sign in sign up
<<
Home , Diimide, Transfer hydrogenation

Metal-free transfer of olefins via dehydrocoupling

Manuel Pérez, Christopher B. Caputo, Roman Dobrovetsky, and Douglas W. Stephan1

Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6

Edited by Richard Eisenberg, University of Rochester, Rochester, NY, and approved June 12, 2014 (received for review April 23, 2014)

A major advance in main-group chemistry in recent years has been cations (EPCs) (22). The species [(C6F5)3PF][B(C6F5)4] 1 proved to the emergence of the reactivity of main-group species that mimics be an effective catalyst for the hydrodefluorination of fluoroalkanes that of transition metal complexes. In this report, the Lewis acidic in the presence of silane (22). We subsequently showed that 1 could phosphonium salt [(C6F5)3PF][B(C6F5)4] 1 is shown to catalyze the effectively catalyze the hydrosilylation of olefins and (23). In dehydrocoupling of silanes with , , phenols, and car- this report, we first exploit the Lewis acidity of 1 to catalyze the boxylic acids to form the Si-E bond (E = N, S, O) with the liberation dehydrocoupling of silane with a variety of species including amines, of H2 (21 examples). This catalysis, when performed in the pres- thiols, phenols, and carboxylic acids, liberating H2. We then show ence of a series of olefins, yields the concurrent formation of the that these intermolecular and intramolecular dehydrocou- products of dehydrocoupling and transfer hydrogenation of the pling reactions can be used for transfer hydrogenation of olefins, olefin (30 examples). This reactivity provides a strategy for metal- thus providing an unprecedented route to metal-free hydrogenation free catalysis of olefin . The mechanisms for both of olefins. catalytic reactions are proposed and supported by experiment and density functional theory calculations. Results and Discussion

Initially, 1.5 mol% of 1,Ph2NH, and Et3SiH were allowed to fluorophosphonium react for 10 h. This afforded near complete conversion to Ph2NSiEt3 (Table 1, entry 1). Interestingly, when Et3SiH was wide range of commodity chemicals, petrochemicals, phar- replaced by ClMe2SiH, the reaction required only 1 h for com- CHEMISTRY Amaceuticals, materials, and foods depend on industrial-scale plete conversion (Table 1, entry 2). In contrast, reactions with hydrogenation reactions. Catalysts for this process are based on more sterically encumbered silanes such as Ph3SiH or PhMe2SiH both heterogeneous transition metals materials and homoge- resulted in an increased reaction time (Table 1, entries 3, 4), neous transition metal complexes. Indeed, it was the discovery of whereas the bulkier silane, iPr3SiH (Table 1, entry 5) precluded Sabatier (1) in the early 20th century that revealed the utility of the reaction. para-Methyl substituted anilines showed a slower amorphous Ni and other metals in hydrogenation processes. reaction time (Table 1, entries 6–8). iPr2NH and aniline (Table 1, Following the dawn of in the 1960s, entries 9, 10) led to catalyst degradation and thus no catalytic homogeneous catalysts were developed principally based on activity. Compound 1 was also very effective in the catalytic precious metals (2–4). Although subsequent modifications and dehydrocoupling of thiophenols (Table 1, entries 11–14) and optimizations have led to numerous highly selective and efficient phenols (Table 1, entries 16–21) with Et3SiH. In general, these catalyst technologies, concerns over cost, natural abundance, and reactions were complete in 1 h with the exception of C6F5SH and toxicity have prompted efforts to uncover catalysts based on C6F5OH, which required a longer time for completion (Table 1, “ ” earth-abundant elements. The principal targets in these efforts entries 15 and 20, respectively). In this prior case (C6F5SH), have been the first-row transition metals. Indeed, the groups heating at 100 °C accelerated the reaction, resulting in complete of Chirik (5, 6), Morris (7), Beller (8), and others (9) have de- conversion in 3 h. In the case when para-methoxy phenol was veloped remarkably active and selective catalysts based on Fe. reacted with the excess of Et3SiH, both dehydrocoupling of Strategies to develop reduction methods using organic or – main-group based reagents have also been explored. For ex- Significance ample, use of Hantzsch esters (10), of arenes (11), or the use of B or Al hydrides are effective, albeit stoi- For more than a century, hydrogenation has been limited to chiometric reductions. More recently, the advent of “frustrated the use of transition metal-based catalysts. With the emerging Lewis pairs” (FLPs) have led to the discovery that combinations focus on the chemistry of earth-abundant elements, the 21st of sterically encumbered donors and acceptors act as catalysts for century has seen a renaissance in main-group chemistry. In this the reductions of , enamines, silylenol ethers, olefins, and work, an electrophilic phosphonium cation is shown to act alkynes (12). as main-group catalyst effecting the dehydrocoupling of si- To uncover hydrogenation strategies using earth-abundant lane and amines, phenols, thiols, and carboxylic acids with elements, we have focused our efforts on Lewis acidic phos- the concurrent release of H2. In addition, performing the reactions phorus species. Although Gudat (13), Burford and coworker in the presence of olefins, dehydrocoupling occurs with simulta- (14), Yoshifuji (15), and Bertrand and coworker (16), among neous hydrogenation of the olefin. This chemistry provides an others, have reported phosphenium cations that demonstrate unprecedented avenue to metal-free transfer hydrogenation Lewis acidity, Radosevich and coworkers (17) have exploited catalysis of olefins. the reaction of the unique planar P(III) species with - borane to give a P(V)H2 derivative that effects the subsequent Author contributions: M.P., C.B.C., R.D., and D.W.S. designed research; M.P., C.B.C., and reduction of diazobenzene. Although the acidity of P(V) has R.D. performed research; M.P., C.B.C., R.D., and D.W.S. analyzed data; and M.P., C.B.C., been exploited previously in ylide reagents (18), Diels–Alder R.D., and D.W.S. wrote the paper. reactions catalysis (19), and addition reactions to polar unsatu- The authors declare no conflict of interest. rates (20), Gabbaï and coworkers (21) have more recently This article is a PNAS Direct Submission. exploited the acceptor capabilities of phosphonium cations, in 1To whom correspondence should be addressed. E-mail: [email protected]. fluoride sensor strategies. In our own efforts, we have recently This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. reported the synthesis of highly electrophilic phosphonium 1073/pnas.1407484111/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1407484111 PNAS Early Edition | 1of5 Downloaded by guest on October 1, 2021 Table 1. Catalytic dehydrocoupling reactions

