Are Phosphines Viable Ligands for Pd-Catalyzed Aerobic Oxidation

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Are Phosphines Viable Ligands for Pd-Catalyzed Aerobic Oxidation Reactions? Contrasting Insights from a Survey of Six Reactions Stephen J. Tereniak, Clark R. Landis, and Shannon S. Stahl ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01009 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Are Phosphines Viable Ligands for Pd-Catalyzed Aerobic Oxidation Reactions? Contrasting Insights from a Survey of Six Reactions Stephen J. Tereniak, Clark R. Landis, and Shannon S. Stahl* Department of Chemistry, University of Wisconsin - Madison, 1101 University Ave, Madison, Wisconsin, 53706, United States Abstract Phosphines are the broadest and most important class of ligands in homogeneous catalysis, but they are typically avoided in Pd-catalyzed aerobic oxidation reactions because of their susceptibility to oxidative degradation. Recent empirical reaction-development efforts have led to a growing number of Pd/phosphine catalyst systems for aerobic oxidative coupling reactions, but few of these studies assess the fate of the phosphine ligand. Here, we assess six different oxidative coupling reactions including the homocoupling of boronic acids, amino- and alkoxycarbonylation reactions, intramolecular C–H annulation, and enantioselective Fujiwara-Moritani C–C coupling. The fate and role of the phosphine, analyzed by 31P NMR spectroscopy throughout the reaction time course in each case, varies in the different reactions. In one case, the phosphine has an inhibitory effect and leads to lower selectivity relative to ligand-free conditions. In other cases, the phosphine ligands have a beneficial effect on the reaction, but undergo oxidative decomposition in parallel with productive catalytic turnover. Inclusion of MnO2 in one of the reactions slows phosphine oxidation by catalyzing disproportionation of H2O2 and thereby supports productive catalytic turnover. Negligible oxidation of the chiral phosphine, (S,S)-chiraphos, is observed during the enantioselective C–C coupling reaction, due to strong chelation of the ligand to PdII. The results of this study suggest that phosphines warrant broader attention as ligands for Pdcatalyzed aerobic oxidation reactions, particularly by implementing strategies identified for ligand stabilization. Keywords: palladium, aerobic, oxidation, catalysis, phosphine, ligand, carbonylation, 1 ACS Paragon Plus Environment

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Introduction Ligand-supported Pd catalysts have been the focus of growing attention for selective aerobic oxidation of organic molecules, with applications including alcohol oxidation, Wacker-type intraand intermolecular functionalization of alkenes, and C–H oxidations.1 Most of the reactions follow a general catalytic mechanism that consists of two redox half-reactions consisting of PdII-mediated oxidation of the organic substrate (SubH2) followed by aerobic oxidation of the reduced Pd0 catalyst (Figure 1). Ancillary ligands have been shown to play important roles in these reactions ranging from controlling chemo-, regio- and stereoselectivity to stabilization of Pd0 and promoting its reoxidation by O2. The majority of these ligands consist of nitrogen donors, including pyridine and bipyridine derivatives and alkylamines (e.g., NEt3, sparteine), and dimethyl sulfoxide, thioethers and N-heterocyclic carbenes (NHCs) have also been used.1d These ligands exhibit (meta)stability under the oxidative reaction conditions and, therefore, are well suited to support the Pd-based catalysts.

LnPdIIX2 H 2O

1/2 O2 + 2 HX

Ln

Pd0

SubH2

Subox + 2 HX

Figure 1. Simplified mechanism for ligand-supported Pd catalyzed aerobic oxidation reactions.

