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A Polystyrene-Cross-Linking Bisphosphine: Controlled Metal Monochelation and Ligand-Enabled First-Row Transition Metal Catalysis Tomohiro Iwai, Tomoya Harada, Hajime Shimada, Kiichi Asano, and Masaya Sawamura ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02988 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 17, 2017

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A Polystyrene-Cross-Linking Bisphosphine: Controlled Metal Monochelation and Ligand-Enabled First-Row Transition Metal Catalysis Tomohiro Iwai,* Tomoya Harada, Hajime Shimada, Kiichi Asano and Masaya Sawamura* Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan ABSTRACT: A polystyrene-cross-linking bisphosphine PS-DPPBz was synthesized through radical emulsion copolymerization between 4-t-butylstyrene as a monomer and tetravinylated 1,2-bis(diphenylphosphino)benzene (DPPBz) as a fourfold cross-linker. The location of the DPPBz bisphosphine moiety at the branching points of the cross-linked network organic polymer allowed controlled bisphosphine monochelation to transition metals under conditions where homogeneous ligands may form bischelated single metal complexes or multinuclear complexes. PS-DPPBz showed high ligand performance in first-row transition metal catalysis, enabling challenging molecular transformations that were not possible through the screening of existing homogeneous ligands. In the Ni-catalyzed amination of aryl chlorides with N-alkyl-substituted primary amines, the substrate scope has been expanded to include 2,6-disubstituted aryl chlorides and Ntertiary-alkyl-substituted primary amines. PS-DPPBz was effective for the Ni-catalyzed C–H/C–O coupling between 1,3-azoles and monocyclic aryl pivalates, which showed poor reactivity in the reported homogeneous catalytic system based on 1,2bis(dicyclohexylphosphino)ethane (DCYPE). The usefulness of the polymer-cross-linking strategy was also demonstrated in alkene hydroboration reactions catalyzed by Cu or Co. KEYWORDS: immobilized phosphines, polystyrene, bisphosphine, first-row transition metals, C–N coupling, C–H/C–O coupling, hydroboration INTRODUCTION A polymer-supported phosphine is a useful material for producing heterogenized transition metal catalysts, which can provide practical benefits in synthetic processes (e.g., catalyst separation and reusability).1 Polystyrene is the most widely used support owing to its facile preparation, and physical, thermal and chemical stability.2,3 However, the use of the polystyrene backbone for increasing the catalytic performance has been overlooked in the design of highly active heterogeneous catalysts.4 In this context, our group previously developed a new type of polystyrene-crosslinking monophosphine PS-TPP (Chart 1).5 Owing to spatial isolation of the P center in the polymer matrix by the threefold crosslinking,6,7 PS-TPP achieved controlled mono-P-ligation toward metals (P/M 1:1) to enable some challenging transition metal catalyses such as Pd-catalyzed cross-coupling of unactivated aryl chlorides and Ir(Rh)-catalyzed heteroatom-directed C(sp3)–H borylation (Chart 2a). In the present work, the concept established by the threefold cross-linking monophosphine PS-TPP has been expanded to a polystyrene-cross-linking bisphosphine ligand for controlled monochelation to transition metals. Thus, a fourfold polystyrenecross-linking bisphosphine ligand (PS-DPPBz, Chart 1) based on 1,2-bis(diphenylphosphino)benzene (DPPBz) exhibited high ligand performance in first-row transition metal catalyses such as Ni-catalyzed amination of aryl chlorides with N-alkyl-substituted primary amines, Ni-catalyzed C–H/C–O coupling between 1,3azoles and aryl pivalates, and alkene hydroboration reactions catalyzed by Cu or Co (Chart 2b). Various experiments including NMR analysis for metal coordination and comparative catalytic studies with various ligands suggested that the enhanced catalytic performances with PS-DPPBz were mainly due to controlled monochelation of the bisphosphine unit and inhibited bischelation to metal centers and/or dimerization of monochelated metal species.

metal catalysis. Owing to their chelating nature, bisphosphines are effective in the construction of precise catalytic environments for various transition metal catalyses. However, monochelation to the metal center (I), bischelation to the metal center (II), or dimerization of monochelate metal species (III) may potentially occur. The latter two species may be inactive and thus cause a decrease in total efficiency of the catalysis (Chart 3a). We envisaged that the formation of II and III (Chart 3a) could be avoided by spatially isolating the bisphosphine units through immobilization on a solid support (Chart 3b). To achieve this, we designed a polystyrene-cross-linking bisphosphine PSDPPBz (Chart 1). The rigid 1,2-bis(diphenylphosphino)benzene (DPPBz) core was adopted as the bisphosphine unit to achieve reliable metal chelation ability in a polymer matrix.8–12 The steric demand in proximity to the P atoms should be only moderate (DPPBz-like) due to the spacer effect of the four aromatic rings on the P atoms, which form a pyramidal-type space; and consequently, the resulting P-P-chelated metal system should provide effective access of substrates for catalysis.

