Suzuki-Miyaura coupling catalyzed by a Ni(II) PNP pincer complex: Scope and mechanistic insights

Inorganica Chimica Acta 504 (2020) 119457

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Research paper

Suzuki-Miyaura coupling catalyzed by a Ni(II) PNP pincer complex: Scope and mechanistic insights Justin Madera, Megan Slattery, Hadi D. Arman, Zachary J. Tonzetich

T

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Department of Chemistry, University of Texas at San Antonio (UTSA), San Antonio, TX 78249, USA

ARTICLE INFO

ABSTRACT

Keywords: Suzuki-Miyaura reaction Cross-coupling Nickel catalysis Pincer complexes

The nickel(II) pincer complex, [NiCl(PhPNP)] (PhPNP = anion of 2,5-bis(diphenylphosphinomethyl)pyrrole), has been employed as a precatalyst for the Suzuki-Miyaura cross-coupling reaction of aryl halides and boronic acids. Both electron-rich and electron-deficient aromatic bromides were found to undergo coupling with boronic acids in modest yield at elevated temperature in the presence of K3PO4·H2O. Preliminary mechanistic studies of the reaction identified a novel species formulated as the boronate complex, [Ni(OB{OH}{2-tolyl})(PhPNP)], which most likely represents a catalyst deactivation pathway. The productive catalytic cycle was found to be most consistent with a Ni(I)/Ni(III) process where the boronic acid serves as both reductant and nucleophile in the presence of base.

1. Introduction Transition metal catalyzed cross-coupling methodologies have dramatically transformed the organic synthesis landscape and now serve as one of the most prevalent means of constructing carbon-carbon bonds [1]. Among the myriad of cross-coupling methods reported in the last few decades, the Suzuki-Miyaura reaction continues to witness the greatest application across both academia and industry [2–4]. Catalysts for this reaction have traditionally centered upon molecular palladium complexes containing strong donor ligands such as phosphines [5,6]. The well-defined nature of these systems has permitted detailed mechanistic studies, which continue to offer new insights into the fundamental aspects of the catalytic cycle [7–10]. Such findings have in turn informed the design of new generations of palladium catalysts further increasing the efficacy of the Suzuki-Miyaura reaction [11,12]. In addition to palladium, nickel catalysts have also been employed with great effect in the Suzuki-Miyaura reaction [13–15]. As with other CeC bond forming processes mediated by nickel, however, mechanistic proposals remain more speculative than those with palladium, and recent work is only now beginning to unravel the unique features of these catalytic systems [16–23]. By in large, application of nickel in the Suzuki-Miyaura reaction has taken the form of simple metal precursors coupled with standard ligands such as phosphines, N-heterocyclic carbenes, and pyridine derivatives [24]. Such catalyst systems have proven effective, but they can present challenges in terms of mechanistic study due to difficulties associated with controlling catalyst speciation, ligand

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stoichiometry, and catalyst deactivation. Beyond simple mono- and bidentate ligands, several groups have also demonstrated successful coupling chemistry with nickel employing kinetically-stabilizing pincer-derived platforms [25–32]. Although such studies remain relatively few by comparison, select pincer systems have demonstrated excellent efficiency across a wide variety of nucleophilic and electrophilic coupling partners [32]. Moreover, the robust nature of pincer complexes makes these species compelling candidates for mechanistic studies [33,34]. With these themes in mind, we turned our consideration to nickel (II) complexes of the pyrrole-based pincer ligands shown in Chart 1 [35–37]. These compounds have already proven competent for both CeC and CeS cross-coupling reactions [38,39]. We therefore reasoned that they might demonstrate efficacy in the Suzuki-Miyaura reaction as well. In this contribution we describe the catalytic behavior of the Ni(II) compounds as precatalysts for the cross-coupling of aryl halides and boronic acids and describe the results of several mechanistic studies aimed at shedding light on the catalytic cycle. 2. Results and discussion 2.1. Catalyst optimization and scope Previous work from our laboratory identified nickel complexes 1 and 2 as effective precatalysts for the Kumada-Corriu type couplings of Grignard reagents and aryl chlorides [38]. We therefore began by

Corresponding author. E-mail address: [email protected] (Z.J. Tonzetich).

https://doi.org/10.1016/j.ica.2020.119457 Received 12 November 2019; Received in revised form 31 December 2019; Accepted 17 January 2020 Available online 23 January 2020 0020-1693/ © 2020 Elsevier B.V. All rights reserved.

