Mechanistic Investigation of the Generation of a Palladium(0) Catalyst

Communication pubs.acs.org/Organometallics

Mechanistic Investigation of the Generation of a Palladium(0) Catalyst from a Palladium(II) Allyl Complex: A Combined Experimental and DFT Study Daniel Ortiz,†,§ Matthias Blug,‡,∥ Xavier-Frédéric Le Goff,‡ Pascal Le Floch,‡,⊥ Nicolas Mézailles,*,‡ and Philippe Maître*,† †

Laboratoire de Chimie Physique, Université Paris Sud, UMR8000 CNRS, Faculté des Sciences, Bât 350, 91405 Orsay Cedex, France Laboratoire de Hétéroéléments et Coordination, Ecole polytechnique, UMR7653 CNRS, 91128 Palaiseau Cedex, France

‡

S Supporting Information *

ABSTRACT: Cross-coupling reactions can be efficiently catalyzed using palladium complexes. The formation of lowcoordinated, highly reactive Pd(0), which is believed to be the catalytic species, is critical. The mechanism of the reduction of a stable and readily available allyl Pd(II) complex into Pd(0) by a combination of K2CO3 and PhB(OH)2 has been studied. We report on the characterization of the associated reactive solution using a combination of density functional theory and experimental methods. First, the stoichiometric reaction of an (allyl)(phosphine)palladium(II) complex with K2CO3 was first investigated using trandem mass spectrometry. A palladium− carbonate complex could be characterized in the electrospray mass spectrum of the reactive solution. Gas-phase infrared spectra of mass-selected complexes have been recorded, giving further information on the coordination mode (κ1) of the carbonate ligand. This structural information derived from spectroscopy is critical because the relative energy of the two κ1- and κ2carbonate isomers is difficult to determine theoretically, presumably because of the charge transfers at play between the carbonate and the palladium. Second, the product of the stoichiometric addition of PhB(OH)2 to this carbonate complex was investigated. Both 31P and 1H NMR data provide compelling evidence for the formation of the desired 14-electron Pd(0) complex.

C

performed recently using either [Pd(NHC)(Py)Cl2]13 or [Pd(allyl)(NHC)(halogen)]14 as a precursor. In the latter case, the presence of a strong base (tBuO−) was required to promote the initial reduction step, following an as yet unsolved mechanism.13 Pd-catalyzed cross-coupling reaction efficiency also strongly depends on the nature of the base (as well as the solvent).15,16 The use of bases milder than alkoxides, more tolerant to common organic functions, is highly desirable but generally requires more stringent conditions. Moreover, while Cs2CO3 was found to be the most suitable base for the first reported cross-coupling reaction using an aryl chloride,17 the use of a less expensive base is also desirable. Blug et al.18 have reported recently that a very high catalytic activity at room temperature in the Suzuki−Miyaura crosscoupling process between chloroarenes and arylboronic acids could be obtained using K2CO3 as the base and the complex [(phosphabarrelene)Pd(allyl)Cl] (1) as a precursor, where phosphabarrelene (L in Scheme 1) is a bulky electron-accepting phosphine ligand. Reactions were conducted at room temperature, which implies an efficient reduction of the Pd(II) starting complex by the tandem “K2CO3/PhB(OH)2” in a stage leading to the catalytic cycle.