Chart 1. Polystyrene-Phosphine Hybrids Used in this Paper

RESULTS AND DISCUSSION Design of polystyrene-cross-linking bisphosphine. Bisphosphines are one of the most common ligands in transition-

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Chart 2. Conceptual View of Metal Catalysts with Polystyrene-cross-linking (a) Monophosphine5 and (b) Bisphosphine

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(CDCl3); δ –13.3 (ppm)].15 Based on the results of combustion elemental analysis for P, the loading of the DPPBz moiety to polystyrene resins was estimated to be 0.12 mmol/g. This value was comparable to a value (0.098 ≈ 0.1 mmol/g) calculated on the basis of the ratio of the used monomers. This calculated value (0.1 mmol/g) was used in metal coordination studies and catalytic applications (vide infra).

Scheme 1. Preparation of PS-DPPBz

Figure 1. TGA profiles of PS-DPPBz (heating rate: 10 °C/min, in air).

Chart 3. Metal Chelation Modes with Bisphosphines

Figure 2. 31P CP/MAS NMR spectrum of PS-DPPBz (spinning rate 8 kHz).

Preparation of polystyrene-cross-linking bisphosphine. PS-DPPBz was synthesized through radical emulsion polymerization with tetravinylated DPPBz (1)9a and 4-t-butylstyrene (1/4-tbutylstyrene 1:60), as shown in Scheme 1. The obtained beads of PS-DPPBz had ordinary swelling properties as a polystyrene resin toward common organic solvents.13 The thermogravimetric analysis (TGA, 10 °C/min, in air) of the dried PS-DPPBz showed that this material was stable below 250 °C (Figure 1).14 The 31P CP/MAS NMR spectrum exhibited a broad signal at δ –17 (ppm) (Figure 2), indicating that its electronic and steric properties are virtually the same as that of a DPPBz molecule [31P NMR

Coordination of a Rh(I) species. To confirm the monochelating property of PS-DPPBz, a cationic Rh(I) complex was employed as a metal source because it was prone to undergo bischelation by two bisphosphine ligands.16 In fact, the reaction of the soluble ligand DPPBz with [Rh(cod)2]BF4 (DPPBz/Rh 1:1) at rt afforded a mixture of mono-DPPBz-chelated complex [Rh(cod)(DPPBz)]BF4 and bis-DPPBz-chelated complex [Rh(DPPBz)2]BF4 with NMR resonances at δ 58.0 and δ 62.9 (ppm), respectively (Figure 3a). When the DPPBz/Rh ratio was 2:1, the DPPBz bisphosphine was completely consumed for exclusive formation of the bischelated complex [Rh(DPPBz)2]BF4 (Figure 3b). The corresponding experiments with the polymer ligand PS-DPPBz showed contrasting behavior. The reaction of PS-DPPBz with [Rh(cod)2]BF4 with a PS-DPPBz/Rh ratio of 1:1

