Chromium- and Cobalt-Catalyzed, Regiocontrolled Hydrogenation of

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Chromium- and Cobalt-Catalyzed, Regiocontrolled Hydrogenation of Polycyclic Aromatic Hydrocarbons: A Combined Experimental and Theoretical Study Bo Han, Pengchen Ma, Xuefeng Cong, Hui Chen, and Xiaoming Zeng J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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Journal of the American Chemical Society

Chromium- and Cobalt-Catalyzed, Regiocontrolled Hydrogenation of Polycyclic Aromatic Hydrocarbons: A Combined Experimental and Theoretical Study Bo Han,†,§,¶ Pengchen Ma,#,¶ Xuefeng Cong,ѱ Hui Chen,*,# and Xiaoming Zeng*,†,‡ †Frontier

Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710054; ‡College of Chemistry, Sichuan University, Chengdu 610064, China; #Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China; §Shaanxi Key Laboratory of Chemical Reaction Engineering, College of Chemistry & Chemical Engineering, Yan'an University, Yan'an 716000, China; ѱDepartment of Chemistry, Northeast Normal University, Changchun 130024, China ABSTRACT: Polycyclic aromatic hydrocarbons are difficult substrates for hydrogenation because of the thermodynamic stability caused by aromaticity. We report here the first chromium- and cobalt-catalyzed, regiocontrolled hydrogenation of polycyclic aromatic hydrocarbons at ambient temperature. These reactions were promoted by low-cost chromium or cobalt salts combined with diimino/carbene ligand and methylmagnesium bromide, and are characterized by high regioselectivity and expanded substrate scope that includes tetracene, tetraphene, pentacene and perylene, which have rarely been reduced. The approach provides a cost-effective catalytic protocol for hydrogenation, is scalable, and can be utilized in the synthesis of tetrabromo- and carboxyl-substituted motifs through functionalization of the hydrogenation product. The systematic theoretical mechanistic modelings suggest that low-valent Cr and Co monohydride species, most likely from zero-valent transition metals, are capable of mediating these hydrogenations of fused PAHs.

1. INTRODUCTION Selective hydrogenation is among the most powerful reactions in modern chemistry because of its central importance in the creation of pharmaceuticals and fundamental feedstock chemicals.1,2 Methods that are used in hydrogenation commonly involve unsaturated compounds such as alkynes, olefins, imines, ketones, and heterocycles.3–7 Compared with single arenes, polycyclic aromatic hydrocarbons (PAHs) have extended π-conjugated systems and the aromaticity of these motifs is usually not completely destroyed in hydrogenation (Scheme 1a).8 Arenophilic precious metals such as Ru, Rh, Pd, and Pt nanoparticles mediated by heterogeneous catalysis dominate the hydrogenation of PAHs.9 In contrast, homogeneous transition-metal catalysis has rarely been developed for PAH hydrogenation.10,11 Given that multiple fused aromatic carbocycles are present in one PAH molecule, regioselectivity has long been a prominent issue, and hydrogenation usually leads to mixed products through the reduction of different carbocycles (Scheme 1b).9,11a–c Notably, PAHs that contain four, five, and even six aromatic carbocycles have rarely been hydrogenated.9d,g,10b,11d Interestingly, these motifs have generated immense concern because of their negative health impacts as pervasive environmental pollutants; such compounds are often produced from the combustion of fossil fuels, traffic exhaust, and wood smoke.12 Regiocontrolled hydrogenation of PAHs would help degrade these harmful compounds and create valuable feedstocks, which are usually difficult to make by common synthetic protocols.

Scheme 1. Transition-Metal-Catalyzed Regioselective Hydrogenation of PAHs

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In recent years, the use of earth-abundant, low-cost, firstrow metal catalysts to replace precious metal catalysts in developing cost-effective hydrogenation protocols has attracted broad interest.13,14 Tremendous advances have been achieved in cobaltand iron-catalyzed hydrogenation.15,16 However, hydrogenation of PAHs using these inexpensive metal catalysts has been rather limited.10b,17 To our knowledge, there is only one report of a hydrogenation reaction that was catalyzed by the group 6 metal chromium, appearing at the early of the 1960s.18 Its reactivity for hydrogenation remains largely unknown. Herein, we report the first chromium- and cobalt-catalyzed hydrogenation of PAHs that was enabled by low-cost metal salts combined with diimino/carbene ligand and organomagnesium reagent (Scheme 1c). These catalyst protocols can be used to control the hydrogenation of one or two terminal aromatic carbocycles of PAHs, thereby achieving high regioselectivities. 2. RESULTS AND DISCUSSION Table 1. The Effect of First-Row Metal Salts and Ligands on the Hydrogenation of Anthracenea

entry

metal salt

ligand

R

2a (%)

3a (%)

1b

CrCl3

none

Ph

trace

ndc

2b

CrCl3

IPr·HCl

Ph

21

ndc

3b

CrCl3

IPr·HCl

Me

37

ndc

4b

CrCl3

CAAC·HCl

Me

30

ndc

5b

CrCl3

2,2-bpy

Me

ndc

ndc

6b

CrCl3

L1

Me

56

trace trace trace

7d

CrCl3

L1/L2/L3

Me

92e

8d

CrCl2

L1

Me

91

9d

(89)f

Cr(acac)3

L1

Me

94

10g

CoCl2

L1

Me

28

46

11g

Co(acac)3

L1

Me

14

60

12g

Co(acac)2

L1

Me

6

79

13g

Co(acac)2

CAAC·HCl

Me

19

43

14g

Co(acac)2

IPr·HCl

Me

ndc

96 (89)e

15f,g

Co(acac)2

IPr·HCl

Me

13

76 ndc ndc

16g

FeCl2

IPr·HCl

Me

ndc

17g

NiBr2

IPr·HCl

Me

ndc

aConditions:

