Ad3P-Catalyzed Suzuki Cross-Coupling

Nov 7, 2017 - Controlled Pd(0)/Ad3P-Catalyzed Suzuki Cross-Coupling ... to be an efficient initiator for controlled Suzuki cross-coupling polymerizati...
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Letter Cite This: ACS Macro Lett. 2017, 6, 1301-1304

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Controlled Pd(0)/Ad3P‑Catalyzed Suzuki Cross-Coupling Polymerization of AB-Type Monomers with Ad3P‑Coordinated Acetanilide-Based Palladacycle Complex as Initiator Jie Dong, Hui Guo, and Qiao-Sheng Hu* Department of Chemistry, College of Staten Island of the City University of New York, Staten Island, New York 10314, United States Ph.D. Program in Chemistry, Graduate Center of the City University of New York, New York, New York 10016, United States S Supporting Information *

ABSTRACT: Controlled Pd(0)-catalyzed Suzuki cross-coupling polymerizations of AB-type monomers with tris(1-adamantyl)phosphine (Ad3P) as the ligand was described. Ad3P-coordinated acetanilide-based palladacycle complex (1) was demonstrated to be an efficient initiator for controlled Suzuki cross-coupling polymerization, affording polymers with narrow Đs and well-controlled end groups. Our study provided an efficient ligand and an efficient initiator for controlled Suzuki cross-coupling polymerizations.

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ver the past decades, Pd(0)-catalyzed Suzuki crosscoupling polymerizations have become some of the most powerful tools for the synthesis of a broad spectrum of conjugated polymers.1,2 The establishment of t-Bu3P as an unique ligand to achieve preference oxidative addition turned Pd(0)-catalyzed cross-coupling polymerizations of AB-type monomers from the step-growth fashion into a chain-growth fashion3 and allowed such cross-coupling polymerizations to occur in a controlled fashion.4−11 By using appropriate t-Bu3Pcontaining initiators, for example, isolated ArPd(t-Bu3P)Br complexes,5,6 in situ generated ArPd(t-Bu3P)Br complexes7,8 or Pd(OAc)2/t-Bu3P system,9 controlled Pd(0)/t-Bu3P-catalyzed cross-coupling polymerizations of AB-type monomers have recently become useful methods to access conjugated polymers with controlled degree of polymerizations, narrow Đ, and welldefined functional chain end groups.3 In our laboratory, since the identification of t-Bu3P as a unique ligand for controlled Pd(0)-catalyzed Suzuki crosscoupling polymerizations,3we have been interested in identifying other monodentate ligands that could exhibit catalytic properties similar to or better than t-Bu3P. Based on our previous preferential oxidative addition study3 and our controlled polymerization study,7 we reasoned that such monodentate ligands likely need to meet the following two criteria: (1) to be as electron-rich as or more electron-donating than t-Bu3P, which would allow to achieve preferential oxidative addition, and (2) to be bulky enough that would allow the reductive elimination step to occur efficiently to minimize chain transfer or termination during the polymerization. Tris(1adamantyl)phosphine (Ad3P) (Figure 1) has recently been reported to be sterically similar to t-Bu3P but more electrondonating than t-Bu3P and Ad3P-coordinated acetanilide-based palladacycle complex 1 is readily accessible (Figure 1).12 We thus reasoned that Ad3P might possess the catalytic properties of t-Bu3P and envisioned that Ad3P might be an efficeient ligand for controlled Pd(0)-catalyzed cross-coupling polymer© XXXX American Chemical Society

Figure 1. Tris(1-adamantyl)phosphine (Ad3P) and Ad3P-Coordinated Acetanilide-Based Palladacycle Complex 1.

