Addition of a B–H Bond across an Amido–Cobalt Bond: CoII–H

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Addition of a B−H Bond across an Amido−Cobalt Bond: CoII−HCatalyzed Hydroboration of Olefins Maofu Pang,†,‡ Chengjuan Wu,† Xuewen Zhuang,† Fanjun Zhang,† Mincong Su,† Qingxiao Tong,*,‡ Chen-Ho Tung,† and Wenguang Wang*,† †

Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, People’s Republic of China ‡ Department of Chemistry, Shantou University, 243 University Road, Shantou, Guangdong 515063, People’s Republic of China S Supporting Information *

ABSTRACT: This paper describes a well-defined cobalt(II) half-sandwich complex bearing a phosphinoaminato ligand, Cp*Co(1,2-Ph2PC6H4NH) (1), that can activate pinacolborane (HBpin) for catalytic terminal hydroboration of olefins. The cooperative cobalt(II)−amido reactivity in 1 enables the B−H bond cleavage, affording the 17-electron cobalt(II) hydride Cp*Co(1,2-Ph2PC6H4NH(Bpin)), abbreviated H1(Bpin), in which the borenium ion is captured by the uncoordinated nitrogen atom of the phosphinoaminato ligand. Hydroboration of the CC bond can be promoted by a heteroatom such as N or O at the β-position of terminal alkenes. The mechanism of such hydroboration was established by various stoichiometric reactions based on the cobalt(II) hydride. With cooperative CoII−N reactivity for the B−H bond cleavage, our catalysis depends on the CoII−H hydride generated by the system itself.

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INTRODUCTION Transition-metal-catalyzed hydroboration of alkenes and alkynes is a particularly powerful method in organic synthesis because it provides valuable organoboron reagents for a range of cross-coupling transformations.1,2 In this type of catalysis, activation of the B−H bond of hydroboranes such as HBpin is a fundamental step, essential for the rational design and optimization of a novel homogeneous catalyst with high activity and selectivity.3 The elementary reaction between an organometallic catalyst and a hydroborane depends upon the nature of the metal center and its ligand scaffold.4 The traditional modes of B−H bond activation have been the domain of precious-metal-based oxidative addition,5 as exemplified by HRh(PiPr3)2(Bcat)Cl6 and [Ir(C5Me5)(PMe3)(Bpin)H],7 and σ-bond metathesis.8 In recent years, use of more earth-abundant metals for homogeneous catalysis has attracted growing interest.9,10 In the context of metal-catalyzed hydroborations, cobalt complexes have been developed to pursue high-efficiency catalysis with excellent selectivity under mild reaction conditions.11 For example, Chirik et al. showed that (pyridyldiimine)cobalt complexes catalyze anti-Markovnikov hydroboration of terminal and internal olefins (Chart 1, I and II).12 Related catalysis has also been reported by the Huang11 and Lu13 groups based on (bipyridyl-phosphine)CoCl2 (IV) and (iminopyridineoxazoline)CoCl2 (V) systems, which are activated in situ with NaBHEt3. Notably, Thomas et al. and Chirik et al. have found that the cobalt(II) precursors (VII and VIII) can be activated by tBuOK14 and LiOMe.15 The B−H bond is almost activated © XXXX American Chemical Society

Chart 1. Examples of Cobalt Complexes for Catalytic Hydroboration of Alkenes by HBpin

Received: February 23, 2018

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DOI: 10.1021/acs.organomet.8b00114 Organometallics XXXX, XXX, XXX−XXX