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Based on previous findings that low-valent chromium species could be formed in situ by reaction of a Cr or Co salt with organomagnesium reagent,19–22 we postulated that such reactive species may chelate with fused aromatic carbocycles through -coordination (A, Scheme 1c),23 thereby facilitating a dearomatic hydride-additive reaction. We commenced our study by choosing anthracene to optimize the hydrogenation conditions. Using 10 mol % of CrCl3 and phenylmagnesium bromide, almost no hydrogenation occurred (Table 1, entry 1). The introduction of an N-heterocyclic carbene ligand bearing IPr or CAAC into the system allowed the hydrogenation to proceed at room temperature, forming partial hydrogenation product 2a in more that 30% yield (entries 3 and 4).24 1,2-Diimino ligand L1, L2, or L3 combined with methylmagnesium bromide gave the same result, leading to 2a in 92% yield (entry 7). Notably, the hydrogenation proceeded with high regioselectivity, with one of terminal carbocycles of anthracene being reduced to give 2a as nearly the sole product. The use of 1.0 equiv of MeMgBr in the hydrogenation led to a decrease in the conversion of 1a to 74% (see Supporting Information). At present, we cannot comment definitively regarding the reason for the use of large amount of MeMgBr for efficient catalyst activation. In addition to activation of the precatalyst,25 one possible role of the extra Grignard that Scheme 2. Chromium-Catalyzed Hydrogenation of PAHs by Regioselective Reduction of One Terminal Carbocyclea

trace

1a (0.2 mmol), metal salt (0.044 mmol), ligand aConditions: 1 (0.2 mmol), CrCl (0.044 mmol), L1 (0.044 (0.044 mmol), RMgBr (0.3 mmol), THF, and H2 (3.5 Mpa), rt, 3 1 24 h. H NMR yield using 1,3,5-trimethoxybenzene as internal mmol), MeMgBr (0.3 mL, 1 M in THF), H2 (5 Mpa), and THF, standard. bCrCl3/ligand (0.02 mmol). cNot detected. dH2 (5 rt, 24 h. Isolated yield. bYield of compound 3. cL3 (0.044 e f g Mpa). Isolated yield. Hg(0) (100 equiv) was added. H2 (8 mmol) was used. dMeMgBr (0.4 mmol), H2 (6 Mpa), 48 h. ACS Paragon Plus Environment Mpa) for 48 h.

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could be envisioned is serving as poison scavenger (e.g. trace amount of moisture and dioxygen that may deactivate metal catalyst) for keeping the reactivity of the active species during hydrogenation. CrCl2 and Cr(acac)3 also showed high reactivity for the hydrogenation (entries 8 and 9). Interestingly, the replacement by CoCl2 led to two-terminal ring-reduced octahydroanthracene 3a as the major product (entry 10). N-Heterocyclic carbene of IPr combined with Co(acac)2 and MeMgBr increased the hydrogenation rate, forming 3a as the sole product in excellent yield (entry 14). However, FeCl2 and NiBr2 did not promote the hydrogenation (entries 16 and 17). The scope of the chromium-catalyzed hydrogenation of PAHs was then examined (Scheme 2). Introducing methyl group into the C-9 and C-10 positions of anthracene had no effect on the hydrogenation; one-terminal carbocyclereduced compounds 2b and 2c were obtained in excellent yields. Noted that using L3 as ligand gave better result than L2 in the reaction of forming 2c, indicating that the R substituent in diimino ligand affects the catalytic activity of chromium in hydrogenation (see Supporting Information). As expected, reaction with 9-phenyl-containing anthracenes Scheme 3. Cobalt-Catalyzed Hydrogenation of PAHs by Regiocontrolled Reduction of Two Terminal Carbocyclesa

