Mechanisms of the CO2 Insertion into (PCP) Palladium Allyl and

Jul 28, 2010 - Fast and reversible insertion of carbon dioxide into zirconocene–alkoxide bonds. A mechanistic study. Alice Brink , Ida Truedsson , A...
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Organometallics 2010, 29, 3521–3529 DOI: 10.1021/om100325v

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Mechanisms of the CO2 Insertion into (PCP) Palladium Allyl and Methyl σ-Bonds. A Kinetic and Computational Study Magnus T. Johnson,† Roger Johansson,† Mikhail V. Kondrashov,† Gideon Steyl,‡ Ma˚rten S. G. Ahlquist,§ Andreas Roodt,‡ and Ola F. Wendt*,† †

Organic Chemistry, Department of Chemistry, Lund University, P.O. Box 124, S-221 00 Lund, Sweden, Department of Chemistry, University of the Free State, Bloemfontein 9300, South Africa, and §Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, S-10691 Stockholm, Sweden

‡

Received April 21, 2010

The reaction of the σ-bonded (PCP)Pd-Me complex (PCP = 2,6-bis[(di-tert-butylphosphino)methyl]phenyl) with CO2 is first-order in palladium and first-order in CO2 with a rate constant ks = 8.9 ( 0.8 M-1 s-1 at 353 K. Activation parameters are ΔHq = 73 ( 7 kJ/mol and ΔSq = -118 ( 19 J/K mol. Based on this and theoretical calculations we propose an SE2 mechanism where the coordinated methyl group attacks a completely noncoordinated carbon dioxide molecule in a bimolecular reaction. The PCPPd-crotyl complex was synthesized in an 65:35 E:Z mixture, and it was shown to react with CO2 to give the complex PCPPd-O(CO)CH(CH3)CHCH2 as a single isomer, where the former γ-carbon has been carboxylated. Theoretical calculations again suggest an SE2 mechanism with a noncoordinated carbon dioxide reacting with the terminal carbon on the allyl group, forming an η2-bonded olefin complex as an intermediate. The rearrangement of this intermediate to the O-bonded product is concluded to be rate determining. The crystal structure of PCPPd-O(CO)C(CH3)2CHCH2 is reported and as well as the solubility of carbon dioxide in benzene-d6 at different pressures and temperatures.

Introduction There has been an increasing interest in utilizing carbon dioxide as a reagent in synthetic chemistry in the last two decades.1 Since carbon dioxide is cheap, abundant, and nontoxic, its use as a C1 source is attractive. Due to the inherent stability of CO2, the process of forming C-C bonds from CO2 is often demanding, but there has been some progress.2,3 However, well-defined stoichiometric insertion reactions into transition metal-carbon σ-bonds are still

relatively uncommon.4,5 Only one example of a palladium methyl σ-bond insertion has been reported, and this employs a PCP pincer ligand where the aromatic backbone exerts a strong trans influence and thereby activates the methyl group.6 The reaction was made catalytic using ZnMe2, cf. Scheme 1. Somewhat more straightforward is the reaction with the allylic analogues, where the insertion of CO2 into a bis-allyl palladium(II) complex resulting in an alkenyl carboxylate was first reported in 1980.7 A catalytic carboxylative coupling was claimed by Shi and Nicholas,8 and this carboxylation was considered to proceed through a (η1-η3)bis-allyl intermediate. It was found that only Pd(0) was active as a catalyst in this reaction. Later, Szabo and co-workers reported that using pincer-ligated palladium, the η3-allyl functionality in the bis-allyl complex can be mimicked, still making the η1-allyl moiety nucleophilic, as shown with a number of electrophiles such as aldehydes and imines.9 Furthermore, we reported a catalytic carboxylation of allylstannanes using palladium pincer complexes and milder conditions than originally reported by Nicholas.10 Allylic

