Nanoporous Gold-Catalyzed Diboration of Methylenecyclopropanes

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Letter

Nanoporous Gold-Catalyzed Diboration of Methylenecyclopropanes via A Distal Bond Cleavage Qiang Chen, Xuan Zhang, Shuo Su, Zhanqiang Xu, Na Li, Yiya Li, Han Zhou, Ming Bao, Yoshinori Yamamoto, and Tienan Jin ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01193 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Catalysis

Nanoporous GoldGold-Catalyzed Diboration of Methylenecyclopropanes via A Distal Bond Cleavage Qiang Chen,† Xuan Zhang,‡ Shuo Su,║ Zhanqiang Xu,‡ Na Li,║ Yiya Li,║ Han Zhou,║ Ming Bao,£ Yoshinori Yamamoto,‡,§ and Tienan Jin*,‡,§,£ †

School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Azaaoba Aramaki Aoba-ku, Sendai, 980-8578, Japan ║ Research Institute of Petroleum Processing, Sinopec, Beijing, 100083, China § Research and Analytical Center for Giant Molecules, Graduate School of Science, Tohoku University, 6-3 Azaaoba Aramaki Aoba-ku, Sendai, 980-8578, Japan £ State Key Laboratory of Fine Chemicals and School of Chemistry, Dalian University of Technology, Dalian 116023, China ‡

ABSTRACT: We have demonstrated that the nanoporous gold (AuNPore) is an efficient catalyst to promote the selective diboration of methylenecyclopropanes (MCPs) in a heterogeneous manner. Notably, the diboration takes place via a regioselective cleavage of the distal bond of the cyclopropane ring exclusively by AuNPore without using any additives. The experimental results and computational studies of the mechanism indicate that the present diboration proceeds through the formation of a relatively stable trimethylenemethane intermediate on the AuNPore surface with increased negative charges on trimethylene carbons, giving rise to the subsequent diboration selectively. KEYWORDS: nanoporous gold catalysis, methylenecyclopropanes, distal bond cleavage, diboration, trimethylenemethane

Nanoporous gold (AuNPore) has recently attracted increasing interests in green heterogeneous catalysis as a consequence of its unique self-supported nanoporous structure, robustness, stability, and simple operation for recovery and reuse (Scheme 1a).1,2 The nanopore network channels are formed by bicontinuous hyperboloid-like ligaments wherein a high density of surface steps and kinks exist with low-coordinated gold atoms that are catalytically active sites in various reactions.3 In addition, compared to the supported metal catalysts, the unsupported metallic feature of AuNPore may help to understand the insight of reaction mechanism, intermediates, and catalytic origins more easily without considering the support-effect in experimental and theoretical studies. In the past decade, we4,5 and other groups6 disclosed that AuNPore is able to dissociate O-O bonds for selective oxidation of alcohols and CO, and H-H and Si-H bonds for selective hydrogenation of various multiple bonds. Regardless of these remarkable catalytic properties of AuNPore in redox reactions, the development of novel catalytic performances for dissociation of diverse chemical bonds to achieve valuable molecular transformations still remains a significant challenge. In our efforts to develop new catalytic performances of AuNPore, we recognized that the B-B bond of bis(pinacolato)diboron (B2(pin)2) can be readily cleaved by AuNPore in the absence of of any additives, giving rise to the steroselective diboration of alkynes (Scheme 1b).7 In light of previous studies, we reasoned that if a certain C-C

single bond and a B-B bond are cleaved by the AuNPore catalyst at the same time, a novel C-B bond-forming reaction may take place to afford synthetically useful organoborons in a heterogeneous process that has never been realized to date.

(a)

-nanopore network channels -no supporting materials -ligament size: ~30 nm -hyperboloid-like ligament -simple recovery and reuse -robust and stable catalyst

(b) R 1

R2 + B2 (pin) 2

R1

cat. AuNPore

R2

toluene, 100 oC

(pin)B

cat. Pt(PPh 3) 4

R1

toluene, 80 oC

R2

cat. AuNPore

R1

B(pin)

toluene, 100 oC

R2

B(pin)

B(pin) B(pin)

(c)

R1 + B2(pin) 2 R2

(d)

R1 + B2(pin) 2 R2

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B(pin)