retains the phenyl substituents intact (2d–n). Terminal aromatic carbocycles without substituents were better reduced compared with the substituted ones (2o–r). Strikingly, two terminal carbocycle rings of tetracene were reduced, allowing the synthesis of 1,2,3,4,7,8,9,10octahydrotetracene (2s). In contrast, hydrogenation of tetraphene formed 8,9,10,11-tetrahydrotetraphene 2t in 89% yield. Because hydrogenation of the central ring in pentacene is more exothermic than reduction of the outer ones, the central ring was hydrogenated to give 6,13dihydropentacene (2u).26 Perylene was amenable to the Crcatalyzed hydrogenation, whereby two carbocycles (A and D) were reduced (2v). Inspired by these results, we then probed the efficiency of the cobalt-catalyzed hydrogenation of PAHs for the buildup of two-terminal carbocycle-reduced motifs. Hydrogenation of anthracenes containing aliphatic or aromatic scaffold at the C-9 position resulted in the products 3b–h in preparatively useful yields (Scheme 3). In most cases, only trace amounts of the mono-terminal ring-reduced compounds were detected. The reaction of sterically hindered 9,10-dimethyl- or phenyl-substituted anthracenes occurred smoothly, with forming octahydroanthracenes 3i and 3j. Two terminal carbocycles in phenanthrene were hydrogenated at relatively high temperature (3k). Interestingly, hydrogenation of tetracene led to an unexpected compound 3l by reducing one central and one terminal carbocycle together with 2s. The reaction with tetraphene hydrogenated the carbocycles of A and D to form 3m with high regioselectivity. Furthermore, 4,5,9,10tetrahydropyrene can be prepared by reduction of the B and D carbocycles of pyrene (3n). Importantly, three carbocycles including A, C, and E in pentacene can be reduced (3o). In contrast, the reaction with fused perylene hydrogenated the A and D carbocycles to form 2v. The protocol can be utilized for the regioselective hydrogenation of bianthracene for the synthesis of four-terminal carbocycle-reduced compound 3p. The kinetic behavior associated with hydrogenation of anthracene was explored. By using chromium catalysis, the hydrogenation proceeded rapidly within 2 h to form 2a in 80% yield (Figure 1a). The reaction profile for cobalt catalysis suggested that hydrogenation of anthracene took place quickly to initially form 2a in 87% yield within 0.25 h, which was consumed by continuous hydrogenation of the second terminal carbocycle to furnish 3a in 66% yield after 1 h (Figure 1b). Introducing a large amount of Hg(0) into the system did not inhibit the Cr- or Co-catalyzed hydrogenation of 1a (Table 1, entries 9 and 15). The chromium- and cobaltcatalyzed hydrogenations are scalable and can be performed on 10 mmol scale to give 71% yield of 2a and 85% yield of 3a, respectively (Scheme 4a). Importantly, the hydrogenation product 3a can be modified, offering a route to prepare

aConditions:

1 (0.2 mmol), Co(acac)2 (0.044 mmol), IPr·HCl (0.044 mmol), MeMgBr (0.3 mL, 1 M in THF), H2 (8 Mpa), and THF, rt, 48 h. Isolated yields. bYield of compound 2. cTrace amount of 2 was detected. d60 C. eThe regioisomer ratio was determined by 1H NMR analysis. ACS Paragon Plus Environment Figure 1. Reaction profile for hydrogenation of anthracene. (a) Chromium catalysis. (b) Cobalt catalysis.


tetrabromo and carboxyl-substituted derivatives 4 and 5 via bromination and oxidation (Scheme 4b). Scheme 4. Gram-Scale Transformation of 3a

Hydrogenation

To gain insight into the mechanistic details associated with the hydrogenation, we studied the dependence of the initial reaction rate on the substrate with the precatalyst of Cr(acac)3/L1 or Co(acac)2/IPr in 1:1 stoichiometric ratio. The initial rate (Δ[2a]/Δt) against the initial concentration of 1a ranging from 0.04 to 0.26 M showed a zero-order dependence of the initial rate on the concentration of 1a under Cr catalysis, indicating that anthracene may not be involved in the rate-determining step of the reaction or at least in the early stage of the hydrogenation (Figure 3a). In contrast, we observed a positive response to the concentration of Cr(acac)3/L1 (0.009–0.026 M) and pressure of hydrogen (3–7 MPa), indicating a first-order dependence of the initial rate on the concentration of Cr(acac)3/L1 and

and

pressure of hydrogen (Figures 3b and 3c). The initial reaction

and

Because of the difficulty in isolation of catalytically relevant transition metal intermediates that are responsible for the hydrogenation of PAHs, alternative strategy by the method of continuous variation (“Job plots”) was utilized to elucidate catalyst stoichiometry of Cr or Co with the related ligand of diketimine L1 or IPr. As shown in Figure 2, conditions C1 and C2 fixed the stoichiometries of CrCl3 and Co(acac)2 with MeMgBr, and varied their mole fraction against the ligands of L1 and IPrHCl, respectively. Both of these conditions gave maximum conversion to the hydrogenated product 2a or 3a in mole fraction of 0.5, indicating that a 1:1 stoichiometry of chromium or cobalt versus the related ligand of L1 or IPr can be considered for the reactive species in the hydrogenation.

Figure 2. Job plots for Crhydrogenation of anthracene (1a).

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Figure 3. Kinetic profile for Cr-catalyzed hydrogenation of 1a. (a) Initial reaction rate against the concentration of 1a. (b) Initial reaction rate against the pressure of hydrogen. (c) Initial reaction rate against the concentration of precatalyst of Cr(acac)3/L1. (d) Initial reaction rate against the concentration of methylmagnesium bromide. rate of showed a positive response to [MeMgBr] at low concentration (Figure 3d), but exhibiting a saturation behavior and reaching a plateau at high concentration (0.17 M). When a similar analysis was carried out with Co catalysis, the initial reaction rates (Δ[3a]/Δt) against the initial concentrations of 1a, the precatalyst of Co(acac)2/IPr and hydrogenation pressure showed clear positive correlations, indicating that first-order dependence of the initial rates on the concentration of anthracene, the precatalyst of Co(acac)2/IPr and hydrogen pressure can be considered (Figures 4a–c). Similarly, a saturation behavior was observed for the initial rate of the hydrogenation with MeMgBr when its concentration became higher than ~0.13 M (Figure 4d). In order to get a better mechanistic understanding for the hydrogenation of fused PAHs reported in this work, density functional theory (DFT) modelings were carried out. The first issue to be clarified is the identity of the metal hydride active species possibly involved in the hydrogenation reactions. The reductive organomagnesium reagent used in the current catalytic systems makes it possible to form low-valent metal

Figure 4. Kinetic profile for Co-catalyzed hydrogenation of 1a in the formation of 3a. (a) Initial reaction rate against the concentration of 1a. (b) Initial reaction rate against the pressure of hydrogen. (c) Initial reaction rate against the concentration of precatalyst of Co(acac)2/IPr·HCl. (d) Initial ACS Paragon Plus Environment Co-catalyzed reaction rate against the concentration of methyl Grignard reagent.