ross-coupling reactions are of outmost importance in C− C, C−N, C−O, and C−S bond forming reactions,1,2 and palladium complexes are among the most powerful catalysts. These reactions proceed through an oxidative addition of an aryl halide to the metal center, which is believed to be the ratelimiting step of the cross-coupling catalytic process.3,4 Lowcoordinated, highly reactive Pd(0) complexes have been proposed to be the catalytic species.4,5 Although 12-electron monoligated palladium species are often considered,6 a 14electron Pd(0) complex7 or an anionic tricoordinated complex such as [Pd0L2Cl]− 8 has also been proposed to be the effective catalyst. In a quest for the development of ever more efficient catalysts, especially able to promote the coupling of aryl chloride derivatives, the generation of the desired highly reactive, coordinatively unsaturated, yet electron-rich Pd(0) species under mild conditions is required. The development and use of bulky electron-rich ligands, such as trialkyl- or diarylphosphines or N-heterocyclic carbenes (NHC),4,13−20 has been critical for this purpose. Several synthetic strategies have been followed, among which are disproportionation reactions of Pd(I) dimers in the presence of sterically hindered, electronrich phosphines.9−12 Generation of a Pd(0) reactive species from a stable and easily available Pd(II) complex offers a very appealing alternative, and efficient cross-coupling reactions have been © 2012 American Chemical Society

Received: May 9, 2012 Published: August 17, 2012 5975

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Scheme 1. Proposed Mechanism for the Reduction of the Pd(II) Complex

Figure 1. Mass spectrum of the reactive solution recorded in negative mode. Inset: a detailed view of the isotopic distribution of the most abundant Pd-containing ion (b) compared against the theoretical isotopic patterns of the complex [(L)Pd(allyl)CO3]− (2) (a).

It is the purpose of the present paper to present the results of a mechanistic study. In particular, the assignment of the coordination mode of the allyl and CO3 moieties of the first intermediate Pd(II) complex 2K is made, using a combination of high-resolution mass spectrometry and infrared spectroscopy. The reduced Pd(0) complex 3 was characterized by NMR techniques and also by the alkene displacement reaction 3 → 4, as shown in Scheme 1. This reduced complex 3 is the point of entry into the catalytic cycle, which allows a likely overall mechanism for the efficient reduction of the precatalyst to be proposed. The stoichiometric reaction of complex 1 with PhB(OH)2 was studied in a first step. As expected, the two compounds did not react. On the other hand, a reaction was observed upon further addition of 1 equiv of K2CO3. The reduction process thus clearly requires the presence of the base. The reactivity of complex 1 toward K2CO3 alone was thus probed. The single new complex 2K was formed quantitatively, as shown by 31P NMR spectroscopy, within 0.5 h. This complex was characterized by the usual NMR techniques. In particular, the five proton signals of the allyl moiety were still observed, but at lower field. Final assignment of the coordination mode of both the allyl and CO3 moieties was obtained using tandem mass spectrometry, which has been shown to provide useful information on organometallic reactive intermediates.19,20 Using intense infrared lasers, infrared multiple photon dissociation (IRMPD) spectra of isolated reactive complexes,21−24 which cannot be structurally characterized otherwise, can be performed within a tandem mass spectrometer. A 10−5 M solution of complex 1 and K2CO3 in toluene/acetonitrile (1/1) was used for electrospray. Mass spectra were recorded in both positive and negative modes using a quadrupole ion trap mass spectrometer (Bruker Esquire 3000+, Bremen, Germany). The mass spectrum recorded in negative mode (Figure 1) displays several mass patterns with the characteristic Pd isotopic pattern, but also metal-free anions. Interestingly, the isotopic patterns of the most abundant Pd-containing ions (m/z 551) nicely match those calculated for the anion 2. The analysis of the mass spectrum recorded in positive mode (Figure 2) suggests that the most abundant m/z 491 ions correspond to a [(phosphabarrelene)(allyl)Pd]+ complex. Infrared spectra of both anionic m/z 551 and cationic m/z 491 complexes (Figure 3) could then be derived. These experiments were performed using with our quadrupole ion trap mass spectrometer coupled to the infrared free electron

Figure 2. Mass spectrum of the reactive solution recorded in positive mode. Inset: a detailed view of the isotopic distribution of the most abundant Pd-containing ion (b) compared against the theoretical isotopic patterns of the [(L)Pd(allyl)]+ ion (a).

