Visible-Light-Enabled Preparation of Palladium Nanoparticles and

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

Visible-Light-Enabled Preparation of Palladium Nanoparticles and Application as Catalysts for Suzuki−Miyaura Coupling Florian Mas̈ ing,† Harald Nüsse,‡ Jürgen Klingauf,‡ and Armido Studer*,† †

Institute of Organic Chemistry, University of Münster, Corrensstrasse 40, 48149 Münster, Germany Institute of Medical Physics and Biophysics, University of Münster, Robert-Koch-Strasse 31, 48149 Münster, Germany

‡

S Supporting Information *

ABSTRACT: Silyl ketones were used for the preparation of palladium nanoparticles (PdNPs) starting with Pd(OAc)2 in dimethylformamide under irradiation with a visible light-emitting diode (LED). Variation of the silyl ketone structure allowed adjustment of the PdNP diameter (1.9 or 5.2 nm). The in situformed PdNPs were further stabilized with polyvinylpyrrolidone and then applied as recyclable catalysts in the Suzuki−Miyaura coupling of arylboronic acids with aryl iodides to obtain substituted biphenyls in excellent yields.

S

Scheme 1. (a) Photoinitiators for Light-Mediated Preparation of Metal Nanoparticles; (b) Photoactive Silyl Ketones Investigated; (c) Novel Concept for PdNP Preparation

ilyl ketones have been successfully applied as visible-light photoinitiators in polymer chemistry.1 Upon irradiation, they undergo homolytic Norrish Type I C−Si or C−C bond cleavage, and the radical fragments generated then initiate the polymerization process.1 Along these lines, visible-light photoinitiators have been successfully applied to photocuring of dental fillings, adhesives, and coatings.1,2 In recent years, several processes for the light-mediated preparation of metal nanoparticles have been developed.3 Scaiano and co-workers first disclosed the potential of using commercially available photoinitiators such as Irgacure-2959 (1) for the preparation of metal nanoparticles (e.g., Ag, Au, Cu) under UV irradiation (Scheme 1a).4 Irradiation of I-2959 leads to Norrish Type I C−C bond cleavage to generate ketyl radicals, which are able to reduce metal salts to the corresponding metal nanoparticles.4 I-2959 was also immobilized into polymers, enabling the in situ preparation of polymercoated Au and Pd nanoparticles by UV irradiation. 5 Furthermore, the commercially available bis(acyl)phosphine oxide (BAPO) photoinitiator 2 has been applied successfully to UV-light-mediated Ag, Au, and Pd nanoparticle synthesis.6 However, expensive special equipment (e.g., UV reactors, quartz glass cuvettes) and harmful UV light are essential in these processes, limiting the universal applicability of these interesting methods. In contrast, the photoinitiator camphorquinone (CQ, 3) allows the preparation of Ag− or Au− polymer nanocomposites under irradiation with blue light.7 Motivated by that study, we aimed to develop a process for the preparation of Pd nanoparticles (PdNPs) using visible-light irradiation, since visible-light sources are cheap, more energy efficient than UV lamps, and easily accessible. The development of a simple and reliable process for PdNP preparation with visible light-emitting diodes (LEDs) should therefore be a significant improvement over existing methods. It is known that © XXXX American Chemical Society

silyl ketones show Norrish Type I activity in the visible spectrum, but surprisingly, they have not been tested for lightmediated metal nanoparticle preparation to date (Scheme 1b).1 Herein we present first results on visible-light-enabled PdNP synthesis using silyl ketones as reagents (Scheme 1c). We also show that the PdNPs thus obtained can be stabilized with commercially available polyvinylpyrrolidone (PVP) and then applied as catalysts for the Suzuki−Miyaura coupling reaction.8 These Pd@PVP catalysts operate at low loading (0.1 mol %) and can be readily recycled. Received: December 14, 2017