izations of AB-type monomers (Scheme 1). Herein, we report our preliminary results on employing Ad3P as the ligand for Scheme 1. Envisioned Ad3P-Coordinated Acetanilide-Based Palladacycle Complex 1 as Initiator for Controlled Suzuki Cross-Coupling Polymerizations of AB-Type Monomers

controlled Pd(0)-catalyzed Suzuki cross-coupling polymerization of AB-type monomers, specifically using Ad3Pcoordinated acetanilide-based palladacycle complex (1)12 as an efficient initiator, to afford conjugated polymer with predictable molecular weights and narrow Đs and well-defined functional chain end groups. Our study began with the polymerization of 7-bromo-9,9dihexylfluoren-2-ylboronic acid pinacol ester by using Ad3Pcoordinated acetanilide-based palladacycle complex 1 as the Received: September 26, 2017 Accepted: November 1, 2017

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DOI: 10.1021/acsmacrolett.7b00759 ACS Macro Lett. 2017, 6, 1301−1304

Letter

ACS Macro Letters initiator. Different common inorganic bases were first tested for the polymerization at room tempature (Table 1, entries 1−4).

Table 2. Pd(0)/Ad3P-Catalyzed Suzuki Cross-Coupling Polymerization of AB-Type Monomers with Complex 1 as Initiatora

Table 1. Condition Screening for Pd(0)/Ad3P-Catalyzed Suzuki Cross-Coupling Polymerization of 7-Bromo-9,9dihexylfluoren-2-ylboronic Acid Pinacol Ester with Complex 1 as Initiatora

entry

base

temp

time (min)

yieldb (%)

Mnc (Đ)

1 2 3 4 5 6 7 8 9 10

K3PO4 K2CO3 Na2CO3 Cs2CO3 K2CO3 K2CO3 K2CO3 (8 equiv) K2CO3 (12 equiv) K2CO3 (15 equiv) K2CO3 (20 equiv)

r.t. r.t. r.t. r.t. 0 0 0 0 0 0

30 30 30 30 30 60 60 60 60 60

95 79 68 88 39 79 64 80 83 85

10700 (1.67) 8000 (1.27) 7600 (1.43) 8100 (1.47) 5000 (1.11) 7700 (1.12) 5700 (1.15) 6900 (1.12) 8200 (1.16) 8100 (1.20)

a

Polymerization condition: monomer (0.2 mmol), 1 (6 mol %), base (2 M aq, 0.5 mL), THF (2.5 mL), room tempearture or 0 °C, 30 or 60 min. bIsolated yield. cAs determined by GPC (PS standards, THF, 40 °C). d5 mL THF were used for the polymerization. a

Polymerization condition: monomer (1 equiv), 1 (6 mol %), K2CO3 (2 M, 0.5 mL), THF (2.5 mL), 0 °C, 60 min, quenched with 4MeOC6H4B(OH)2. bIsolated yield. cAs determined by GPC (PS standards, THF, 40 °C). dQuenched with 4-FC6H4B(OH)2. e9 mol % of 1 was used. fThe polymerization was carried out at room tempearture for 90 min. g9 mol % of 1 was used. h12 mol % of 1 was used. i4 mol % of 1 was used, reaction time: 2 h. j2 mol % of 1 was used.