yrtnE H-E enaliS ,T h ,dleiY *%

1Ph2 HN tE 3SiH 10 >99

2Ph2 HN eMlC 2SiH 1 >99

3Ph2 HN hP 3SiH 20 >99

4Ph2 HN eMhP 2SiH 48 >99

5Ph2NH iPr3SiH 96 0

6 p-Me(C6H4)2NH Et3SiH 30 >99

7 p-Me(C6H4)2NH ClMe2SiH 16 >99

8 p-Me(C6H4)2NH Ph3SiH 36 40

9 iPr2 HN tE 3SiH 48 0

10 PhNH2 Et3SiH 48 0

11 HShP tE 3SiH <1 >99

12 p-Me(C6H4)SH Et3SiH <1 >99

13 p-Cl(C6H4)SH Et3SiH <1 >99

14 p-F(C6H4 HS) tE 3SiH <1 >99

15 C6F5 HS tE 3SiH 168 >99

† 16 HOhP tE 3SiH 2 >99

† 17 o-(Me)2(C6H3)OH Et3SiH 2 >99

† 18 p-OMe(C6H4)OH Et3SiH 18 >99

† 19 p-Me(C6H4)OH Et3SiH 3 >99

† 20 C6F5 HO tE 3SiH 24 >99

21 p-C8H17(C6H4)CO2HEt3SiH 1 >99

Conditions: To a solution of the catalyst (1.5 mol%) in C6D5Br or CD2Cl2 (1.0 mL) were added silane (1.1 Eq) and E-H (1.0 Eq) at 25 °C. *Yields measured by 1H-NMR. † 1.0 mol% of catalyst was used.