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Scheme 1. Nano-structure of AuNPore Catalyst and Diboration of Alkynes and MCPs. (a) SEM image of nanoporous gold (AuNPore) catalyst. (b) Our previous study: AuNPore-catalyzed diboration of alkynes via a B-B bond cleavage. (c) Pt-Catalyzed homogeneous diboration of MCPs via a proximal bond cleavage by Miyaura et al. (d) This study: AuNPore-catalyzed diboration of MCPs via a distal bond cleavage. Methylenecyclopropanes (MCPs) are highly strained but relatively stable molecules, which show diverse reactivities through the cleavage of a proximal or a distal C-C single bond of the cyclopropane ring and the reaction with a C-C double bond.8 Their highly strained structures and intriguing bonding characteristics make them versatile synthetic intermediates for the study of various selective molecular transformations. Among them, the diboration of MCPs through the regioselective ring-opening reaction is one of the attractive method for the preparation of the synthetically useful organodiborons. Although some homogeneous Pd-catalyzed ring-opening reactions of MCPs via a distal bond cleavage, such as silylcyanation,9a silaboration,9b hydrocarbonation,9c and hydroamination9d have been reported, the diboration of MCPs with B2(pin)2 catalyzed by a Pt catalyst has been demonstrated to proceed via a proximal bond cleavage (Scheme 1c).9e To the best of our knowledge, the diboration of MCPs via a distal bond cleavage has never been reported to date. Herein, we report the first example of the diboration of MCPs with B2(pin)2 through the cleavage of a distal C-C bond of the cyclopropane ring by a unique heterogeneous catalyst of AuNPore (Scheme 1d). Moreover, the theoretical and experimental studies indicate that the distinct regioselectivity of this ring-opening reaction is due to the formation of a stable trimethylenemethane (TMM) intermediate on the gold surface with increased negative charges on the trimethylene carbons. Table 1. Screening of Various Catalysts for Diboration of MCPs.a

entry

catalyst

2a (%)b

1

AuNPore

94 (91,c 82d)

2 3

AuNPs/TiO2e AuAg alloy

11 0

4

AuCl

0

5

PtNPore

0

6

PdNPore

0

7

AgNPore

0

8

CuNPore

0

9f

AuNPore

82

10g

AuNPore

74

a Reaction conditions: 1a (1 mmol), B2(pin)2 (0.5 mmol), catalyst (10 mol%), toluene (0.5 mL), 100 oC, 16 h. b 1H NMR

yield determined using CH2Br2 as an internal standard. c Second use. d Third use. e The average particle size of AuNPs (Au nanoparticles) supported on TiO2 is around 5 nm. f The mole ratio of 1a to B2(pin)2 was 1:1. g The mol ratio of 1a to B2(pin)2 was 1:2.

Based on our previous diboration of alkynes,7 we first examined the catalytic activity of AuNPore in the reaction of (cyclopropylidenemethylene)dibenzene (1a) with B2(pin)2 in tolune at 100 oC (Table 1, entry 1). The AuNPore catalyst was prepared following our previously reported method by dealloying of Au30Ag70 alloy in nitric acid (70%) at room temperature for 18 h.5a The scanning electronic microscopy (SEM) image shown in Scheme 1a clearly indicates its nanoporous structure with the average ligament size of ~30 nm. We were pleased to find that the AuNPore was an efficient and selective heterogeneous catalyst for promoting the diboration of 1a, affording the corresponding 1,3-diboron product 2a as a sole isomer in 94% 1H NMR yield. Moreover, the structure of 2a indicated that the reaction takes place through a regioselective distal bond cleavage of the cyclopropane ring. The AuNPore catalyst can be easily recovered by filtration, which showed a good recyclability without significant decreasing the activity after reusing additional two times (entry 1). The use of gold nanoparticles on TiO2 (AuNPs/TiO2) (SI, Figure S2)10 as a heterogeneous catalyst resulted in partial decomposition of 1a along with a low yield (11%) of 2a (entry 2). The Au30Ag70 alloy and the homogeneous AuCl did not exhibit any catalytic activity (entries 3 and 4). These results indicated that the nanostructured gold materials are superior catalysts for the present diboration. Surprisingly, other nanoporous metal catalysts, such as nanoporous platinum (PtNPore),11 nanoporous palladium (PdNPore),12 nanoporous silver (AgNPore),13 nanoporous copper (CuNPore)14 were examined to be totally inactive (entries 5-8), indicating the unique catalytic activity of AuNPore for the present diboration reaction. We also found that the ratio of 1a to B2(pin)2 plays an important role for achieving a high yield of 2a. For example, compared to the high yield of the reaction using a 2:1 ratio of 1a to B2(pin)2 (entry 1), the use of 1.5:1 (84%), 1:1 (82%), 1:1.5 (78%), and 1:2 (74%) obviously decreased the yields (entries 9 and 10, SI, Table S2). In addition, the leaching experiments and ICP-MS analysis clearly showed that no gold atoms were leached into the reaction solution (SI, Scheme S1), indicating that the present diboration proceeds in a heterogeneous process. The current catalytic diboration can be applied to the gram scale process using 2.7 g of 1a and 1.66g of B2(pin)2, giving the corresponding product 2a in a slightly lower isolated yield of 80%. Moreover, the resulting 1,3-diboron product 2a was readily converted into the corresponding 1,3-diol 3a in 90% yield under standard oxidation conditions (eq 1).