Phosphines have found widespread use as ancillary ligands in other classes of Pd-catalyzed reactions, 2 but they are typically avoided in the present oxidation reactions owing to their susceptibility to formation of phosphine oxides under aerobic conditions. In the 1960s, two groups reported that Pd(PPh3)4 reacts with O2 to afford the !2-peroxopalladium(II) complex, (Ph3P)2Pd(O2), and further showed that Pd(PPh3)4 catalyzes efficient aerobic oxidation of 2 ACS Paragon Plus Environment

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triphenylphosphine to triphenylphosphine oxide (eq 1),3,4 and similar reactivity has been reported with Pd(OAc)2 as the catalyst.5 Nevertheless, several aerobic Pd catalytic reactions have been reported with phosphine-based ligands.6 The fate of the phosphine under the catalytic conditions has seldom been probed,6j however, raising questions about not only the fate but also the potential role of this ligand. Answers to these questions have important implications for the field of Pdcatalyzed aerobic oxidation chemistry because phosphines represent a very broad and versatile class of ligands that are seldom used to support these reactions.

Ph3P + 1/2 O2

cat. Pd(PPh3)4

! 500 TON

Ph3P=O

(1)

In the present study, we examine six previously reported examples of Pd-catalyzed aerobic oxidation reactions that employ phosphine ligands. These reactions include two examples of oxidative homocoupling of boronic acid derivatives (Figure 2A),6b,e alkoxycarbonylation of an alkyne C–H bond and aminocarbonylation of an arylboronic acid (Figure 2B),6c,q aromatic C–S bond formation via intramolecular C–H functionalization (Figure 2C),6f and an asymmetric oxidative C–C coupling reaction (Figure 2D).6d In each case, the fate of the phosphine has been analyzed throughout the reaction by 31P NMR spectroscopy. Phosphine oxidation is found to occur in some, but not all, of these reactions. In one case, the phosphine is detrimental to the reaction outcome, with better performance achieved in the absence of an ancillary ligand. The diverse roles and fates of the phosphine ligand in this set of representative reactions shows that phosphine ligands warrant further consideration in Pd-catalyzed aerobic oxidation reactions, even though the oxidative sensitivity of this ligand class remains an important consideration.

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cat. Pd/L1 or L2

A. 2 Ar–B(OR)2

B. R–M/H + CO + H–Nu

H

O2

NR2

CO2Me

L1

NR2 N CO2Me

cat. Pd/L3 O2

PPh2

Ph2P

Nu S

O2

D. Ar–B(OR)2 +

PPh3

R

cat. Pd/L1

S

N H

O

cat. Pd/L1

M = B(OH)2

C.

Ar–Ar

O2

L2

Ar *

PPh2

Ph2P L3

Figure 2. The reaction classes investigated in this study: (A) homocoupling of arylboronic acids or esters, (B) oxidative alkoxy- and aminocarbonylation, (C) oxidative annulation of an aromatic C–H bond, and (D) asymmetric Fujiwara-Moritani (oxidative Heck) coupling.

Results and Discussion The first reactions investigated consisted of aerobic oxidative homocoupling of arylboronic acid derivatives that afford symmetrical biaryls. Many reactions of this type have been reported,7 and, in 2006, Amatore and Jutand presented a mechanistic study of Pd-catalyzed homocoupling of three different 4-X-phenylboronic acids (X = MeO, H, CN) to the corresponding biphenyls.6e This study revealed the involvement of (Ph3P)2Pd(O2) in transmetalation of the aryl group from boron to Pd, and this peroxo species was shown to be an effective precatalyst. The fate of PPh3 under the catalytic reaction conditions was not reported, however. In order to probe this issue, the homocoupling of 4-methoxyphenylboronic acid 1 to 4,4’-dimethoxybiphenyl 2 was analyzed by 1

H and 31P NMR spectroscopy (Figure 3). The reaction proceeds with nearly complete conversion

of 1 within 2 h, affording 2 and 4-methoxyphenol 3 in a net yield of 86% (65% and 21% yields, respectively). Partial phosphine oxidation occurs in parallel with the productive reaction, with 24% conversion of the initial PPh3 (1.2 mol% with respect to substrate) to the phosphine oxide at the


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employed (Ph3P)2Pd(O2) as a well-defined Pd catalyst precursor, and reaction 6 is excluded because asymmetric catalysis is not possible without the ancillary ligand.

