Article pubs.acs.org/Organometallics
C−CN Bond Cleavage Using Palladium Supported by a Dippe Ligand Lloyd Munjanja,† Coralys Torres-López,† William W. Brennessel, and William D. Jones* Department of Chemistry, University of Rochester, Rochester, New York 14627, United States S Supporting Information *
ABSTRACT: The (dippe)palladium(0) fragment generated from [(dippe)Pd(μ-H)]2 (1) has been shown to form an intermediate η2-nitrile complex with acetonitrile (dippe)Pd(η2C,N-CH3CN-BEt3) (2a) in the presence of BEt3 [(dippe = bisdiisopropylphosphino)ethane)]. On introducing a solution of 2a to 1 equiv of BPh3, rapid formation of (dippe)Pd(η2-C,NCH3CNBPh3) (2a′) is observed. Heating 2a′ at 100 °C in THF-d8 results in the C−CN activation product 3a′, (dippe)Pd(CH3)(CNBPh3). Reaction of 1 with benzonitrile in the presence of BEt3 gives the C−CN activation product (dippe)Pd(Ph)(CN-BEt3) (3b) exclusively. The complexes 2a, 2a′, 3a′, and 3b were characterized by 1H, 31P{1H}, and 13C{1H} NMR spectroscopy, elemental analysis, IR spectroscopy, and X-ray diffraction.
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Scheme 1. Formation of 2a from the Reaction of [(dippe)Pd(μ-H)]2 (1) with Acetonitrile at 23 °C
nterest in the carbon−carbon bond activation of alkyl, allyl, and aryl nitriles has risen due to potential industrial1 and organic synthesis applications.2 Recently, the complexes [(dippe)Ni(μ-H)] 2 and [(dippe)PtH] 2 (dippe = bisdiisopropylphosphino)ethane) have been employed in stoichiometric activation of nitriles in our group in collaboration with Garcı ́a.3,4 These complexes furnish a reactive 14-electron (dippe)M0 (M = Ni, Pt) fragment that can cleave the C−CN bond in the nitriles. Garcı ́a and co-workers have since expanded [(dippe)Ni(μ-H)]2 studies by providing the first catalytic example of low-valent nickel additions of alcohols to aryl, alkyl, and heteroatomic nitriles.5 As an extension to the studies with group 10 hydride dimers in our group, we wanted to investigate the palladium hydride dimer, [(dippe)Pd(μ-H)]2 (1), in the activation of the C−CN bond in nitriles. Complex 1 has been shown to be effective in cleaving the carbon−sulfur bonds in thiophenes and thioethers.6 Herein, we report Lewis acid assisted C−CN cleavage to form (dippe)palladium(II) intermediate complexes that are crucial in cross-coupling, hydrodecyanation, and cycloaddition reactions.7 Complex 1, [(dippe)Pd(μ-H)]2,6,8 is generated in situ when a (dippe)PdCl2 suspension in THF is treated with 2 equiv of potassium triethylborohydride with the loss of KCl/BEt3 (Scheme 1). After filtration to remove the KCl salt, the filtrate is treated with an excess of acetonitrile, causing a color change from intense dark red to light transparent orange. A pair of doublet resonances at δ 75.63 (d, 2JP‑P = 25.0 Hz) and 61.77 (d, 2 JP‑P = 25.0 Hz) is observed in the 31P{1H} NMR spectrum, consistent with asymmetrically η2-coordinated (dippe)Pd(0) complexes.9 The 13C{1H} NMR spectrum of 2a in THF shows a distinct downfield shift of the CN to δ 149.70 compared to δ 117 in free acetonitrile. The 11B{1H} NMR spectrum shows a resonance at δ −13.2, the region for BEt3 complexes.10 Attempts to isolate 2a as a solid by removing solvent in © XXXX American Chemical Society