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was conducted in CH2Cl2 at rt for 2 h. Filtration to remove a colorless solution, washing, and removal of volatiles gave orange beads. The solid-state 31P CP/MAS NMR spectrum of the beads indicated the formation of a new Rh-bound DPPBz species, which gave a signal at δ 53 (ppm) accompanied by significant spinning side bands (Figure 4a). Although the high-field signal of the spinning side bands overlapped the region of the free bisphosphine signal [δ –17 (ppm)], most of the bisphosphine moieties including those deep inside the beads were accessible for the metal complexation. Notably, even with excess bisphosphine toward the Rh(I) complex (PS-DPPBz/Rh 2:1), [Rh(cod)(PSDPPBz)]BF4 existed as the virtually sole Rh-bound P species, along with the free bisphosphine (Figure 4b). Thus, the polystyrene-cross-linking strategy appeared to be effective to realize controlled bisphosphine monochelation to metals owing to the spatial isolation of the coordination center in the polymer matrix. Effect of the bridge between the two P atoms: Coordination of a Pd(II) species. The following NMR experiments elucidated that the bridge between the two P atoms must be sufficiently rigid for maintaining the chelating ability of the bisphosphine as a polymer cross-linker. The 31P CP/MAS NMR spectrum obtained from PS-DPPBz and excess PdCl2(cod) in THF at rt (PSDPPBz/Pd 1:2) showed only one signal of a bisphosphinemonochelate Pd complex PdCl2(PS-DPPBz) at δ 61 (ppm) (Figure 5a).17 In contrast, the use of a fourfold cross-linked polystyrene PS-DPPE (Chart 1) based on a more flexible bisphosphine 1,2-bis(diphenylphosphino)ethane (DPPE) instead of PS-DPPBz gave two well-separated signals at δ 63 and δ 33 (ppm) (Figure 5b), which correspond to a bisphosphine-monochelate Pd complex PdCl2(PS-DPPE) and a mono-P-ligated (non-chelate) Pd complex Pd2Cl4(L)2(PS-DPPE) (L = donor ligands), respectively (see Supporting Information for 31P NMR studies with EtPh2P as a model ligand, Figure S8). The P–C–C–P tetrad of the DPPE core in PS-DPPE must adopt a thermally favored s-trans conformation as prepared, and the structural change to the s-cis conformation, which is required for metal chelation, seems to be inhibited due to the rigidity of the cross-linked polymer matrix.

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Figure 4. 31P CP/MAS NMR spectra obtained from PS-DPPBz and [Rh(cod)2]BF4 [PS-DPPBz/Rh (a) 1:1 and (b) 2:1] (spinning rate 8 kHz).

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Figure 3. 31P NMR spectra in CDCl3 obtained from DPPBz and [Rh(cod)2]BF4 [(a) DPPBz/Rh 1:1 and (b) 2:1].

Figure 5. 31P CP/MAS NMR spectra obtained (a) from PSDPPBz and PdCl2(cod) (PS-DPPBz/Pd 1:2), and (b) from PSDPPE and PdCl2(cod) (PS-DPPE/Pd 1:2) (spinning rate 8 kHz).

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First-row transition metal catalysis. First-row transition metal catalysis has attracted great attention as a strategy for environmentally benign organic synthesis because of their high earth abundance and low toxicity.18 However, homogeneous catalytic systems with first-row transition metals often suffer from facile ligand dissociation, aggregation, and/or disproportionation of metal complexes. We expected that the polystyrene-cross-linking bisphosphine PS-DPPBz should be useful for eliminating problems associated with aggregation and disproportionation reactions that demand proximity of different catalytic centers, thus realizing highly efficient first-row transition metal catalysis. Nickel-catalyzed amination of aryl chlorides with N-alkylsubstituted primary amines. As the initial first-row transition metal catalysis to test the feasibility of our idea described above, we adopted Ni-catalyzed amination of unactivated aryl chlorides with N-alkyl-substituted primary amines,19 for which (BINAP)Ni(η2-NC-Ph) was identified to be an exceptionally effective catalyst precursor by Hartwig and co-workers (Chart 4).20 Mechanistic studies by Hartwig’s group indicated the importance of controlled Ni monochelation by the BINAP bisphosphine ligand (A) to form a catalytically active species that reacts with aryl chlorides. In addition, they identified four catalytically inactive species, (BINAP)2Ni(0) (B), [(BINAP)NiI(µ2-Cl)]2 (C), (amine)2NiAr(Cl) (D), and a nickelacycle complex (E), the formation of which limited the catalytic activity and scope of the reaction. Specifically, the reaction of 4-chloroanisole (2a), which was deactivated by an electron-donating p-MeO group, with noctylamine (3a) required relatively high catalyst loading (4 mol%) to obtain an acceptable yield of coupling product 4a.20–22 We envisioned that the use of PS-DPPBz could suppress the formation of the inactive species B and C (Chart 4), both of which have two bisphosphine ligands. Moreover, the rigid DPPBz core should be favorable for suppressing the ligand degradation due to P–C bond cleavage as in the formation of E.