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higher yields, although somewhat surprisingly the reaction between 2tolylboronic acid and 4-nitrobromobenzene (entry 9) failed to produce any cross-coupled product. The incompatibility of such a substrate in the coupling reaction suggests that the nitro group might engage in deleterious side-reactivity with the nickel catalyst. We also attempted the cross-coupling reaction of bromocyclohexane (entry 14), which resulted in low but quantifiable yields of the corresponding product. 2.2. Mechanistic studies Chart 1.

The success of precatalyst 1 for the Suzuki-Miyaura reaction next prompted us to investigate the potential mechanism of the catalytic cycle. Such studies are relatively scarce for nickel catalysts in general and essentially absent for pincer compounds in particular. Given the additional spectroscopic handle afforded by the methyl group of the 2tolylboronic acid, we elected to focus on this nucleophile for the majority of our studies. To begin, we examined the stoichiometric reactivity of isolated 1 with nucleophile and electrophile at elevated temperature in the absence of base (Scheme 1). In both instances, no reactivity was observed including decomposition of the precatalyst as judged by 1H NMR spectroscopy. Likewise, no reaction was found to take place when stoichiometric amounts of 1 were combined simultaneously with nucleophile and electrophile in the absence of base. Repeating the reactions in Scheme 1 in the presence of K3PO4·H2O led to several new species as judged by 1H NMR spectroscopy. When 2.4 equivalents of 2-tolylboronic acid were allowed to react with 1, a new nickel species was apparent in addition to unreacted 1. Toluene and 2,2′-dimethylbiphenyl were also detected signifying that apparent protodeboronation and nucleophile homocoupling had taken place. While we have been unable to isolate the new nickel species in sufficient purity for unambiguous characterization, spectroscopic evidence points to its identity as the boronate complex, 3 (Scheme 2a). To bolster this assignment, we were able to generate a species with identical spectroscopic features through the reaction of [Ni(N{SiMe3}2)(PhPNP)] and 2-tolylboronic acid (Scheme 2b and see Supporting Information). Such a protonolysis strategy has also been employed to prepare a related Rh boronate complex [40]. The possibility that the putative boronate species could be the 2-tolyl complex, [Ni(2-tolyl)(PhPNP)] was

screening these two compounds for catalytic activity in the SuzukiMiyaura coupling of phenylboronic acid and 4-haloanisoles (Table 1). Optimization of the catalytic reaction conditions demonstrated that compound 1 was superior to 2 as a precatalyst with aryl bromides proving the most efficient as coupling partners. The reaction was found to proceed with highest yields when performed at 130 °C in toluene with potassium phosphate monohydrate as base. These conditions are similar to those reported by Kirchner and coworkers for another pincerbased nickel catalyst system [32]. In contrast to the findings of Kirchner, however, only boronic acids were effective nucleophiles in the present system, with esters and borate salts failing to generate any coupled species (Table 1, entries 14–16). With optimized conditions in hand, we next examined the efficacy of 1 for the Suzuki-Miyaura coupling reaction across a series of electrophilic and nucleophilic substrates. As displayed in Table 2, the catalyst system demonstrates broad compatibility for a variety of electrondonating and electron-releasing aryl bromides. Introduction of steric bulk at the 2-position of the aryl bromide resulted in reduced yield (entry 5), although modest quantities of coupled product were still isolated. A coupling reaction with naphthyl triflate (entry 2) also proved moderately successful, consistent with the ability of various halides (Table 1) and pseudohalides to undergo cross-coupling. Boronic acids other than PhB(OH)2 (entries 6–13) likewise exhibited activity for cross-coupling with 2-tolylboronic acid performing as well as the parent phenyl nucleophile. For the series of nucleophiles examined, aryl bromides containing electron-withdrawing substituents afforded slightly Table 1 Reaction optimization for the Suzuki-Miyaura coupling with complexes 1 and 2.