*To whom correspondence should be addressed. E-mail: ola. [email protected]. (1) (a) Riduan, S. N.; Zhang, Y. Dalton Trans. 2010, 39, 3347–3357. (b) Sakakura, T.; Choi, J.-C.; Yasuda, H. Chem. Rev. 2007, 107, 2365–2387. (c) Louie, J. Curr. Org. Chem. 2005, 9, 605–623. (d) Gibson, D. H. Chem. Rev. 1996, 96, 2063–2095. (e) Yin, X.; Moss, J. R. Coord. Chem. Rev. 1999, 181, 27–59. (2) (a) Shimizu, K.; Takimoto, M.; Sato, Y.; Mori, M. Org. Lett. 2005, 7, 195-197, and references therein. (b) Louie, J.; Gibby, J. E.; Farnworth, M. V.; Tekavec, T. N. J. Am. Chem. Soc. 2002, 124, 15188– 15189. (3) (a) Ukai, K.; Aoki, M.; Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2006, 128, 8706–8707. (b) Takaya, J.; Tadami, S.; Ukai, K.; Iwasawa, N. Org. Lett. 2008, 10, 2697–2700. (c) Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2008, 130, 15254–15255. (d) Yeung, C. S.; Dong, V. M. J. Am. Chem. Soc. 2008, 130, 7826–7827. (e) Ochiai, H.; Jang, M.; Hirano, K.; Yorimitsu, H.; Oshima, K. Org. Lett. 2008, 10, 2681–2683. (f ) Williams, C. M.; Johnson, J. B.; Rovis, T. J. Am. Chem. Soc. 2008, 130, 14936–14937. (g) Kobayashi, K.; Kondo, Y. Org. Lett. 2009, 11, 2035–2037. (4) (a) Darensbourg, D. J.; Kudaroski Hankel, R.; Bauch, C. G.; Pala, M.; Simmons, D.; White, J. N. J. Am. Chem. Soc. 1985, 107, 7463– 7473. (b) Darensbourg, D. J.; Gr€otsch, G.; Wiegreffe, P.; Rheingold, A. L. Inorg. Chem. 1987, 26, 3827–3830. (5) Mankad, N. P.; Gray, T. G.; Laitar, D. S.; Sadighi, J. P. Organometallics 2004, 23, 1191–1193.

(6) Johansson, R.; Jarenmark, M.; Wendt, O. F. Organometallics 2005, 24, 4500–4502. (7) Hung, T.; Jolly, P. W.; Wilke, G. J. Organomet. Chem. 1980, 190, C5. (8) (a) Shi, M.; Nicholas, K. M. J. Am. Chem. Soc. 1997, 119, 5057– 5058. (b) Franks, R. J.; Nicholas, K. M. Organometallics 2000, 19, 1458– 1460. (9) (a) Solin, N.; Kjellgren, J.; Szab o, K. J. Angew. Chem., Int. Ed. 2003, 42, 3656–3658. (b) Solin, N.; Kjellgren, J.; Szabo, K. J. J. Am. Chem. Soc. 2004, 126, 7026–7033. (10) Johansson, R.; Wendt, O. F. J. Chem. Soc., Dalton Trans. 2007, 4, 488–492.

r 2010 American Chemical Society

Published on Web 07/28/2010

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Organometallics, Vol. 29, No. 16, 2010 Scheme 1

stannanes are well known to react with electrophiles in a reaction often referred to as the metallo-ene reaction, in which the allylic π-electrons are involved in an uncatalyzed or Lewis acid-catalyzed pericyclic reaction.11 This was shown not to occur in our system since the carboxylation does not take place without the catalyst. The same type of cyclic mechanism was proposed in the carboxylation of allenes using a (PSiP)Pd pincer catalyst.3c Compared to other insertion reactions into transition metal carbon bonds, there are very few studies on the mechanistic features of carboxylation of metal-carbon(sp3) bonds4,12 and, to our knowledge, none involving isolated σ-allyl Pd complexes. Herein we report on a detailed mechanistic investigation based on both experimental and theoretical results.

