Are Phosphines Viable Ligands for Pd-Catalyzed Aerobic Oxidation

Mar 21, 2018 - Phosphines are the broadest and most important class of ligands in homogeneous catalysis, but they are typically avoided in Pd-catalyze...
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Research Article Cite This: ACS Catal. 2018, 8, 3708−3714

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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 Avenue, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: Phosphines are the broadest and most important class of ligands in homogeneous catalysis, but they are typically avoided in Pdcatalyzed 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 have assessed 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 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 Pd-catalyzed aerobic oxidation reactions, particularly by implementing strategies identified for ligand stabilization. KEYWORDS: palladium, aerobic, oxidation, catalysis, phosphine, ligand, carbonylation

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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 intra- and 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

under the oxidative reaction conditions and, therefore, are well suited to support the Pd-based catalysts. 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 (η2-peroxo)palladium(II) complex (Ph3P)2Pd(O2) and further showed that Pd(PPh3)4 catalyzes efficient aerobic oxidation of 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 Pd-catalyzed aerobic oxidation chemistry because phosphines represent a very broad and versatile class of ligands that are seldom used to support these reactions.

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

important roles in these reactions ranging from controlling chemo-, regio-, and stereoselectivity to stabilizing 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). Dimethyl sulfoxide, thioethers, and N-heterocyclic carbenes (NHCs) have also been used.1d These ligands exhibit (meta)stability © XXXX American Chemical Society

Received: March 13, 2018 Revised: March 21, 2018 Published: March 21, 2018 3708

DOI: 10.1021/acscatal.8b01009 ACS Catal. 2018, 8, 3708−3714

Research Article

ACS Catalysis cat. Pd(PPh3)4

Ph3P + 1/2O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ph3PO ≥ 500 TON

(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

Figure 3. Reaction time course for the oxidative homocoupling of 4methoxyphenylboronic acid (1) to 4,4′-dimethoxybiphenyl (2), showing the formation of products and oxidized phosphine ligand. Conditions: 37.0 mM 1, 5 mol % (Ph3P)2Pd(O2) (1.8 mM), 1 atm O2, CDCl3, room temperature.

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 end of the reaction. The remaining PPh3 is coordinated to PdII species observed in solution at the end of the reaction.8 A similar reaction was reported by Yoshida and co-workers for the oxidative homocoupling of arylboronic esters using Pd(OAc)2 as the Pd source and 1,3-bis(diphenylphophino)propane (dppp) as the ancillary ligand.6e Homocoupling of the 1,3-propanediol-derived 4-fluorophenylboronic ester (4) to 4,4′-difluorobiphenyl (5) was monitored by 19F and 31P NMR spectroscopy (Figure 4). 4-Fluorophenol (6), together with small amounts of the protodeboronation product, fluorobenzene (7), were observed as byproducts in yields of 16% and 2%, respectively. The reaction exhibits a short induction period, after which approximately half of the dppp undergoes oxidation to the bis phosphine oxide in parallel with biphenyl formation; however, the oxidative homocoupling continues even when

Figure 2. Reaction classes investigated in this study: (A) homocoupling of arylboronic acids or esters; (B) oxidative alkoxyand aminocarbonylation; (C) oxidative annulation of an aromatic C− H bond; (D) asymmetric Fujiwara−Moritani (oxidative Heck) coupling.

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 31 P 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 show 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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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 1H and 31 P NMR spectroscopy (Figure 3). The reaction proceeds with nearly complete conversion of 1 within 2 h, affording 2 and 4methoxyphenol (3) in a net yield of 86% (65% and 21% yields,

Figure 4. Reaction time course for the oxidative homocoupling of 4fluorophenylboronic acid ester (4) to 4,4′-difluorobiphenyl (5) and the phenol byproduct 6, showing the formation of products and oxidized phosphine ligand. Conditions: 307 mM 4, 3 mol % Pd(OAc)2 (9.0 mM), 4.5 mol % dppp (13.9 mM), 1 atm O2, DMSO, 80 °C. 3709