vacuo results in 2a decomposing to [(μ-dippe)Pd]2,11 which has the characteristic signal at δ 33 in the 31P{1H} NMR spectrum. However, X-ray quality crystals grew from the THF solution of 2a at room temperature overnight (Figure 1). Complex 2a approaches square-planar coordination with an RMS deviation from planarity of 0.032 Å. The dippe bite angle of 87.27° is consistent with those in related (dippe)Pd(0) complexes.6,12 Complex 2a shows an elongated C1−N1 bond of 1.217 Å compared to the CN triple bond of 1.153 Å13 in free acetonitrile. On the basis of both the NMR spectroscopy and the X-ray diffraction, we observe the coordination of BEt3, generated from KHBEt3 reaction with (dippe)PdCl2, on the nitrile nitrogen of complex 2a (Scheme 1, Figure 1). Lewis acids have been known to coordinate to the nitrogen lone pair electrons in nitriles via an η1 fashion.14 Their use in Received: April 15, 2016
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DOI: 10.1021/acs.organomet.6b00304 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Figure 1. ORTEP drawing of (dippe)Pd(η2-C,N-CH3CNBEt3) (2a). Ellipsoids are shown at the 50% probability level. All the hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles [deg]: Pd(1)−C(1) 1.993(10); Pd(1)−N(1) 2.101(8); Pd(1)−P(1) 2.253(3); C(1)−N(1) 1.217(15); C(1)−C(2) 1.488(17); N(1)−B(1) 1.602(14); P(1)−Pd(1)−P(2) 87.27(14); C(1)−Pd(1)−N(1) 34.5(4); C(1)−N(1)−B(1) 146.4(11).
Figure 2. ORTEP drawing of (dippe)Pd(η2-C,N-CH3CN-BPh3) (2a′). Ellipsoids are shown at the 50% probability level. All the hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles [deg]: Pd(1)−C(1) 2.014(5); Pd(1)−N(1) 2.103(5); Pd(1)−P(1) 2.2602(15); Pd(1)−P(2) 2.3402(14); N(1)−C(1) 1.229(6); N(1)−B(1) 1.579(7); P(1)−Pd(1)−P(2) 88.05(5); C(1)−Pd(1)−N(1) 34.64(18); C(1)−N(1)−B(1) 145.6(4).
conjunction with transition-metal catalysts can improve reaction rates, product selectivity, and extend catalyst lifetimes either by increasing steric bulk on the catalyst prior to coordination to the nitrile or by changing the charge distribution on the nitrogen or the boron atom.15 With this knowledge, we hypothesized that a stronger Lewis acid would stabilize η2-coordinated product 2a so we could isolate it as a solid and further explore C−CN activation. To test this hypothesis, we added 1 equiv of BPh3 to a solution of 2a in THF and observed formation of a new product in the 31P{1H} spectrum. The resonances for 2a disappeared, giving rise to doublet resonances at δ 76.87 (d, 2JP‑P = 18.4 Hz) and 60.00 (d, 2 JP‑P = 18.4 Hz). The 1H NMR spectrum shows a doublet at δ 2.09 (4JH‑P = 6.2 Hz) for the methyl group of the nitrile. The product is assigned as the BPh3 adduct, (dippe)Pd(η2-C,NCH3CN-BPh3) 2a′, isolated as a light orange solid (eq 1). Phosphorus 2JP‑P coupling constants observed for 2a′ are typical for asymmetric (dippe)Pd(II) and rather unusual for (dippe)Pd(0) complexes.8 The significant differences between 2JP‑P coupling values between 2a (2JP‑P = 25.0 Hz) and 2a′ (2JP‑P = 18.4 Hz) can be attributed to the steric bulk and increased electronic withdrawing ability of the BPh3 versus BEt3 moiety as shown by the single crystal structure of 2a′ in Figure 2. The RMS deviation from planarity is 0.057 Å for 2a′. The BPh3 adduct complex 2a′ demonstrates a slightly increased dippe bite angle of 88.05° compared to 2a at 87.27°.