Experimental Section General Procedures and Materials. All experiments were carried out under an atmosphere of argon or nitrogen using standard Schlenk or high vacuum line techniques13 unless otherwise noted. All solvents were distilled under vacuum directly to the reaction vessel from sodium/benzophenone ketyl radical, except THF and dichloromethane, which were dried over calcium hydride. Dry carbon dioxide was obtained by passing it through a P2O5 column and a -78 °C trap on the high-vacuum line. Carbon dioxide was purchased from AGA Gas AB. NMR experiments were carried out using J. Young NMR tubes. All chemicals were purchased from either Acros or Sigma-Aldrich. 1H, 13C, and 31P NMR experiments were recorded on a Varian Unity INOVA 500 spectrometer operating at 499.77 MHz (1H) using C6D6 unless otherwise stated. Complexes 26,14 and 610 and silver carboxylates15 were prepared according to literature procedures. Chemical shifts are given in ppm downfield from TMS using residual solvent peaks (1H and 13C NMR) or H3PO4 as reference. Multiplicities are abbreviated as follows: (s) singlet, (d) doublet, (t) triplet, (q) quartet, (m) multiplet, (br) broad, (v) virtual. Elemental analyses were performed by H. Kolbe Microanalytisches Laboratorium, M€ ulheim an der Ruhr, Germany. Modified Synthesis of 2,6-Bis[(di-tert-butylphosphino)methyl]benzene, PCPtBuH (1). To a Strauss flask with R,R-dibromo meta-xylene (1 g, 3.79 mmol) was distilled MeOH (30 mL). After complete dissolution, di-tert-butylphosphine (15.6 mL, 10 wt % in (11) Davies, A. G. Organotin Chemistry, 2nd ed.; Wiley-VCH Verlag GmbH & Co. KGaA, 2004; Chapter 9, pp 140-142. (12) (a) Hartwig, J. F.; Bergman, R. G.; Andersen, R. A. J. Am. Chem. Soc. 1991, 113, 6499–6508. (b) Sakaki, S.; Ohkubo, K. Organometallics 1989, 8, 2970–2973. (13) Burger, B. J.; Bercaw, J. E. In New Developments in the Synthesis, Manipulation and Characterization of Organometallic Compounds, Vol. 357; Wayda, A.; Darensbourg, M. Y., Ed.; American Chemical Society,: Washington DC, 1987. (14) Johnson, M. T.; Cetina, M.; Rissanen, K.; Wendt, O. F. Acta Crystallogr. 2010, E66, m675. (15) Bonner, W. A.; DeGraw, J., Jr. J. Chem. Educ. 1962, 39, 639–640.