Entry

Precatalyst Loading

Base/equiv.

Solvent

Temp. (°C)

ArX

PhB(OH)2 equiv.

Isolated Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13a 14b 15c 16d

5 5 4 5 5 5 5 5 5 5 2 1 5 5 5 5

anhyd. K3PO4/3.0 anhyd. K3PO4/3.0 anhyd. K3PO4/2.0 anhyd. K3PO4/3.0 anhyd. K3PO4/1.5 anhyd. K3PO4/2.5 anhyd. K3PO4/3.0 KOH/3.0 KOtBu/4.0 K3PO4·H2O/2.5 K3PO4·H2O/2.5 K3PO4·H2O/2.5 K3PO4·H2O/2.5 K3PO4·H2O/3.0 K3PO4·H2O/3.0 K3PO4·H2O/3.0

THF THF 1:1 THF/tol. toluene toluene toluene toluene toluene 1:1 THF/tol. toluene toluene toluene toluene toluene toluene toluene

23 23 130 130 130 130 130 130 130 130 130 130 130 130 130 130

Cl Cl Cl Cl Br Br I Cl Cl Br Br Br Br Cl Cl Cl

1.4 1.4 1.4 1.4 1.4 1.8 1.4 1.4 1.4 2.0 2.0 2.0 2.0 1.4 1.4 1.4

0 0 21 10 42 59 39 10 0 85 82 35 67 0 0 0

a b c d

mol% mol% mol% mol% mol% mol% mol% mol% mol% mol% mol% mol% mol% mol% mol% mol%

1 2 1 2 1 1 1 1 1 1 1 1 1 1 1 1

Reaction time 3 h. K(PhBF3) used in place of PhB(OH)2, yield assayed by GC–MS. PhB(pin) used in place of PhB(OH)2, yield assayed by GC–MS. PhB(MIDA) used in place of PhB(OH)2, yield assayed by GC–MS. 2

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Table 2 Substrate scope for Suzuki-Miyaura couplings using complex 1.a

Entry

Electrophile

Nucleophile

Isolated Yield (%)

1

93

2b

55

3c

98

4

95

5

55

6b

80

7c

92

8

89

9

0

10

17

11

47

12

65

13

32

14

a b c

C8H17Br (1-bromooctane)

11

Reaction conditions are 1.0 equiv. electrophile, 1.4 equiv. nucleophile, and 2.5 equiv. K3PO4·H2O. Catalyst loading 6 mol%. Catalyst loading 2.5 mol%.

ruled out through independent synthesis of the latter and comparison to its spectroscopic features (see Supporting Information). Inclusion of 4-bromoanisole to the reaction shown in Scheme 2a afforded a small amount of cross-coupled product in addition to both toluene and 2,2′-dimethylbiphenyl. Four PNP nickel species were also detected by 1H NMR spectroscopy. Of these four species, two corresponded to complexes 1 and 3. The remaining two compounds were identified as the anisyl complex, [Ni(C6H4OMe)(PhPNP)] (4, Scheme 3), and the bromide complex, [NiBr(PhPNP)]. The identity of compound 4 was confirmed through independent synthesis from 1 and 4-anisylmagnesium bromide. The solid-state structure of 4 obtained from crystallographic analysis is displayed in Fig. 1. The appearance of compound 3 in stoichiometric reactions of 1 with 2-tolylboronic acid and 4-bromoanisole suggested its possible

Scheme 1. Stoichiometric reactions of 1 with nucleophile and electrophile in the absence of base.