Garcı ́a and co-workers reported inhibition of C−CN activation of CH3CN in the presence of BPh3. In the reaction of [(dippe)Ni(μ-H)]2 with CH3CN, they observed exclusive formation of the BPh3 adduct (dippe)Ni(η2-C,N-CH3CNBPh3), which did not undergo further C−CN activation even after heating at 100 °C overnight, thus showing increased thermal stability compared to 2a.3c Consequently, this observation demonstrates an inhibition of C−CN activation by addition of a stronger Lewis acid notably stabilizing the η2nitrile acetonitrile Ni(0) complex. In another report, palladium complexes of the form P2Pd(R)(CN) show enhanced reductive elimination rates of RCN with the addition of Lewis acids.14a To test if the introduction of BPh3 inhibits or favors C−CN activation of acetonitrile in our studies, we heated a THF-d8 solution of 2a′, (dippe)Pd(η2-C,N-CH3CN-BPh3), at 100 °C (eq 2). In 2 h, the complete disappearance of resonances for 2a′ is observed and the formation of a new pair of doublet resonances is seen with 2JP‑P coupling constants of 18.7 Hz in the 31P{1H} NMR spectrum indicative of a C−CN activation product, 3a′ (Figure 3, eq 2). The 1H NMR spectrum shows a distinctive doublet of doublets (3JH‑P = 6.1, 4.8 Hz) resonance upfield for the methyl moiety at δ 0.611 for complex 3a′. In addition to 3a′ resonances, there is formation of other unidentified phosphorus product as shown in the 31P{1H} spectrum, which is in a 1:1 ratio with 3a′ (Figure 3). On
Heating complex 2a to initiate C−CN cleavage results in decomposition, while photolysis of 2a results in C−CN cleavage product 3a. However, the C−CN cleavage product is unstable and decomposes easily, making the characterization challenging.16
Figure 3. Products {complex 3a′ + (dippe)Pd hydride} from the heating of complex 2a′ at 100 °C for 2 h. B
DOI: 10.1021/acs.organomet.6b00304 Organometallics XXXX, XXX, XXX−XXX
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Organometallics checking the 1H NMR hydride region, a triplet of triplets is observed centered at δ −3.98 (2J H‑P = 68, 10 Hz). While the identity of this hydride-containing product could not be determined, this species is likely to have formed from the C− H activation of the acetonitrile.17 A dimeric palladium hydride is proposed, such as Pd2(dippe)2(μ-H)(μ-CH2CNBPh3).
In a separate experiment, reaction of complex 1 with benzonitrile affords the C−CN activation product 3b instantly at room temperature (Scheme 2). Complex 3b, (dippe)PdScheme 2. Formation of 3b from the Reaction of [(dippe)Pd(μ-H)]2 (1) with Benzonitrile at 23 °C
A small quantity of X-ray quality crystals grew out of the THF-d8 solution, revealing a monomeric C−CN activated complex 3a′ (Figure 4). Complex 3a′ shows an RMS deviation
(Ph)(CN-BEt3), was isolated as a yellowish white solid. The P{1H} NMR spectrum of 3b shows a pair of doublet resonances at δ 76.09 (d, 2JP‑P = 18.4 Hz) and 70.43 (d, 2JP‑P = 18.3 Hz) consistent with asymmetric (dippe)Pd(II) complexes.6 X-ray quality crystals for complex 3b were grown from a THF solution layered with pentane overnight at room temperature (Figure 5) to confirm a monomeric planar (dippe)Pd(II) complex. 31
Figure 4. ORTEP drawing of (dippe)Pd(CH3)(CN-BPh3) (3a′). Ellipsoids are shown at the 50% probability level. All the hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles [deg]: Pd(1)−C(1) 2.019(6); Pd(1)−C(2) 2.086(7); Pd(1)−P(1) 2.2593(15); Pd(1)−P(2) 2.3038(16); N(1)−C(1) 1.146(7); N(1)− B(1) 1.578(8); P(1)−Pd(1)−P(2) 87.12(6); C(1)−Pd(1)−C(2) 87.4(3).