Johnson et al. hexanes, 7.58 mmol) was added slowly inside a glovebox. The solution was stirred for 24 h at room temperature, sodium methoxide (0.41 g, 7.58 mmol) was added, and the stirring was continued for 4 h. The methanol was evaporated, dichloromethane (30 mL) was distilled into the flask, and the sodium bromide was separated through swivel-frit filtration under vacuum. The dichloromethane was evaporated to give 1.33 g (89%) of white crystals. Spectroscopic data were in agreement with the literature.16 Modified Synthesis of PCPtBuPdMe (3). Method A. Trifluoroacetate complex 2 (460 mg, 0.75 mmol) was placed in a Strauss flask to which THF (25 mL) was distilled. To this solution was added slowly ZnMe2 (0.67 mL, 0.80 mmol, 1.2 M in toluene). After 14 h, 2,20 -bipyridyl (125 mg, 0.80 mmol) was added, whereafter the precipitate that immediately formed was removed by swivel-frit filtration under vacuum. The solvent was evaporated and the precipitate washed with pentane. Evaporation left 386 mg (60%) of a light gray solid. Method B. In a glovebox, ligand 1 (320 mg, 0.81 mmol) was placed together with fresh (TMEDA)PdMe2 (205 mg, 0.81 mmol) in a Strauss flask. Benzene (15 mL) was distilled directly into the reaction vessel, and the mixture was left stirring at room temperature for 16 h. Evaporation of the volatiles in high vacuum left 284 mg (68%) of a light gray solid. In both cases 1H and 31P NMR spectra were in agreement with the literature.6 Synthesis of PCPtBuPdCH2CHCHCH3 (7). To a stirred solution of chloride complex 6 (400 mg, 0.75 mmol) in diethyl ether was added slowly crotyl magnesium chloride (0.63 mL, 1.6 M in THF, 1.0 mmol, E:Z 80:20) at room temperature, whereafter the mixture was heated to 70 °C for 7 h. Swivel-frit filtration under vacuum with thorough washing with diethyl ether and subsequent evaporation left a pale yellow solid. The product was extracted with pentane and filtered. The volatiles were removed under vacuum, leaving a pale yellow solid (266 mg, 64%). E-isomer: 1H NMR δ 7.15-7.20 (overlapping m, 3H), 6.48 (m, 1H), 5.48 (m, 1H), 3.27 (overlapping vt, 4H, J = 3.5 Hz), 2.62 (m, 2H), 1.96 (d, 3H, J = 6 Hz), 1.22 (vt, 36H, J = 6.5 Hz); 13C{1H} NMR δ 149.85 (t, J = 9 Hz), 144.53 (s), 128.1 (overlapped), 124.69 (br s), 120.82 (overlapping t, J = 9 Hz), 112.12 (s), 39.83 (overlapping vt, J = 11 Hz), 35.44 (overlapping vt, J = 6 Hz), 29.99 (overlapping vt, J = 3 Hz), 19.04 (s), 8.00 (t, J = 7.4 Hz); 31P{1H} NMR δ 71.67. Z-isomer: 1H NMR δ 7.15-7.20 (overlapping m, 1H), 6.58 (m, 1H), 5.42 (m, 1H), 3.28 (vt, 4H, J = 3.5 Hz), 2.50 (m, 2H), 2.01 (d, 3H, J = 7 Hz), 1.22 (vt, 36H, J = 6.3 Hz); 13C{1H} NMR δ 149.92 (t, J = 9 Hz), 143.78 (s), 128.1 (overlapped), 124.69 (br s), 120.85 (overlapping t, J = 9 Hz), 112.99 (s), 39.81 (overlapping vt, J = 11 Hz), 35.30 (overlapping vt, J = 6.6 Hz), 29.95 (overlapping vt, J = 3 Hz), 14.10 (s), 2.29 (t, J = 8 Hz); 31P{1H} δ 71.15. Synthesis of PCPtBuPdOCOCH(CH3)CHCH2 (8). Reaction of 7 with CO2: In a J. Young NMR tube, 6 mg of the isomeric E,Zmixture 7 was dissolved in 0.6 mL of C6D6. The tube was pressurized with 4 atm of dry CO2. At room temperature the single product carboxylate 8 was obtained with full conversion within a few minutes. Anal. Found: C, 58.90; H, 8.92; O, 5.11 Calcd for C29H50O2P2Pd: C, 58.14; H, 8.41; O, 5.34. 1H NMR δ 7.02 (t, 1H, J = 8 Hz), 6.91 (d, 2H, J = 8 Hz), 6.60 (m, 1H), 5.19 (m, 1H), 5.10 (m, 1H), 3.38 (m, 1H), 2.93 (vt, 4H, J = 4 Hz), 1.59 (d, 3H, J = 7 Hz), 1.27 (vt, 36H, J = 6.5 Hz); 13C{1H} NMR δ 177.23 (s), 155.11 (s), 151.82 (t, J = 8 Hz), 143.44 (s), 124.98 (s), 122.44 (t, J = 8 Hz), 111.61 (s), 48.13 (s), 34.89 (vt, J = 6 Hz), 33.85 (vt, J = 8 Hz), 29.27 (vt, J = 2 Hz), 18.19 (s); 31P{1H} NMR δ 73.01 Substitution Reaction. Complex 6 (20 mg, 37 μmol) was placed together with silver 2-methyl-3-butenoate (12 mg, 56 μmol) in a Strauss flask covered with aluminum foil. THF (5 mL) was added through distillation, and the mixture was stirred at 70 °C for 1 h. After being cooled to room temperature, the solvent was (16) The original procedure can be found in: Gusev, D. G.; Madott, M.; Dolgushin, F. M.; Lyssenko, K. A.; Antipin, M. Y. Organometallics 2000, 19, 1734–1739.