3

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Scheme 2. Generation of the putative boronate complex, 3, from 2-tolylboronic acid.

Scheme 3. Single turnover experiment with precatalyst 1.

for this reduction process is displayed in Scheme 4. Such a reaction would also explain why no catalytic activity was observed for boronate esters and why both toluene and 2,2′-dimethylbiphenyl are observed as reaction products during catalysis. Given the observation of compound 5 from the reaction of 1 and boronic acid, we reasoned that it, and not the boronate 3, may be ultimately responsible for the catalytic behavior. In support of this hypothesis, prior work by our group has established the reductive capacity of 5 toward carbon halogen bonds [35]. To confirm the importance of the bimetallic Ni(I) complex 5, we next examined its stoichiometric reactivity with 4-bromoansiole. Gratifyingly, spectroscopic analysis of the reaction demonstrated that a mixture of [NiBr(PhPNP)] and 4 is produced (Scheme 5). This finding supports the notion that electrophile activation occurs from Ni(I), which is itself generated by the reaction of precatalyst 1 with boronic acid in the presence of base. Indeed, use of isolated 5 in place of 1 as a precatalyst in the reaction of 2-tolylboronic acid with 4-bromoanisole (i.e. entry 6, Table 2) led to comparable isolated yield (65%) of the coupled product. Compound 3 therefore most likely represents an inactive form of the catalyst that is produced by subsequent reaction of 5 with boronic acid. In line with this reasoning, monitoring of catalytic reactions by 1H NMR spectroscopy demonstrated increasing formation of 3 over time. Despite these observations, however, we cannot at present completely rule out a productive role for the boronate complex in catalysis. Considering the stoichiometric reactivity displayed by precatalyst 1 and bimetallic Ni(I) complex 5, we propose the mechanism for the Suzuki-Miyaura coupling shown in Scheme 6. This catalytic cycle incorporates many of the observations discussed in the preceding section and features a Ni(I)/Ni(III) redox couple, which has similarly been put forward in several other nickel systems [41–43]. Reduction of precatalyst 1 by ArB(OH)2 generates 5 and aryl radicals, which may

Fig. 1. Thermal ellipsoid drawing of the solid-state structure of 4. Hydrogen atoms omitted for clarity. Bond distances and angles can be found in the Supporting Information.

intermediacy in the CeC bond forming process. Consequently, we examined the reaction of in situ generated 3 with both 4-bromoanisole and complex 4. In both cases, prolonged heating at 80 °C failed to produce any new organic species or nickel complexes as judged by 1H NMR spectroscopy thereby arguing against a direct role for 3 in the catalytic cycle. To gain further insight into the processes responsible for the formation of 3 we attempted a scaled-up synthesis according to the reaction in Scheme 2a. During the course of the reaction we were surprised to find that the bimetallic Ni(I) species, 5, was also produced in roughly 30%. Compound 5 has been reported previously by our laboratory and others although its preparation typically requires treatment with strong reductants or hydride sources [37,38]. In the present system, the reaction of boronic acid and base with compound 1 represents a novel synthetic route to Ni(I). A plausible reaction scenario 4

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Scheme 4. Proposed formation of compound 5 from 1 and boronic acid.

Scheme 5. Electrophile activation with compound 5.

Scheme 6. Mechanistic proposal for the cross-coupling reaction. 5

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dimerize or undergo H-atom abstraction from solvent in the absence of other productive pathways. Compound 5 then reacts with electrophile to generate 4 and additional Ni(II) halide. Aryl radical capture by 4 produces a Ni(III) species primed to undergo reductive elimination and produce the cross-coupled product. Boronate complex 3 is formed by reaction of boronic acid with compound 5 and may represent a deactivation pathway or dormant state of the catalyst that can re-enter the cycle.