from planarity of 0.061 Å. To understand the distribution and relationship of palladium hydrides and the C−CN activation product 3a′, complex 2a′ was heated at different temperatures in separate experiments.18 The reactions were followed by 31 1 P{ H} and 1H NMR spectroscopy. We observe that, at lower temperatures, it takes 2a′ longer times to completely convert to 3a′ and the palladium hydride. However, the ratio of palladium hydride to 3a′ is significantly decreased at lower temperatures. At 65 °C, the ratio of 3a′ to palladium hydride is 1:0.35, whereas, at 100 °C, it is 1:1. On the other hand, photolysis of 2a′ is complete in 24 h but yields the lowest ratio of (dippe)palladium hydride to 3a′ (0.19:1). Interestingly, taking the photolyzed sample and heating it to 100 °C for 2 h results in a ratio of 1:1 for 3a and palladium hydride.19 This shows that the palladium hydride complex can be formed from the C−CN activated product. No effect is observed on the product ratio after lowering the temperature, signifying that this is not a rapidly equilibrating process.
Figure 5. ORTEP drawing of (dippe)Pd(Ph)(CN-BEt3) (3b). Ellipsoids are shown at the 50% probability level. All the hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles [deg]: Pd(1)−C(1) 2.016(4); Pd(1)−C(2) 2.065(3); Pd(1)−P(1) 2.287(9); Pd(1)−P(2) 2.321(9); N(1)−C(1) 1.143(4); N(1)−B(1) 1.598(5); P(1)−Pd(1)−P(2) 86.82(3); C(1)−Pd(1)−C(2) 88.64(13).
Surprisingly, unlike with [(dippe)Ni(μ-H)]2, in which (dippe)Ni0(η2-C,N-PhCN) and (dippe)NiII(Ph)(CN) are in equilibrium,3a herein, we observe exclusive formation of C−CN cleavage product 3b. The presence of BEt3 might be facilitating this rapid formation of 3b. In summary, we have demonstrated that [(dippe)Pd(μ-H)]2 (1) can react with acetonitrile to furnish η2-nitrile complexes (dippe)Pd(η2-C,N-CH3CNBEt3) (2a) or (dippe)Pd(η2-C,NCH3CNBPh3) (2a′) in the presence of BEt3 or BPh3, respectively. Under photochemical and thermal conditions, C
DOI: 10.1021/acs.organomet.6b00304 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Dankwardt, J. W. Tetrahedron Lett. 2003, 44, 1907−1910. (dd) Nakai, K.; Kurahashi, T.; Matsubara, S. Org. Lett. 2013, 15, 856−859. (8) For a similar Pd hydride dimer, see: Fryzuk, M. D.; Lloyd, B. R.; Clentsmith, G. K. B.; Rettig, S. J. J. Am. Chem. Soc. 1994, 116, 3804− 3812. (9) Schager, F.; Bonrath, W.; Pörschke, K- R.; Kessler, M.; Kruger, C.; Seevogel, K. Organometallics 1997, 16, 4276−4286. (10) Free triethylborane in THF is observed at δ 70 externally referenced to boron trifluoride diethyl etherate (0 ppm). (11) Reid, S. M.; Fink, M. J. Organometallics 2001, 20, 2959−2961. (12) (a) Krause, J.; Pluta, C.; Pörschke, K.-R.; Goddard, R. J. Chem. Soc., Chem. Commun. 1993, 1254−1256. (b) Krause, J.; Haack, K.-J.; Pörschke, K.-R.; Gabor, B.; Goddard, R.; Pluta, C.; Seevogel, K. J. Am. Chem. Soc. 1996, 118, 804−821. (c) Goddard, R.; Hopp, G.; Jolly, P. W.; Krüger, C.; Mynott, R.; Wirtz, C. J. Organomet. Chem. 1995, 486, 163−170. (d) Schager, F.; Seevogel, K.; Pörschke, K.