We found that potassium carbonate was the best base, affording the polymer with 79% yield and a Đ of 1.27 (Table 1, entry 2). To narrow the Đ, we then carried out the polymerization at 0 °C. We found although only 39% yield was observed after 30 min, a Đ of 1.11 was obtained (Table 1, entry 5). Extending the polymerization time to 60 min afforded the polymer with 79% yield and a Đ of 1.12 (Table 1, entry 6). These results suggested that Ad3P was indeed an efficient ligand for the controlled Suzuki cross-coupling polymerization of AB-type monomers, except that the polymerzaion with Ad3P as the ligand occurred slower than the polymerization with t-Bu3P as the ligand.7d We have also briefly examined the impact of the amounts of base on the polymerization and found 10−12 equiv of potassium carbonate resulted in better control of the polymerization, as evidenced by a narrow Đ of 1.12 of the formed polymer (Table 1, entries 6−10). Other AB-type monomers were examined with complex 1 as initiator for the controlled polymerization and our results are listed in Table 2. We found that the polymerization proceed smoothly for these AB-type monomers with complex 1 as initiator, affording polymers with narrow Đs (Table 2, entries 2−5). We also found that when 7-iodo-9,9-dihexylfluoren-2ylboronic acid pinacol ester was used as the monomer, the polymerization occurred very slowly (Table 2, entry 6), likely because of the slow transmetalation process for Ar−PdI species.13 Carrying out the polymerization at room temperature afforded the polymer with 81% yield (Table 2, entry 7). In addition, we found that by using different amounts of the initiator, the molecular weight of the polymer could be adjusted with a slight fluctuation of the Đ (Table 2, entries 1, 8−10). When 2 mol % of complex 1 was used for the polymerization, a low conversion (27%) was observed (Table 2, entry 11), supporting that the polymerzaion was slower with Ad3P as the ligand than the polymerization with t-Bu3P as the ligand.

The relationship between the monomer conversion and the molecular weight of the generated polymer was examined to understand the polymerization behavior by using Ad3Pcoordinated palladacycle complex 1 as initiator. A linear relationship was observed between them with almost the same Đs for polymers of different conversions (Figure 2), a characteristic of chain-growth polymerization. This result suggested that the controlled polymerization with complex 1 as the initiator likely occurred via a chain-growth fashion. Matrix-assisted laser desorption ionization-time of flight (MALDI-tof) mass spectrometry was used to analyze the end groups of polymer products (Table 2). There was only one series of peaks being observed in MALDI-tof spectra, which corresponded to the polymers with a 2-aminophenyl group as one end group (from initiator 1) and p-methoxyphenyl or pfluorophenyl group as the other end group (from the quenching reagent Ar’B(OH)2; see Supporting Information, Figure S1). These results indicated that the end groups were well-controlled when complex 1 was employed as initiator for Suzuki cross-coupling polymerization with AB-type monomers. The results that all polymers contain the 2-aminophenyl group from the initiator supported that all the polymers were grown from the initiator 1 via a chain-growth fashion. In summary, based on the consideration that Ad3P are electronically and sterically similar to t-Bu3P, we demonstrated that Ad3P was an efficient ligand for controlled Pd(0)-catalyzed 1302

DOI: 10.1021/acsmacrolett.7b00759 ACS Macro Lett. 2017, 6, 1301−1304

ACS Macro Letters

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Suzuki cross-coupling polymerizations of AB-type monomers. With Ad3P-coordinated acetanilide-based palladacycle complex 1 as the initiator, controlled Suzuki cross-coupling polymerization occurred efficiently, affording polymers with narrow Đs (1.12−1.18) and well-defined end groups. Our study provided an efficient ligand and an efficient initiator for controlled Suzuki cross-coupling polymerizations. Our study also suggested that complex 1 might be a useful initiator for other controlled crosscoupling polymerizations.11 In addition, our study paved the road for us to investigate the use of other bulky monophosphines as ligands for controlled cross-coupling polymerizations. Work toward these directions is currently underway.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00759. Experimental details and characterization data (PDF).

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REFERENCES

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Figure 2. Pd(0)/Ad3P-Catalyzed Suzuki Cross-Coupling Polymerization of 7-Bromo-9,9-dihexylfluoren-2-ylboronic acid Pinacol Ester with Palladacycle Complex 1 as the Initiator (see Supporting Information for details).

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qiao-Sheng Hu: 0000-0003-1466-8995 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank the NSF (CHE1507839) for funding. ABBREVIATIONS Pd(0), palladium(0); Ad3P, tris(1-adamantyl)phosphine; Đ, polydispersity index 1303

DOI: 10.1021/acsmacrolett.7b00759 ACS Macro Lett. 2017, 6, 1301−1304

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DOI: 10.1021/acsmacrolett.7b00759 ACS Macro Lett. 2017, 6, 1301−1304