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Organometallics by oxidative addition of HBpin to the Co(I) compound (III)16 or via σ-bond metathesis based on low-valent cobalt hydride or alkyl active species.11a,12 In addition to metal-centered activation, the H−B bond can also be activated by cooperative metal−ligand reactivity.17 In such cases, the metal center accepts the hydride while the basic site of the ligand captures the remaining borenium ion.18 This strategy has been successfully applied to Ru-19 and Ir-based20 systems for catalytic hydroborations. However, the examples of cobalt-based systems are sparse. It is noteworthy that Stradiotto, Turculet et al. have recently reported a threecoordinate (N-phosphinoamidinate)(amido)cobalt(II) precatalyst (IX) for terminal hydroboration of branched alkenes.21 By the synergism of cobalt−amido reactivity, the systems operate in the absence of external activation. Generation of CoII−H species by direct addition of a hydroborane to a cobalt(II) entity for catalytic hydroboration has not been previously reported. In this paper, we report a cobalt(II) half-sandwich complex (1) bearing a phosphinoaminato ligand that, with HBpin, achieves catalytic terminal hydroboration of olefins. The reaction is initiated by addition of the B−H bond across an N−Co bond (Scheme 1), affording

Figure 1. Structure of 1 showing 50% probability ellipsoids. For clarity, hydrogen atoms are omitted, and the two phenyl groups bonded at the phosphorus atom are drawn as lines. Selected bond distances (Å): Co−N, 1.857(4); Co−P, 2.161(1).

Table 1. Cobalt(II)-Catalyzed Hydroboration of Olefinsa

Scheme 1. Addition of HBpin across an N−Co Bond To Generate CoII−H for Catalytic Hydroborations

a 17-electron cobalt(II) hydride, H1(Bpin), that is responsible for the catalysis. On the basis of the isolation and characterization of such a well-defined cobalt(II) hydride, the mechanism of catalytic hydroboration was established by various stoichiometric reactions.

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RESULTS AND DISCUSSION Synthesis and Characterization of 1. By the reaction of [Cp*CoCl]2 with lithium (2-(diphenylphosphino)phenyl)amide at −78 °C in THF, 1 was prepared. After evaporation of the reaction mixture, the product was extracted into toluene and isolated in high yield as a pink solid. The magnetic moment of 2.02 μB established for 1 by the Evans method is consistent with a low-spin d7 configuration (μeff = 2.2 μB).22 Crystallographic analysis confirmed its structure as a five-coordinate neutral cobalt(II) compound, in which the phosphinoaminato group is chelated at the cobalt center with a 17-electron configuration (Figure 1). Hydroboration of Alkenes. The activity of 1 as a hydroboration catalyst was demonstrated with unactivated aliphatic alkenes (Table 1). Terminal alkenes such as vinylcyclohexane, 1-hexene, 1-octene, and 1-isopentene (entries 1−4) underwent anti-Markovnikov hydroboration with HBpin. High-yield conversions were achieved in 4 h with only 2 mol % of catalyst at 40 °C. The corresponding terminal alkylboronic esters were obtained in ∼90% yields. Internal alkenes, including trans- and cis-alkenes, are all amenable to this transformation, affording the isomerization−hydroboration products, again in good yields (entries 5−8). For a mixture of 1-hexene, cis-3-

a

Reaction conditions: olefin substrate (0.5 mmol), HBpin (0.5 mmol), and 1 (2 mol %) were stirred in C6D6 (0.6 mL) at 40 °C. bIsolated yields are provided.

hexene, and trans-2-hexene, only a single product was obtained, the terminal alkylboronic ester (3b, entries 2, 5, and 6). Endocyclic alkenes such as cyclohexene and norbornylene, which are more challenging in hydroboration reactions, required prolonged reaction times but were ultimately converted to the desired products (3f, 52%; 3g, 43%). Stepwise Hydroboration of CC and CN Bonds. Chemoselective hydroboration of alkene substrates in the presence of carbonyl or imine functional groups is always challenging.16,23 We were therefore encouraged to find that 1 catalyzed selective hydroboration of N-allylimines (Table 2). With 1 mol % of 1, the reaction of N-allylbenzylideneamine and B

DOI: 10.1021/acs.organomet.8b00114 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 3. “Heteroatom-Promoted” Hydroboration of Alkenesa

Table 2. Cobalt(II)-Catalyzed Stepwise Hydroboration of CC and CN Bondsa,b

a

Reaction conditions: olefin substrate (0.5 mmol), HBpin (0.5 mmol), and 1 (1 mol %) were stirred in C6D6 (0.6 mL) at room temperature. Isolated yields are provided. bTwo equivalents of HBpin was used.