DOI: 10.1021/acscatal.8b01009 ACS Catal. 2018, 8, 3708−3714

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ACS Catalysis phosphine oxidation concludes. Species with both dppp and κ1dppp-O ligands (three P donors per Pd center) are observed in the 31P NMR spectrum during the induction period (see Figure S5), reflecting the use of excess phosphine in the catalyst mixture, which consists of a 1.5:1 dppp:Pd(OAc)2 ratio.9 The induction period in Figure 4 could suggest that phosphine oxidation is required before the reaction takes place, and as a control experiment, the homocoupling of 4 was investigated in the absence of dppp. A comparison of the reaction time courses in the presence and absence of phosphine (Figure 5) reveals that the reaction is more effective in the

Figure 6. Reaction time course for aminocarbonylation of 4methoxyphenylboronic acid (1) with N-benzylmethylamine (8) to afford the N-benzyl-N-methylbenzamide 9, with phenol 3 as a byproduct, showing the formation of products and oxidized phosphine. Conditions: 96.9 mM 1, 131 mM 8, 5 mol % (Ph3P)2PdCl2 (4.8 mM), 5 mol % Cu powder (4.8 mM), 1 atm 2:1 CO:O2, DMSO, 80 °C.

The second oxidative carbonylation reaction consists of Pd(OAc)2/PPh3-catalyzed synthesis of ynoate esters via alkoxycarbonylation of 1-alkynes, reported by Yamamoto and co-workers.6c The time course of the reaction of 4fluorophenylacetylene (10) with CO and methanol-d4 to give the desired ynoate ester 11, together with diyne 12 as a byproduct, was analyzed by 19F and 31P NMR spectroscopy (Figure 7). Products 11 and 12 were generated in 76% and 9% yields, respectively. Relatively fast substrate and phosphine oxidation occurred at the start of the reaction, until approximately 50% of PPh3 was converted to Ph3PO, after which substrate and phophine oxidation continued in parallel, but at a slower rate.10 A control experiment, conducted in the absence of PPh3, led to only a 6% yield of 11, a 3% yield of 12,

Figure 5. Comparison of product formation time courses for the oxidative homocoupling of 4-fluorophenylboronic acid ester (4) in the presence (green) and absence (orange) of dppp ligand. Reaction conditions are the same as those given in Figure 4.

absence of dppp. No induction period is observed, the rate is faster, and the reaction exhibits higher yield (92%) and selectivity (i.e., only 5% yield of the phenol byproduct 6 and 1% yield of fluorobenzene was observed) in the absence of the phosphine ligand. Phosphine ligands have been used in several aerobic oxidative carbonylation reactions,6c,g−i,l,m,o,q,r and two were selected to probe the fate of the phosphine. The first involves (Ph3P)2PdCl2-catalyzed synthesis of benzamides via the aminocarbonylation of arylboronic acids with CO and secondary amines, reported by Jiao and co-workers.6q The reaction of 4-methoxyphenylboronic acid (1) with Nbenzylmethylamine (8) under an atmosphere composed of CO and O2 (2:1) was monitored by 1H and 31P NMR spectroscopy (Figure 6). The desired oxidative carbonylation reaction proceeds in parallel with oxidation of the phophine ligand, but only partial phosphine oxidation occurs under the reaction conditions (∼40% of the original PPh3). The aminocarbonylation product 9 and byproduct 4-methoxyphenol (3) are generated in 73% and 26% yields, respectively, at the end of the reaction. Several Pd-bound PPh3 resonances are detected in the 31P NMR spectrum during and after completion of the reaction. A reaction performed in the absence of PPh3 led to a substantially lower yield of 9 (23%), with the phenol byproduct 3 formed in 10% yield.

Figure 7. Reaction time course for the alkoxycarbonylation of 4fluorophenylacetylene (10) with CO and MeOD-d4 to the alkynyl ester to afford the ynoate ester 11 and diyne 12, showing the formation of products and oxidized phosphine ligand. Conditions: 45.8 mM 10, 2.2 M MeOD-d4 (0.45 mL), 10 mol % Pd(OAc)2 (4.6 mM), 20 mol % PPh3 (9.2 mM), 1 atm 1:1 CO:O2, DMF, room temperature. 3710