Chart 4. Ni-catalyzed Amination of Aryl Chlorides with NAlkyl-substituted Primary Amines reported by Hartwig’s Group20

Initially, the reaction between 2a and 3a was conducted at 50 °C with PS-DPPBz/Ni(cod)2 or (BINAP)Ni(η2-NC-Ph) systems with 1 mol% Ni loading (Chart 5). Both of the catalytic systems induced the amination, but with comparable low yields of 4a (90% purity, 15 mmol), 1 (0.1375 g, 0.25 mmol) and AIBN (0.050 g, 0.30 mmol) in chlorobenzene (3 mL) was degassed by three freeze-pump-thaw cycles, and was injected into the rapidly stirred aqueous solution. The mixture was stirred at 80 °C for 18 h. After being cooled to rt, the mixture was filtered through a glass adaptor equipped with a cotton plug, washed successively with H2O, MeOH, toluene, THF and MeOH, and dried in vacuo at 100 °C for 12 h to give the polymer PS-DPPBz as white beads (2.30 g, 90 wt%). The obtained beads were passed through a series of sieves, and the beads with 250–710 µm size in diameter were used for catalytic reactions. A Typical Procedure for Ni-catalyzed Amination of Aryl Chlorides with N-Alkyl-substituted Primary Amines (Table 1, Entry 1). In a nitrogen-filled glove box, PS-DPPBz (37.5 mg, 0.00375 mmol, 1.5 mol%), toluene (0.72 mL) and a solution of Ni(cod)2 (0.69 mg, 0.0025 mmol, 1 mol%) in toluene (0.28 mL) were placed in a 10-mL glass tube containing a magnetic stirring bar. After stirring at rt for 5 min, NaOtBu (36.0 mg, 0.375 mmol), 4-chloroanisole (2a, 30.6 µL, 0.25 mmol) and n-octylamine (3a, 62.1 µL, 0.375 mmol) were added successively. The tube was sealed with a screw cap and was removed from the glove box. The mixture was stirred at 80 °C for 20 h. After being cooled to rt, the mixture was filtered through a Celite pad (eluting with Et2O). The volatiles were removed under reduced pressure, and an internal standard (1,2-diphenylethane) was added to determine the yield of 4-methoxy-N-octylaniline (4a, 93% yield). The crude product was purified by silica gel column chromatography (hexane/EtOAc 100:0 to 95:5) to give 4a as a pale yellow oil (53.6 mg, 0.228 mmol, 91% yield). A Typical Procedure for Ni-catalyzed C–H/C–O Coupling between Benzoxazole and Aryl Pivalates (Table 2, Entry 1). In a nitrogen-filled glove box, PS-DPPBz (60 mg, 0.0060 mmol, 6 mol%), m-xylene (0.5 mL) and a solution of Ni(cod)2 (1.4 mg, 0.005 mmol, 5 mol%) in m-xylene (0.5 mL) were placed in a 10mL glass tube containing a magnetic stirring bar. After stirring at rt for 10 min, Cs2CO3 (48.9 mg, 0.15 mmol), phenyl pivalate (7b, 26.7 mg, 0.15 mmol) and benzoxazole (6a, 11.9 mg, 0.1 mmol) were added. The tube was sealed with a screw cap, and was removed from the glove box. The mixture was stirred at 150 °C for 24 h. After being cooled to rt, the mixture was filtered through a short silica pad (eluting with EtOAc). The volatiles were removed under reduced pressure, and an internal standard (1,1,2,2tetrachloroethane) was added to determine the yield of 2phenylbenzo[d]oxazole (8b, 83% yield). The crude product was purified by preparative thin-layer chromatography (silica gel, hexane/EtOAc 10:1) to give 8b as a white solid (16.6 mg, 0.085 mmol, 85% yield). A Typical Procedure for Cu-catalyzed Alkene Hydroboration with Pinacolborane (Table 3, Entry 1). In a nitrogen-filled glove box, PS-DPPBz (30 mg, 0.0030 mmol, 1.5 mol%), THF (0.5 mL) and a solution of Cu(OAc)2 (0.36 mg, 0.0020 mmol, 1 mol%) in THF (0.5 mL) were placed in a 10-mL glass tube containing a magnetic stirring bar. The tube was sealed with a screw cap and was removed from the glove box. The mixture was stirred at 80 °C for 10 min. After being cooled to rt, the volatiles were removed under vacuum. The mixture was moved to the glove box again. Toluene (1 mL), 4-phenyl-1-butene (9a, 26.4 mg, 0.20 mmol) and pinacolborane (10, 30.7 mg, 0.24 mmol) were added to the mixture. The tube was sealed with a screw cap and was removed from the glove box. The mixture was stirred at 80 °C for 2 h. After being cooled to rt, the mixture was filtered through a Celite pad (eluting with Et2O). The volatiles were removed under reduced pressure, and an internal standard (1,1,2,2-