4.3. Materials Compounds 1, 2, and 5 were prepared by published procedures or slight modifications thereof [35,37,38]. All other reagents, including boronic acids/esters, were purchased from commercial suppliers and used as received. 4.4. General procedure for Suzuki-Miyaura reactions A Schlenk flask was charged with catalyst (1–5 mol%), electrophile (1.0 equiv.), boronic acid (1.4 equiv.), K3PO4·H2O (2.5 equiv.) and 15 mL of toluene. The mixture was heated to 130 °C and allowed to stir for 16 h under an atmosphere of nitrogen. After this time, the reaction was allowed to cool and quenched with 10 mL of a saturated aqueous oxalic acid solution. The resulting biphasic mixture was diluted with 20 mL of diethyl ether and the organic layer separated and washed with water. The organic layer was dried over sodium sulfate, filtered, and concentrated to dryness. Cross-coupled products were then purified by silica gel chromatography using 5–10% ethyl acetate in hexanes. 1H NMR spectroscopy of the isolated products in CDCl3 was used to confirm identity. Generation of [Ni(OB{OH}{2-tolyl})(PhPNP)] (3) and [NiI2(PhPNP)2] (5). To a Schlenk flask was added 350 mg (0.64 mmol) of 1, 89 mg (0.65 mmol) of 2-tolylboronic acid, and 350 mg (1.52 mmol) of K3PO4·H2O. The solids were suspended in 20 mL of toluene and the flask sealed with a rubber septum. The flask was removed from the glovebox and heated to 75 °C with stirring for 16 h. After this time, the flask was quickly returned to the glovebox and the reaction mixture was filtered through a plug of Celite while still warm. The filtrate was evaporated to dryness and extracted 3 times with 10 mL of diethyl ether. The remaining ether-insoluble residue is compound 5, of which 102 mg (32%) was isolated. The soluble fraction in the ether extracts contains 3 along with other unidentified byproducts. Evaporation of the ether permitted isolation of crude 3, but further purification protocols were unsuccessful. Selected NMR data for 3: 1H (500 MHz, C6D6) δ 7.30 (m, 8 Ph), 6.96 (m, 4 Ph), 6.91 (m, 8 Ph), 6.72 (app d, 1 tolylH), 6.64 (s, 2 pyr-CH), 3.67 (app t, 4 CH2P), 2.04 (s, 3 tolylMe); 31P (202 MHz): δ 34.7. [Ni(C6H4OMe)(PhPNP)], 4. A round bottom flask was charged with 104 mg (0.19 mmol) of 1 and 15 mL of THF. To the solution was added dropwise 1.1 mL of a 0.345 M solution (0.38 mmol) of 4-anisylmagnesium bromide in THF. The mixture was allowed to stir at room temperature for 30 min during which time the color changed from orange-red to pale brown. All volatiles were removed exhaustively and the remaining residue was extracted into 10 mL of toluene. The toluene solution was filtered through a plug of Celite and then evaporated to dryness. The resulting residue was washed with pentane and collected by filtration to afford 109 mg (92%) of a yellow solid. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a concentrated benzene solution. NMR: 1H (500 MHz, C6D6) δ 7.32 (m, 8 Ph), 7.12 (d, J = 8.5 Hz, 2 ArH), 6.97 (app t, 4 Ph), 6.91 (app t, 8 Ph), 6.69 (d, J = 8.5 Hz, 2 ArH), 6.64 (s, 2 pyr-CH), 3.66 (app t, 4 CH2P), 3.35 (s, 3 OMe); 31P (202 MHz) δ 34.0. [Ni(N{SiMe3}2)(PhPNP)]. To a round bottom flask was added 123 mg (0.22 mmol) of 1 and 41 mg (0.22 mmol) of NaN(SiMe3)2. The solids were dissolved in 15 mL of toluene and the reaction mixture was sealed with a rubber septum and brought out of the glovebox. The resulting solution was stirred at 60 °C for 16 h during which time the color darkened. All volatiles were removed in vacuo and the remaining residue was washed with 10 mL of pentane and isolated by filtration to afford 60 mg (40%) of a blue solid. NMR: 1H (500 MHz, C6D6): δ 7.67 (m, 8 Ph), 7.06 (m, 12 Ph), 6.35 (s, 2 pyr-CH), 3.28 (app t, 4 CH2P), 0.36 (s, 18 SiMe3); 31P (202 MHz) δ 4.76. Anal. Calcd for C36H44N2NiP2Si2: C, 63.44; H, 6.51; N, 4.11. Found: C, 62.28; H, 6.66; N, 3.67. Analogous synthesis of the cyclohexyl derivative, [Ni(N{SiMe3})2)(CyPNP)] was carried out beginning from 2. Slow cooling of a saturated pentane

