structural diversity of catalysts capable of effecting metal-free imine hydrogenation, the group of Repo15 developed the innovative FLP based on a piperidine moiety tethered to an electrophilic borane center, C5H6Me4NH(CH2C6H4)BH(C6F5)2 (4; Scheme 5). This socalled “molecular tweezer” is highly effective as a hydrogenation catalyst for reductions of imines and enamines using 4 mol % catalyst, at 110 °C and 2 atm of H2.15 It is also noteworthy that 4 reduced sterically unencumbered imine substrates, presumably because the steric congestion about boron precludes adduct formation. The scope of substrates reduced by FLP catalysts was expanded by the Erker group to include silyl enol ethers. This feat was initially accomplished using the FLP combination of bis-phosphine C10H6(PPh2)2 and B(C6F5)3, which under H2 affords the salt, [C10H6(PPh2)2H][HB(C6F5)3] 5 (Scheme 5).16 The FLP catalyzes hydrogenation of silyl enol ethers under 2 atm of H2 at 25 °C, although 20 mol % of 5 was employed. In a related manner, the bis-borane Lewis acid C10H6(B(C6F5)2)2 (6; Scheme 5)17 was employed by Berke and co-workers to reduce imines under 15 atm of H2 at 120 °C. Interestingly, the adjacent boron centers act in concert to provide a “super Lewis acidic activation pathway.” The tolerance of FLP systems to the presence of functional groups during hydrogenation catalysis was evaluated for 1 and 2.13 In these experiments, standard imine reductions were conducted in the presence of an added functionalized molecule. In this fashion, the FLP catalysts were shown to remain active in the presence of naphthalene, bulky ethers, n-hexyl acrylate, bulky amines, and alkyl or aryl halides. In contrast, the catalyst activity was dramatically reduced in the presence of sterically unprotected functionalities, including PhNMe2, tBuNH2, carbamate esters, ketones, or aldehydes. Interestingly, these catalysts were not active in the presence of 2,4,6-Me3C6H2OH, although they did tolerate bulkier 2,6-tBu2C6H3OH. Taken together, these results imply that first-generation FLP catalysts have functional group tolerance limited to either softer or sterically encumbered donors.

Scheme 6. Borenium ion-catalyzed enamine hydrogenation

H2 in combination with a proton acceptor, and operates as an effective hydrogenation catalyst for imines and enamines at room temperature under 100 bar of H2 pressure. The mechanism of action is thought to be directly analogous to that described for electrophilic boranes (Scheme 6). The Lewis acidity at B is imparted by the cationic charge on the complex rather than by the presence of electron-withdrawing substituents. In early efforts to evaluate the commercial potential of FLP catalysts, optimization of the conditions for imine reductions showed that substrates devoid of oxygen-containing contaminants could be reduced using as little as 0.1 mol % 1 at 130 °C and 120 atm of H2.13 In an alternative strategy to the use of ultrapure substrates, so-called “scrubbers” such as TiBAL or DiBAL could be exploited.27 In this case, addition of 5 mol % of these scrubbers to a solution of the substrate prior to addition 387



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(Scheme 8). Related reductions could be also performed on tetracene and tetraphene PAHs using triarylphosphine bases.

of the FLP catalyst enabled hydrogenation catalysis using commercial grade solvents, substrates and hydrogen gas without the need for rigorous exclusion of water or other impurities. This low-cost solution greatly enhanced the potential for metal-free catalysts as a commercially viable option in hydrogenation catalysis.

Scheme 8. FLP hydrogenation of PAHs

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OLEFINS, ALKYNES, AND POLYAROMATICS Efforts to expand the scope of substrates for FLP reductions to olefins took a dramatic turn in 2012. While reduction of an alkenyl-borane derived from carboboration was reported,28 it was subsequently discovered that combinations of 2 and Ph2P(C6F5) or other weakly basic phosphines or amines effected the activation of H2 at low temperature.29 The observation of [(C6F5)PHPh2][HB(C6F5)3] 12 (Scheme 7)

Concurrent with the discovery of FLP-catalyzed olefin hydrogenation was a report by Wang et al. that Piers’ borane, HB(C6F5)2, could catalyze these processes in the absence of an added base.34 In this case, the reaction mechanism is markedly different from the FLP-catalyzed processes, as it is based on a catalytic cycle of hydroboration and hydrogenolysis steps (Scheme 9). This mechanism is analogous to that reported

