Metal-Free Phosphine Oxide Reductions Catalyzed by B(C6F5)3 and

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Metal-Free Phosphine Oxide Reductions Catalyzed by B(C6F5)3 and Electrophilic Fluorophosphonium Cations Meera Mehta,†,‡ Isaac Garcia de la Arada,†,‡ Manuel Perez,† Digvijay Porwal,§ Martin Oestreich,*,§ and Douglas W. Stephan*,† †

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S3H6, Canada Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 115, 10623 Berlin, Germany

§

S Supporting Information *

ABSTRACT: The hydrosilylation/reduction of tertiary and secondary phosphine oxides to phosphines is catalyzed by B(C6F5)3 or electrophilic fluorophosphonium cations (EPCs). B(C6F5)3 is an effective catalyst for phosphine oxide reduction using (EtO)3SiH, PhSiH3, and Ph2SiH2 at elevated temperature (105 °C), while EPCs effect the same reduction at significantly lower temperature with PhSiH3 as reducing agent, allowing for good functional-group tolerance.

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INTRODUCTION Phosphines are central reagents in many classical organic transformations such as the Mitsunobu, Wittig, Rauhut− Currier, and Appel reactions.1−6 Furthermore, phosphines represent a large proportion of classic donor ligands employed in organometallic chemistry. While classic synthetic routes to phosphines include the use of phosphides as nucleophiles or chlorophosphines as electrophiles,7 more recent developments have included metal-catalyzed phosphination of aryl halides, alkenes, and alkynes.8−12 An alternative and important approach involves the reduction of phosphine oxides to phosphines.13 This latter approach is particularly interesting to industry, as a number of large-scale processes generate phosphine oxides as byproducts, making their recycling an avenue for the monetization of an otherwise waste material. Known methods for the reduction of phosphine oxides typically require the use of LiAlH4,14 DiBAL−H,15 or Cl3SiH.16,17 While hydrosilane reduction of phosphine oxides (such as Ph3PO) is used commercially, these reactions do require high temperatures and long reaction times.16,17 Electrochemical reductions of phosphine oxides have been reported,16−18 and the Buchwald,19 Lawrence,20 Lin,21 Lemaire,22,23 and Beller24 groups and others25 have reported the catalytic hydrosilylation of phosphine oxides mediated by Ti(Oi-Pr)4 and Cu(OTf)2. More recently, Beller and co-workers have also reported an example of a metal-free system for phosphine oxide reduction.26 Pietrusiewicz and co-workers have recently reported the use of BH 3 to effect the reduction of hydroxyalkylphosphines.27 In recent work, we have uncovered the ability of frustrated Lewis pairs (FLPs) to effect the metal-free catalytic hydrogenation of ketones and aldehydes.28−30 These carbonyl reductions exploit the electrophilic borane B(C6F5)3 (1, Figure 1, top). It should be noted that the corresponding hydro© XXXX American Chemical Society

Figure 1. Electrophilic borane and phosphonium cation salts.

silylation of carbonyls was established two decades ago by Piers and co-workers.31−36 In related work, we have also recently established a family of mono- and dicationic electrophilic phosphonium cations (EPCs). These compounds proved to be remarkably strong Lewis acids37 and, hence, effective catalysts for a number of reactions. [(C6F5)3PF][B(C6F5)4] and [(SIMes)PFPh2][B(C6F5)4]2 (SIMes = C3H4(NC6H3Me3)2; 3 and 2, Figure 1, top) 38 are potent catalysts for the hydrosilylation of alkenes and alkynes,39 dehydrocoupling transfer hydrogenation of alkenes,40 imines, and ketones,41 and the hydrodefluorination of fluoroalkanes.37 EPCs have also been reported to catalyze the hydrodeoxygenation of ketones under mild conditions.42 It is interesting that the Lewis acids 1−3 effect carbonyl reduction despite the fact that the nature of the Lewis acidity in these species differs. In the case of 1 the vacant p orbital is the source of Lewis acidity, whereas for EPCs it is attributable to the σ* orbital oriented opposite the P−F Received: February 25, 2016