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ACS Catalysis To further investigate the catalytic performance of AuNPore, we investigated the substrate scope with respect to the MCPs bearing substituents at the alkenyl terminus (Scheme 2). The diaryl-substituted MCPs having electronrich or electron-poor groups such as methyl, fluorine, and chlorine on the phenyl ring furnished the corresponding 1,3-diboron products 2b-e in good to high yields with negligible electronic effects on the catalytic efficiency under the method A (2:1 mole ratio of 1 to B2(pin)2). By comparison, much lower yields of 2b-e were obtained using the mole ratio of 1:2 of 1 to B2(pin)2 (method B). We also optimized the mole ratio of MCPs to B2(pin)2 using the alkylsubstituted MCP of 1l (SI, Table S2). In sharp contrast, the use of the method B (1:2 of 1l to B2(pin)2) gave the highest 1H NMR yield of 2l (78%), while the use of 1:1.5 (73%), 1:1 (71%), 1.5:1 (62%), and 2:1 (60%, method A) resulted in decreased yields. Similarly, (1cyclopropylideneethyl)benzene 1f, the dialkyl-substituted MCPs of 1g and 1h, and the monoalkyl- (1m) and monophenyl-substituted (1n) MCPs under the method B afforded higher yields of the corresponding 1,3-diborons compared to the method A. Similar to the diaryl-substituted MCPs, the dialkyl-substituted MCPs bearing relatively bulky substituents, such as dihexyl (1i), cyclohexyl (1j), and cycloheptyl (1k) produced higher yields of the corresponding products 2i-k under the method A than the method B. It seems that the method A is favorable for the sterically bulky diaryl- and dialkyl-substituted MCPs, while the sterically less hindered dialkyl- and monosubstituted MCPs are suitable for the method B. We found that the diaryl-substituted MCPs are more stable than the alkylsubstituted MCPs on AuNPore. For example, the reaction of 1a under the standard conditions in the absence of B2(pin)2 produced a mixture of unidentified products with the recovery of 1a in 46% yield, while 1f, 1i, and 1m were completely consumed, resulting in a mixture of unknown products with serious decomposition of the starting MCPs (vide infra for 1l). We suppose that the sterically bulky MCPs may weaken the interaction between AuNPore and MCPs, and hence a higher concentration of MCPs (method A) is required to enhance their interaction with AuNPore. The diaryl-substituted MCPs usually afford much higher yields of the 1,3-diboron products under both methods A and B compared to the alkyl-substituted MCPs, which are ascribed to the high stability of the proposed trimethylenemethane (TMM) intermediate (Int_A, Figure 1d) via delocalization of negative charges over the aromatic rings. In contrast, the TMM intermediates arising from the sterically less hindered MCPs are rather unstable, which could rapidly react together or react with MCPs to give a mixture of products. Therefore, an excess amount of B2(pin)2 (method B) is required to increase the reaction probability between the corresponding TMM intermediates and B2(pin)2 while inhibit the side reactions. Overall, in all cases the corresponding diboron isomers forming from the proximal bond cleavage have never been observed, demonstrating the remarkable catalytic performance of AuNPore for the regioselective ring-opening diboration of MCPs.

anistic insight into this unprecedented regioselective ring opening pathway. According to the recent reports on theoretical studies of AuNPore using Au(111) as a model system,15 we simulated the adsorption behaviour and the ring-opening pathways of MCP on the Au(111) surface. The DFT calculations indicated that MCP prefers to adsorb on the top position of Au(111) from the C=C double bond with the adsorption energy of -50.34 kJ/mol (Ads) (Figures 1a and 2, and SI Figure S3 and Table S3), which is similar to the reported adsorption conformation of propene on Au (111).16 Moreover, the net Hirshfeld charge of the adsorbed neutral MCP is calculated to be -0.04e (Figure 1a), indicating the slight electron transfer from the Au atom to MCP. To computationally understand the fact that the preference for the distal bond cleavage over the proximal bond cleavage on AuNPore, the energy difference of two dissociation pathways of the cyclopropane ring was calculated on Au(111) as shown in Figure 2. The corresponding activation energy barriers for dissociation of a distal bond and a proximal bond are 12.2 kJ/mol (TS1) and 52.10 kJ/mol (TS2), respectively. As a result, the dissociation of the distal bond is favored by 39.85 kJ/mol over the dissociation of the proximal bond, indicating that the cleavage of a distal bond is more favorable than a proximal bond on Au (111). In comparison, the bond dissociation energy values of the distal and proximal bonds of the original MCP are calculated to be as large as 109.62 kJ/mol and 258.76 kJ/mol (SI, Table S4), respectively, implying the strong bond dissociation performance of gold for the cyclopropyl ring of MCP. Particularly, the optimized intermediate (Int_A) with respect to the distal bond cleavage is energetically 178.08 kJ/mol more stable than the intermediate (Int_B) with respect to the proximal bond cleavage (Figures 1d, 1e, 2). Consequently, the DFT calculation results are well consistent with our experimental outcome.