the OH group and the methoxy moiety was replaced by silyl these bases interact stronger with the Lewis acid 1 than Et3SiH. fragments; however, one equivalent of silane leads selectively At the same time, sterically demanding silanes slow or even stop to the product of dehydrocoupling of the phenol (entry 18). the dehydrocoupling reaction (Table 1, entry 5), which is con- This observation is similar to that reported for the hydro- sistent with the proposed role of hypervalent silicon species. silylation of methoxy-α-methylstyrenes (23). The carboxylic acid Furthermore, the use of silane in excess (10.0 Eq) with respect to p-C8H17(C6H4)CO2H and Et3SiH gave the corresponding ester the pTol2NH, accelerates the reaction, whereas using a large in less than 1 h (entry 21). excess of dramatically increases the reaction time. This The mechanism for these dehydrocoupling reactions is thought supports the view that equilibrium involving binding of silane or to be analogous to that previously proposed for 1 catalyzed olefin aniline to P(V) is competitive, and the dehydrocoupling reaction hydrosilylation (22). Thus, the initial step involves activation of the proceeds via silane activation. Si-H bond by the fluorophosphonium Lewis acid 1.Backsideat- The quest for theoretical insight regarding the dehydro- tack of the LUMO at Si center by the Lewis base (N, O, S) coupling reaction of silane and amine with the Lewis acid 1 generates transient hypervalent silicon species that has hydridic prompted preliminary gas-phase density functional theory (DFT) (Si-H) and protic (E-H,E= N, O, S) hydrogens prompting loss calculations at the WB97XD/def2TZV (27, 28) level of theory of dihydrogen (Fig. 1). This mechanism is conceptually similar using the cation of 1,Me3SiH, and Ph2NH. Phosphonium cation to related Lewis acid mediated hydrosilylations described by interacts with both Me3SiH and Ph2NH resulting in the Piers and coworkers (24), Oestreich and coworker (25), and Me3SiH∙∙∙P and Ph2(H)N∙∙∙P distances of 2.3 and 3.5 Å, re- Gevorgyan, Yamamoto, and coworkers (26). Furthermore, the spectively. Both interactions are exothermic by 15.19 (ΔG = − proposed mechanism is consistent with the experimental ob- −1.8) and 23.35 (ΔG = −6.3) kcal·mol 1, respectively. This sug- −4 servation that basic amines and sterically less encumbered gests that these species exist in an equilibrium (Keq = 5 × 10 ), anilines do not dehydrocouple with Et3SiH, suggesting that slightly favoring the amine–phosphonium interaction, which was

2of5 | www.pnas.org/cgi/doi/10.1073/pnas.1407484111 Pérez et al. Downloaded by guest on October 1, 2021 olefins (E)-α-methylstilbene (Table 2, entry 18) and 1-phenyl-2,2- diphenylethylene in less than 2 h (Table 2, entry 19). The reduction of 3,3-dimethylbut-1-ene using p-Cl(C6H4)SH and p-Cl(C6H4)OH as the proton sources yielded 76% and 69% of 2,3-dimethylbutane, respectively (Table 2, entries 20 and 21 results from methyl mi- gration). Methylenecyclohexane and 1-methylcyclohex-1-ene were also successfully reduced to methylcyclohexane in good yields (Table 2, entries 22–26). The olefinic unit of dibutyl 2-methylene- succinate was also hydrogenated using arylthiols and silane at 25 °C, displaying compatibility with unsaturated esters (Table 2, entries 27–29). Finally, using 2-methylenesuccinic acid with Et3SiH in catalytic presence of 1, resulting in reduction of the internal . Subsequent hydrolysis of the silyl-ester affords the corresponding acid in 89% isolate yield (Table 2, entry 30). It is noteworthy that, in all cases, neither hydrogenation nor dehydro- coupling is observed in the absence of the fluorophosphonium Fig. 1. Proposed mechanistic pathways for dehydrocoupling of silane and 1 amine and transfer hydrogenation of olefins (experiment: R3Si = Et3Si, catalyst . ClMe2Si, Ph3Si, PhMe2Si; Ar2NH = Ph2NH, p-Me(C6H4)2NH; calculations: R3Si = These reactions provide access to both dehydrocoupling products Me3Si; Ar = Ph). Gibbs free energies and enthalpies in parentheses for every as well as the derived from olefin hydrogenation. It is also step are provided in kilocalories per mole. important to note that these reactions are highly chemoselective, providing exclusively the products of dehydrocoupling and transfer hydrogenation with no hydrosilylation of olefin byproducts. In ad- also observed experimentally. Coordination of amine to the phos- dition, these reactions exhibit impressive tol- phonium bound silane generating a transient five-coordinate silicon erance, proceeding in the presence of aryl-halides, amines, thiols, ΔH = − ΔG = − intermediate is also exothermic with 37.1 ( 23.7) ethers, and ester fragments. · −1 + kcal mol , which yields (C6F5)3P(F)H and [Ph2N(H)SiMe3] in The mechanism for these transfer hydrogenations is thought to ΔH = · −1 a slightly endothermic ( 2.8 kcal mol ), but exergonic proceed by the intervention of the olefin in dehydrocoupling re-