The extent of phosphine oxidation shows more diverse behavior among the reactions. Reactions 1–3 show only partial oxidation of the phosphine ligand during the reaction, and

31

P

NMR spectroscopic data from these reactions show that the unoxidized phosphines are coordinated to Pd. Moreover, in each of these cases, phosphine oxidation no longer occurs when the reaction is complete. Although multiple pathways are available for oxidation of phosphines in the presence of Pd,3,5,17 these data suggest that reactive intermediates generated during catalyst turnover are responsible for ligand oxidation. Probable intermediates consist of peroxide species generated during two-electron oxidation of Pd0 by O2, such as Pd(O2), Pd-OOH, H2O2, and X2BOOH species.18 Reactions 4 and 5 resemble reactions 1 and 3 in that phosphine oxidation proceeds in parallel with active catalytic turnover, but they differ in that phosphine ligand oxidation is nearly complete by the end of the reaction. Reaction 6 is unique and noteworthy among the reactions investigated here, as virtually no oxidation of the chelating chiral phosphine ligand is observed. Once again, the 31P NMR spectroscopic data show that the ligand present is entirely coordinated to Pd. This observation, together with the complementary observations from reactions 1–3, suggests that restriction of ligand lability, e.g., via chelation, represents a strategy to enhance phosphine stability under the reaction conditions. Reaction 2 also features a diphosphine, but dppp forms a less stable chelate than chiraphos. A similar concept underlies the successful use of NHC ligands in Pd-catalyzed aerobic oxidation reactions, as free NHCs undergo rapid oxidation to the corresponding ureas in the presence of O2.19 Pd-catalyzed aerobic oxidative carbonylation reactions commonly employ phosphine ligands.6c,g-i,l-m,o,q-r The two reactions of this type in the present study (reactions 3 and 4) show a clear beneficial effect of the phosphine ligand (e.g., Figure 10), even though phosphine oxidation 13 ACS Paragon Plus Environment

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occurs during the reaction. CO is a good reductant for PdII (e.g., forming CO2 in the presence of water),11a,b,20 and it can nucleate the formation of reduced Pd clusters,21 ultimately generating Pd black. 22 The relatively "hard" nitrogen donor ligands commonly used in other classes of Pdcatalyzed aerobic oxidation reactions are probably less effective in stabilizing Pd0 or competing with CO to prevent aggregation of Pd, thereby accounting for the prevalent use of phosphines in these reactions.23,24 A final highlight from this study concerns the beneficial effect of MnO2 in the Pd(PPh3)4catalyzed intramolecular C–H functionalization reaction (reaction 5). Sigman and coworkers had previously reported the use of MnO2 as an additive in the coupling of organostannanes and styrene under aerobic conditions with Pd[(–)-sparteine)]Cl2 as the catalyst.25 The reaction was conducted in the presence of a 16-fold excess of sparteine relative to the Pd catalyst, and MnO2 was proposed to catalyze H2O2 disproportionation12 and to slow the rate of background oxidation of sparteine. This concept has been rarely used in Pd-catalyzed aerobic oxidation reactions, but the observations of Batey and coworkers and the associated data in Figure 8 suggest that this approach could be a particularly valuable tactic to mitigate oxidative decomposition of phosphines in such reactions.