presence of 2 equiv of HBpin, allylamine was fully converted to the doubly borylated compound (7b) in 15 min. A similar reaction with a secondary or a tertiary N-allylamine gave the expected hydroborated products (7c,d) in perfect yields within 30 min. However, the hydroboration reactions became slow in the presence of an external amine. For instance, the yield of 7d decreased to 33% when the catalysis was carried out with the addition of 0.2 equiv of diethylamine into the system. Interestingly, 4-pentenenitrile afforded the CC borylated product (7e), indicating that the catalyst was not deactivated by the cyano group. In these cases, the presence of the nitrogen atom actually promoted the alkene hydroborations, which were achieved under milder conditions in comparison with the simple olefins in Table 1. Furthermore, the presence of an oxygen atom also promoted such cobalt-catalyzed alkene hydroboration, as demonstrated by the formation of 7f,g in high yields. Mechanistic Studies. For typical cobalt-based hydroborations, the catalysis involves B−H bond cleavage via oxidative addition or σ-bond metathesis by Co(I) active species.11,12,16 To understand the mechanism of our cobaltoperated system, several stoichiometric reactions were carefully examined. Although complex 1 is inactive toward olefins, it reacts readily with HBpin, affording the cobalt(II) hydride H1(Bpin). By treatment of a benzene solution of 1 with HBpin, the reaction was indicated by an instantaneous color change from pink to green. The product H1(Bpin) was identified by ESI-MS (Figure S3 in the Supporting Information). In its IR spectrum, H1(Bpin) exhibits the νCo−H band at 1902 cm−1, comparable to the CoII−H frequency of 1935 cm−1 for HCo(PMe3)4Cl.25 H1(Bpin) is also paramagnetic with the measured magnetic moment μeff = 2.62 μB, which is consistent with the 2.01 μB observed for [HCo(P(CH2CH2PPh2)3]PF6.26 Crystallographic analysis reveals the two-legged piano-stool complex Cp*CoH(P-NH(Bpin)). The Me5C5− ring and the P atom are coordinated to the Co(II) center, while the N atom is uncoordinated and connected to the borenium moiety (Figure 2). The hydride does not interact with the borylated amine, and

a

Reaction conditions: olefin substrate (0.5 mmol) and 1 (1 mol %) were stirred in C6D6 (0.6 mL) at room temperature. Yields are based on 1H NMR analysis. bOne equivalent of HBpin (0.5 mmol) was used for selective CC bond hydroborations (5), and 2 equiv of HBpin was used for the fully hydroborated product 5′.

1 equiv of HBpin produced the terminal boronic ester (5a) exclusively in 10 min at room temperature. With an extra 1 equiv of HBpin added into the system, the substrate was converted to the fully hydroborated product (5a′) in 4 h. In the case of benzaldimines, both electron-donating and electronwithdrawing substituents at the para position of the phenyl ring are all compatible with the highly selective stepwise hydroboration of CC and CN bonds (5a−5e, 5a′−5e′). Significantly, the variation of the substituents in the imine moiety from aryl to cyclohexyl or ferrocenyl also allowed regioselective hydroboration of the CC bonds by 1 equiv of HBpin in perfect yields (5f,g). In Rh-based catalytic systems, the amide group in the alkene substrates was found to facilitate the CC bond hydroboration, achieving “carbonyl-directed” hydroboration.24 Binding of the substrate carbonyl to rhodium during the catalysis improves the level of regioselectivity. In our case of regioselective hydroboration of the CC bonds for Nallylimines, the imine moiety may also serve as a “directing” group. The hypothesis of substrate−catalyst interactions through nitrogen−cobalt interaction was further tested by the results of hydroboration of N-allylamines (see below). “Heteroatom-Promoted” Hydroboration of Alkenes. Using 1 mol % of 1, terminal alkenes containing a nitrogen or oxygen atom at the β position all underwent rapid hydroboration under the mild catalytic conditions (Table 3). The yields for these terminal boronic esters with functional groups are good to excellent. In addition to imine groups, amine groups were also found to accelerate alkene hydroboration. For example, 1 catalyzed the hydroboration of allyamine to γaminoboronic ester (7a). Crystallographic characterization of 7a revealed the formation of a spirocyclic amine−borane. In the C