Scheme 7. Reactions related to FLP hydrogenation of olefins

Scheme 9. Olefin hydroboration/hydrogenolysis

at −80 °C demonstrates the formation of the highly acidic phosphonium cation. This cation was subsequently shown capable of protonating 1,1-disubstituted olefins. The resulting carbocation then abstracted hydride from the hydridoborate anion, effecting the reduction of the olefin substrate, although these reactions were sluggish. Paradies et al. trapped such a carbocationic intermediate using the FLP (2-F-C6H4)2PPh/1 and α-methylstyrene in the presence of H2.30 Further exploiting this advance, Paradies et al. realized an innovative approach to improving the substrate scope available for FLP-catalyzed olefin hydrogenation. Electron-deficient αnitroolefins and acrylates were hydrogenated by combinations of B(2,6-F 2 −C 6 H 3 ) 3 with lutidine or collidine. 31 The mechanism for hydrogenations of electron-deficient alkenes stands in contrast to reductions of electron-rich olefins, as initial hydride delivery from B to the β-position was followed by delivery of proton from N to the α-position. In a closely related finding, we showed that hydrogenation of 1,1-diphenylethylene can be accomplished by combinations of 1 with simple dialkyl ethers (Scheme 7).32 It is noteworthy that, although diethyl ether is known to form an adduct with 2, pressurizing a mixture of 1 and Et2O with 4 atm of HD showed complete isotope scrambling in 15 min. Such facile H2 activation proved effective for the catalytic hydrogenation of 1,1-diphenylethylene, whereas in the absence of ether only the Friedel−Crafts-type dimerization product is observed. The same FLP systems that are capable of catalyzing olefin hydrogenations also proved effective toward chemoselective partial reductions of polycyclic aromatic hydrocarbons (PAHs).33 For instance, combinations of 1 with Et2O or Ph2P(C6F5) were found capable of hydrogenating anthracene and its derivatives to the analogous 9,10-dihydroanthracenes

by DeWitt et al.35 in the 1960s using alkylborane catalysts35 and forcing conditions (140 °C, 6 bar of H2, 20 mol % catalyst loading, >72 h). Repo et al. have recently discovered an ingenious approach to the metal-free, stereoselective partial hydrogenations of alkynes to cis-alkenes using an amine-tethered borane catalyst.36 Until this discovery, such reactivity could only be carried out with metallic catalysts such as Lindlar’s. This newfound FLP methodology represents a viable alternative to this system for a variety of alkyl/aryl-disubstituted internal alkynes under mild conditions. Mechanistically, the linked tertiary amine-triarylborane FLP activates H2 to generate an ammonium-hydridoborate zwitterion. Protonolysis of a perfluoroaryl group generates an amine-hydridoborane, which effects hydroboration of alkyne substrate. Heterolysis of H2 by the resulting amine-alkenylborane produces a new ammonium-hydridoborate (Scheme 10). Subsequent protonolysis liberates the cis-alkene to regenerate the catalyst.

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N-HETEROCYCLES AND ANILINES In the presence of a catalytic amount of 2 the N-containing rings of quinoline and related N-heterocycles are easily reduced by H2.37 However, in 2012, it was discovered that heating 1:1 mixtures of sterically bulky anilines and B(C6F5)3 under H2 for 48 h led to reductions of the aromatic rings to give cyclohexylammonium salts (Scheme 11).38 These remarkable transformations were shown to be tolerant of a variety of substituents both at N and on the aromatic ring. Moreover, related aromatic imines could also be reduced, with both the imine and arene undergoing hydrogenation. Interestingly, an N388



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Scheme 12. Reductions of N-Heterocycles

Scheme 10. Metal-free alkyne hydrogenation

Scheme 11. Reduction of N-tert-butylaniline to the tertbutylcyclohexylammonium salt platinum group metals. In addition, current U.S. governmentregulated levels of acceptable residues are at least 100 times higher for main group elements than for many transition metals.41 While these prospects seem attractive in terms of reduced costs of the catalyst and process, FLP catalysts remain significantly slower than many of those based on transition metals. In addition, current FLP catalysts are effective for a limited range of substrates, do not tolerate impurities in the substrate, and lack broad functional group tolerance. Thus, while these systems present a significant new strategy for hydrogenation catalysis, these systems require much further development to achieve commercial viability. Nonetheless, in the 7 years since the original discovery of metal-free activation of hydrogen, main-group catalysts have demonstrated efficacy toward hydrogenations of imines, aziridines, enamines, N-heterocycles, anilines, olefins, alkynes, and PAHs, and to a lesser extent aldehydes and ketones. While in some cases, the conditions for catalysis have been optimized and strategies developed to deal with catalyst poisoning by impurities, the development of metal-free hydrogenation catalysis continues to warrant active exploration. Recent efforts have expanded Lewis acidic catalysts for hydrogenations beyond highly electrophilic boranes. While it is truly exciting to witness the evolution of new options for hydrogenation catalysis emerging from main group chemistry, this rapidly evolving field is currently targeting catalysts that offer improved activities, chemo-, regio-, or stereoselectivities. These pending developments will undoubtedly produce economically viable, metal-free hydrogenation technology that will evolve from laboratory research into the realm of commercial application.