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reduction of O2 in the catalytic mechanism.11,12 The data in Figure 8B are consistent with such a role, whereby MnO2catalyzed disproportionation of H2O2 slows oxidation of the phophine ligand under the reaction conditions. A separate experiment in which Pd(PPh3)4 was replaced with Pd2(dba)3 as a source of Pd0 (i.e., 1.5 mol % Pd2(dba)3·CHCl3/10 mol % MnO2) resulted in no conversion of starting material, further demonstrating the utility of the PPh3 ligand. Finally, we analyzed an enantioselective aerobic Fujiwara− Moritani reaction reported by Mikami and co-workers.6d The reaction of 4-trifluoromethylphenylboronic acid (15) and methyl 1-cyclopentene-1-carboxylate (16) with Pd(OAc)2/ (S,S)-chiraphos (L3 in Figure 2) as the catalyst affords the oxidative coupling product 17 in 46% ee.6d The reaction was monitored by 19F and 31P NMR spectroscopy (Figure 9). A

and rapid generation of Pd black. No Pd black was observed in the reaction with PPh3 until nearly all of the phosphine was oxidized at the end of the reaction. Triphenylphosphine was also used in an intramolecular C−H functionalization method reported by Batey and co-workers for the synthesis of benzothiazoles from arylthioureas.6f A number of different phosphine ligands were tested in the original study, and it was speculated that PPh3 might be the most effective because it is more resistant to oxidation than other phosphines, but the fate of PPh3 was not directly analyzed. The optimized catalyst system consisted of a combination of 3 mol % Pd(PPh3)4 and 10 mol % MnO2. The oxidative annulation of the N-(4-fluorophenyl)thiourea 13 to the benzothiazole 14 was monitored by 19F and 31P NMR spectroscopy (Figure 8A).

Figure 9. Substrate and phosphine oxidation time course of the enantioselective Fujiwara−Moritani reaction of 4-trifluoromethylphenylboronic acid (15) with methyl 1-cyclopentene-1-carboxylate (16) with a Pd(OAc)2/(S,S)-chiraphos catalyst. Conditions: 336 mM 15, 220 mM 16, 5 mol % Pd(OAc)2 (30 mM), 5.5 mol % (S,S)-chiraphos (33 mM), 1 atm O2, DMF, 50 °C.

Figure 8. Reaction time course for intramolecular C−H oxidation involving converstion of arylthiourea 13 to benzothiazole 14, showing the formation of the product and oxidized phosphine with Pd(PPh3)4 as the catalyst in the presence (A) and absence (B) of cocatalytic MnO2. Conditions: 83.5 mM 13, 3 mol % Pd(PPh3)4 (2.5 mM), CH3CN, 1 atm O2, 80 °C; (A) only, 10 mol % MnO2 (8.5 mM).

linear time course was observed for the formation of product 17, and several byproducts were also observed (for example, PhCF3 derived from hydrodeboronation of the boronic acid 15). Virtually no oxidation of the chelating phosphine ligand was detected. Use of the Wisconsin High-Pressure NMR Reactor (WiHP-NMRR),13 which allows continuous delivery of gases to liquid solutions in an NMR tube, enabled in situ 31P NMR analysis of the catalytic species present throughout the reaction time course. The data (Figure S25) reveal the presence of both [((S,S)-chiraphos)2Pd](OAc)2 (18)14 and ((S,S)chiraphos)Pd(4-CF3C6H4)(OAc) (19)15,16 in a 1.6:1 18:19 ratio during catalytic turnover. This survey provides insights into the role and fate of phosphine ligands in six different Pd-catalyzed aerobic oxidative coupling reactions: (1) homocoupling of arylboronic acids catalyzed by Pd(PPh3)4 or (Ph3P)2Pd(O2) (2) homocoupling of arylboronic 1,3-propanediol esters catalyzed by Pd(OAc)2/dppp (3) aminocarbonylation of arylboronic acids catalyzed by (Ph3P)2PdCl2 (4) alkoxycarbonylation of 1-alkynes catalyzed by Pd(OAc)2/PPh3 (5) C−H annulation of arylthioureas catalyzed by Pd(PPh3)4

Formation of the product 14 exhibited a linear reaction time course. Oxidation of PPh3 proceeded relatively rapidly to Ph3PO until approximately half of the phosphine was consumed, after which it slowed but ultimately consumed approximately 90% of the PPh3 by the end of the reaction. The decrease in rate of formation of Ph3PO correlates with depletion of unligated PPh3, which is detected by 31P NMR spectroscopy through the first 60 min of the reaction (Figure S18). Pd-coordinated PPh3 persists until the substrate is fully consumed. A reaction conducted in the absence of the MnO2 cocatalyst was also analyzed (Figure 8B). Under these conditions, rapid and nearly complete oxidation of PPh3 to Ph3PO was observed during the first 30 min. Only a 12% yield of product was observed, with no further product formation occurring after consumption of the PPh3 ligand (Figure 8B). In the original report, the authors considered different possible roles for the MnO2 cocatalyst, including its ability to promote disproportionation of H2O2 generated from the two-electron 3711