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reactive, but any triorganosilane showed minimal conversion. In turn, several alkoxy-substituted hydrosilanes, (EtO)3SiH in particular, participated in the deoxygenation; (MeSiHO)n did not react though. From this survey, three hydrosilanes, PhSiH3, Ph2SiH2, and (EtO)3SiH, emerged as useful for the 1-catalyzed hydrosilylation of phosphine oxides in near-quantitative yields. In a similar fashion, to a solution of 2 (2.0 mol %) in C6D5Br were added an excess of the indicated hydrosilane (3.0 equiv) and Ph3PO sequentially. Unlike with 1 as catalyst, PhSiH3 now enabled exhaustive deoxygenation even at 45 °C. Ph2SiH2 was again less reactive. At 100 °C, representative triorganosilanes such as Ph3SiH, Et3SiH, and i-Pr3SiH lead to low or no conversion. The more reactive alkoxy-substituted hydrosilanes (EtO)2MeSiH and (Me2SiH)2O did not react at 60 °C but afforded high conversion at 100 °C; (MeSiHO)n performed poorly. We finally tested the EPC 3 with those hydrosilanes that had shown the highest reactivity in combination with Lewis acid 1 or 2: PhSiH3, (EtO)3SiH, (EtO)2MeSiH, and (Me2SiH)2O. Interestingly, 3 would catalyze only the Ph3PO reduction with PhSiH3 as reducing agent, presumably reflecting the reduced steric congestion and the increased hydridicity of the silane. Low loadings of both 2 and 3 (2.0 mol %) mediate this deoxygenation at 45 °C, whereas 1 requires more forcing temperatures (ideally 105 °C) and substantially higher catalyst loading (10 mol %). Negligible conversion was seen in the absence of any catalyst. An examination of the scope of the phosphine oxide reduction was undertaken employing 1/PhSiH3 (Table 2).

bond (Figure 1, bottom). To further probe the utility of these differing classes of Lewis acids, herein we examine the use of both boranes and fluorophosphonium cations in the catalytic hydrosilylation of a broad range of phosphine oxides.

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RESULTS AND DISCUSSION The borane 1 and the EPCs 2 and 3 were used as catalysts for the deoxygenation of Ph3PO employing various hydrosilanes at different temperatures (Table 1). Using catalytic amounts of 1, Table 1. Catalytic Reductions of Ph3PO to Ph3P

cat.

mol %

hydrosilanea

t (h)

T (°C)

convb (%)c

1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3

2.0 5.0 10 10 10 10 10 5.0 10 10 10 10 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

PhSiH3 PhSiH3 PhSiH3 Ph2SiH2 Ph2MeSiH Et3SiH i-Pr3SiH (EtO)3SiH (EtO)3SiH (Me2SiH)2O (Me3SiO)2MeSiH (MeSiHO)n PhSiH3 PhSiH3 PhSiH3 Ph2SiH2 Ph3SiH Et3SiH i-Pr3SiH (EtO)2MeSiH (EtO)2MeSiH (Me2SiH)2O (Me2SiH)2O (MeSiHO)n (MeSiHO)n PhSiH3 PhSiH3 (EtO)3SiH (EtO)2MeSiH (Me2SiH)2O PhSiH3 (EtO)2MeSiH (Me2SiH)2O

20 20 20 20 20 20 20 20 20 20 20 20 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24

45 105 105 105 105 105 105 105 105 105 105 105 45 60 100 100 100 100 100 60 100 60 100 100 45 45 100 100 100 100 45 100 100