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tetrachloroethane) was added to determine the yield of 4,4,5,5tetramethyl-2-(4-phenylbutyl)-1,3,2-dioxaborolane (11a, 92% yield). The crude product was purified by silica gel column chromatography (hexane/EtOAc 95:5) to give 11a as a colorless oil (41.0 mg, 0.158 mmol, 79% yield). A Typical Procedure for Co-catalyzed Alkene Hydroboration with Pinacolborane (Eq 2). In a nitrogen-filled glove box, PS-DPPBz (30 mg, 0.0030 mmol, 1.5 mol%), a solution of CoI2 (0.63 mg, 0.0020 mmol, 1 mol%) in THF (0.13 mL), and THF (0.27 mL) were placed in a 10-mL glass tube containing a magnetic stirring bar. After stirring at rt for 5 min, NaBEt3H in THF (1 M, 10 µL, 0.01 mmol, 5 mol%) was added, and the resulting mixture was stirred for an additional 10 min. α-Methylstyrene (9c, 23.7 mg, 0.20 mmol) and pinacolborane (10, 30.7 mg, 0.24 mmol) were added successively. The tube was sealed with a screw cap and was removed from the glove box. The mixture was stirred at 25 °C for 16 h. The mixture was filtered through a Celite pad (eluting with Et2O). The volatiles were removed under reduced pressure, and an internal standard (1,1,2,2tetrachloroethane) was added to determine the yield of 4,4,5,5tetramethyl-2-(2-phenylpropyl)-1,3,2-dioxaborolane (11c, 78% yield). The crude product was purified by silica gel column chromatography (hexane/EtOAc 100:0 to 95:5) to give 11c as a colorless oil (31.0 mg, 0.126 mmol, 63% yield, contaminated with trace amounts of impurities).

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ASSOCIATED CONTENT Supporting Information. Experimental details and characterization data for new compounds (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

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Corresponding Author * [email protected] * [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Number JP25810056 to Young Scientists (B) to T.I. and by ACT-C and CREST from JST, and JSPS KAKENHI Grant Number JP15H05801 in Precisely Designed Catalysts with Customized Scaffolding to M.S. T.H. thanks the JSPS fellowship for young scientists. Support from Tosoh Organic Chemical Co., Ltd. is gratefully acknowledged. We thank Prof. Kenta Kokado (Hokkaido University) for the TGA measurement.