DOI: 10.1021/acs.organomet.8b00114 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Interestingly, 7d was produced when extra HBpin was added to the reaction solutions (eq 2, Figure S8). In the presence of HBpin, the reaction of 6d with D1(Bpin) generated by treatment of 1 with DBpin gave unambiguously the deuteride product (d1-7d) according to 1H NMR spectroscopic and GC-MS analysis. ESI-MS studies for the reaction solution indicated that the organocobalt species is H1(Bpin), and the formation of d1-7d was also verified (Figure 3). In contrast, H1(Bpin) reacts with 6d and DBpin, giving 7d

Figure 2. Structure of H1(Bpin) showing 50% probability ellipsoids. Selected bond distances (Å): Co−H, 1.37(3); Co−P, 2.115(2); N−B, 1.416(3).

its position was refined. The Co−H distance of 1.37(3) Å is reasonable when it is compared to 1.34(8) Å for HCo(PCy3)2(BH4)27 and 1.42 Å for HCo(PPh3)3(BH3CN).28 On the basis of H1(Bpin) formed by addition of HBpin to 1, we propose a Co(II)-based cycle for alkene hydroboration (Scheme 2). The CC bond of the substrate is proposed to Scheme 2. Proposed Catalytic Cycle for “HeteroatomPromoted” CC Bond Hydroboration

Figure 3. ESI-MS spectroscopic analysis of the reaction solutions of D1(Bpin) with 6d and HBpin.

rather than d1-7d (eq 3, Figure S11). The kinetic analysis was performed for the catalytic hydroboration of 6d, which showed that the reaction rate is first order with respect to HBpin (Figure S12). Especially, a kinetic isotope effect (KIE, Figure S13) of 3.08 was obtained by using DBpin, indicating that the B−H bond cleavage is the rate-determining step in the catalysis. Control experiments showed that each of the three components is necessary for the formation of 7d. Hence, it is reasonable to argue that the catalytic reaction proceeds via an intermolecular pathway (path b) by the reaction of XI with extra amounts of hydroboranes.

interact with the metal center of CoII−H (X). When the substrate contains a heteroatom (X = N, O), substrate−catalyst interactions increase the reaction rate significantly.24d Such an interaction facilitates 1,2-insertion of CoII−H into terminal olefins, leading to the formation of a cobalt(II)−alkyl species (XI). There are two possible pathways for the formation of hydroborated products from XI. The intramolecular pathway (path a) involves the recovery of the N−Co bond releasing the product and proceeds back to compound 1. The second pathway (path b) involves reaction of XI with another HBpin molecule, affording the organocobalt product H1(Bpin). More stoichiometric reactions were conducted at room temperature to further probe the mechanism of “heteroatompromoted” hydroboration. In the stoichiometric reaction of H1(Bpin) with N,N-dimethylallylamine (6d), the hydroborated product (7d) was not observed (eq 1). These results indicate that the intramolecular pathway (path a) can be excluded.

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CONCLUSIONS Generating CoII−H species through direct activation of a simple hydroborane by cooperative metal−ligand reactivity is attractive. The catalysis established in such a manner requires no additives, because the borane is the activator. The addition of the B−H bond across the Co−N bond is driven in part by the stability of the N−B(pin) bond. Although several cobalt(II) hydrides have been structurally identified,25−28 the reactivity of such complexes has rarely been reported.29 H1(Bpin) displays excellent activity for olefin hydroborations. It should be noted that examples of cobalt-catalyzed isomerization−hydroboration D