Conclusion Phosphine ligands have been evaluated empirically in a wide range of Pd-catalyzed aerobic oxidation reactions, but the present study provides the first broader assessment of the viability of these ligands. The data show that the role and fate of the phosphine ligands varies for different reactions. In most cases, however, the phosphine exhibited a beneficial effect on the reactions. Of the six reactions investigated here, all but one were more effective in the presence of the phosphine ligand. On the other hand, oxidation of phosphine ligand was found to proceed in parallel with the desired catalytic reaction in five of the six reactions. This metastablity will be especially important 14 ACS Paragon Plus Environment

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to consider in larger scale applications when lower catalyst loadings and higher turnover numbers are needed. The asymmetric C–C coupling reaction with (S,S)-chiraphos as an ancillary ligand is noteworthy because virtually no phosphine oxidation was observed during the reaction. This result shows that strong coordination/chelation of the phosphine ligand can slow or eliminate ligand decomposition. This strong-coordination strategy and broader use of MnO2 as an additive represent two important opportunities to expand the utility of phosphine ligands in Pd-catalyzed aerobic oxidation reactions. In light of the widespread availability of phosphine ligands, including enantioselective variants, these insights have important implications for future developments in this field. Qualitative insights of the type described here are relatively straightforward to acquire, and they provide valuable insights into the fate and role of the phosphine ligands. The field would benefit significantly if at least this minimal level of analysis is included in future reports of Pdcatalyzed aerobic oxidation reactions that employ phosphine ligands.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details for syntheses of compounds, data acquisition, and spectroscopic data, including Figures S1-S32, are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] 15 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful for financial support from the NIH through an NRSA fellowship for SJT (F32 GM119214). Additional support for this project was provided by the NSF through the CCI Center for Selective C–H Functionalization (CHE-1205646 and CHE-1700982). The WiHP-NMRR is supported by the Dow Chemical Company. We acknowledge the NSF (CHE-9709065) and the Paul and Margaret Bender Fund for support of the NMR facility.

References


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Nuclear Magnetic Resonance Spectroscopy.” Rev. Sci. Instrum. 2015, 86, 104101-1-1041019. 14. For examples of [bis-(chelating phosphine)PdII]+ complexes, see: (a) Luo, H.-K.; Kou, Y.; Wang, X.-W.; Li, D.-G. Studies on Palladium-Bisphosphine Catalyzed Alternating Copolymerization of CO and Ethylene.” J. Mol. Catal. A: Chem. 2000, 151, 91-113. (b) Bianchini, C.; Mantovani, G.; Meli, A.; Oberhauser, W.; Brüggeller, P.; Stampfl, T. Novel Diphosphine-Modified Palladium Catalysts for Oxidative Carbonylation of Styrene to Methyl Cinnamate.” J. Chem. Soc., Dalton Trans. 2001, 690-698. (c) Marson, A.; van Oort, A. B.; Mul, W. P. In Situ Preparation of Palladium Diphosphane Catalysts. Eur. J. Inorg. Chem. 2002, 3028-3031. (d) Bianchini, C.; Lee, H. M.; Meli, A.; Oberhauser, W.; Peruzzini, M.; Vizza, F. Ligand and Solvent Effects in the Alternating Copolymerization of Carbon Monoxide and Olefins by Palladium–Diphosphine Catalysis. Organometallics 2002, 21, 16-33. (e) Mooibroek, T. J.; Bouwman, E.; Lutz, M.; Spek, A. L.; Drent, E. NMR Spectroscopic Studies of Palladium(II) Complexes of Bidentate Diphenylphosphane Ligands with Acetate and Tosylate Anions: Complex Formation and Structures. Eur. J. Inorg. Chem. 2010, 298-310. 15. Moncarz, J. R.; Brunker, T. J.; Glueck, D. S.; Sommer, R. D.; Rheingold, A. L. Stereochemistry of Palladium-Mediated Synthesis of PAMP–BH3: Retention of Configuration at P in Formation of Pd–P and P–C Bonds. J. Am. Chem. Soc. 2003, 125, 1180-1181. 16. See Sections IIIK, IV, and V in the Supporting Information for details. 17. In addition to pathways involving activated oxygen species, Pd-mediated P–O coupling with water and/or acetate as the source of oxygen are also known. For leading references, see: (a) Ozawa, F.; Kubo, A.; Hayashi, T. Generation of Tertiary Phosphine-Coordinated Pd(0) Species from Pd(OAc)2 in the Catalytic Heck Reaction. Chem. Lett. 1992, 21, 2177-2180. (b) Amatore, C.; Jutand, A.; M’Barki, M. A. Evidence of the Formation of Zerovalent Palladium from 21 ACS Paragon Plus Environment