A

DOI: 10.1021/acs.orglett.7b03892 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

irradiation of Pd(OAc)2 in DMF for 3 h, documenting the importance of both the silyl ketone and light for successful NP preparation.11 Irradiation (λ = 420 nm) of a mixture of 4, Pd(OAc)2, (2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), and DMF for 30 min provided benzoyl-TEMPO derived from trapping of benzoyl radicals by TEMPO, showing that homolytic Norrish Type I C−Si bond cleavage had occurred. This indicates that the metal salt reduction process likely follows a radical pathway.1 However, Pd salt reduction by a siloxycarbene intermediate generated by a light-induced [1,2]Brook rearrangement currently cannot be ruled out.1,12 To test the catalytic activity of these PdNPs, they were used as catalysts in Suzuki−Miyaura cross-coupling of phenylboronic acid (6a) with iodobenzene (7a) to form biphenyl (8aa).8 The activity of Pd@5PVP (1.9 ± 0.5 nm) was investigated first, and 8aa was obtained in 83% isolated yield after 3 h at 50 °C with 0.5 mol % catalyst loading (Table 1, entry 1). Surprisingly, the

PdNP formation was studied with easily accessible phenyl(trimethylsilyl)methanone (4) and bis(dimethyl(phenyl)silyl)methanone (5) as photoactive reagents (see the Supporting Information (SI)). Acyl silane 4 exhibits UV/vis absorption in the λ = 350−450 nm region (λmax = 418 nm), whereas the UV/ vis absorption of bis(silyl) ketone 5 is red-shifted to the λ = 425−575 nm region (λmax = 544 nm) (see the SI for UV/vis spectra). For nanoparticle preparation, acyl silane 4 or bis(silyl) ketone 5 was diluted in dimethylformamide (DMF), and Pd(OAc)2 was added under an argon atmosphere. The molar ratio of the silyl ketones with respect to Pd(OAc)2 was set as 40:1. The reaction mixture was then irradiated with LEDs at λ = 420, 462, or 520 nm under vigorous stirring. When acyl silane 4 was used, irradiation at λ = 420 nm for 30 min provided a brown solution, indicating the formation of PdNPs. Indeed, transmission electron microscopy (TEM) revealed the formation of small nanoparticles with a mean diameter of 5.2 ± 0.9 nm (Figure 1a). The oxidation state of the PdNPs was

Table 1. Suzuki−Miyaura Coupling with Pd@PVP Catalysts and Recycling Experimentsa

Figure 1. TEM images of the prepared PdNPs: (a) Pd@4 and (b) Pd@5.

investigated by X-ray photoelectron spectroscopy (XPS), which showed the characteristic binding energies for Pd0 (Pd 3d5/2 at 335.1 eV and Pd 3d3/2 at 340.4 eV), proving the complete photochemical reduction of Pd ions to Pd0 (see the SI).9 When bis(silyl) ketone 5 was used, a dark-violet reaction mixture resulted after 1 h of irradiation at λ = 462 or 520 nm. TEM analysis revealed in both cases that small nanoparticles were formed. Under irradiation with blue light, PdNPs with a mean diameter of 1.9 ± 0.6 nm were obtained, and irradiation with green light resulted in PdNPs with a similar diameter of 1.9 ± 0.5 nm (Figure 1b). Notably, the beginning of PdNP formation under LED irradiation was observed rapidly after 5−10 min using 4 and 5. The in situ-formed “naked” PdNPs prepared with ketones 4 (Pd@4) and 5 (Pd@5) showed high stability in DMF. However, after evaporation of the solvent, aggregation to larger particles was noted. Therefore, the PdNPs were further stabilized by addition of commercially available PVP (molar mass = 10 000 g/mol).10 The PdNP−polymer hybrids Pd@ 4PVP and Pd@5PVP could be readily isolated by solvent evaporation without aggregation. Dynamic light scattering (DLS) measurements indicated that these PdNPs were coated with a thin PVP layer (∼2.0 nm for Pd@4PVP and ∼1.8 nm for Pd@5PVP). To document the necessity of light for nanoparticle preparation, the reactions with 4 and 5 were repeated in the dark. As expected, formation of PdNPs was not observed after 30 min of stirring in these cases. With 5, however, NP formation was observed after more than 30 min of stirring in the dark. Furthermore, no nanoparticles were generated upon