aryl aziridine also underwent reductive ring-opening followed by reduction of the N-bound arene. It is noteworthy that, while these reactions require the stoichiometric combination of aniline and borane, they do in fact consume 4 equiv of H2. In an analogous fashion, the complete aromatic reduction has been extended to sterically encumbered pyridines and Nheterocycles.39 For example, lutidine, 2-phenylpyridine, and 2,6-diphenylpyridine were reduced to the saturated substituted piperidinium salts. Similarly, substituted quinolines were fully reduced to the saturated cyclic ammonium salts. A particularly interesting example is the hydrogenation of 7,8-benzoquinoline, where the “pyridine” and “aniline” rings are reduced while the remaining arene ring remains untouched (Scheme 12). It is interesting to speculate that such aromatic reductions present a synthetic approach that could be extended to specific natural products derived from N-heterocycles. Certainly, functionalization of aromatic systems by conventional crosscoupling methods is readily achieved. Subsequent aromatic reduction can be then envisioned as a route to the derivatized N-heterocyclic ammonium salts. While this avenue represents a potentially unique shortcut to such products, the presently employed achiral catalyst, B(C6F5)3, will not enable enantioselective reductions.

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FUTURE CONSIDERATIONS While hydrogenation catalysts have been known for almost 100 years since the work of Sabatier,40 the development of FLP chemistry has been a major advance in this seemingly mature field, extending the nature of hydrogenation catalysts into the realm of main group compounds. The prospect of metal-free hydrogenation catalysis offers many potential advantages. Among the most attractive features is the substantially lower cost of earth-abundant p-block elements relative to the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest. 389