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Pd(OAc)2 and Triphenylphosphine. Organometallics 1992, 11, 3009-3013. (c) Grushin, V. V.; Alper, H. Alkali-Induced Disproportionation of Palladium(II) Tertiary Phosphine Complexes, [L2PdC12], to LO and Palladium(0). Key Intermediates in the Biphasic Carbonylation of ArX Catalyzed by [L2PdC12]. Organometallics 1993, 12, 1890-1901. (d) Amatore, C.; Carré, E.; Jutand, A.; M’Barki, M. A. Rates and Mechanism of the Formation of Zerovalent Palladium Complexes from Mixtures of Pd(OAc)2 and Tertiary Phosphines and Their Reactivity in Oxidative Additions. Organometallics 1995, 14, 1818-1826. (e) Amatore, C.; Jutand, A.; Thuilliez, A. Formation of Palladium(0) Complexes from Pd(OAc)2 and a Bidentate Phosphine Ligand (dppp) and Their Reactivity in Oxidative Addition. Organometallics 2001, 20, 32413249. (f) Fors, B. P.; Krattiger, P.; Strieter, E.; Buchwald, S. L. Water-Mediated Catalyst Preactivation: An Efficient Protocol for C–N Cross-Coupling Reactions. Org. Lett. 2008, 10, 3505-3508. (g) Wei, C. S.; Davies, G. H. M.; Soltani, O.; Albrecht, J.; Gao, Q.; Pathirana, C.; Hsiao, Y.; Tummala, S.; Eastgate, M. D. The Impact of Palladium(II) Reduction Pathways on the Structure and Activity of Palladium(0) Catalysts. Angew. Chem., Int. Ed. 2013, 52, 58225826. (h) Amatore, C.; El Kaïm, L.; Grimaud, L.; Jutand, A.; Meignié, A.; Romanov, G. Kinetic Data on the Synergetic Role of Amines and Water in the Reduction of PhosphineLigated Palladium(II) to Palladium(0). Eur. J. Org. Chem. 2014, 4709-4713. (i) Ji, Y.; Plata, E. R.; Regens, C. S.; Hay, M.; Schmidt, M.; Razler, T.; Qiu, Y.; Geng, P.; Hsiao, Y.; Rosner, T.; Eastgate, M. D.; Blackmond, D. G. Mono-Oxidation of Bidentate Bis-phosphines in Catalyst Activation: Kinetic and Mechanistic Studies of a Pd/Xantphos-Catalyzed C–H Functionalization. J. Am. Chem. Soc. 2015, 137, 13272-13281. 18. For a mechanistic study of phosphine oxidation with (R3P)2Pt(O2), see, Sen, A.; Halpern, J. Role of Transition Metal-Dioxygen Complexes in Catalytic Oxidation. Catalysis of the Oxidation of Phosphines by Dioxygen Adducts of Platinum. J. Am. Chem. Soc. 1977, 99, 833722 ACS Paragon Plus Environment