− CHEMISTRY (ΔG = −9.3 kcal·mol 1) step. The subsequent reaction of these action pathway. Thus, rather than the elimination of H2,the two species to yield the dehydrocoupling product is driven by the + transient [Ar2N(H)SiR3] protonates the olefin, generating car- liberation of H2 even though the reaction is somewhat endo- bocation that subsequently abstracts the hydride from (C F ) P(F) −1 6 5 3 thermic and endergonic (ΔH = 43.5 and ΔG = 34.1 kcal·mol ). H (Fig. 1), yielding the and dehydrocoupling product. This Overall, this catalytic cycle is computed to be slightly exo- view of the mechanism is supported by the observation that these −1 thermic (ΔH = −6.0 kcal·mol ) with only a small overall ΔG transfer hydrogenations are favored by the 1,1-disubstituted olefins. − · −1 of 0.6 kcal mol . InthecaseoftBuCH=CH2 (Table 2, entries 19 and 20), pro- This dehydrocoupling mechanism is reminiscent of that de- tonation prompts methyl migration, also generating tertiary carbo- scribed for analogous B(C6F5)3 catalyzed reaction in the classic cation. To support the proposed mechanism, the catalysis using work from Piers and coworkers (29), which subsequently fur- Et3SiD in combination with a proton source was performed, leading ther illuminated and expanded by the groups of Oestreich (30), exclusively to deuteration of the more electrophilic position of the Brook (31), Rubinsztajn (32), and Paradies (33). In the context of olefin (SI Appendix). This is consistent with a Markovnikov de- dehydrocoupling, it is also important to note the pioneering work livery of proton to a less substituted carbon, forming tertiary from the Tilley and Manners research groups, who exploited carbocation followed by the reaction of the latter with deuteride. transition metal catalysts for this purpose (34, 35), whereas Hill DFT calculations at the WB97XD/def2TZV (27, 28) level of et al. (36) described Zn, Mg, Ca, and Sr catalyzed preparations theory for hydrogenation of 1,1-diphenylethylene show that the + of amino silanes and Baba and coworker (37) have used InBr3 to protonation of the 1,1-diphenylethylene by [Ph2N(H)SiMe3] is − dehydrocouple silanes and carboxylic acids (Table 1 and Fig. 1). endothermic [ΔH = 14.1 (ΔG = 13.7) kcal·mol 1], whereas the The above dehydrocoupling reactions proceed with the liber- subsequent hydride abstraction from (C6F5)3P(F)H affording ation of H2. We queried the possibility of capturing this H2 by 1,1-diphenylethane and regenerating catalyst is almost thermal − the addition of an olefin. Thus, 1,1-diphenylethylene was added neutral (ΔH = 1.1 and ΔG = 0.0 kcal·mol 1). Overall, concurrent p to a mixture of -Me(C6H4)2NH, Et3SiH, or Ph3SiH and 1.5 mol% transfer hydrogenation with dehydrocoupling is energetically 1 −1 of ; heating this mixture to 100 °C gave after 5 h the dehydro- favorable to loss of H2 as it requires 28.3 kcal·mol less energy, − coupled product and 1,1-diphenylethane with 60% and >99% and lower by 20.5 kcal·mol 1 Gibbs free energy. This was also yield, respectively (Table 2, entries 1 and 2). Reactions occurred supported by the competition experiment in which the dehydro- at room temperature; however, the reaction times and conversions coupling reaction in the presence of 1,1-diphenylethylene was ob- improved at elevated temperatures (SI Appendix). Transfer hydro- served to be significantly faster than in its absence (SI Appendix). genation is not limited to anilines, as substituted thiophenols, Transfer hydrogenations of , , and imines phenols, and carboxylic acids are competent proton sources as well, can be achieved by the classic Meerwein–Ponndorf–Verley pro- resulting in complete reduction of the olefin in less than 1 h (Table tocol, catalyzed by a variety of transition metal complexes and 2, entries 3–8). In a similar fashion, the olefin, 1-methyl-4-(prop- more recently by FLPs (38). In contrast, transfer hydrogenation 1-en-2-yl) was catalytically reduced using either ditolyl- of olefins has received much less attention. Homogeneous amine or tolylthiophenol in combination with Et3SiH, affording catalysts based on Fe (39) and Ru (40) were demonstrated in yields up to 75%. The aniline reactions were done at 100 °C, the 1970s, whereas the groups of Berke (41), Peters (42), and whereas the reductions using were performed at 25 °C (Table Manners (43) have more recently reported the transfer hydro- 2, entries 9–12). 2-Methylstyrene was reduced in up to 67% yields genation of olefins using Re-, Co-, and Rh-based catalysts with under similar conditions using thiophenols as the proton source at ammonia- as the H2 source. Heterogeneous catalysts for 25 °C (Table 2, entries 13 and 14), whereas (E)-α-methylstilbene transfer hydrogenation of olefins include supported Ni, Pd, or was hydrogenated in high yields using thiophenol or carboxylic Ir and have exploited isopropanol, , ammonium – acids (Table 2, entries 15 17). Interestingly, the carboxylic acid formate, and glycerol as sources of H2 (44, 45). Catalyst-free C6F5CO2HwithEt3SiH resulted in complete reduction of internal reductions of olefins have also been reported, although this