3. Conclusions In this contribution, we have demonstrated the efficacy of the pyrrole-based pincer complex, 1, for the catalytic Suzuki-Miyaura coupling of boronic acids and aryl halides. The catalyst system is successful across of series of different electrophilic coupling partners but requires the use of boronic acid as the nucleophile source. The robust nature of the PNP-Ni system has also permitted preliminary mechanistic investigation into the catalytic pathway. Stoichiometric reactivity of the precatalyst and other putative intermediates such as 3 and 5, is most consistent with a cycle involving shuttling of the catalyst between Ni(I) and Ni(III) forms with the boronic acid playing a crucial role as both reductant and nucleophile. Formation of the putative boronate species (3) via reaction of Ni(I) with boronic acid appears to be a means of catalyst deactivation, although it remains possible that this species plays other roles in the catalysis. This mechanistic proposal differs substantially from many in the literature which favor Ni(0) as the oxidation state responsible for oxidative addition of the electrophile. These differences may reflect in large part the negative charge on the pincer ligand in the present system which disfavors formation of zerovalent nickel. 4. Experimental 4.1. General comments Manipulations of air- and moisture-sensitive materials were performed under an atmosphere of nitrogen gas using standard Schlenk technique or in an Vacuum Atmospheres glovebox under an atmosphere of purified nitrogen. Tetrahydrofuran, diethyl ether, pentane, dichloromethane, and toluene were purified by sparging with argon and passage through two columns packed with 4 Å molecular sieves. Benzene‑d6 was dried over sodium ketyl and vacuum-distilled prior to use. 1H NMR spectra were recorded on Varian spectrometer operating at 300 or 500 MHz (1H) and referenced to the residual protium resonance of the solvent (δ 7.16). Elemental analyses were performed by the CENTC facility at the University of Rochester. 4.2. Crystallography Crystals suitable for X-ray diffraction were mounted, using Paratone oil, onto a nylon loop. All data were collected at 98(2) K using a Rigaku AFC12/Saturn 724 CCD fitted with Mo Kα radiation (λ = 0.71075 Å). Low-temperature data collection was accomplished with a nitrogen cold stream maintained by an X-Stream low-temperature apparatus. Data collection and unit cell refinement were performed using the CrystalClear software [44]. Data processing and absorption correction, giving minimum and maximum transmission factors, were accomplished with CrysAlisPro [45] and SCALE3 ABSPACK [46], respectively. The structure, using Olex2 [47], was solved with the ShelXT structure solution program using direct methods and refined (on F2) with the ShelXL refinement package using full-matrix, least-squares techniques [48,49]. All nonhydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atom positions were determined by geometry and refined by a riding model. 6

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solution of [Ni(N{SiMe3}2)(CyPNP)] at −30 °C produced crystals suitable for X-ray diffraction (see Supporting Information).

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CRediT authorship contribution statement Justin Madera: Investigation, Formal analysis, Writing - original draft. Megan Slattery: Investigation. Hadi D. Arman: Investigation. Zachary J. Tonzetich: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors thank the Welch Foundation (AX-1772 to Z.J.T.) for financial support. NMR facilities at UTSA are supported by a grant from NSF (CHE-1625963). The authors also acknowledge Mr. Michael Trimble for experimental assistance through support from the ACS Project SEED Program. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ica.2020.119457. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

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