Figure 3. IR spectra of the complexes (a−c) [(L)Pd(allyl)(CO3)]− (2) and (d, e) [(L)Pd(allyl)]+ (L = phosphaberrelene). Experimental IRMPD spectra (a and d) are compared against calculated IR absorption spectra. In the case of [(L)Pd(allyl)(CO3)]−, two structures characterized by the coordination mode of the carbonate (κ1 (b), κ2 (c)) were considered.

laser in Orsay, France.23,24 Mass-selected electrosprayed ions were irradiated with the IR-FEL beam for 250 ms, and the infrared induced photofragmentation efficiency was recorded at each fixed wavelength. MS2 mass spectra were recorded at each photon energy, which was scanned from 1200 through 1800 cm−1 by steps of ∼4 cm−1. Experimental IRMPD spectra were compared with calculated IR absorption spectra of candidate structures for the [(L)Pd(allyl)(CO3)]− and [(L)Pd(allyl)]+ complexes. Harmonic vibrational spectra were determined using the B3PW91 functional using a polarized valence doubleζ quality basis set, as described previously25 using the Gaussian03 package.26 5976

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1000−2000 cm−1 energy range. As can be seen in Figure 3d, three IR bands were observed at 1284, 1457, and 1591 cm−1 for the [(L)Pd(allyl)]+ fragment. A comparison with the calculated IR absorption spectrum (Figure 3e) of this complex shows that, as in the case of the anionic [(L)Pd(allyl)(CO3)]− (2), IRinduced photodissociation is only observed when the laser is tuned in resonance with the strongly IR active modes of the complex. Comparing experimental and theoretical intensities can be delicate, since the IR-induced dissociation of the massselected ion is based on a multiple photon absorption process.21 An enhanced IRMPD efficiency, for instance, can be observed in resonance with nearly degenerate modes. This could explain the large IRMPD intensity observed at 1284 cm−1 (several relatively strongly IR active methyl umbrella modes), in comparison to those observed at 1457 and 1591 cm−1, corresponding to CH bending and CC stretching modes of the phosphobarrelene ligand, respectively. Note that a Pd−allyl π interaction can typically be characterized by CH2 wagging modes at ∼950−1050 cm−1 (of relatively weak intensity), but this spectral region was not scanned. Having isolated and characterized this first intermediate, 2K, an unreduced Pd(II) complex, the isolation of the other intermediates was then attempted. When 1 equiv of PhB(OH)2 was added to complex 2K in solution, an instantaneous color change from pale to intense yellow was observed, concomitant with the formation of a white precipitate. The 31P NMR spectrum showed the formation of the single new complex 3, characterized by a singlet at −22.1 ppm (Δδ = −11 ppm from 2K). Complex 3 being a Pd(0) complex was evidenced in the 1 H NMR spectrum. Indeed, only three signals for olefinic protons are observed at 4.95, 3.82, and 3.17 ppm. This chemical shifts are typically indicative of strong back-donation from the Pd(0) center to the π-coordinated 2-propenylbenzene. Unfortunately, it was not possible to further characterize complex 3 by 13C NMR because of its relatively fast decomposition at room temperature, leading to the formation of Pd precipitate and free phosphabarrelene ligand. This decomposition was not unexpected, since complex 3 features a Pd center only stabilized by a phosphine-like ligand and an alkene. Only a few examples of such 14-electron complexes are known in the literature, and they all feature two donating bulky ligands.30 Note that, using charge tags, the observation of otherwise neutral species, such as the neutral Pd(0) complex 3, could have been attempted as shown in recent electrospray ionization mass spectrometry studies of cross-coupling reactions.31 We have, however, relied on the chemical reactivity to obtain further evidence for the formation of the reduced Pd(0) complex 3 using the displacement of the alkene by a diene, dvds (Scheme 1). The quantitative formation of a novel complex, 4, was observed as evidenced by 31P NMR (−28.8 ppm, Δδ = −8.7 ppm from 3). Furthermore, the formation of free 2-propenylbenzene was proved by NMR spectroscopy. Complex 4 was isolated and fully characterized. Final proof of the structure of complex 4, a 16-electron Pd(0) species, was obtained by an X-ray crystal analysis (Figure 4). To conclude, a mechanistic study aimed at understanding the reduction process from an air- and water-stable allyl−Pd(II) complex into a highly reactive, electron-rich Pd(0) species was carried out. In a first step, the reaction of complex 1 with K2CO3 yields the complex [(L)Pd(allyl)(CO3)]−K+ (2K). The combination of ESI-MS/MS and gas-phase IR spectroscopy on an integrated experiment allowed for the characterization of the structure of the isolated complex 2 as featuring η3-allyl and κ1-