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(14) Thermal degradation of commercially available non-cross-linked polystyrene (average Mw ~280,000) began at 200 °C in air: Peterson, J. D.; Vyazovkin, S.; Wight, C. A. Macromol. Chem. Phys. 2001, 202, 775–784. (15) McFarlane, H. C. E.; McFarlane, W. Polyhedron 1999, 18, 2117–2127. The upfield shift (∆δ ca. –4 ppm) is reasonable as electron-donating effects of the p-alkyl substituents of the Pphenyl groups. (16) (a) Anderson, M. P.; Pignolet, L. H. Inorg. Chem. 1981, 20, 4101–4107. (b) Tani, K.; Yamagata, T.; Tatsuno, Y.; Yamagata, Y.; Tomita, K.; Akutagawa, S.; Kumobayashi, H.; Otsuka, S. Angew. Chem., Int. Ed. Engl. 1985, 24, 217–219. (c) Marshall, W. J.; Aullón, G.; Alvarez, S.; Dobbs, K. D.; Grushin, V. V. Eur. J. Inorg. Chem. 2006, 2006, 3340–3345. (17) After filtration of the polymer-bound Pd(II) complex, the measurement of weight of unreacted PdCl2(cod) in the filtrate allowed us to estimate the DPPBz loading on the polystyrene resin, which was assumed to be 0.11 mmol/g comparable to the calculated value based on the ratio of the used monomers (0.1 mmol/g). (18) (a) Catalysis without Precious Metals, Bullock, R. M., Ed.; Wiley-VCH: Weinheim, 2010. (b) Chirik, P.; Morris, R. Acc. Chem. Res. 2015, 48, 2495–2495. (c) Dunetz, J. R.; Fandrick, D.; Federsel, H.-J. Org. Process Res. Dev., 2015, 19, 1325– 1326. See also: (d) Egorova, K. S.; Ananikov, V. P. Angew. Chem., Int. Ed. 2016, 55, 12150–12162. (19) Selected reviews on transition-metal catalyzed C–N coupling reactions: (a) Hartwig, J. F. Acc. Chem. Res. 2008, 41, 1534– 1544. (b) Surry, D. S.; Buchwald, S. L. Chem. Sci. 2011, 2, 27– 50. (c) Lundgren, R. J.; Stradiotto, M. Aldrichimica Acta 2012, 45, 59–65. (d) Beletskaya, I. P.; Cheprakov, A. V. Organometallics 2012, 31, 7753–7808. (e) Bariwal, J.; Van der Eycken, E. Chem. Soc. Rev. 2013, 42, 9283–9303. (f) Valente, C.; Pompeo, M.; Sayah, M.; Organ, M. G. Org. Process Res. Dev. 2014, 18, 180–190. (g) Marín, M.; Rama, R. J.; Nicasio, M. C. Chem. Rec. 2016, 16, 1819–1832. (h) Ruiz-Castillo, P.; Buchwald, S. L. Chem. Rev. 2016, 116, 12564–12649. (20) Ge, S.; Green, R. A.; Hartwig, J. F. J. Am. Chem. Soc. 2014, 136, 1617–1627. (21) Other selected reports of Ni-catalyzed amination of aryl chlorides with N-alkyl-substituted primary amines: (a) Wolfe, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1997, 119, 6054–6058. (b) Desmarets, C.; Schneider, R.; Fort, Y. J. Org. Chem. 2002, 67, 3029–3036. (c) Kampmann, S. S.; Skelton, B. W.; Wild, D. A.; Koutsantonis, G. A.; Stewart, S. G. Eur. J. Org. Chem. 2015, 2015, 5995–6004. Recently, Stradiotto and co-workers reported that a sterically demanding and electron-poor bisphosphine allowed the efficient Ni-catalyzed amination of various aryl electrophiles, including aryl chlorides, with N-alkyl-substituted primary amines (for the reaction between 2a and 3a, 5 mol% Ni, 60 °C, 85%): (d) Lavoie, C. M.; MacQueen, P. M.; Rotta-Loria, N. L.; Sawatzky, R. S.; Borzenko, A.; Chisholm, A. J.; Hargreaves, B. K. V.; McDonald, R.; Ferguson, M. J.; Stradiotto, M. Nat. Commun. 2016, 7, 11073. (e) Clark, J. S. K.; Lavoie, C. M.; MacQueen, P. M.; Ferguson, M. J.; Stradiotto, M. Organometallics 2016, 35, 3248–3254. (22) Recently, Cu-catalyzed C–N coupling with aryl chlorides was reported. See: Zhou, W.; Fan, M.; Yin, J.; Jiang, Y.; Ma, D. J. Am. Chem. Soc. 2015, 137, 11942–11945. (23) Hatakeyama, T.; Hashimoto, T.; Kondo, Y.; Fujiwara, Y.; Seike, H.; Takaya, H.; Tamada, Y.; Ono, T.; Nakamura, M. J. Am. Chem. Soc. 2010, 132, 10674–10676. (24) See Supporting Information for details of ligand effects (Table S2). (25) (a) Green, R. A.; Hartwig, J. F. Angew. Chem., Int. Ed. 2015, 54, 3768–3772. (b) Borzenko, A.; Rotta-Loria, N. L.; MacQueen, P. M.; Lavoie, C. M.; McDonald, R.; Stradiotto, M. Angew. Chem., Int. Ed. 2015, 54, 3773–3777.