60 98 (91) >99 (89) 89 99 50 5 0 0 5 95 5 99 10 8 >99 (87) >99 15 10 10 0 99 >99 >99 60 >97 50 >99 (83) >99 >99 (85) >99 (35) 4/96 84/16 50/4 >99 (48) 45/55 >99 (24) 80 80 >99 >99 30 >99 77 >99 (71) 17 > 99 >99 (83) 51 >99 19/23 95 0 0 >99 (28) >99 >99 >99 15 20 30 77 20 17 25

product Et3P Ph2PH Ph2PH Ph2(C6F5)P (C6F5)3P dppm dppm(O)/dppm dppm(O)/dppm binap(O)/binap binap dppb(O)/dppb dppb (4-CF3C6H4)3P (MeOCH2)Ph2P [4-t-BuC(O)C6H4]Ph2P [4-HO(O)CC6H4]Ph2P [2-Me2NC6H4]Ph2P

Ph2(2-py)P tolN(Ph2PO)(Ph2P)/tolN(Ph2P)2 tolN(Ph2P)2

S,S-(CH2CHPh)2PH Ph2PH Ph2PH

Ph3P Ph3P

a

All reactions were performed on a 0.11−0.16 mmol scale in C6D5Br at the indicated temperature for 24 h with PhSiH3 (3.1 equiv) as reducing agent. bDetermined by 31P NMR spectroscopy; isolated yields given in parentheses.

catalyst 3 at 100 °C and stands in contrast to the catalysis with 1 at 105 °C. Ph2(C6F5)PO and (C6F5)3PO were both reduced to the corresponding phosphine at 45 °C using either catalyst. Increased loadings of 2 or 3 (5.0 instead of 2.0 mol %) along with higher temperatures were required to generate dppm [from dppm(O)2 ] and binap [from binap(O)2 ] more effectively. Only traces of the monooxides were seen with these bisphosphine dioxides. Conversely, reduction of dppb(O)2 with 2 furnished a 45:55 mixture of dppb(O) and desired dppb, while the use of catalyst 3 resulted in quantitative reduction to dppb. While the fluorophosphonium cations 2 and 3 have been reported to be active for hydrodefluorination of

with the lesser polarization of the PS bond and the formation of the weaker Si−S compared to the Si−O bond.44,45 The fluorophosphonium salts 2 and 3 in combination with PhSiH3 proved to be efficient catalysts for the reduction of a series of phosphine oxides (Table 3). Et3PO was quantitatively reduced to Et3P at 60 °C using 3, while 2 afforded just 70% conversion under the same setup. The secondary phosphine oxide Ph2(H)PO was efficiently reduced by both catalysts at 45 °C to afford Ph2PH. Ph2(Cl)PO was reduced and dehalogenated, also yielding Ph2PH. This stands in contrast to that seen with 1 and reflects the greater Lewis acidity of the fluorophosphonium cations. This was particularly effective with C

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Scheme 1. (a) Si−H Bond Activation by η1 Coordination of Main-Group Lewis Acids and (b) Simplified Catalytic Cycle of the EPC-Promoted Deoxygenationa