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Journal of the American Chemical Society Scheme 5. Coordination Number Preference of Lowvalent Cr-L1 and Co-IPr Systems Explored by Calculated Ligand Binding Thermodynamics (in kcal/mol)a IPr

Co

THF

0

Co

IPr

G = 6.8

IPr

Co

CH 3

I

Co

CH 3

Co

IPr

G = 5.0

G = 3.9

I

Co

N N

0

Cr

N

CH 3

L1

I

Cr

THF

G = – 1.2

Cr

N

N

I

N

THF

N THF

G = – 12.8

Cr

N

THF

(c)

THF

CH 3

I

(b)

CH 3 Co

IPr

THF 0

THF

THF

CH 3

N

CH 3

Co

IPr

THF N

0

THF

N

(a)

CH 3

CH 3 THF

IPr

THF

THF

THF N

0

Co

IPr

THF 0

THF IPr

THF THF

THF 0

THF

THF

THF 0

THF

N

CH 3

N

0

Cr

THF

CH 3 THF

I

Cr

(d)

THF CH 3

(e)

THF

aRed

cross means that ligand (THF) cannot coordinate to the metal center. species in situ from chromium and cobalt salts. Some previous work supported generation of Cr(0) and Co(0) species under such reductive condition.19,20,27 The formation of active Cr or Co species in a different oxidation state after the addition of methyl Grignard was observed by X-ray photoelectron spectroscopy (XPS) analysis (see Supporting Information for details). In addition, here we take Cr(I) and Co(I) species also into consideration for comparative purpose. Our experiments of continuous variation for speciation of catalyst have confirmed that the coordination ratios of the added ligand (IPr/L1) to the corresponding metal (Co/Cr) are both 1:1. Based on these results, to know coordination number preference of the Cr and Co systems, the additional DFT calculations summarized in Scheme 5 further indicate that: (a) with one monodentate IPr ligand already coordinated, both Co(0) and Co(I) can have two more ligands (Me or THF) at most, but favoring one more only to form Scheme 6. Calculated Thermodynamics (in kcal/mol) of Transmetalation and Metal Hydride Formation from Reactions of Grignard reagent and H2 with the Low-valent (a) Cr-L1 System, (b) Co-IPr System (a) MeMgBr

N N

0

Cr

N N

N MeMgBr

N

THF

0

Cr

I

Cr

N N

THF

I

IPr

+ H2

THF

+

H2

0

MeMgBr

IPr

IPr

+

Co THF 0

Co

CH3

+

I

Co THF

+

G = 48.2

I

IPr Co

CH3

N

H2 H2

+

H2

+

H2

H

H

II

Cr

G = -18.4

N

G  -9.8 G = -16.2

G  4.0 G = -17.4

+

THF

(1a)

THF THF

0

Cr

H III

Cr

H

THF

I

Cr

+

CH 4

IPr

IPr

(a)

H2

N

II

I

Cr

N

THF

-

H

N

H C H 15.2 3

H N N

N

0

Cr

H2

N

HH L1

H

4.5

-1.1

CH3

H

0

Cr

N

H H

0

Cr

N

CH4

TSCD

H2

TSBC -12.3

-18.4

CH3

B N N

N

G

N

(kcal/mol)

+

THF

(3a)

H2

+

+

Co H

IPr Co H

0

0

Cr

5.2 N

0

Cr

N N

H

-15.0

C H H

0

Cr

N N

CH 4

0.0

(4a)

THF

(1b)

N

I

Cr

N N

I

H

H

Cr

H C 21.3 H3

THF 10.1

quartet

Co H

I

THF

0.0

N

Co H 2

N

quintet

H

0

N

G (kcal/mol)

(b) L1

H IPr

After obtaining the thermodynamically accessible candidates of the active low-valent metal hydride species, we next explored their reactivity towards the hydrogenation of fused PAHs. For this purpose, anthracene was selected as the hydrogenation substrate. Extensive calculations were carried out to search for reaction pathways of hydrogenation of the first C=C double bond on the terminal aromatic carbocycle of anthracene, and the corresponding results for the two chromium monohydride are shown in Figure 5. We can see

N

(2a)

+

THF

With these coordination preference of the low-valent Cr-L1 and Co-IPr systems in hand, we then systematically investigated the thermodynamics of their hydride formations from H2. As shown in Scheme 6a (reactions 1a and 3a), for the Cr system, generating dihydride species via oxidative addition (OA) of H2 to neither Cr(0) nor Cr(I) is thermodynamically favorable. On the contrary, as shown in reactions 2a and 4a of Scheme 6a, transmetalation from Grignard MeMgBr to Cr to form methyl Cr complex, and then producing monohydride species via hydrogenolysis of the methyl Cr complex, either Cr(0) or Cr(I), is thermodynamically accessible. Therefore, for the low-valent Cr-L1 system of either Cr(0) or Cr(I), monohydride species seems a reasonable candidate for active species in hydrogenation of fused PAHs. Similar to the Cr-L1 system, as shown in Scheme 6b, the low-valent Co-IPr system also bears thermodynamically accessible transmetalation from Grignard MeMgBr, which combined with the following hydrogenolysis, tends to form monohydride species (reactions 2b and 4b) for both Co(0) and Co(I).