Density functional theory (DFT) calculations were carried out using the parent MCP for simplicity to gain mech-

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R1 + B2(pin) 2 R2 1

AuNPore (10 mol%) toluene, 100 oC

A: 1 (1 mmol), B 2(pin) 2 (0.5 mmol) B: 1 (0.5 mmol), B2(pin) 2 (1 mmol)

1a, R 1 = R2 = C 6H5 1b, R 1 = 4-Me-C 6H4, R 2 = C6H 5 1c, R 1 = R2 = 4-Me-C 6H 4 1d, R 1 = 4-Cl-C 6H4 , R 2 = C6 H 5 1e, R 1 = R2 = 4-F-C 6H 4 1f, R 1 = C 6H 5, R2 = Me 1g, R 1 = R2 = n-C 3H 7

R1

B(pin)

2

(c)

(d)

1h, R 1 = R 2 = n-C 4H 9 1i , R 1 = R2 = n-C 6H 13 1j , R 1-R 2 = -(CH2)51k, R1 -R 2 = -(CH 2)61l , R 1 = CH2CH 2C 6H 5, R 2 = H 1m , R 1 = n-C 7H15, R 2 = H 1n, R 1 = C 6H 5, R2 = H

B(pin)

B(pin)

B(pin)

B(pin)

2b, 86% (48 h, A) 72% (48 h, B)

2c, 62% (60 h, A) 50% (60 h, B)

F

B(pin)

B(pin)

B(pin)

B(pin)

2d, 63% (60 h, A) 55% (60 h, B)

B(pin) Me

F 2e, 81% (48 h, A) 69% (48 h, B)

B(pin)

2f, 30% (40 h, B) 20% (40 h, A)

n-C 3H 7

B(pin)

n-C 4H 9

B(pin)

n-C 6H 13

B(pin)

n-C 3H 7

B(pin)

n-C 4H 9

B(pin)

n-C 6H 13

B(pin)

2g, 58% (40 h, B) 50% (40 h, A)

2h, 53% (40 h, B) 34% (40 h, A)

B(pin)

B(pin)

B(pin)

B(pin)

2j, 38% (60 h, A) 25% (60 h, B) n-C 7H 15

(b)

2

B(pin)

Cl

(a)

B(pin)

R

B(pin)

2a, 86% (16 h, A) 70% (16h, B)

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B(pin) B(pin)

2m, 73% (40 h, B) 54% (40 h, A)

2k, 43% (60 h, A) 35% (60 h, B) Ph

2i, 45% (60 h, A) 20% (60 h, B) Ph B(pin) B(pin) 2l, 70% (40 h, B) 54% (40 h, A)

B(pin)

(e)

B(pin) 2n, 50% (40 h, B) 41% (40 h, A)

Scheme 2. Substrate Scope for Diboration of MCP. a Isolated yields are shown. The calculation indicated that the optimized Int_A formed by a distal bond cleavage is composed of a TMM structure binding on Au(111) with three identical C-C bond length of 1.445 Å as shown in Figure 1d. In the past three decades, the reactive TMM species has been well studied in versatile synthetic application functioning as a diradical and a 1,3-dipolar fragment in cycloaddition and 1,3-bifunctionalization.8,17 The most striking difference of the TMM in Int_A is that each trimethylene carbon atom possesses a negative Hirshfeld charge of -0.124 with a positive charge of +0.024 at the central carbon. Furthermore, the Hirshfeld charges on the trimethylene carbon atoms are much negative compared with two distal carbon atoms of the adsorbed MCP in Ads (-0.083) and hence this TMM intermediate is expected to be favorable for the nucleophilic attack.

Figure 1. Optimized adsorption and dissociation conformations of MCP on Au(111), Hirshfeld charges (red), and C-C bond lengths (blue). The gray and white balls represents the C atom and the H atom, respectively. (a) Ads. (b) TS1. (c) TS2. (d) Int_A. (e) Int_B.