-R.; Kessler, M.; Krüger, C. J. Am. Chem. Soc. 1996, 118, 13075−13076. (13) Kratochwill, A.; Weidner, J. U.; Zimmermann, H. Ber. Dtsch. Bunsen-Ges. 1973, 77, 408. (14) (a) Tolman, C. A.; Seidel, W. C.; Druliner, J. D.; Domaille, P. J. Organometallics 1984, 3, 33−38. (b) Brunkan, N. M.; Brestensky, D. M.; Jones, W. D. J. Am. Chem. Soc. 2004, 126, 3627−3641. (c) AcostaRamírez, A.; Muñoz-Hernández, M.; Jones, W. D.; García, J. J. Organometallics 2007, 26, 5766−5769. (15) (a) Huang, J.; Haar, C. M.; Nolan, S. P.; Marcone, J. E.; Moloy, K. G. Organometallics 1999, 18, 297−299. (b) McKinney, R. J.; Nugent, W. A. Organometallics 1989, 8, 2871−2875. (c) Swartz, B. D. The Investigation of C-CN Cleavage of Allyl- and Aryl-nitriles by [(dippe)NiH]2 and [(dippe)PtH]2. Ph.D. Dissertation, University of Rochester, Rochester, NY, 2009. (e) Purcell, K. F.; Drago, R. S. J. Am. Chem. Soc. 1966, 88, 919−924. (f) Nakao, Y.; Ebata, S.; Yada, A.; Hiyama, T.; Ikawa, M.; Ogoshi, S. J. Am. Chem. Soc. 2008, 130, 12874−12875. (16) See the Supporting Information for details of the photolysis experiment. (17) (a) Ateşin, T. A.; Li, T.; Lachaize, S.; Brennessel, W. W.; García, J. J.; Jones, W. D. J. Am. Chem. Soc. 2007, 129, 7562−7569. (b) Oertel, A. M.; Ritleng, V.; Chetcuti, M. J.; Veiros, L. F. J. Am. Chem. Soc. 2010, 132, 13588−13589. (c) Evans, M. E.; Li, T.; Jones, W. D. J. Am. Chem. Soc. 2010, 132, 16278−16284. (18) Experiments were conducted at 65 °C, 100 °C, and at room temperature for photolysis. See details in the Supporting Information. (19) Note that heating for longer times > 24 h or increasing the temperature to 125 °C does not change this ratio of 1:1 once it is reached.
2a′ undergoes C−CN cleavage to yield 3a′, (dippe)Pd(CH3)(CNBPh3). Contrary to the Garcı ́a3c and Nolan15a observations, the Lewis acid BPh3 favors C−CN activation of acetonitrile with the (dippe)Pd system, perhaps due to a stronger Pd−CN bond. Reaction of 1 with benzonitrile yields the C−CN activation product, (dippe)Pd(Ph)(CN-BEt3) 3b, exclusively. Formation of a (dippe)Pd(η2-C,N-PhCNBEt3) intermediate is not observed. We are currently investigating effects of Lewis acids in the C−CN bond cleavage in alkyl nitriles as well as their effect in establishing the equilibrium between (dippe)Pd(CN)(Ph) and (dippe)Pd(η2-C,N-PhCN).
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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.organomet.6b00304. Experimental procedures, product characterization data, and X-ray crystallographic data for 2a, 2a′, 3a′, and 3b (CCDC# 1474230−1474233) (PDF) X-ray crystallographic data for 2a, 2a′, 3a′, and 3b (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
† These authors (L.M., C.T.-L.) contributed equally. 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 This work was supported in part by the NSF under grant CHE1360985 and in part by the DOE grant DE-86ER13569. C.T.-L. thanks the NSF CCI Center for Enabling New Technologies through Catalysis (CENTC), CHE-1205189, for REU support. L.M. thanks Professor Ellen Matson for helpful discussions.
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REFERENCES
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DOI: 10.1021/acs.organomet.6b00304 Organometallics XXXX, XXX, XXX−XXX