D 0

Cr

HH

H H

H

N

-

H

N

N

G = -17.4 THFMgBr

N N

CH3

G = -9.7 THFMgBr

G = -19.6

+

(b) MeMgBr

G = 18.2

N N

+ H2

THF Cr

H2

-

CH3

G = -8.1 THFMgBr

+

THF

G = -4.3 THFMgBr

THF

two-coordinate structure (Scheme 5a, b, c); (b) with one bidentate L1 ligand coordinated, both Cr(I) and Cr(0) can have two more ligands at most to form four-coordinate structure, which is more favorable in energy than threecoordinate structure, especially for the Cr(I) system (Scheme 5e).

N N

I

Cr

CH3 N

CH 4

(2b)

+

THF

(3b)

+

CH 4

(4b)

I

Cr

H

H

HH

H

14.0 6.5

D(I)

TSBC(I)

-6.1

I

TSCD(I)

H2

B(I) N

+

Cr 5.6

H2

THF CH3

N

CH4

I

N

H H

Cr

N N

-11.5

N

C(I)

N

I

Cr

H

H

I

Cr

H H

H H

H

+

Figure 5. Calculated reaction profiles for hydrogenation of anthracene mediated by monohydride intermediate in the lowvalent Cr-L1 system with (a) Cr(0) and (b) Cr(I) center.



that the Cr(0) and Cr(I) hydrides share similar pathways in (a)

H H

0

Co G (kcal/mol) H 3C 16.0

IPr 0

Co H

H2

H H

-9.2

doublet 0

IPr

H2

TSEF

-16.2

IPr

H

CH 4

0.0

H 3C Co

II

Co

IPr

H

-16.2

H

H

-10.6 TSFG

F

E

0

0

IPr

H Co

H

Co IPr

-31.4

G

H

0

Co H H

H

(b)

H

G (kcal/mol) H 3C H2

Co

I

I

IPr

Co H

21.5

Co

IPr

H H H H 9.5 H2

TSEF(I) IPr

H

IPr

H

H

2.0

0.0 H 3C

Co

IPr

H

CH 4

triplet I

I

H

-17.4

E(I) I

H Co

IPr

TSFG(I)

-12.4

-16.5

F(I)

G(I)

I

I

Co IPr

Co IPr

H

H H

H

H

H

Figure 6. Calculated reaction profiles for hydrogenation of anthracene mediated by monohydride intermediate in the low-valent Co-IPr system with (a) Co(0) and (b) Co(I) center.

anthracene hydrogenation. Taking the Cr(0) system as an example, hydrogenolysis of methyl Cr complex could readily render three-coordinate chromium monohydride species B, with activation barrier of 15.2 kcal/mol in the Cr(0)-L1 system (note that binding of anthracene to Cr before hydrogenolysis of methyl group is unfavorable in reaction because H2 would then dissociate from metal center). Migratory insertion of C=C of anthracene substrate into Cr-H then occur via TSBC, to overcome an effective barrier of 17.3 kcal/mol, resulting the semi-hydrogenated intermediate C. The C=C hydrogenation is accomplished by the second hydrogenolysis of C, with a barrier 16.8 kcal/mol via TSCD. Comparing Cr(0) and Cr(I) reaction pathways depicted in Figure 5a and 5b, it is clear that Cr(0) is much more reactive than Cr(I), due to the much higher reaction barrier (25.5 kcal/mol) for the second hydrogenolysis process in the Cr(I) system. Therefore, our computational results suggest that hydrogenation of fused PAHs by the Cr-L1 system is more likely to be mediated by zero-valent chromium hydride species. Notably, based on the most favorable pathway in Figure 5a, the experimental regioselectivity can be reproduced. For example, the barriers of the regioselectivity-determining steps, i.e., first H transfer in 9,10-addition and second H transfer in 1,4-addition of anthracene, none of which is observed in experiments, are higher than the barriers via TSBC and TSCD by 4.0 and 3.7 kcal/mol, respectively. For low-valent cobalt system, we explored pathways based on the two monohydride candidates bearing more favorable formation thermodynamics (reactions 2b, 4b in Scheme 6b).