DOI: 10.1021/acs.organomet.8b00114 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Yield: 105 mg (80%). ESI-MS: calcd for D1(Bpin)+, 599.2519; found, 599.2505. Anal. Calcd for C34H42DBO2NPCo: C, 68.12; H, 7.40; N, 2.34. Found: C, 68.21; H, 7.28; N, 2.19. Synthesis of N-Allylimines. In a typical experiment, anhydrous sodium sulfate (1.0 g) and allylamine (11.0 mmol, 1.1 equiv) were added to a stirred solution of the aldehyde (10.0 mmol, 1.0 equiv) in CH2Cl2 (12 mL). After 12 h, the reaction mixture was filtered through a sintered-glass funnel. The solvent was evaporated, and the residue was kept under high vacuum for 3 h. The product was obtained as a colorless oil and analyzed by a 1H NMR spectrum (Supporting Information). General Procedure for the Hydroboration of Olefins. Complex 1 (4.7 mg, 10 μmol) was dissolved in C6D6 (0.6 mL) in a J. Young NMR tube. Alkene (0.5 mmol) was then added, followed by HBpin (73 μL, 0.5 mmol). The reaction at 40 °C was monitored by 1 H NMR until complete conversion of the reactants. The product was isolated by chromatography on silica gel with EtOAc/hexane as eluent. General Procedure for “Heteroatom-Promoted” Hydroboration of Alkenes. In a J. Young NMR tube, the substrate (0.5 mmol) and 1 (2.3 mg, 5 μmol) were dissolved in 0.6 mL of C6D6. Then, HBpin (73 μL, 0.5 mmol) was added. The reaction at room temperature was monitored by 1H NMR until complete conversion of the substrate. The product was isolated by chromatography on silica gel with EtOAc/hexane as eluent. Stoichiometric Reaction of N,N-Dimethylallylamine (6d), H1(Bpin), and HBpin. A scintillation vial containing a magnetic stir bar was charged with 0.100 g of N,N-dimethylallylamine (6d) and a stoichiometric amount of H1(Bpin). The reaction mixture was stirred at room temperature and monitored by GC-MS and ESI-MS spectroscopy. The hydroborated product 7d was not observed. Subsequently, 1 equiv of HBpin was added to the mixture, and the production of 7d was analyzed by GC-MS and ESI-MS spectroscopy (Figures S7 and S8). Deuteroboration Experiments. In a N2-filled glovebox, a scintillation vial was charged with 0.100 g of N,N-dimethylallylamine in C6H6 (10 mL) and a stoichiometric amount of H1(Bpin) (or D1(Bpin)) and then with 1 equiv of DBpin (or HBpin). The vial was capped, and the mixture was stirred at the room temperature for 1 h. The mixture was divided into two portions. One portion was filtered through a plug of silica gel and then analyzed by 2H NMR spectroscopy (Figure S9). The remaining portion was analyzed by ESI-MS spectroscopy (Figures S10 and S11).

of internal alkenes have recently been reported by the groups of Chirik12a,15b and Stradiotto.21 In comparison to the reported catalytic systems, our CoII−H-based system proceeds at 40 °C and with a longer reaction time for olefin hydroborations; however, the catalysis is able to provide the terminal hydroborated products in good to excellent yields. The work was extended to chemoselective hydroboration of N-allylimines. We propose that CC bond hydroborations are promoted by the heteroatoms in N-allylimines, allylamines, allyl ethers, and allyl esters. In particular, the addition of an external small amine or ether, i.e. diethylamine and diethyl ether, slows down the hydroboration reactions. Stoichiometric reactions were carried out to establish the catalytic mechanism of heteroatom-promoted CC hydroboration based on the CoII−H. Our study demonstrates the possibility of using cobalt(II) hydride catalysts in hydroboration reactions.