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ACS Catalysis

Figure 2. Energy diagram of the ring-opening pathways of MCP on Au(111).

The adsorption energy of B2(pin)2 on Au(111) was also calculated to be -20.40 kJ/mol, which is much weak than that of MCP probably due to the steric hindrance of the pinacol moiety (SI, Figure S4). This physisorption interaction results in a slight change of the B-B bond length from the intrinsic length of 1.704 Å to 1.706 Å. Although this weak interaction between B2(pin)2 and Au(111) could not fully explain the fact of the B-B bond cleavage on the Au(111) surface,18 our previous study on the cross diboration of alkynes with different diboron reagents clearly demonstrates the B-B bond cleavage by AuNPore. Additionally, a very slight change of the Hirshfeld charge of the B atoms between intrinsic (+0.139) and adsorbed (+0.137) B2(pin)2 should not considerably alter its electrophilicity.19 On the basis of the experimental and theoretical studies, the plausible reaction mechanism is outlined in Scheme 3. The MCP 1a adsorbs on AuNPore from the C=C double bond, which may induce a selective distal bond cleavage to form a relatively stable TMM intermediate binding on AuNPore with negative charges at the three trimethylene carbons and a positive charge at the central carbon. At the same time, the homolytic dissociation of B2(pin)2 takes place on AuNPore, affording the corresponding boron electrophile species binding with AuNPore, which reacts with sterically less hindered two nucleophilic methylene carbons to afford the 1,3-diboron product 2a.

(pin)B B(pin) homolytic dissociation

mation of the electronically distinct TMM species (eq 2). If the ring-opening diboration of 1o proceeds through the TMM intermediate (Int_A), the reaction is expected to give the product 2n, a formal diboration product of 1n, due to the steric effect (Scheme 2). As expected, the diboration of 1o with B2(pin)2 under the standard AuNPore-catalyzed conditions afforded the desired product 2n as a sole isomer in 52% yield. In addition, although the reactions of most MCPs with AuNPore in the absence of B2(pin)2 gave a mixture of unidentified products, we were able to obtain the desired diene product 3l arising from the distal bond cleavage as a major isomer in the reaction of the monoalkyl-substituted MCP 1l with AuNPore catalyst (eq 3). Furthermore, (E)-(2-cyclopropylvinyl)benzene was totally inactive for the current diboration, indicating the inert reactivity of the C=C bond for diboration and hence the important role of the MCP structure. Thus, the mechanism through the C=C bond boration of 1o followed by the ringopening is unlikely. These experimental outcomes strongly support our TMM intermediate mechanism.

Ph + B 2(pin) 2

AuNPore (10 mol%)

Ph

B(pin) (2)

toluene, 100 °C, 24 h

B(pin)

1o

2n, 52%

Ph

Ph

AuNPore (10 mol%)

(3)

toluene, 100 oC, 16 h 1l

3l, 35%

In conclusion, we have demonstrated that AuNPore is a unique catalyst for the regioselective ring-opening diboration of MCPs in a heterogeneous process. To the best of our knowledge, this is the first successful catalytic diboration of MCP through a distal bond cleavage, which afforded various new organic 1,3-diborons in good to high yields with exclusive selectivity. The DFT calculations and experimental evidences with respect to the distal bond cleavage of MCP strongly support the involvement of an electronically distinct TMM intermediate arising from an energetically favorable distal bond dissociation transition state. The increased negative charges on the trimethylene carbons in the TMM intermediate gives rise to the nucleophilic attack to B2(pin)2, affording the corresponding 1,3diboron product. The present study is expected to open new avenues for developing novel catalytic performances of AuNPore as well as the TMM chemistry.

AuNPore Ph Ph Ph

distal bond cleavage Ph δPh

δ+

δ+

δ+

pin B

δPh

AuNPore

AuNPore

Ph

ASSOCIATED CONTENT

pin B + δ

1a

B(pin)

Ph

B(pin) 2a

Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. Experimental and computation details, and analytic data and 1H, 13C NMR spectra of all reaction products.

Scheme 3. Plausible Reaction Mechanism.

AUTHOR INFORMATION

According to calculations, we carried out the diboration of (2-methylenecyclopropyl)benzene (1o) bearing a phenyl group on the cyclopropane ring to support the for-

*E-mail: [email protected]

Corresponding Author

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant No.JP16H01000 in Precisely Designed Catalysts with Customized Scaffolding. All calculations were performed with the Materials Studios (MS) 2016, a software developed by Biovio group.

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