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Our extensive calculations indicate that the kinetically most favorable mechanism of anthracene hydrogenation is mediated also by Co(0) monohydride species, as shown in Figure 6. After generation of the monohydride species E from hydrogenolysis of methyl Co(0) species, migratory insertion of C=C of anthracene into Co-H then occur to produce the semi-hydrogenated intermediate F, which only needs to overcome a small effective barrier of 7.0 kcal/mol via TSEF. From F, the second hydrogenolysis process occurs via TSFG to generate the C=C hydrogenated product, with a barrier of 5.6 kcal/mol. Thus, similar to the Cr(0) system (Figure 5a), the first incorporation of H into C=C of anthracene is slightly harder than the second one. Compared to the calculated pathway of Co(0) hydride species shown in Figure 6a, our calculations on Co(I) hydride species (Figure 6b) show that the corresponding pathway is kinetically less favorable, by bearing higher reaction barriers of 19.4 and 21.9 kcal/mol for the first and second H transfer processes. Therefore, our calculations suggest that hydrogenation of fused PAHs by the Co-IPr system is more likely to be mediated by cobalt(0) hydride species formed from H2 hydrogenolysis with zero-valent cobalt species. Importantly, the smaller barriers (7.0 and 5.6 kcal/mol) of the most favorable reaction pathway for the Co-IPr system (Figure 6a) in comparison with those (17.3 and 16.8 kcal/mol) of the most favorable reaction pathway for Cr-L1 system (Figure 5a), suggest that the Co-IPr system is more reactive than the CrL1 system in hydrogenation of fused PAHs. This trend is consistent with our experimental finding of higher hydrogenation reactivity of the Co system than the Cr system, therefore lending more credence to the current theoretical modelings. 3. CONCLUSIONS In summary, we have developed efficient chromium and cobalt catalyst systems for the hydrogenation of fused PAHs to give partially saturated compounds, by regiocontrolled reduction of one or two terminal aromatic carbocycles. Lowcost chromium and cobalt salts combined with diimino/carbene ligand and methylmagnesium bromide allowed the hydrogenation to proceed effectively at ambient temperature. The approach provides a cost-effective method for catalytic hydrogenation and can be used to address regioselectivity issues associated with hydrogenation of PAHs such as substituted anthracenes, tetracene, tetraphene, pentacene, perylene, and bianthracene. The systematic theoretical modelings imply that low-valent Cr and Co monohydride species, most likely from zero-valent transition metals, are capable of mediating these hydrogenation of fused PAHs. 4. EXPERIMENTAL SECTION General Procedure for Chromium-Catalyzed Regioselective Hydrogenation of PAHs. THF (2 mL) was added to a dry tube containing PAH (0.2 mmol), CrCl3 (0.044 mmol, 7 mg) and diketimine ligand of L1 or L3 (0.044 mmol) under nitrogen atmosphere. After stirring at room temperature for 5 min, MeMgBr (0.3 mL, 1 M in THF) was added dropwise slowly by using a syringe at 0 oC. The resulting mixture was stirred for 30 min, and the tube was


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quickly moved to a high-pressure autoclave. The reaction mixture was stirred under the atmosphere of H2 (5 Mpa) at room temperature for 24 h. After quenching with saturated NH4Cl/H2O (2 mL), the crude product was extracted with EtOAc (3  4 mL). The combined organic phases were dried over anhydrous Na2SO4 and concentrated under vacuum. The crude product was purified by silica gel chromatography to give the desired hydrogenation product. General Procedure for Cobalt-Catalyzed Hydrogenation of PAHs by Regiocontrolled Reduction of Two Terminal Carbocycles. In a dried tube were placed PAH (0.2 mmol), Co(acac)2 (0.044 mmol, 11 mg), IPr·HCl (0.044 mmol, 19 mg) and THF (2 mL) under nitrogen atmosphere. After stirring at room temperature for 5 min, MeMgBr (0.3 mL, 1 M in THF) was added dropwise by the use of a syringe, and the resulting mixture was stirred at 0 oC for 30 min. After quickly moving to a high-pressure autoclave, the reaction mixture was stirred under the atmosphere of H2 (8 Mpa) at room temperature or 60 oC for 48 h. After quenching with saturated NH4Cl/H2O (2 mL), the crude product was extracted with EtOAc (3  4 mL). The combined organic layer was dried over anhydrous Na2SO4 and concentrated under vacuum. The crude product was purified by silica gel chromatography to give the product.

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ASSOCIATED CONTENT Supporting Information. Detailed optimization data; experimental procedures; characterization data of all new compounds, detailed optimized geometries, and computational details and results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mails: [email protected] (X.Z.) [email protected] (H.C.)

Author Contributions ¶B.

Han and P. Ma contributed equally.

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT Support for this work by NSFC (21873103, 21833011, 21572175, 21702028), SCU, the Institute of Chemistry, Key Project of Natural Science of Shaanxi Provincial Education Department (No. 16JS121), and Industrial Public Relation Project of Science & Technology Bureau of Yan’an City (No. 2015KG-01) is gratefully acknowledged. We thank the Reviewers for the valuable comments and suggestions in the improvement of the quality of this manuscript.