Pérez et al. PNAS Early Edition | 3of5 Downloaded by guest on October 1, 2021 Table 2. Transfer hydrogenation of olefins with concurrent dehydrocoupling catalysis

yrtnE H-E R3SiH T, °C T, h Yield, %*

Ph2C = CH2

1 p-Me(C6H4)2 HN tE 3SiH 100 5 60

2 p-Me(C6H4)2 HN hP 3SiH 100 6 >99

3 p-Me(C6H4 HS) tE 3SiH 25 <1 >99

4 p-Cl(C6H4 HS) tE 3SiH 25 <1 >99 (94)

5 p-F(C6H4 HS) tE 3SiH 25 <1 >99 (96)

6 p-MeO(C6H4 HO) tE 3SiH 25 <1 >99 (93)

7 p-C8H17(C6H4)CO2HEt3SiH 25 1 >99

8C6F5CO2H tE 3SiH 25 1 >99 (94)

p-MeC6H4(Me)C = CH2

9 p-Me(C6H4)2 HN tE 3SiH 100 3 56

† 10 p-Me(C6H4)2 HN tE 3SiH 100 3 74

11 p-Me(C6H4 HS) tE 3SiH 25 <1 75

‡ 12 p-Me(C6H4 HS) tE 3SiH 25 <1 70

Ph(Me)C = CH2

13 p-Me(C6H4 HS) tE 3SiH 25 <1 67

14 p-F(C6H4 HS) tE 3SiH 25 <1 61 Ph(Me)C = CHPh

15 C6F5 HS tE 3SiH 100 3 >99 (98)

16 p-C8H17(C6H4)CO2HEt3SiH 25 1 55

17 p-C8H17(C6H4)CO2HPh3SiH 25 3 90

18 C6F5CO2H tE 3SiH 25 2 >99 (99)

Ph2C = CHPh

§ 19 C6F5CO2H tE 3SiH 25 2 >99 (97)

t-BuCH = CH2

20 p-Cl(C6H4 HS) tE 3SiH 25 1 76

21 p-Cl(C6H4 HO) tE 3SiH 25 3 69

(CH2)5C = CH2 (methylenecyclohexane)

22 p-Cl(C6H4 HS) tE 3SiH 25 2 60

23 p-Cl(C6H4 HO) tE 3SiH 25 3 77

C6H10Me (1-methylcyclohex-1-ene)

24 p-Cl(C6H4 HS) tE 3SiH 25 12 50

25 p-Cl(C6H4 HO) tE 3SiH 25 1 65

26 C6F5CO2H tE 3SiH 25 2 >99

BuO2CCH2(BuO2C)C = CH2

27 p-Me(C6H4 HS) tE 3SiH 25 1 >99 (95)

28 p-Cl(C6H4 HS) tE 3SiH 25 1 89

29 p-F(C6H4 HS) tE 3SiH 25 1 98 (96)