As mentioned above, our objective was to characterize the coordination mode of the carbonate ligand in complex 2. The experimental IRMPD spectrum is given in Figure 3a, together with the calculated IR spectra of the κ1 and κ2 carbonate complexes (Figure3b,c, respectively). The experimental spectrum of [(L)Pd(allyl)(CO3)]− (2) has only two strong features at 1248 and 1645 cm−1 (Figure 3a). These two experimental features nicely match with the strongly IR active bands predicted at 1258 and 1642 cm−1 for the symmetric and asymmetric carbonate “free CO” stretches, respectively, of the κ1 isomer. On the basis of literature data,27 the strongly IR active carbonate CO stretches are very sensitive to the coordination mode. In the present case, while the κ1-carbonate complex is predicted to have two intense IR CO stretches (Figure 3b), the κ2-carbonate isomer is characterized by only one very strongly IR active CO stretching mode, which is predicted at ∼1715 cm−1. No strong IRMPD signal is observed in this spectral range. It should be noted, however, that a small band is observed at ∼1700 cm−1 which may reveal a small population of this κ2 isomer. On the basis of these results, it is safe to conclude that the coordination mode of the carbonate in complex 2 is κ1. While the κ1 carbonate isomer is observed experimentally, the κ2-carbonate isomer is predicted to be more stable using hybrid density functional (B3PW91 and B3LYP) and different basis sets. At the level of theory (B3PW91 and a polarized valence double-ζ quality basis set) used to determine the IR absorption spectra, the isomer corresponding to the κ2carbonate was found to be lower in energy (0 K enthalpy) than the κ1-carbonate isomer by 31 kJ/mol. The relative energy is similar (38 kJ/mol) using the B3LYP functional. Progressively extending the basis set to a polarized valence triple-ζ set and adding diffuse functions on the main-group elements did not significantly change the relative energies of the two isomers. A single-point energy calculation using the B3LYP functional and the largest basis set (polarized TZP and diffuse functions), for example, predicts that the κ2-carbonate isomer is 33 kJ/mol more stable than the κ1-carbonate isomer. One possible explanation for the apparent discrepancy between theory and experiment could be related to the wellknown difficulty of describing electronic states involving charge transfer at the DFT level that has been shown earlier.28 In the present case, the charge transfers are likely to be larger in the κ2-carbonate than in the κ1-carbonate isomer, and this is thus a typical case for which hybrid density functionals such as B3LYP and B3PW91 are not adapted for predicting the binding energy of the carbonate to the palladium center. As suggested recently, functionals such as M06-2X29a are better suited to describe charge transfers.29b Using this functional and the same basis set, the energy ordering of the two states changes significantly, and in contrast with the B3LYP or B3PW91 results, the κ1carbonate isomer is predicted to be lower in energy than the κ2carbonate isomer by 3 kJ/mol (see the Supporting Information). To conclude, the [(L)Pd(allyl)(CO3)]− complex (2) seems to have two competing isomers, and the corresponding relative energy is difficult to determine theoretically. The structural information derived from the gasphase IR spectrum of this complex (2) thus provides useful information on the structure of the lowest energy isomer of this isolated molecular ion. In order to further support the structural assignment of complex 2, the IRMPD spectrum of the cationic [(L)Pd(allyl)]+ complex has also been recorded (Figure 3d) in the 5977