entry

Pd@PVP

cycle

time (h)

yield of 8aa (%)b

1 2 3 4 5 6c 7d 8f

Pd@5PVP Pd@4PVP Pd@4PVP Pd@4PVP Pd@4PVP Pd@4PVP Pd@4PVP Pd@PVP

1 1 2 3 4 1 1 1

3 1 1 1.5 1.5 1 4 0.5

83 93 93 83 98 94 92 90

a

Reaction with 0.2 mmol of iodobenzene and 0.3 mmol of phenylboronic acid. bIsolated yields after column chromatography. c Reaction with 1 mmol of iodobenzene and 1.5 mmol of phenylboronic acid. dReaction with 0.1 mol % Pd@4PVP. fThe Pd@PVP catalyst was prepared according to a literature prcedure.8d,15

larger Pd@4PVP (5.2 ± 0.9 nm) showed higher catalytic activity, and 8aa was obtained in 93% yield after 1 h under otherwise identical conditions (Table 1, entry 2).13 Possible reasons for the lower reactivity of Pd@5PVP might be stronger adsorption of reactive intermediates on the NP surface or deactivation of the catalyst by fragments derived from the bis(silyl) ketone 5.13 Notably, Pd@4PVP was readily recycled. After completion of the reaction, the EtOH/H2O solvent was evaporated, and 8aa was extracted with 3:1 pentane/CH2Cl2. Pd@4PVP was not soluble in pentane/CH2Cl2 and could be dissolved in EtOH after extraction of 8aa. By means of this simple protocol, product 8aa was obtained in 83−98% yield over four cycles (Table 1, entries 2−5). Leaching experiments were also conducted. After 0.5 h, the Pd@4PVP catalyst was precipitated, and the Pd content of the supernatant was analyzed with inductively coupled plasma mass spectrometry (ICP-MS) (see the SI). The Pd content was found to be 0.1 ppm. When this “leached Pd” was applied as the catalyst, no biphenyl was formed after 1 h at 50 °C, and only a 30% yield of biphenyl was obtained after 3 days. Both experiments indicate that Pd@4PVP rather than leached Pd is the catalyst in this reaction.8,14 B


Letter

Organic Letters

bromophenylboronic acid (6b) with 7a gave 4-bromobiphenyl (8ba) in poor yield (29%). However, the chlorinated and fluorinated boronic acids 6c and 6d provided the corresponding biphenyls 8ca (99%) and 8da (94%) in excellent yields, showing that the bromo substituent in 6b is detrimental to the reaction outcome. Formyl- and acyl-substituted boronic acids 6e and 6f were transformed in excellent yields to the targeted biphenyls 8ea (98%) and 8fa (94%). 2-Tolylboronic acid (6g) afforded biphenyl derivative 8ga in nearly quantitative yield. Sterically hindered 2,6-dimethylphenylboronic acid (6h) could also be coupled with 7a to give biphenyl 8ha, although a lower yield was noted (66%). The reaction was slow (18 h), and the catalyst loading had to be increased to 1 mol %. Finally, 3,4(methylenedioxy)phenylboronic acid (6i) was transformed to biphenyl 8ia (96%). In summary, a novel method for the preparation of PdNPs with silyl ketone photoinitiators was developed. Simple mixing of silyl ketones with Pd(OAc)2 in DMF and subsequent irradiation with visible light provided small PdNPs. Depending on the photoinitiator used, the diameter of the PdNPs could be adjusted to 1.9 or 5.2 nm. The PdNPs were further stabilized with commercially available polyvinylpyrrolidone, and the palladium−polymer hybrid materials obtained were carefully characterized by TEM, XPS, DLS, NMR, and IR. The Pd@PVP hybrid material was also applied as a catalyst for Suzuki− Miyaura coupling of various boronic acids with a series of aryl iodides.

