Letter pubs.acs.org/OrgLett
On-Surface Heck Reaction of Aryl Bromides with Alkene on Au(111) with Palladium as Catalyst Ke-Ji Shi,† Chen-Hui Shu,† Cheng-Xin Wang, Xin-Yan Wu, He Tian, and Pei-Nian Liu* Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials, State Key Laboratory of Chemical Engineering and School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China S Supporting Information *
ABSTRACT: The on-surface Heck reaction of aryl bromides with terminal alkene has been achieved for the first time. With palladium as the catalyst, cross-coupling of porphyrin-derived aryl bromides with terminal alkene proceeds with high selectivity on an Au(111) surface, as determined by scanning tunneling microscopy at the single molecular level. Density functional theory calculations suggest that the on-surface Heck reaction proceeds via debromination of aryl bromide, addition to the CC bond, and elimination of hydrogen, ultimately affording the cross-coupling product.
O
Scheme 1. Heck Reaction on Au(111) with Pd as Catalyst
n-surface reactions, defined as the processes by which organic reactions take place on surfaces, have attracted considerable attention and contribute new perspectives to organic chemistry.1 Ever since the first pioneering work in 2007,2 numerous conventional organic reactions in solution have been achieved on surfaces, including Ullmann coupling;2,3 homocoupling of alkanes,4 alkenes,5 and alkynes;6 imine formation;7 condensation of boron acids;8 Bergman cyclization;9 acylation;10 cycloaddition of acetyls;11 cyclodehydrogenation;12 and azide−alkyne reactions.13 On-surface homocoupling reactions, especially Ullmann coupling of aryl halides, have been used extensively to construct diverse macromolecular systems, including polymeric chains,14 hyperbranched oligomers,15 graphene ribbons,16 porous molecular networks,17 2D covalent organic frameworks,18 and other stuctures.19 Cross-coupling reactions in solution are extremely powerful tools for creating C−C single bonds and generating molecular diversity. They allow highly selective joining of organic moieties A and B into A−B20 while avoiding formation of A−A or B−B. Because most of the organic molecules contain consecutive C−C bonds, these reactions are tremendously useful in the synthesis of organic compounds, including pharmaceuticals and conjugated organic materials. For example, the Heck reaction, which involves cross-coupling of aryl halides with terminal alkenes, allows substitution at CC bonds, affording a diversity of useful alkene molecules (Scheme 1).21 © 2017 American Chemical Society
Realizing cross-coupling reactions such as the Heck reaction on surfaces may allow the construction of a variety of nanostructures that cannot be generated via homocoupling, for example, organic functional materials with a D−π−A structure. However, developing on-surface cross-coupling reactions is Received: March 22, 2017 Published: May 16, 2017 2801
DOI: 10.1021/acs.orglett.7b00855 Org. Lett. 2017, 19, 2801−2804
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Organic Letters
assemble side by side to form a close-packed array (Figure 1b). Most A1 molecules (green ellipse) were scattered among arrays of B molecules (white ellipse), while some porphyrin molecules aggregated into small patches. We tried to couple A1 and B by annealing the sample to 150 °C for 20 min. However, STM results showed no Heck reaction product P1; instead, alkene B desorbed from the Au(111) surface and porphyrin A1 selfassembled on the surface (see Figure S1). These results show that the Au(111) surface by itself does not effectively promote the cross-coupling reaction of aryl bromide and alkene. To enhance the reactivity of the reactants on Au(111), we dosed Pd onto Au(111) (0.09 ML) to act as catalyst in the Heck reaction of aryl bromide A1 with alkene B. After loading Pd onto the sample containing molecules A1 and B at room temperature and then annealing to 150 °C for 20 min, porphyrin molecules P1 with two tails were generated and packed together closely to form islands (Figure 1c). Porphyrins lined up one by one at a separation of 1.38 ± 0.05 nm (Figure 1d), suggesting that van der Waals forces drive the alignment.24 The porphyrins were arranged with their biphenyl groups on alternating sides. The length of each biphenyl side chain was 1.04 ± 0.05 nm, and the distance between two side chains was 0.62 ± 0.05 nm. These results suggest that the biphenyl chains did not interact strongly. The full length of the molecule along the two tails was 4.08 ± 0.05 nm, consistent with the DFT-calculated length of 4.04 nm for molecule P1. The length of the molecule along the two phenyl substitutions was 1.78 ± 0.05 nm, consistent with the DFT-calculated length of 1.81 nm. Note that the image of P1 simulated by DFT calculation was shown in the corner of Figure 1d, which is identical with the STM image of P1. P1 selfassembled into a periodic structure with the unit