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(26) 21.0% of the loaded Ni was detected in the solution phase by ICP-AES analysis. See Supporting Information for details (Table S3). (27) Formation of catalytically inactive 2:1 bisphosphine-Ni(0) species bound to the polystyrene resin is also conceivable as one of the catalyst deactivation pathways. (28) Effects of monomer ratio in PS-DPPBz (1/4-t-butylstyrene 1:0 to 1:60) towards the reaction efficacy were also investigated for the reaction at 80 °C over 6 h. The organic polymers with higher DPPBz loading (1:0 to 1:30) showed lower catalytic activity than that with the standard polymer concentration (1:60). See Supporting Information for details (Table S4). (29) Air sensitivity of polystyrene-bound bisphosphine-Ni(0) complexes hampered spectroscopic analysis of coordination properties of the polystyrene-cross-linking bisphosphines to Ni(cod)2. (30) (a) Phan, N. T. S.; Sluys, M. V. D.; Jones, C. W. Adv. Synth. Catal. 2006, 348, 609–679. (b) Crabtree, R. H. Chem. Rev. 2012, 112, 1536–1554. (31) In some cases, isolated products (4o, 4q and 4s) were contaminated with small amounts of the corresponding imines (1%, 2% and 2% yields, respectively), as estimated by 1H NMR analyses. (32) The (PS-DPPBz)-Ni catalyst system competes with the recently reported Pd-catalyzed C–N coupling employing bulky biarylbased monophosphines as supporting ligands in reaction efficacy and substrate scope: Ruiz-Castillo, P.; Blackmond, D. G.; Buchwald, S. L. J. Am. Chem. Soc. 2015, 137, 3085–3092. (33) Muto, K.; Yamaguchi, J.; Itami, K. J. Am. Chem. Soc. 2012, 134, 169−172. (34) Selected reviews on transition-metal-catalyzed cross-coupling reactions with inert C–O electrophiles: (a) Rosen, B. M.; Quasdorf, K. W.; Wilson, D. A.; Zhang, N.; Resmerita, A.-M.; Garg, N. K.; Percec, V. Chem. Rev. 2010, 111, 1346−1416. (b) Yu, D.-G.; Li, B.-J.; Shi, Z.-J. Acc. Chem. Res. 2010, 43, 1486−1495. (c) Li, B.-J.; Yu, D.-G.; Sun, C.-L.; Shi, Z.-J. Chem. Eur. J. 2011, 17, 1728−1759. (d) Mesganaw, T.; Garg, N. K. Org. Process Res. Dev. 2013, 17, 29−39. (e) Kozhushkov, S. I.; Potukuchi, H. K.; Ackermann, L. Catal. Sci. Technol., 2013, 3, 562−571. (f) Tobisu, M.; Chatani, N. Acc. Chem. Res. 2015, 48, 1717−1726. (35) Later, Yamaguchi, Itami and co-workers reported that 3,4bis(dicyclohexylphosphino)thiophene was an effective ligand for the Ni-catalyzed C–H/C–O coupling between 1,3-azoles and aryl carbamates: Muto, K.; Hatakeyama, T.; Yamaguchi, J.; Itami, K. Chem. Sci. 2015. 6, 6792–6798. (36) (a) Muto, K.; Yamaguchi, J.; Lei, A.; Itami, K. J. Am. Chem. Soc. 2013, 135, 16384−16387. See also: (b) Hong, X.; Liang, Y.; Houk, N. K. J. Am. Chem. Soc. 2014, 136, 2017−2025. (c) Lu, Q.; Yu, H.; Fu, Y. J. Am. Chem. Soc. 2014, 136, 8252−8260. (d) Xu, H.; Muto, K.; Yamaguchi, J.; Zhao, C.; Itami, K.; Musaev, D. G. J. Am. Chem. Soc. 2014, 136, 14834−14844. (37) Attempts to reuse of the (PS-DPPBz)-Ni system were unsuccessful, probably due to partial decomposition of catalytically active species during the reaction. (38) The effects of PS-DPPBz in the reaction of more reactive naphthyl pivalates (7a) were comparable with those of DCYPE. Specifically, the (PS-DPPBz)-Ni system (5 mol% Ni) catalyzed the C–H/C–O coupling between 6a and 7a with Cs2CO3 in 1,4dioxane at 120 °C over 24 h to give 8a in 82% yield.

(39) Recently, Ni-catalyzed α-arylation of ketones with aryl pivalates using BINAP-type ligands was reported. See: Cornella, J.; Jackson, E. P.; Martin, R. Angew. Chem., Int. Ed. 2015, 54, 4075– 4078.