HO2CCH2(HO2C)C = CH2

¶ 03 — tE 3SiH 25 20 96 (89)

Conditions: To a solution of silane (1.0 Eq), R-H (1.0 Eq), and olefin (1.0 Eq) in C6D5Br or CD2Cl2 (1.0 mL) was added the catalyst (1.5 mol%). *Yields determined by 1H-NMR spectroscopy (isolated yield). †2.0 Eq of olefin were used. ‡ To a solution of R-H (1.0 Eq) and olefin (1.0 Eq) in C6D5Br (1.0 mL) were added a mixture of silane (1.0 Eq) and the catalyst (1.5 mol%). §For complete conversion 1.5 Eq of reagents were used with respect to the olefin. { Isolated yield of the corresponding acid.

required the generation of the energetic diimide from Summary monohydrate and O2 (46). Although FLP catalysts have been In conclusion, we have described a general and efficient pro- shown to effect olefin hydrogenation, the present system, which cedure for dehydrocoupling of silanes with anilines, phenols, exploits concurrent dehydrocoupling, is (to our knowledge) thiophenols, and benzoic acids catalyzed by EPCs. Moreover, the first main-group system to mediate transfer hydrogenations we showed that these reactions can be used for in situ trans- of olefins. fer hydrogenation of olefins. Thus, the present work further

4of5 | www.pnas.org/cgi/doi/10.1073/pnas.1407484111 Pérez et al. Downloaded by guest on October 1, 2021 demonstrates the high versatility of fluorophosphonium cations 25 °C. The reaction was monitored by NMR or TLC until completion. Yield 1 in catalysis providing a main-group catalyst for the transfer hy- was determined by H-NMR spectroscopy. For isolated yields, the reaction drogenation of olefins. was quenched with a diluted solution of NaHCO3 and the mixture was extracted with CH2Cl2. The organic solution was dried over MgSO4, filtered, Methods and evaporated. The crude was diluted with hexane and filtered over silica gel; products were eluted with hexane and Et2O for dibutyl 2-methyl- General Procedure for Dehydrocoupling Reactions. To a solution of the catalyst enesuccinate. The quality of the catalyst is again essential for the successful – [(C6F5)3PF][B(C6F5)4](1.0 1.5 mol%) in C6D5Br or CD2Cl2 (0.34 M) was added completion of the reaction. the corresponding silane (1.0–1.2 Eq) and RH (R = Ar2N, ArS, ArO, ArCO2)(1.0 Eq) at 25 °C. The reaction was monitored by NMR analysis until the reaction Supplementary Data. Details of the syntheses, spectroscopic data are in- was complete. Yield was determined by 1H-NMR spectroscopy. Freshly pre- cluded in the SI Appendix. pared catalyst was used and resulted in the optimal yields. ACKNOWLEDGMENTS. We thank the Natural Sciences and Engineering General Procedure for Olefin Transfer-Hydrogenation. To a solution of silane Research Council (NSERC) of Canada for financial support. D.W.S. is grateful = – (1.0 Eq), RH (R Ar2N, ArS, ArO, ArCO2) (1.0 Eq) and olefin (1.0 1.2 Eq) was for the award of a Canada Research Chair. C.B.C. is grateful for the award added the [(C6F5)3PF][B(C6F5)4](1.5mol%)inC6D5Br or CD2Cl2 (0.5 M) at of an NSERC Postgraduate Scholarship and a Walter C. Sumner Fellowship.