cell dimensions of 2.73 ± 0.05 nm along the long side, 1.38 ± 0.05 nm along the short side, and an angle of 107°. The simulated packing of four molecules of P1 was shown in Figure 1d, which fit well with the experimental image. These results demonstrate that under our experimental conditions the aryl bromide group of A1 underwent Heck reaction with the CC group of alkene B to form product P1. This on-surface cross-coupling reaction showed excellent selectivity, and no obvious homocoupling products of A1 or dimerization product of B were observed. The reaction of the full monolayer of monomer B was also carried out under the same reaction conditions, but no dimerization product was observed, similar to the reported result on Au(111).5 These results show that Pd as catalyst on Au(111) allows efficient Heck reaction of aryl bromide with alkene, while the Ullmann reaction of aryl bromide and dehydrogenative dimerization of alkene are prohibited. Note that the metalation of Pd into porphyrin ring of P1 might occur, although it could not be determined by the STM image.25 To verify the high reactivity and selectivity of this on-surface, Pd-catalyzed Heck reaction, we attempted the reaction with porphyrin molecule A2 containing four phenyl bromide groups. After deposition on Au(111) at room temperature, A2 molecules formed a close-packed monolayer structure (Figure 2a). Each A2 molecule had four neighbors, and the distance between adjacent porphyrins was 1.58 ± 0.05 nm, suggesting that van der Waals forces drive the alignment.24 Then B molecules were dosed onto the surface at approximately −20 °C. The two different molecules tended to assemble side-by-side into a close-packed structure (Figure 2b). Finally, Pd was loaded onto the surface containing molecules A2 (blue ellipse) and B (white ellipse) held at room temperature. The sample was annealed to 150 °C for 20 min, generating porphyrin molecules P2 with four tails, which
challenging because it requires inhibiting homocoupling of identical precursor molecules while efficiently promoting crosscoupling of different precursor molecules. On-surface crosscoupling reactions have so far been limited to cross-coupling of porphyrin bromide with aryl bromide22 and Sonogashira reaction of alkynes with aryl halides,23 but the selectivity of both reactions is not satisfactory. In the on-surface Sonogashira reactions of phenylacetylene with phenyl halides, homocoupled species are the major products on Au(111),23a Au(100),23b Ag(100)23c surfaces. This is because the two reactants strongly prefer to form homomolecular islands on the surface instead of intermixing; as a result, cross-coupling occurs only at island boundaries.23a Here, we report the first on-surface Heck reaction of aryl bromides with alkene on Au(111) with Pd as catalyst. Using scanning tunneling microscopy (STM) at single-molecule resolution, we investigated the reactions of 5,15-bis(4bromophenyl)-10,20-diphenylporphyrin (A1) and 5,10,15,20tetrakis(4-bromophenyl)porphyrin (A2) with 4-vinyl-1,1′-biphenyl (B). Both reactions formed the cross-coupling products highly selectively, with no obvious formation of homocoupling products. Density functional theory (DFT) calculations suggest that the reaction proceeds via debromination of aryl bromide and addition to the CC bond and elimination of hydrogen, ultimately affording the cross-coupling product. In our approach, A1 was deposited onto a pristine surface of Au(111) held at room temperature inside a commercial ultrahigh vacuum (UHV) system (base pressure: ∼ 2 × 10−10 mbar) equipped with a variable-temperature STM (SPECS, Aarhus 150). The STM image revealed a close-packed monolayer structure in which A1 molecules adopted the same orientation (Figure 1a). Each A1 molecule had four neighbors, and the distance between two porphyrins was 1.87 ± 0.05 nm, suggesting that van der Waals forces drive the alignment.24 Next, we dosed B molecules onto the sample at approximately −20 °C. The STM image showed that B molecules tended to
Figure 1. (a) Self-assembly of A1 molecules on Au(111) at room temperature (12 × 12 nm2, −2.0 V, −0.06 nA). (b) Self-assembly of A1 and B molecules on Au(111) at −20 °C (20 × 20 nm2, −2.5 V, −0.08 nA). (c) Heck reaction of A1 and B on Au(111) after dosing Pd and annealing at 150 °C (26 × 26 nm2, −2.0 V, −0.08 nA). (d) Zoomed-in image of the Heck reaction products (8 × 8 nm2, −2.0 V, −0.11 nA). 2802
DOI: 10.1021/acs.orglett.7b00855 Org. Lett. 2017, 19, 2801−2804
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Figure 2. (a) Self-assembly of A2 molecules on Au(111) at room temperature (39 × 39 nm2, inset 7 × 7 nm2, −2.0 V, −0.08 nA). (b) Selfassembly of A2 and B molecules on Au(111) at −20 °C (13 × 13 nm2, −2.5 V, −0.06 nA). (c) Heck reaction of A2 and B on Au(111) after dosing with Pd and annealing at 150 °C (34 × 34 nm2, −2.0 V, −0.08 nA). (d) Zoomed-in image of Heck reaction products (10 × 10 nm2, −2.0 V, −0.07 nA).