C(sp3)−F bonds,37 (4-CF3C6H4)3PO was chemoselectively reduced in the presence of excess PhSiH3 to (4-CF3C6H4)3P, leaving the CF3 group untouched. This reflects the preferential reactivity of phosphine oxide over C−F bond activation in the presence of silane. Similarly, (MeOCH2)Ph2PO was fully reduced to (MeOCH2)Ph2P despite the previous reports of the ability of catalysts 2 and 3 to activate the C−O bonds of ethers.42 Moreover, [4-t-BuC(O)C6H4]Ph2PO and [4-HO(O)CC6H 4]Ph2PO were converted into the corresponding phosphines, reducing neither the carbonyl nor the free carboxyl group. These observations are consistent with the greater Lewis basicity of the phosphine oxides, prompting their preferential reduction. It also suggests that the generated phosphines inhibit both C(sp3)−F activation and C−O bond cleavage, respectively. This view was tested in a competition experiment where an equimolar mixture of Ph3PO and Ph2CO was exposed to 2 mol % of 2 and Et3SiH. This experiment showed no reaction. Changing to PhSiH3, selective reduction of the phosphine oxide is observed (see the SI). The protocol for catalytic phosphine oxide reduction was also tolerant of an amino group, as [2-Me2NC6H4]Ph2PO was reduced quantitatively by PhSiH3 in the presence of 2.0 mol % of 2 at 100 °C or 2.0 mol % of 3 at 50 °C. Similarly, 2.0 mol % of 3 at 100 °C was effective for the reduction of Ph2(2-py)PO. In the case of the bidentate amido-linked bisphosphine dioxide tolN(Ph2PO)2, clean reduction to tolN(Ph2P)2 was found using 3 as catalyst at 100 °C; with 2, a mixture of the starting dioxide, the monooxide, and the free bisphosphine was obtained. Interestingly, neither catalyst was effective for the reduction of (OCH2CH2N)3PO. Both the five-membered cyclic phosphinic acid S,S-[(CH2CHPh)2P](O)OH and the acyclic acid Ph2P(O)OH are reduced to S,S-[(CH2(CHPh)2PH] and Ph2PH, respectively, using either 2 or 3 as catalyst at 45 °C. In contrast, the acylphosphine oxide [MesC(O)]Ph2PO was reluctant to deoxygenate, yielding Ph2PH in 20% yield with 2 at 100 °C and 30% yield with 3 at 45 °C. As seen before with 1/PhSiH3, the extension of this protocol to Ph3PS was a challenge. However, 5.0 mol % of 2 at 100 °C promoted the desulfurization in 77% yield; catalyst 3 was not competent in this transformation. Unfortunately, analogous efforts to reduce the phosphinimine Ph3PNSiMe3 at 100 °C and 5.0 mol % catalyst loading were not met with success. The common mechanism of these phosphine oxide reductions is thought to be analogous to those proposed for hydrosilylations mediated by B(C6F5)3 131−33,46 and EPCs 2 and 339,41 (Scheme 1a). As an example, a general catalytic cycle is depicted for monocationic [EPC]+ 3 (Scheme 1b). Initial interaction of the hydrosilane R′3SiH with the Lewis-acidic phosphorus center prompts weakening of the Si−H bond (EPC···H···SiR′3, Scheme 1b), thereby facilitating nucleophilic attack at the silicon atom by phosphine oxide R3PO. Hydride transfer yields transient R3P(H)OSiR′3 together with 3, and both react further with another equivalent of R3′SiH to afford [R3P(H)(OSiR′3)2]+. This is believed to dissociate to give the weakly Lewis-basic disiloxane (R′3Si)2O and Brønsted-acidic phosphonium cation [R3PH]+.47 Subsequent protonation of the EPC−H by the phosphonium liberates free phosphine and H2 and regenerates the [EPC]+. It is noted that experimentally H2 evolution (bubbles) is observed. This mechanism is closely related to that initially described for the hydrosilylation of ketones by Piers31 and subsequently unambiguously confirmed by Oestreich and co-workers.34

a

Counteranion omitted for clarity.

Given that 2 exhibits significantly higher solubility than 3, the reduction of Ph3PO with 2 as catalyst was monitored by 31P NMR spectroscopy over time, showing the consumption of the phosphine oxide as well as the appearance of the phosphine (see the SI). However, it is noteworthy that a weak resonance attributable to the phosphonium cation 2 is seen throughout. The corresponding 19F NMR data confirmed the persistent presence of 2 over the course of the reaction. Nonetheless, upon completion of the reaction, in addition to 2, minor amounts of the difluorophosphorane [(SIMes)PF2Ph2] and the phosphine [(SIMes)PPh2] were observed. In an independent reaction, 2 in the presence of PhSiH3 alone was shown to generate both [(SIMes)PF2Ph2] and [(SIMes)PPh2]. This suggests that PhSiH3 also acts to partially reduce the EPC in the absence of phosphine oxide. In contrast, the stability of 2 in the presence of Et3SiH, forming an equilibrium mixture of 2 and the corresponding hydrosilane adduct, was previously reported. Analogous reactivity was observed with 3 (cf. Scheme 1a). The above results illustrate that borane 1 and fluorophosphonium cations 2 and 3 are effective catalysts for the reduction of a variety of phosphine oxides in the presence of silane. The initial screening revealed that PhSiH3 was most suitable for effective reduction, while the catalyst comparison showed that the EPCs 2 and 3 were generally more effective and operated at lower catalyst loadings and frequently at lower temperatures. These Lewis-acid catalysts effect these reductions in what is now referred to as an FLP-type mechanism in which the Lewis acid and phosphine oxide act on the silane. The superior reactivity of 2 and 3 is consistent with the greater Lewis acidity in comparison to 1.37 D