1. Sabatier P (1926) How I have been led to the direct hydrogenation method by metallic 26. Gevorgyan V, Rubin M, Benson S, Liu JX, Yamamoto Y (2000) A novel B(C6F5)3-cata- catalysts. Ind Eng Chem 18(10):1005–1008. lyzed reduction of and cleavage of aryl and alkyl ethers with hydrosilanes. 2. Hallman PS, Evans D, Osborn JA, Wilkinson G (1967) Selective catalytic homogeneous hy- J Org Chem 65(19):6179–6186. drogenation of terminal olefins using tris(triphenylphosphine)hydridochlororuthenium(II); 27. Grimme S (2006) Semiempirical GGA-type density functional constructed with a long- hydrogen transfer in exchange and isomerisation reactions of olefins. Chem Commun range dispersion correction. J Comput Chem 27(15):1787–1799. 1967(7):305–306. 28. Chai JD, Head-Gordon M (2008) Long-range corrected hybrid density functionals with 3. Boerner A, Holz J (2004) Homogeneous hydrogenations: Olefin hydrogenations. damped atom-atom dispersion corrections. Phys Chem Chem Phys 10(44):6615–6620. Transition Metals for , eds Beller M, Bolm C (Wiley, New York), 2nd 29. Blackwell JM, Foster KL, Beck VH, Piers WE (1999) B(C6F5)3-catalyzed silation of al- – Ed, Vol 2, pp 3 13. cohols: A mild, general method for synthesis of silyl ethers. J Org Chem 64(13): 4. Pettinari C, Marchetti F, Martini D (2004) Metal complexes as hydrogenation catalysts. 4887–4892. Comprehensive Coordination Chemistry II, eds McCleverty JA, Meyer TJ (Elsevier, 30. Hermeke J, Mewald M, Oestreich M (2013) Experimental analysis of the catalytic cycle Oxford), Vol 9, pp 75–139. of the borane-promoted reduction with hydrosilanes: Spectroscopic detection 5. Friedfeld MR, et al. (2013) Cobalt precursors for high-throughput discovery of base of unexpected intermediates and a refined mechanism. J Am Chem Soc 135(46): metal asymmetric alkene hydrogenation catalysts. Science 342(6162):1076–1080. 17537–17546. CHEMISTRY 6. Trovitch RJ, Lobkovsky E, Bill E, Chirik PJ (2008) Functional group tolerance and 31. Brook MA, Grande JB, Ganachaud F (2011) New synthetic strategies for structured substrate scope in bis(imino) catalyzed alkene hydrogenation. Organo- silicones using B(C F ) . Adv Polym Sci 235:161–183. metallics 27(7):1470–1478. 6 5 3 32. Cella J, Rubinsztajn S (2008) Preparation of polyaryloxysilanes and polyaryloxysiloxanes 7. Zuo W, Lough AJ, Li YF, Morris RH (2013) Amine(imine)diphosphine iron catalysts by B(C F ) catalyzed polyetherification of dihydrosilanes and bis-phenols. Macro- for asymmetric transfer hydrogenation of ketones and imines. Science 342(6162): 6 5 3 – 1080–1083. 41(19):6965 6971. 8. Jagadeesh RV, et al. (2013) Nanoscale Fe O -based catalysts for selective hydroge- 33. Greb L, Tamke S, Paradies J (2014) Catalytic metal-free Si-N cross-dehydrocoupling. 2 3 – nation of nitroarenes to anilines. Science 342(6162):1073–1076. Chem Commun (Camb) 50(18):2318 2320. 9. Bullock RM (2013) Chemistry. Abundant metals give precious hydrogenation perfor- 34. Sadow AD, Tilley TD (2005) Synthesis and characterization of scandium silyl complexes ’ mance. Science 342(6162):1054–1055. of the type Cp*2ScSiHRR . sigma-Bond metathesis reactions and catalytic dehydro- 10. Hantzsch A (1881) Condensationprodukte aus Aldehydammoniak und Ketoniartigen genative silation of . J Am Chem Soc 127(2):643–656. Verbindungen. Chem Ber 14:1637–1638. 35. Waterman R (2013) Mechanisms of metal-catalyzed dehydrocoupling reactions. Chem 11. Birch AJ (1944) A new reagent for primary and secondary aliphatic amines. J Chem Soc Rev 42(13):5629–5641. Soc 1944:314–315. 36. Hill MS, Liptrot DJ, MacDougall DJ, Mahon MF, Robinson TP (2013) Hetero-dehy- 12. Stephan DW, Erker G (2013) Frustrated lewis pair mediated hydrogenations. Top Curr drocoupling of silanes and amines by heavier alkaline earth catalysis. Chem Sci 4(11): Chem 332:85–110. 4212–4222. 