Furthermore, the reaction was carried out on a 1 mmol scale, and 8aa was isolated in 94% yield (Table 1, entry 6). The coupling also proceeded well at a lower catalyst loading (0.1 mol %), affording 8aa in 92% yield; however, reaction time had to be extended to 4 h in that run (Table 1, entry 7). In addition, the activity of the photochemically generated PdNPs was compared with the activity of Pd@PVP (DLS diameter = 5.4 ± 1.7 nm) prepared according to a traditional literature procedure.8d,15 The “traditional” particles turned out to be slightly more active, affording 8aa in 90% isolated yield after 0.5 h (Table 1, entry 8). However, the method used to prepare these established NPs is more time-consuming and elaborate compared with the method introduced herein. Unfortunately, bromobenzene and chlorobenzene did not react with 6a to form 8aa using these PdNP catalysts (the experiments were conducted for 16 h at 80 °C with 1 mol % Pd@4PVP or Pd@5PVP). Next, the substrate scope was investigated using Pd@4PVP as the catalyst at 0.5 mol % loading (Scheme 2). Aryl iodides Scheme 2. Substrate Scopea

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03892. Detailed experimental procedures and spectroscopic data for all products (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Armido Studer: 0000-0002-1706-513X Notes

The authors declare no competing financial interest.

■

a

Reaction with 0.2 mmol of aryl iodide and 0.3 mmol of arylboronic acid. Isolated yields after column chromatography are presented. b Reaction at 50 °C. cReaction with 1 mol % catalyst.

ACKNOWLEDGMENTS Sebastian Lamping (Institute of Organic Chemistry, University of Münster) is acknowledged for performing XPS measurements. Oliver Bolle Bauer (Institute of Inorganic and Analytical Chemistry, University of Münster) is acknowledged for performing ICP-MS experiments. We thank the Deutsche Forschungsgemeinschaft (SFB 858) for funding this work.

7b−h were systematically varied, and coupling with 6a provided the substituted biphenyls 8ab−ah. The cross-coupling of 6a with 4-iodotoluene (7b) afforded 4-methylbiphenyl (8ab) in 96% yield. The constitutional isomers of iodoanisole (7c−e) were readily converted to the corresponding 4-, 3-, and 2methoxybiphenyls (8ac−ae) in excellent yields (90−93%). Also, electron-deficient aryl iodides bearing ethyl ester (7f), nitro (7g), and nitrile (7h) functional groups could be coupled to give the substituted biphenyls 8af−ah in good to excellent yields (80−98%). Arylboronic acids 6b−i were then systematically varied, keeping 7a as the coupling partner. The reaction of 4-

■

REFERENCES

(1) (a) Graff, B.; Klee, J. E.; Fik, C.; Maier, M.; Fouassier, J. P.; Lalevée , J. Macromol. Rapid Commun. 2017, 38, 1600470. (b) Mitterbauer, M.; Haas, M.; Stüger, H.; Moszner, N.; Liska, R. Macromol. Mater. Eng. 2017, 302, 1600536. (c) Bouzrati-Zerelli, M.; Kirschner, J.; Fik, C. P.; Maier, M.; Dietlin, C.; Morlet-Savary, F.; Fouassier, J. P.; Becht, J.-M.; Klee, J. E.; Lalevée, J. Macromolecules 2017, 50, 6911−6923. C