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Author Contributions

CONCLUSION In summary, the present work demonstrates that both B(C6F5)3 (1) and EPCs 2 and 3 are efficient catalysts for the reduction of a broad range of phosphine oxides in the presence of hydrosilanes. This reactivity further affirms that EPCs are highly effective Lewis-acid catalysts. Notably, chemoselective reduction of phosphine oxides was observed in the presence of other reducible functional groups such as halogens, ethers, ketones, and carboxylic acids. It is noteworthy that in a very recent publication Werner et al. have demonstrated similar chemoselective phosphine oxide reductions using Brønsted-acid catalysts.48 We are continuing to develop and study these metal-free phosphorus-based Lewis-acid catalysts for a broad range of applications in organic transformations.

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‡

M. Mehta and I. Garcia de la Arada contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.W.S. gratefully acknowledges NSERC of Canada for financial support and the award of a Canada Research Chair. This research was supported by the German Academic Exchange Service (predoctoral fellowship to D.P., 2014−2018) and the Cluster of Excellence Unifying Concepts in Catalysis of the Deutsche Forschungsgemeinschaft (EXC 314/2). M.O. is indebted to the Einstein Foundation (Berlin) for an endowed professorship.

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EXPERIMENTAL SECTION

General considerations regarding spectrometers and analyses as well as spectroscopic characterization of the products of catalysis are detailed in the Supporting Information. General Procedure for Reduction of Phosphine Oxides. Protocol 1. In a glovebox, an oven-dried 1 mL screw-capped sealed tube with a magnetic stir bar was charged with 1 (5.0−10 mol %), phenylsilane (61 mg, 0.56 mmol, 3.5 equiv), and the indicated phosphine oxide (0.16 mmol, 1.0 equiv). Toluene (0.50 mL) was added to the mixture. The tube was sealed and transferred to an oil bath preheated at 105 °C outside the glove box. After 20 h, the reaction mixture was cooled to room temperature. The reaction tube was then transferred back to the glovebox, and the reaction mixture was passed through a small plug of silica gel using degassed ethyl acetate (HPLC grade) and collected in a glass vial. The crude material was subjected to GLC analysis or 31P NMR analysis to determine the conversion (with respect to the starting material). The vial was then tightly sealed with a rubber septum, and the solvents were evaporated under high vacuum. Once the solvent was completely evaporated, the vial was purged with nitrogen gas and transferred to the glovebox. The crude material was passed through a small plug of silica gel and rinsed multiple times with degassed cyclohexane (HPLC grade); in selected cases purification was done by flash column chromatography on silica gel using cyclohexane/ethyl acetate as eluent if no reoxidation was observed. The collected solvent was evaporated under high vacuum to obtain the analytically pure title compounds. Protocol 2. All reactions were carried out under identical conditions on a 0.1−0.2 mmol scale. In a glovebox, the respective catalyst (2 or 3) (2 mol %, 2:4 mg, 3:4 mg) was added to a solution of PhSiH3 in C6D5Br (0.7 mL). The respective substrate (3:0.11 mmol, 2:0.16 mmol) was then added in one equivalence. The reaction mixture was transferred to an NMR tube, sealed, and heated in the reported conditions. Often the evolution of H2 gas could be observed. The reaction could be monitored and characterized by 1H NMR, 13C NMR, and 31P NMR. Upon completion the mixture was cooled to room temperature. Solvent was removed, the residue was washed with hexanes, and the result was eluted with dichloromethane through silica, affording the final product.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00158. Additional experimental and spectral data (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. E

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