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(3) Grushin, V. V.; Alper, H. Chem. Rev. 1994, 94, 1047−1062. (4) Lewis, A. K. D.; Caddick, S.; Cloke, F. G. N.; Billingham, N. C.; Hitchcock, P. B.; Leonard, J. J. Am. Chem. Soc. 2003, 125, 10066− 10073. (5) Littke, A. F.; Dai, C. Y.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 4020−4028. (6) Stambuli, J. P.; Buhl, M.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 9346−9347. (7) Portnoy, M.; Milstein, D. Organometallics 1993, 12, 1655−1664. (8) Amatore, C.; Jutand, A. Acc. Chem. Res. 2000, 33, 314−321. (9) Littke, A. F.; Fu, G. C. J. Org. Chem. 1999, 64, 10−11. (10) Hooper, M. W.; Utsunomiya, M.; Hartwig, J. F. J. Org. Chem. 2003, 68, 2861−2873. (11) Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc. 1999, 121, 1473−1478. (12) Stambuli, J. P.; Kuwano, R.; Hartwig, J. F. Angew. Chem., Int. Ed. 2002, 41, 4746−4748. (13) (a) Calimsiz, S.; Sayah, M.; Mallik, D.; Organ, M. G. Angew. Chem., Int. Ed. 2010, 49, 2014−2017. (b) Sase, S.; Jaric, M.; Metzger, A.; Malakhov, V.; Knochel, P. J. Org. Chem. 2008, 73, 7380−7382. (c) Calimsiz, S.; Organ, M. G. Chem. Commun. 2011, 47, 5181−5183. (d) Organ, M. G.; Avola, S.; Dubovyk, I.; Hadei, N.; Kantchev, E. A. B.; O Brien, C.; Valente, J. C. Chem. Eur. J. 2006, 12, 4749−4755. (14) (a) Marion, N.; Navarro, O.; Mei, J. G.; Stevens, E. D.; Scott, N. M.; Nolan, S. P. J. Am. Chem. Soc. 2006, 128, 4101−4111. (b) Navarro, O.; Marion, N.; Mei, J. G.; Nolan, S. P. Chem. Eur. J. 2006, 12, 5142− 5148. (15) Diebolt, O.; Braunstein, P.; Nolan, S. P.; Cazin, C. S. Chem. Commun. 2008, 3190−3192. (16) Rousseaux, S.; Davi, M; Sofack-Kreutzer, J; Pierre, C.; Kefalidis, C. E.; Clot, E.; Fagnou, K.; Baudoin, O. J. Am. Chem. Soc. 2010, 132, 10706−10716. (17) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 1998, 37, 3387− 3388. (18) Blug, M.; Guibert, C.; Le Goff, X. F.; Mezailles, N.; Le Floch, P. Chem. Commun. 2009, 201−203. (19) Plattner, D. A. Int. J. Mass Spectrom. 2001, 207, 125−144. (20) Chen, P. Angew. Chem., Int. Ed. 2003, 42, 2832−2847. (21) MacAleese, L.; Maitre, P. Mass Spectrom. Rev. 2007, 26, 583− 605. (22) Bakker, J. M.; Besson, T.; Lemaire, J.; Scuderi, D.; Maitre, P. J. Phys. Chem. A 2007, 111, 13415−13424. (23) Prazeres, R.; Glotin, F.; Insa, C.; Jaroszynski, D. A.; Ortega, J. M. Eur. Phys. J. D 1998, 3, 87−93. (24) Mac Aleese, L.; Simon, A.; McMahon, T. B.; Ortega, J. M.; Scuderi, D.; Lemaire, J.; Maitre, P. Int. J. Mass Spectrom. 2006, 249, 14−20. (25) Blug, M.; Doux, M.; Le Goff, X.; Maitre, P.; Ribot, F.; Le Floch, P.; Mézailles, N. Organometallics 2009, 28, 2020−2027. (26) Frisch, M. J. et al. Gaussian 03; Gaussian, Inc., Wallingford, CT, 2004. (27) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed.; Wiley-Interscience: New York, 1997; Part B. (28) (a) Tozer, D. J.; Amos, R. D.; Handy, N. C.; Roos, B. O.; Serrano-Andres, L. Mol. Phys. 1999, 97, 859−868. (b) Drew, A.; HeadGordon, M. J. Am. Chem. Soc. 2004, 126, 4007−4016. (29) (a) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157−167. (b) Steinmann, S. N.; Piemontesi, C.; Delachat, A.; Corminboeuf, C. J. Chem. Theory Comput. 2012, 8, 1629−1640. (30) Otsuka, S.; Yoshida, T.; Matsumoto, M.; Nakatsul, K. J. Am. Chem. Soc. 1976, 15, 5850−5858. (31) (a) Vikse, K. L.; Henderson, M. A.; Oliver, A. G.; McIndoe, J. S. Chem. Commun. 2010, 46, 7412−7414. (b) Schade, M. A.; Feckenstem, J. E.; Knochel, P.; Koszinowski, K. J. Org. Chem. 2010, 75, 6848−6857.