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EXPERIMENTAL SECTION

General Information. All manipulations were conducted under a N2 atmosphere on Schlenk techniques or in a glovebox, unless otherwise stated. All reagents were purchased from Sigma-Aldrich and used without further purification; d1-pinacolborane (DBpin)30 was synthesized according to literature procedures. Et2O, pentane, hexane, toluene, benzene and THF were purified using a Glass Contour solvent purification system consisting of neutral alumina, copper catalyst, and activated molecular sieves and then passed through an inline, 2 μm filter immediately before being dispensed. C6D6, toluene-d8, and THF-d8 were dried over CaH2 and purified by vacuum transfer. 1,2-Ph2PC6H4NH231 and [Cp*CoCl]232 were prepared according to published methods. NMR spectra were recorded on a Bruker Avance 500 spectrometer in J. Young NMR tubes at room temperature. 1H and 13C NMR chemical shifts are referenced to the proton signal of the deuterated solvent. Carbons that are directly attached to boron atoms were not observed due to quadrupolar relaxation. 11B NMR is referenced to the signal of H3BO3. Infrared spectra were recorded on a PerkinElmer FTIR Spectrum Two spectrometer. GC-MS spectra were obtained on a Shimadzu GCMS-QP2010 SE spectrometer. Single crystals with appropriate dimensions were chosen under an optical microscope and quickly coated with high-vacuum grease (Dow Corning Corp.) to prevent decomposition. Intensity data and cell parameters were recorded at 100 or 173 K on a Bruker Apex II single-crystal diffractometer, employing Mo Kα or Cu Kα radiation and a CCD area detector. Preparation of Cp*Co(1,2-Ph2PC6H4NH) (1). n-BuLi (2.5 mol/ L) (0.29 mL, 0.72 mmol) was added to a solution of 1,2Ph2PC6H4NH2 (200 mg, 0.72 mmol) in THF (20 mL) at −78 °C. After it was stirred for 6 h, the solution was warmed to room temperature, and then [Cp*CoCl]2 (165 mg, 0.36 mmol) was added. The mixture immediately turned from yellow to rose Bengal. The solvent was removed under vacuum, and the residue was extracted with hexane (3 × 10 mL). After recrystallization from hexane at −30 °C, compound 1 was obtained as red microcrystals. Yield: 298 mg (88%). ESI-MS: calcd for [1]+, 470.1447; found, 470.1437. Anal. Calcd for C28H30CoNP: C, 71.48; H, 6.43; N, 2.98. Found: C, 71.52; H, 6.48; N, 3.07. μeff = 2.02 μB (THF-d8, 23 °C). Preparation of Cp*CoH(1,2-Ph2PC6H4NH(Bpin)) (H1(Bpin)). In a N2-filled glovebox, pinacolborane (34 mg, 0.26 mmol) was added to a solution of 1 (100 mg, 0.22 mmol) in THF (10 mL). The mixture turned yellow-green. The solvent was then removed under vacuum and the residue was extracted with hexane (3 × 10 mL). H1(Bpin) was obtained as pale yellow crystals by recrystallization from Et2O at −30 °C. Yield: 109 mg (83%). ESI-MS: calcd for H1(Bpin)+, 598.2456; found, 598.2441. Anal. Calcd for C34H43BO2NPCo: C, 68.24; H, 7.24; N, 2.34. Found: C, 68.36; H, 7.32; N, 2.23. FT-IR (νCo−H): 1902 cm−1. μeff = 2.62 μB (THF-d8, 23 °C). Compound D1(Bpin) was prepared by the reaction of 1 and DBpin following the same procedure as in the preparation of H1(Bpin).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00114. Detailed synthesis procedures, characterization data such as NMR and IR spectra, and crystallographic data (PDF) Accession Codes

CCDC 1561201, 1561234, and 1561243 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Q.T.: [email protected]. *E-mail for W.W.: [email protected]. ORCID

Qingxiao Tong: 0000-0002-8125-9684 Chen-Ho Tung: 0000-0001-9999-9755 E

DOI: 10.1021/acs.organomet.8b00114 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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Wenguang Wang: 0000-0002-4108-7865 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Thousand Plan” Youth Program, the Natural Science Foundation of China (21402107, 91427303, and 51673113). We thank Prof. Di Sun for assistance with the X-ray crystallography and Prof. Zhenghu Xu for discussions.

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DOI: 10.1021/acs.organomet.8b00114 Organometallics XXXX, XXX, XXX−XXX