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Journal of the American Chemical Society (15) Selected examples of cobalt-catalyzed hydrogenation: (a) Friedfeld, M. R.; Shevlin, M.; Margulieux, G. W.; Campeau, L.-C.; Chirik, P. J. Cobalt-Catalyzed Enantioselective Hydrogenation of Minimally Functionalized Alkenes: Isotopic Labeling Provides Insight into the Origin of Stereoselectivity and Alkene Insertion Preferences. J. Am. Chem. Soc. 2016, 138, 3314–3324. (b) Adam, R.; CabreroAntonino, J. R.; Spannenberg, A.; Junge, K.; Jackstell, R.; Beller, M. A General and Highly Selective Cobalt-Catalyzed Hydrogenation of N-Heteroarenes under Mild Reaction Conditions. Angew. Chem., Int. Ed. 2017, 56, 3216–3220. (c) Mukherjee, A.; Srimani, D.; Chakraborty, S.; Ben-David, Y.; Milstein, D. Selective Hydrogenation of Nitriles to Primary Amines Catalyzed by a Cobalt Pincer Complex. J. Am. Chem. Soc. 2015, 137, 8888–8891. (d) Friedfeld, M. R.; Margulieux, G. W.; Schaefer, B. A.; Chirik, P. J. Bis(phosphine)cobalt Dialkyl Complexes for Directed Catalytic Alkene Hydrogenation. J. Am. Chem. Soc. 2014, 136, 13178–13181. (e) Monfette, S.; Turner, Z. R.; Semproni, S. P.; Chirik, P. J. Enantiopure C1-Symmetric Bis(imino)pyridine Cobalt Complexes for Asymmetric Alkene Hydrogenation. J. Am. Chem. Soc. 2012, 134, 4561–4564. (f) Chen, F.; Topf, C.; Radnik, J.; Kreyenschulte, C.; Lund, H.; Schneider, M.; Surkus, A.-E.; He, L.; Junge, K.; Beller, M. Stable and Inert Cobalt Catalysts for Highly Selective and Practical Hydrogenation of CN and C=O Bonds. J. Am. Chem. Soc. 2016, 138, 8781–8788. (g) Yu, R. P.; Darmon, J. M.; Milsmann, C.; Margulieux, G. W.; Stieber, S. C. E.; DeBeer, S.; Chirik, P. J. Catalytic Hydrogenation Activity and Electronic Structure Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes. J. Am. Chem. Soc. 2013, 135, 13168–13184. (h) Tokmic, K.; Markus, C. R.; Zhu, L.; Fout, A. R. Well-Defined Cobalt(I) Dihydrogen Catalyst: Experimental Evidence for a Co(I)/Co(III) Redox Process in Olefin Hydrogenation. J. Am. Chem. Soc. 2016, 138, 11907–11913. (16) Selected examples of iron-catalyzed hydrogenation: (a) Jagadeesh, R. V.; Surkus, A.-E.; Junge, H.; Pohl, M.-M.; Radnik, J.; Rabeah, J.; Huan, H.; Schünemann, V.; Brückner, A.; Beller, M. Nanoscale Fe2O3-Based Catalysts for Selective Hydrogenation of Nitroarenes to Anilines. Science 2013, 342, 1073–1076. (b) Zuo, W.; Lough, A. J.; Li, Y. F.; Morris, R. H. Amine(imine)diphosphine Iron Catalysts for Asymmetric Transfer Hydrogenation of Ketones and Imines. Science 2013, 342, 1080–1083. (c) Trovitch, R. J.; Lobkovsky, E.; Bill, E.; Chirik, P. J. Functional Group Tolerance and Substrate Scope in Bis(imino)pyridine Iron Catalyzed Alkene Hydrogenation. Organometallics 2008, 27, 1470–1478. (17) (a) Nador, F.; Moglie, Y.; Vitale, C.; Yus, M.; Alonso, F.; Radivoy, G. Reduction of polycyclic aromatic hydrocarbons promoted by cobalt or manganese nanoparticles. Tetrahedron 2010, 66, 4318–4325. (b) Nelkenbaum, E.; Dror, I.; Berkowitz, B. Reductive hydrogenation of polycyclic aromatic hydrocarbons catalyzed by metalloporphyrins. Chemosphere 2007, 68, 210–217. (18) Sloan, M. F.; Matlack, A. S.; Breslow, D. S. Soluble Catalysts for the Hydrogenation of Olefins. J. Am. Chem. Soc. 1963, 85, 4014–4018. (19) (a) Cong, X.; Tang, H.; Zeng, X. Regio- and Chemoselective Kumada–Tamao–Corriu Reaction of Aryl Alkyl Ethers Catalyzed by Chromium Under Mild Conditions. J. Am. Chem. Soc. 2015, 137, 14367–14372. (b) Cong, X.; Fan, F.; Ma, P.; Luo, M.; Chen, H.; Zeng, X. Low-Valent, High-Spin Chromium-Catalyzed Cleavage of Aromatic Carbon– Nitrogen Bonds at Room Temperature: A Combined Experimental and Theoretical Study. J. Am. Chem. Soc. 2017, 139, 15182−15190.