Figure 4. ORTEP plot of complex 4 (thermal ellipsoids at the 50% probability level). Selected bond lengths (Å) and angles (deg): Pd1− P1 = 2.3527(5), Pd1−C20 = 2.174(2), Pd1−C21 = 2.196(2), Pd1− C26 = 2.193(2), Pd1−C27 = 2.213(2); P1−Pd1−C20 = 107.43(6), P1−Pd1−C21 = 144.55(5), P1−Pd1−C26 = 124.39(5), P1−Pd1− C27 = 87.61(5), P1−C1−Si1 = 117.8(1), P1−C5−Si2 = 124.3(1).

CO3− moieties. In a second step, complex 2K is efficiently reduced at room temperature upon addition of PhB(OH)2 via the coupling of the allyl and phenyl moieties, to form the highly reactive Pd(0) complex 3. Beyond the mechanistic study, we present here a very efficient process, using a weak base compatible with most organic substrates and a boronic acid, to generate a catalytically active 14-electron Pd(0) complex.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Text, figures, tables, and a CIF file giving experimental details, X-ray crystallographic data, and additional characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected] (N.M.); [email protected] (P.M.). Present Addresses §

Université d’Evry Val d’Essonne, Laboratoire Analyze et Modélisation pour la biologie et l’environnement, LAMBE UMR8587 CNRS. Tel: +033(0)169477655. E-mail: daniel. [email protected]. ∥ Evonik Industries AG, PB 18, Paul-Baumann-Straße 1, 45772 Marl, Germany. Notes

The authors declare no competing financial interest. ⊥ Deceased on March 17th, 2010.

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ACKNOWLEDGMENTS We thank the CNRS (Centre National de la Recherche Scientifique) and the Ecole Polytechnique for financial support of this work. Financial support from the TGE FT-ICR (CNRS) for conducting the research is gratefully acknowledged.

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DEDICATION This article is dedicated to Prof. P. Le Floch, an inspiring chemist and a missed friend. REFERENCES

(1) Christmann, U.; Vilar, R. Angew. Chem., Int. Ed. 2005, 44, 366− 374. (2) Tang, W. J.; Capacci, A. G.; Wei, X. D.; Li, W. J.; White, A.; Patel, N. D.; Savoie, J.; Gao, J. J.; Rodriguez, S.; Qu, B.; Haddad, N.; Lu, B. Z.; Krishnamurthy, D.; Yee, N. K.; Senanayake, C. H. Angew. Chem., Int. Ed. 2010, 49, 5879−5883. 5978

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