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8339. 19. Rogers, M. M.; Stahl, S. S. N-Heterocyclic Carbenes as Ligands for High-Oxidation-State Metal Complexes and Oxidation Catalysis. Top. Organomet. Chem. 2007, 21, 21-46. 20. See, for example: (a) Moiseev, I. I.; Stromnova, T. A.; Vargaftig, M. N.; Mazo, G. J.; Kuz’mina, L. G.; Struchkov, Y. T. New Palladium Carbonyl Clusters: X-Ray Crystal Structure of [Pd4(CO)4(OAc)4]!(AcOH)2. J. Chem. Soc., Chem. Commun. 1978, 27-28. (b) Stromnova, T. A.; Vargaftik, M. N.; Moiseev, I. I. Mechanism of Reaction of Palladium(II) Carboxylates with Carbon Monoxide in Nonaqueous Media. J. Organomet. Chem. 1983, 252, 113-120. (c) Sergeev, A. G.; Neumann, H.; Spannenberg, A.; Beller, M. Synthesis and Catalytic Applications of Stable Palladium Dioxygen Complexes. Organometallics 2010, 29, 33683373. (d) Bruk, L. G.; Temkin, O. N.; Abdullaeva, A. S.; Timashova, E. A.; Bukina, E. Y.; Odintsov, K. Y.; Oshanina, I. V. Coupled Processes in Carbon Monoxide Oxidation: Kinetics and Mechanism of CO Oxidation by Oxygen in PdX2–Organic Solvent–Water Systems. Kinet. Catal. 2010, 51, 678-690. (e) Ragaini, F.; Larici, H.; Rimoldi, M.; Caselli, A.; Ferretti, F.; Macchi, P.; Casati, N. Mapping Palladium Reduction by Carbon Monoxide in a Catalytically Relevant System. A Novel Palladium(I) Dimer. Organometallics 2011, 30, 2385-2393. (f) Willcox, D.; Chappell, B. G. N.; Hogg, K. F.; Calleja, J.; Smalley, A. P.; Gaunt, M. J. A General Catalytic #–C–H Carbonylation of Aliphatic Amines to #-Lactams.” Science 2016, 354, 851-857. 21. For a related reduction of a PdII peroxo by tBuNC, generating a PdI cluster, see: Tereniak, S. J.; Stahl, S. S. Mechanistic Basis for Efficient, Site-Selective, Aerobic Catalytic Turnover in Pd-Catalyzed C–H Imidoylation of Heterocycle-Containing Molecules. J. Am. Chem. Soc. 2017, 139, 14533-14541. 22. Evidence for homoleptic Pd(CO)n (1 0 n < 4) species, which are thermally unstable under 23 ACS Paragon Plus Environment

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ambient conditions, was obtained by cryogenic matrix isolation techniques. See the following leading references: (a) Darling, J. H.; Ogden, J. S. Infrared Spectroscopic Evidence for Palladium Tetracarbonyl. Inorg. Chem. 1972, 11, 666-667. (b) Huber, H.; Kündig, P.; Moskovits, M.; Ozin, G. A. Tetracarbonyls of Palladium and Platinum in Low-Temperature Matrices. Nat. Phys. Sci. 1972, 235, 98-100. 23. For a study of Pd black formation in allylic substitution chemistry, see: Tromp, M.; Sietsma, J. R. A.; van Bokhoven, J. A.; van Strijdonck, G. P. F.; van Haaren, R. J.; van der Eerden, A. M. J.; van Leeuwen, P. W. N. M.; Koningsberger, D. C. Deactivation Processes of Homogeneous Pd Catalysts using in situ Time Resolved Spectroscopic Techniques. Chem. Commun. 2003, 128-129. 24. Cabrera-Pardo, J. R.; Trowbridge, A.; Nappi, M.; Ozaki, K.; Gaunt, M. J. Selective Palladium(II)-Catalyzed Carbonylation of Methylene #-C–H Bonds in Aliphatic Amines. Angew. Chem., Int. Ed. 2017, 56, 11958-11962. 25. Gligorich, K. M.; Cummings, S. A.; Sigman, M. S. Palladium-Catalyzed Reductive Coupling of Styrenes and Organostannanes under Aerobic Conditions. J. Am. Chem. Soc. 2007, 129, 14193-14195.


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Are Phosphines Viable Ligands for Pd-Catalyzed Aerobic Oxidation

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