Review

Biographies

(5) Hantzsch, A. Chem. Ber. 1881, 14, 1637−1638. (6) Birch, A. J. J. Chem. Soc. 1944, 430−436. (7) Meerwein, H.; Schmidt, R. J. Liebigs Ann. Chem. 1925, 444, 221− 238. (8) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124−1126. (9) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880− 1881. (10) McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46, 4968−4971. (11) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008, 1701−1703. (12) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46, 8050−8053. (13) Stephan, D. W.; Greenberg, S.; Graham, T. W.; Chase, P.; Hastie, J. J.; Geier, S. J.; Farrell, J. M.; Brown, C. C.; Heiden, Z. M.; Welch, G. C.; Ullrich, M. Inorg. Chem. 2011, 50, 12338−12348. (14) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Fröhlich, R.; Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072−5074. (15) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.; Leskelä, M.; Repo, T.; Pyykkö, P.; Rieger, B. J. Am. Chem. Soc. 2008, 130, 14117−14118. (16) Wang, H. D.; Fröhlich, R.; Kehr, G.; Erker, G. Chem. Commun. 2008, 5966−5968. (17) Jiang, C.; Blacque, O.; Berke, H. Chem. Commun. 2009, 5518− 5520. (18) Eros, G.; Mehdi, H.; Papai, I.; Rokob, T. A.; Kiraly, P.; Tarkanyi, G.; Soos, T. Angew. Chem., Int. Ed. 2010, 49, 6559−6563. (19) Axenov, K. V.; Kehr, G.; Fröhlich, R.; Erker, G. J. Am. Chem. Soc. 2009, 131, 3454−3455. (20) Schwendemann, S.; Tumay, T. A.; Axenov, K. V.; Peuser, I.; Kehr, G.; Fröhlich, R.; Erker, G. Organometallics 2010, 29, 1067−1069. (21) Chen, D.; Wang, Y.; Klankermayer, J. Angew. Chem., Int. Ed. 2010, 49, 9475−9478. (22) Chen, D.; Schmitkamp, M.; Franciò, G.; Klankermayer, J.; Leitner, W. Angew. Chem., Int. Ed. 2008, 47, 7339−7341. (23) Chen, D.; Klankermayer, J. Chem. Commun. 2008, 2130−2131. (24) Ghattas, G.; Chen, D.; Pan, F.; Klankermayer, J. Dalton Trans. 2012, 41, 9026−9028. (25) Heiden, Z. M.; Stephan, D. W. Chem. Commun. 2011, 47, 5729−5731. (26) Farrell, J. M.; Hatnean, J. A.; Stephan, D. W. J. Am. Chem. Soc. 2012, 134, 15728−15731. (27) Thomson, J. W.; Hatnean, J. A.; Hastie, J. J.; Pasternak, A.; Stephan, D. W.; Chase, P. A. Org. Process Res. Dev. 2013, 17, 1287− 1292. (28) Reddy, J. S.; Xu, B. H.; Mahdi, T.; Frohlich, R.; Kehr, G.; Stephan, D. W.; Erker, G. Organometallics 2012, 31, 5638−5649. (29) Greb, L.; Oña-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. Angew. Chem., Int. Ed. 2012, 51, 10164−10168. (30) Greb, L.; Tussing, S.; Schirmer, B.; Ona-Burgos, P.; Kaupmees, K.; Lokov, M.; Leito, I.; Grimme, S.; Paradies, J. Chem. Sci. 2013, 4, 2788−2796. (31) Greb, L.; Daniliuc, C. G.; Bergander, K.; Paradies, J. Angew. Chem., Int. Ed. 2013, 52, 5876−5879. (32) Hounjet, L. J.; Bannwarth, C.; Garon, C. N.; Caputo, C. B.; Grimme, S.; Stephan, D. W. Angew. Chem., Int. Ed. 2013, 52, 7492− 7495. (33) Segawa, Y.; Stephan, D. W. Chem. Commun. 2012, 48, 11963− 11965. (34) Wang, Y.; Chen, W.; Lu, Z.; Li, Z. H.; Wang, H. Angew. Chem., Int. Ed. 2013, 52, 7496−7499. (35) Dewitt, E. J.; Trapasso, L. E.; Ramp, F. L. J. Am. Chem. Soc. 1961, 83, 4672. (36) Chernichenko, K.; Madarász, Á .; Pápai, I.; Nieger, M.; Leskelä, M.; Repo, T. Nat. Chem. 2013, 5, 718−723. (37) Geier, S. J.; Chase, P. A.; Stephan, D. W. Chem. Commun. 2010, 46, 4884−4886.

Lindsay J. Hounjet attended the University of Saskatchewan, where he received a B.Sc. with High Honours in 2006 before undertaking his graduate studies at the University of Alberta as a Queen Elizabeth II Doctoral Scholar. He completed his Ph.D. under the direction of Martin Cowie in 2011 and then joined the Stephan group at the University of Toronto.

Douglas W. Stephan Doug Stephan, (Ph.D., University of Western Ontario, 1980). After NATO postdoctoral studies with R. H. Holm at Harvard in 1980−1982, he became an Assistant Professor at the University of Windsor, ultimately being appointed Full Professor in 1992. In 2008, he took up a Canada Research Chair and Professorship at the University of Toronto. Author of 350 scientific articles and over 80 patents, Stephan’s research exploits fundamental studies to target innovative technologies for transition metal and main group catalysis. His most notable work has included the development of catalysts for polymerization, hydrogenation, and metathesis, as well as “Frustrated Lewis Pairs” and electrophilic phosphonium cations.

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ACKNOWLEDGMENTS D.W.S. is grateful to NSERC of Canada for continued support and for the award of a Canada Research Chair. In addition, D.W.S. acknowledges the many contributions of students and postdoctoral fellows over the years that has made this work possible.

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REFERENCES

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(38) Mahdi, T.; Heiden, Z. M.; Grimme, S.; Stephan, D. W. J. Am. Chem. Soc. 2012, 134, 4088−91. (39) Voss, T.; Mahdi, T.; Otten, E.; Fröhlich, R.; Kehr, G.; Stephan, D. W.; Erker, G. Organometallics 2012, 31, 2367−2378. (40) Sabatier, P.; Senderens, J.-B. C. R. Hebd. Sci. Seances Acad. Sci. 1899, 1173−1176. (41) In US Parmacopeial Convention: http://www.usp.org/usp-nf/ key-issues/elemental-impurities, 2013.

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Hydrogenation by Frustrated Lewis Pairs: Main Group Alternatives to

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