packed closely into islands (Figure 2c). The measured length of molecule P2 was 4.02 ± 0.05 nm, similar to the length of molecule P1 (Figure 2d). The image of P2 simulated by DFT calculations was also shown in the corner of Figure 2d, similar to the STM image of P2. The porphyrins lined up one by one with a separation of 2.70 ± 0.05 nm and interchain separation of 0.63 ± 0.05 nm. These results indicate that the biphenyl chains did not interact strongly. They also indicate that the aryl bromide group of A2 underwent Heck reaction with the CC group of alkene B to form product P2. To unravel the mechanism of the Heck reaction catalyzed by Pd on Au(111), DFT calculations were performed using simplified benzene bromide and styrene as the reactants. We analyzed debromination and coupling steps using the climbing image nudged elastic band (CI-NEB) technique implemented in the plane wave-based Vienna ab initio simulation package (VASP).26 For the first step of debromination (Figure 3a), the calculated reaction barrier energy Ebarrier (ETS1 − EIS1) is 0.83 eV on Au(111) with a Pd adatom as catalyst. In contrast, Ebarrier in the absence of Pd adatom is 1.02 eV,27 consistent with our observations that the Pd adatom is an effective catalyst of debromination.28 Although the calculated reaction energy Ereact (EINT1 − EIS1) of debromination is 0.44 eV, the benzene radical can be further stabilized by another Pd adatom with an exothermal energy of −0.86 eV (see Figure S5). In the subsequent coupling step (Figure 3b), the benzene radical stabilized by the Pd adatom and an isolate styrene were set as IS2, and the styrene approached the radical by coordination with Pd adatom to form the initial state IS3. Then the benzene radical underwent addition to the head atom of the CC bond in the alkene to form intermediate INT2 via transition state TS2. The calculated reaction energy Ebarrier (ETS2 − EIS3) of this step was small (0.44 eV), and the resulting radical intermediate INT2 was also stabilized by the Pd adatom. Subsequent elimination of the H atom (marked as red) in INT2 gave the final Heck product with Ebarrier (ETS3 − EINT2) of 0.86 eV and Ereact (EFS − EIS3) of
Figure 3. DFT-calculated energy diagrams for (a) debromination and (b) coupling of Heck reaction on Au(111) with Pd as catalyst. Below the energy diagrams are shown top and side views of the initial state (IS), transition state (TS), intermediate state (INT), and final state (FS) of the reactions. In panel (b), the red atom is the H atom eliminated from the intermediate.
−0.54 eV. These calculations show that the on-surface Heck reaction is exothermal. The calculation of similar Ebarrier for elimination and debromination suggests that once the aryl radical forms on the surface, it can react smoothly with alkene to generate the Heck reaction product through addition and elimination steps. Moreover, the debrominated radicals from porphyrin-derived bromides A1 and A2 are less stable and have much higher adsorption energy in comparison with alkene B. As a result, the radicals generated from A1 or A2 prefer to react with the fast-moving alkene B instead of homocoupling with another radical. This is consistent with the experimental observation of the high selectivity of Heck reaction. It is noteworthy that the DFT-calculated barrier energy of the dimerization of alkene on more active Cu(110) surface was reported as 1.35 eV,5 illustrating the dimerization of alkene might be more difficult than Heck reaction. In summary, we describe here for the first time an on-surface Heck reaction of aryl bromides with terminal alkene, which we analyze using UHV-STM at single molecular level. Pd as catalyst is essential on Au(111) to promote highly selective crosscoupling of porphyrin-derived aryl bromides A1 and A2 with alkene B to afford the products P1 and P2. DFT calculations suggest that the reaction proceeds via debromination of aryl bromide, addition to alkene and elimination of hydrogen. This 2803
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protocol may shed light to the development of more on-surface cross-coupling reactions with high selectivity and may be applied in the preparation of nano electronic materials and devices on ultraclean solid surfaces in the future.
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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.orglett.7b00855. General procedures for the STM experiments and DFT calculations; supplemental STM and calculation data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
He Tian: 0000-0003-3547-7485 Pei-Nian Liu: 0000-0003-2014-2244 Author Contributions †
K.-J.S. and C.-H.S. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Prof. Nian Lin (Hong Kong University of Science & Technology) for help with this work. This work was supported by the National Natural Science Foundation of China (Project Nos. 21672059, 21421004, 21561162003, and 21372072), the Program for Eastern Scholar Distinguished Professor, the Fundamental Research Funds for the Central Universities, and the Programme of Introducing Talents of Discipline to Universities (B16017).
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DOI: 10.1021/acs.orglett.7b00855 Org. Lett. 2017, 19, 2801−2804