13. Gudat D (2010) Diazaphospholenes: N-Heterocyclic between molecules 37. Motokura K, Baba T (2012) An atom-efficient synthetic method: Carbosilylations of and Lewis pairs. Acc Chem Res 43(10):1307–1316. , alkynes, and cyclic acetals using Lewis and Bronsted acid catalysts. Green 14. Burford N, Ragogna PJ (2002) New synthetic opportunities using Lewis acidic phos- Chem 14(3):565–579. phines. Dalton Trans (23):4307–4315. 38. Farrell JM, Heiden ZM, Stephan DW (2011) Metal-free transfer hydrogenation catal- – 15. Yoshifuji M (2009) Product class 3: Phosphenium salts. Sci Synth 42:63 69. ysis by B(C6F5)3. Organometallics 30(17):4497–4500. 16. Guerret O, Bertrand G (1997) Trigonal planar phosphorus cations. Acc Chem Res 39. Nishiguchi T, Fukuzumi K (1971) Homogeneous transfer-hydrogenation of olefins – 30(12):486 493. catalyzed by FeII, CoII, and NiII complexes: o- and p-dihydroxybenzene as hydrogen 17. Dunn NL, Ha M, Radosevich AT (2012) Main group redox catalysis: Reversible P(III)/P(V) donors. Chem Commun 1971(3):139–140. – redox cycling at a phosphorus platform. J Am Chem Soc 134(28):11330 11333. 40. Ohkubo K, Aoji T, Hirata K, Yoshinaga K (1976) Enantioselective 18. Wittig G, Schöllkopf U (1954) Über Triphenyl-phosphin-methylene als Olefinbildende of secondary carbinols by (II) chiral complexes in the transfer Reagenzien I. Chem Ber 87:1318–1330. hydrogenation of olefins. Inorg Nucl Chem Lett 12(11):837–842. 19. Terada M, Kouchi M (2006) Novel metal-free Lewis acid catalysis by phosphonium 41. Jiang Y, Hess J, Fox T, Berke H (2010) Rhenium hydride/boron Lewis acid cocatalysis of salts through hypervalent interaction. Tetrahedron 62(2-3):401–409. alkene hydrogenations: Activities comparable to those of precious metal systems. 20. Sereda O, Tabassum S, Wilhelm R (2010) Lewis acid organocatalysts. Top Curr Chem J Am Chem Soc 132(51):18233–18247. 291:349–393. 42. Lin TP, Peters JC (2013) Boryl-mediated reversible H activation at cobalt: Catalytic 21. Hudnall TW, Kim YM, Bebbington MWP, Bourissou D, Gabbaï FP (2008) Fluoride ion 2 hydrogenation, dehydrogenation, and transfer hydrogenation. J Am Chem Soc chelation by a bidentate phosphonium/borane Lewis acid. J Am Chem Soc 130(33): 135(41):15310–15313. 10890–10891. 43. Leitao EM, Jurca T, Manners I (2013) Catalysis in service of main group chemistry 22. Caputo CB, Hounjet LJ, Dobrovetsky R, Stephan DW (2013) Lewis acidity of organo- fluorophosphonium salts: Hydrodefluorination by a saturated acceptor. Science offers a versatile approach to p-block molecules and materials. Nat Chem 5(10): – 341(6152):1374–1377. 817 829. 23. Pérez M, Hounjet LJ, Caputo CB, Dobrovetsky R, Stephan DW (2013) Olefin isomeri- 44. Johnstone RAW, Wilby AH, Entwistle ID (1985) Heterogeneous catalytic transfer hy- zation and hydrosilylation catalysis by Lewis acidic organofluorophosphonium salts. drogenation and its relation to other methods for reduction of organic-compounds. J Am Chem Soc 135(49):18308–18310. Chem Rev 85(2):129–170.

24. Parks DJ, Blackwell JM, Piers WE (2000) Studies on the mechanism of B(C6F5)3-cata- 45. Blaser HU, et al. (2003) Selective hydrogenation for fine chemicals: Recent trends and lyzed hydrosilation of carbonyl functions. J Org Chem 65(10):3090–3098. new developments. Adv Synth Catal 345(1-2):103–151.

25. Rendler S, Oestreich M (2008) Conclusive evidence for an SN2-Si mechanism in the 46. Pieber B, Martinez ST, Cantillo D, Kappe CO (2013) In situ generation of diimide from B(C6F5)3-catalyzed hydrosilylation of carbonyl compounds: Implications for the re- hydrazine and oxygen: Continuous-flow transfer hydrogenation of olefins. Angew lated hydrogenation. Angew Chem Int Ed Engl 47(32):5997–6000. Chem Int Ed Engl 52(39):10241–10244.

Pérez et al. PNAS Early Edition | 5of5 Downloaded by guest on October 1, 2021