DOI: 10.1021/acscatal.8b01009 ACS Catal. 2018, 8, 3708−3714

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Figure 10), even though phosphine oxidation 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 Pd-catalyzed 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)4-catalyzed intramolecular C−H functionalization reaction (reaction 5). Sigman and co-workers 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 co-workers 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.

(6) asymmetric coupling of arylboronic acids and alkenes catalyzed by Pd(OAc)2/(S,S)-chiraphos. These reactions exhibit varied behavior with respect to the contribution and stability of the phosphine ligands. Of the four reactions tested in the presence and absence of a phosphine ligand, the yields of three reactions improved significantly with use of a phosphine (Figure 10), demonstrating a beneficial contribution of the ligand to catalyst performance. Reaction 2, however, is inhibited by the chelating diphosphine ligand.

Figure 10. Comparison of yields observed for reactions 2−5 in the presence (solid red) and absence (slashed blue) of a phosphine ligand. Reaction 1 is not included in this summary because it 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.

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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 vary 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 metastability will be especially important 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 Pd-catalyzed aerobic oxidation reactions that employ phosphine ligands.

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 31P 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 in comparison to 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.,

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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/acscatal.8b01009. 3712

DOI: 10.1021/acscatal.8b01009 ACS Catal. 2018, 8, 3708−3714

Research Article

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Synth. Catal. 2005, 347, 1569−1575. (e) Adamo, C.; Amatore, C.; Ciofini, I.; Jutand, A.; Lakmini, H. Mechanism of the PalladiumCatalyzed Homocoupling of Arylboronic Acids: Key Involvement of a Palladium Peroxo Complex. J. Am. Chem. Soc. 2006, 128, 6829−6836. (f) Joyce, L. L.; Batey, R. A. Heterocycle Formation via PalladiumCatalyzed Intramolecular Oxidative C−H Bond Functionalization: An Efficient Strategy for the Synthesis of 2-Aminobenzothiazoles. Org. Lett. 2009, 11, 2792−2795. (g) Liu, Q.; Li, G.; He, J.; Liu, J.; Li, P.; Lei, A. Palladium-Catalyzed Aerobic Oxidative Carbonylation of Arylboronate Esters under Mild Conditions. Angew. Chem., Int. Ed. 2010, 49, 3371−3374. (h) Zhang, H.; Liu, D.; Chen, C.; Liu, C.; Lei, A. Palladium-Catalyzed Regioselective Aerobic Oxidative C−H/N−H Carbonylation of Heteroarenes under Base-Free Conditions. Chem. Eur. J. 2011, 17, 9581−9585. (i) Wu, X.-F.; Neumann, H.; Beller, M. Palladium-Catalyzed Oxidative Carbonylative Coupling Reactions of Arylboronic Acids with Styrenes to Chalcones under Mild Aerobic Conditions. Chem. - Asian J. 2012, 7, 282−285. (j) Jurčík, V.; Schmid, T. E.; Dumont, Q.; Slawin, A. M. Z.; Cazin, C. S. J. [Pd(NHC)(PR3)] (NHC = N-Heterocyclic Carbene) Catalysed Alcohol Oxidation using Molecular Oxygen. Dalton Trans. 2012, 41, 12619−12623. (k) Girard, S. A.; Hu, X.; Knauber, T.; Zhou, F.; Simon, M.-O.; Deng, G.