(20) (a) Lee, P.-S.; Fujita, T.; Yoshikai, N. Cobalt-Catalyzed, Room-Temperature Addition of Aromatic Imines to Alkynes via Directed C–H Bond Activation. J. Am. Chem. Soc. 2011, 133, 17283–17295. (b) For a review of low-valent cobalt catalysis, see: Gao, K.; Yoshikai, N. Low-Valent Cobalt Catalysis: New Opportunities for C–H Functionalization. Acc. Chem. Res. 2014, 47, 1208–1219. (21) For report of forming low-valent chromium species by 2ereduction of the CrCl2 or CrCl3 complex with Grignard reagent or Mg, see: (a) Mock, M. T.; Chen, S.; O’Hagan, M.; Rousseau, R.; Dougherty, W. G.; Kassel, W. S.; Bullock, R. M. Dinitrogen Reduction by a Chromium(0) Complex Supported by a 16-Membered Phosphorus Macrocycle. J. Am. Chem. Soc. 2013, 135, 11493–11496. (b) Albahily, K.; Shaikh, Y.; Sebastiao, E.; Gambarotta, S.; Korobkov, I.; Gorelsky, S. I. Vinyl Oxidative Coupling as a Synthetic Route to Catalytically Active Monovalent Chromium. J. Am. Chem. Soc. 2011, 133, 6388−6395. (22) Selected examples of Cr catalysis using Grignard reagents: (a) Murakami, K.; Ohmiya, H.; Yorimitsu, H.; Oshima, K. Chromium-Catalyzed Arylmagnesiation of Alkynes. Org. Lett. 2007, 9, 1569–1571. (b) Steib, A. K.; Kuzmina, O. M.; Fernandez, S.; Flubacher, D.; Knochel, P. Efficient Chromium(II)-Catalyzed Cross-Coupling Reactions between Csp2Centers. J. Am. Chem. Soc. 2013, 135, 15346–15349. (c) Li, Y.; Deng, G.; Zeng, X. Chromium-Catalyzed Regioselective Hydropyridination of Styrenes. Organometallics 2016, 35, 747–750. (d) Steib, A. K.; Kuzmina, O. M.; Fernandez, S.; Malhotra, S.; Knochel, P. Chemoselective Chromium(II)–Catalyzed Cross–Coupling Reactions of Dichlorinated Heteroaromatics with Functionalized Aryl Grignard Reagents. Chem. Eur. J. 2015, 21, 1961–1965. (e) Kuzmina, O. M.; Knochel, P. RoomTemperature Chromium(II)-Catalyzed Direct Arylation of Pyridines, Aryl Oxazolines, and Imines Using Arylmagnesium Reagents. Org. Lett. 2014, 16, 5208–5211. (f) Yan, J.; Yoshikai, N. Chromium-catalyzed migratory arylmagnesiation of unactivated alkynes. Org. Chem. Front. 2017, 4, 1972–1975. (g) Yan, J.; Yoshikai, N. Phenanthrene Synthesis via Chromium-Catalyzed Annulation of 2-Biaryl Grignard Reagents and Alkynes. Org. Lett. 2017, 19, 6630– 6633. (h) Chen, C.; Liu, P.; Luo, M.; Zeng, X. Kumada Arylation of Secondary Amides Enabled by Chromium Catalysis for Unsymmetric Ketone Synthesis under Mild Conditions. ACS Catal. 2018, 8, 5864−5868. (23) (a) Shuster, V.; Gambarotta, S.; Nikiforov, G. B.; Budzelaar, P. H. M. Heterometallic Aluminum–Chromium Phenazine and Thiophenazine Complexes. Formation of a Tetranuclear Chromium(I) Sandwich Complex. Organometallics 2013, 32, 2329–2335; (b) Miller, J. S.; O'Hare, D. M.; Chakraborty, A.; Epstein, A. J. Ferromagnetically coupled linear electrontransfer complexes. Structural and magnetic characterization of [Cr(6-C6MexH6-x)2][TCNE] (x = 0, 3, 6) and S = 0 [TCNE]22-. J. Am. Chem. Soc. 1989, 111, 7853–7860. (24) Zhao, D.; Candish, L.; Paul, D.; Glorius, F. N-Heterocyclic Carbenes in Asymmetric Hydrogenation. ACS Catal. 2016, 6, 5978–5988. (25) Selected examples of cobalt catalysis using large amounts of Grignard reagents: (a) Gao, K.; Lee, P.-S.; Fujita, T.; Yoshikai, N. Cobalt-Catalyzed Hydroarylation of Alkynes through Chelation-Assisted C−H Bond Activation. J. Am. Chem. Soc. 2010, 132, 12249–12251. (b) Ohmiya, H.; Wakabayashi, K.; Yorimitsu, H.; Oshima, K. Cobalt-catalyzed cross-coupling reactions of alkyl halides with aryl Grignard reagents and their application to sequential radical cyclization/crosscoupling reactions. Tetrahedron 2006, 62, 2207–2213. (c)



Wakabayashi, K.; Yorimitsu, H.; Oshima, K. CobaltCatalyzed Tandem Radical Cyclization and Cross-Coupling Reaction: Its Application to Benzyl-Substituted Heterocycles. J. Am. Chem. Soc. 2001, 123, 5374–5375. (d) Czaplik, W. M.; Mayer, M.; Jacobi von Wangelin, A. Direct Cobalt-Catalyzed Cross-Coupling Between Aryl and Alkyl Halides. Synlett 2009, 2931–2934. (e) Kauffmann, T. Nonstabilized Alkyl Complexes and Alkyl–Cyano–Ate Complexes of Iron(II) and Cobalt(II) as New Reagents in Organic Synthesis. Angew. Chem., Int. Ed. 1996, 35, 386–403. (26) Wu, J. I.; Wannere, C. S.; Mo, Y.; Schleyer, P. v. R.; Bunz, U. H. F. 4n π Electrons but Stable: N,NDihydrodiazapentacenes. J. Org. Chem. 2009, 74, 4343–4349. (27) For a recent example of low-valent Co-catalyzed hydrogenation using zinc as reductant, see: Friedfeld, M. R.; Zhong, H.; Ruck, R. T.; Shevlin, M.; Chirik, P. J. CobaltCatalyzed Asymmetric Hydrogenation of Enamides Enabled by Single-Electron Reduction. Science 2018, 360, 888–893.


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Chromium- and Cobalt-Catalyzed, Regiocontrolled Hydrogenation of

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