Letter

Organic Letters (2) Jö ckle, P.; Schweigert, C.; Lamparth, I.; Moszner, N.; Unterreiner, A.-N.; Barner-Kowollik, C. Macromolecules 2017, 50, 8894−8906. (3) Grzelczak, M.; Liz-Marzán, L. M. Chem. Soc. Rev. 2014, 43, 2089−2097. (4) (a) McGilvray, K. L.; Decan, M. R.; Wang, D.; Scaiano, J. C. J. Am. Chem. Soc. 2006, 128, 15980−15981. (b) Marin, M. L.; McGilvray, K. L.; Scaiano, J. C. J. Am. Chem. Soc. 2008, 130, 16572−16584. (c) Scaiano, J. C.; Stamplecoskie, K. G.; Hallett-Tapley, G. L. Chem. Commun. 2012, 48, 4798−4808. (5) (a) Mäsing, F.; Mardyukov, A.; Doerenkamp, C.; Eckert, H.; Malkus, U.; Nüsse, H.; Klingauf, J.; Studer, A. Angew. Chem., Int. Ed. 2015, 54, 12612−12617. (b) Mäsing, F.; Wang, X.; Nüsse, H.; Klingauf, J.; Studer, A. Chem. - Eur. J. 2017, 23, 6014−6018. (6) (a) Mäsing, F.; Nüsse, H.; Klingauf, J.; Studer, A. Org. Lett. 2017, 19, 2658−2661. (b) Balan, L.; Melinte, V.; Buruiana, T.; Schneider, R.; Vidal, L. Nanotechnology 2012, 23, 415705. (c) Buruiana, E. C.; Chibac, A. L.; Buruiana, T.; Melinte, V.; Balan, L. J. Nanopart. Res. 2013, 15, 1335. (7) (a) Yagci, Y.; Sangermano, M.; Rizza, G. Polymer 2008, 49, 5195−5198. (b) Yagci, Y.; Sangermano, M.; Rizza, G. Macromolecules 2008, 41, 7268−7270. (8) For selected publications, see: (a) Balanta, A.; Godard, C.; Claver, C. Chem. Soc. Rev. 2011, 40, 4973−4985. (b) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett. 2000, 2, 2385−2388. (c) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340− 8347. (d) de Luna Martins, D.; Alvarez, H. M.; Aguiar, L. C. S. Tetrahedron Lett. 2010, 51, 6814−6817. (9) Wojcieszak, R.; Ghazzal, M. N.; Gaigneaux, E. M.; Ruiz, P. Catal. Sci. Technol. 2014, 4, 738−745. (10) Koczkur, K. M.; Mourdikoudis, S.; Polavarapu, L.; Skrabalak, S. E. Dalton Trans. 2015, 44, 17883−17905. (11) (a) Pastoriza-Santos, I.; Liz-Marzán, L. Adv. Funct. Mater. 2009, 19, 679−688. (b) Pastoriza-Santos, I.; Liz-Marzán, L. M. Langmuir 2002, 18, 2888−2894. (12) (a) Brook, A. G.; Harris, J. W.; Lennon, J.; El Sheikh, M. J. Am. Chem. Soc. 1979, 101, 83−95. (b) Zhang, H.-J.; Priebbenow, D. L.; Bolm, C. Chem. Soc. Rev. 2013, 42, 8540−8571. (13) Li, Y.; Boone, E.; El-Sayed, M. A. Langmuir 2002, 18, 4921− 4925. (14) (a) Ellis, P. J.; Fairlamb, I. J. S.; Hackett, S. F. J.; Wilson, K.; Lee, A. F. Angew. Chem., Int. Ed. 2010, 49, 1820−1824. (b) Lee, A. F.; Ellis, P. J.; Fairlamb, I. J. S.; Wilson, K. Dalton Trans. 2010, 39, 10473− 10482. (15) Bradley, J. S.; Millar, J. M.; Hill, E. W. J. Am. Chem. Soc. 1991, 113, 4016−4017.

D


Visible-Light-Enabled Preparation of Palladium Nanoparticles and

Recommend Documents