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(40) (a) Noh, D.; Chea, H.; Ju, J.; Yun, J. Angew. Chem., Int. Ed. 2009, 48, 6062–6064. (b) Noh, D.; Yoon, S. K.; Won, J.; Lee, J. Y.; Yun, J. Chem. Asian J. 2011, 6, 1967–1969. (c) Feng, X.; Jeon, H.; Yun, J. Angew. Chem., Int. Ed. 2013, 52, 3989–3992. (d) Lee, S.; Li, D.; Yun, J. Chem. Asian J. 2014, 9, 2440–2443. (e) Lee, H.; Lee, B. Y.; Yun, J. Org. Lett. 2015, 17, 764–766. (f) Xi, Y.; Hartwig, J. F. J. Am. Chem. Soc. 2016, 138, 6703–6706. See also: (g) Grigg, R. D.; Van Hoveln, R.; Schomaker, J. M. J. Am. Chem. Soc. 2012, 134, 16131–16134. (41) Mechanistic studies on the Cu-catalyzed hydroboration of styrene with HBpin: Won, J.; Noh, D.; Yun, J.; Lee, J. Y. J. Phys. Chem. A, 2010, 114, 12112–12115. (42) Related review of catalytic transformations using copper(I) hydride species: (a) Rendler, S.; Oestreich, M. Angew. Chem., Int. Ed. 2007, 46, 498–504. (b) Díez-González, S.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 349–358. (c) Deutsch, C.; Krause, N. Chem. Rev. 2008, 108, 2916–2927. (d) Lipshuz, B. H. Synlett 2009, 509–524. (e) Fujihara, T.; Semba, K.; Terao, J.; Tsuji, Y. Catal. Sci. Technol. 2014, 4, 1699–1709. (f) Pirnot, M. T.; Wang, Y.-M.; Buchwald, S. L. Angew. Chem., Int. Ed. 2016, 55, 48–57. See also for a review of copper(I) hydride clusters: (g) Dhayal, R. S.; van Zyl, W. E.; Liu, C. W. Acc. Chem. Res. 2016, 49, 86–95. (43) A related paper from our group on copper(I) hydride catalyzed transformation with a silica-supported monophosphine ligand: Kawamorita, S.; Yamazaki, K.; Ohmiya, H.; Iwai, T.; Sawamura, M. Adv. Synth. Catal. 2012, 354, 3440−3444. (44) Hartwig and a co-worker showed that the [(S)-DTBM-Segphos]Cu system gave high enantioselectivity (98% ee) in the hydroboration of trans-4-octene with HBpin, but the reactivity of this simple alkene was much lower than that of the alkenes bearing electron-withdrawing functional groups at a homoallylic position (See ref 40f). (45) After stirring the mixture of PS-DPPBz and Cu(OAc)2 in THF at rt, the color of the solution phase remained blue, indicating in-

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sufficient metal complexation. In contrast, heating the mixture at 80 °C caused almost complete coloring of the beads to greenish blue over few minutes. For the catalytic reaction, toluene appeared to be the more effective solvent than THF. Selected examples of the copper(I) hydride catalysis using bulky ligands: (a) Lipshutz, B. H.; Noson, K.; Chrisman, W. J. Am. Chem. Soc. 2001, 123, 12917–12918. (b) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Organometallics 2004, 23, 3369–3371. (c) Fujihara, T.; Semba, K.; Terao, J.; Tsuji, Y. Angew. Chem., Int. Ed. 2010, 49, 1472–1476. See also ref 42d. (a) Zaidlewicz, M.; Meller, J. Tetrahedron Lett. 1997, 38, 7279– 7282. (b) Zaidlewicz, M.; Meller, J. Main Group Metal Chem. 2000, 23, 765–771. Recent reports on Co-catalyzed alkene hydroboration with HBpin: (a) Obligacion, J. V.; Chirik, P. J. J. Am. Chem. Soc. 2013, 135, 19107–19110. (b) Zhang, L.; Zuo, Z.; Leng, X.; Huang, Z. Angew. Chem., Int. Ed. 2014, 53, 2696–2700. (c) Zhang, L.; Zuo, Z.; Wan, X.; Huang, Z. J. Am. Chem. Soc. 2014, 136, 15501–15504. (d) Chen, J.; Xi, T.; Ren, X.; Cheng, B.; Guo, J.; Lu, Z. Org. Chem. Front. 2014, 1, 1306–1309. (e) Ruddy, A. J.; Sydora, O. L.; Small, B. L.; Stradiotto, M.; Turculet, L. Chem. Eur. J. 2014, 20, 13918–13922. (f) Palmer, W. N.; Diao, T.; Pappas, I.; Chirik, P. J. ACS Catal. 2015, 5, 622–626. (g) Scheuermann, M. L.; Johnson, E. J.; Chirik, P. J. Org. Lett. 2015, 17, 2716–2719. (h) Zhang, L.; Huang, Z. J. Am. Chem. Soc. 2015, 137, 15600–15603. (i) Reilly, S. W.; Webster, C. E.; Hollis, T. K.; Valle, H. U. Dalton Trans. 2016, 45, 2823–2828. (j) Zuo, Z.; Yang, J.; Huang, Z. Angew. Chem., Int. Ed. 2016, 55, 10839–10843. (k) Zhang, H.; Lu, Z. ACS Catal. 2016, 6, 6596– 6600. See also, ref 10d. The recovered (PS-DPPBz)-Co system by decantation showed decreased catalytic activity in the second run (~40%).

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