-J.; Li, C.J. Pd-Catalyzed Synthesis of Aryl Amines via Oxidative Aromatization of Cyclic Ketones and Amines with Molecular Oxygen. Org. Lett. 2012, 14, 5606−5609. (l) Shi, R.; Lu, L.; Zhang, H.; Chen, B.; Sha, Y.; Liu, C.; Lei, A. Palladium/Copper-Catalyzed Oxidative C−H Alkenylation/ N-Dealkylative Carbonylation of Tertiary Anilines. Angew. Chem., Int. Ed. 2013, 52, 10582−10585. (m) Ren, L.; Jiao, N. Pd/Cu-Cocatalyzed Aerobic Oxidative Carbonylative Homocoupling of Arylboronic Acids and CO: A Highly Selective Approach to Diaryl Ketones. Chem. Asian J. 2014, 9, 2411−2414. (n) Chen, S.; Liao, Y.; Zhao, F.; Qi, H.; Liu, S.; Deng, G.-J. Palladium-Catalyzed Direct Arylation of Indoles with Cyclohexanones. Org. Lett. 2014, 16, 1618−1621. (o) Shi, R.; Zhang, H.; Lu, L.; Gan, P.; Sha, Y.; Zhang, H.; Liu, Q.; Beller, M.; Lei, A. (E)-α,β-Unsaturated Amides from Tertiary Amines, Olefins and CO via Pd/Cu-Catalyzed Aerobic Oxidative N-Dealkylation. Chem. Commun. 2015, 51, 3247−3250. (p) Shi, Y.; Wang, Z.; Cheng, Y.; Lan, J.; She, Z.; You, J. Oxygen as an Oxidant in Palladium/CopperCocatalyzed Oxidative C−H/C−H Cross-Coupling between Two Heteroarenes. Sci. China: Chem. 2015, 58, 1292−1296. (q) Ren, L.; Li, X.; Jiao, N. Dioxygen-Promoted Pd-Catalyzed Aminocarbonylation of Organoboronic Acids with Amines and CO: A Direct Approach to Tertiary Amides. Org. Lett. 2016, 18, 5852−5855. (r) Tu, Y.; Yuan, L.; Wang, T.; Wang, C.; Ke, J.; Zhao, J. Palladium-Catalyzed Oxidative Carbonylation of Aryl Hydrazines with CO and O2 at Atmospheric Pressure. J. Org. Chem. 2017, 82, 4970−4976. (7) For examples using Pd catalysts with phosphine ancillary ligands, see ref 6b and the following: (a) Moreno-Mañas, M.; Pérez, M.; Pleixats, R. Palladium-Catalyzed Suzuki-Type Self-Coupling of Arylboronic Acids. A Mechanistic Study. J. Org. Chem. 1996, 61, 2346−2351. (b) Aramendía, M. A.; Lafont, F.; Moreno-Mañas, M.; Pleixats, R.; Roglans, A. Electrospray Ionization Mass Spectrometry Detection of Intermediates in the Palladium-Catalyzed Oxidative SelfCoupling of Areneboronic Acids. J. Org. Chem. 1999, 64, 3592−3594. (c) Wong, M. S.; Zhang, X. L. Ligand Promoted Palladium-Catalyzed Homo-Coupling of Arylboronic Acids. Tetrahedron Lett. 2001, 42, 4087−4089. (8) See Figure S2 in the Supporting Information for details. (9) See section IIIB in the Supporting Information for details. (10) See section IIIF in the Supporting Information for analysis of 31 P NMR spectroscopic data. (11) Homogeneous Pd catalysts have been used for production of H2O2 under biphasic conditions and in fundamental studies of welldefined peroxopalladium(II) complexes. See the following leading references: (a) Bianchi, D.; Bortolo, R.; D’Aloisio, R.; Ricci, M. Biphasic Synthesis of Hydrogen Peroxide from Carbon Monoxide, Water, and Oxygen Catalyzed by Palladium Complexes with Bidentate Nitrogen Ligands. Angew. Chem., Int. Ed. 1999, 38, 706−708. (b) Bortolo, R.; Bianchi, D.; D’Aloisio, R.; Querci, C.; Ricci, M. Production of Hydrogen Peroxide from Oxygen and Alcohols,

Experimental details for syntheses of compounds, data acquisition, and spectroscopic data, including Figures S1−S32 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for S.S.S.: [email protected]. ORCID

Clark R. Landis: 0000-0002-1499-4697 Shannon S. Stahl: 0000-0002-9000-7665 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the NIH through an NRSA fellowship for S.J.T. (F32 GM119214). Additional support for this project was provided by the NSF through the CCI Center for Selective C−H Functionalization (CHE1205646 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.

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REFERENCES

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DOI: 10.1021/acscatal.8b01009 ACS Catal. 2018, 8, 3708−3714