Article evaporated and the palladium carboxylate was extracted with benzene (3  2 mL). The solution was filtered, the benzene was evaporated, and the product was recrystallized from heptane to give colorless crystals (10 mg, 45%). Synthesis of PCPtBuPdOCOC(CH3)2CHCH2 (9). Complex 6 (20 mg, 37 μmol) was placed with silver 2,2-dimethyl-3-butenoate (12 mg, 56 μmol) in a Strauss flask covered with aluminum foil. THF (5 mL) was added through distillation, and the mixture was stirred at 70 °C for 1 h. After being cooled to room temperature, the solvent was evaporated and the palladium carboxylate was extracted with benzene (3  2 mL). The solution was filtered, the benzene was evaporated, and the product was recrystallized from heptane (22 mg, 96%). Found: C, 59.82; H, 8.87; O, 5.39. Calcd for C30H52O2P2Pd: C, 58.77; H, 8.55; O, 5.22. 1H NMR δ 7.02 (t, J = 8.0 Hz, 1H), 6.91 (d, J = 7.5 Hz, 2H), 6.76 (dd, 3J = 17.5 Hz, 3J = 10.5 Hz, 1H), 5.19 (dd, 3J = 17.5 Hz, 2J = 2.0 Hz, 1H), 5.08 (dd, 1 J = 10.5 Hz, 2J = 2.0 Hz, 1H), 2.93 (vt, J = 4 Hz, 1H), 1.62 (s, 6H), 1.26 (vt, J = 7 Hz, 36H); 13C{1H} NMR δ 178.8 (s), 154.9 (s), 151.8 (t, J = 10.1 Hz), 125.0 (s), 122.4 (t, J = 10.2 Hz), 108.7 (s), 46.88 (s), 34.93 (vt, J = 7.1 Hz), 33.88 (vt, J = 10.6 Hz), 29.11 (s), 26.37 (s), 23.10 (s); 31P NMR δ 72.83. Kinetic Investigation. To evaluate the solubility of carbon dioxide, 13C NMR spectra were recorded in benzene at different temperatures using various CO2 pressures. Toluene was used as an internal standard, and the integral of the p-carbon at 125.68 ppm was compared with that of the CO2 signal at 124.88 ppm. A relaxation delay of 60 seconds was used, and decoupling was on only during acquisition. To evaluate the insertion reaction, complex 3 was dissolved in C6D6 in a J. Young NMR tube. The C6D6 was degassed by the freeze-pump-thaw method and then pressurized with the appropriate amount of dry CO2. The sample was inserted into the prethermostated NMR spectrometer, and kinetic traces were obtained by recording 1H NMR spectra at various times. There was no fluctuation of the peak widths, and thus peak heights were used in the kinetic evaluation. Crystallography. XRD-quality crystals of 9 were obtained through recrystallization from heptane. Intensity data were collected with an Oxford Diffraction Xcalibur 3 system using ω-scans and Mo KR (λ = 0.71073 A˚) radiation.17 The data were extracted and integrated using Crysalis RED.18 The structure was solved by direct methods and refined by full-matrix least-squares calculations on F2 using SHELXTL 5.1.19 Non-H atoms were refined with anisotropic displacement. All hydrogen atoms were constrained to parent sites, using a riding model. All crystallographic data are available in CIF format and are given in the Supporting Information. Crystal data: C30H52O2P2Pd, M = 612.25, monoclinic, a = 11.5131(3) A˚, b = 17.9918(4) A˚, c = 15.9809(5) A˚, β = 105.175(3)°, V = 3194.88(26) A˚3, space group P21/n, Z = 4, 31 976 reflections measured, 11 169 unique (Rint = 0.027), which were used in all calculations. The final wR(F2) was 0.088 (all data), and the R(F ) was 0.034 (I > 2σ(I )) using 178 parameters. CCDC reference number 771139. Computational Details. All calculations were performed using the B3LYP20 type density functional theory as implemented in Jaguar 7.5,21 except the final energy corrections, which were performed with M06.22 For geometry optimizations, solvation energy, and frequency calculations the LACVP** core potential and basis set was used, while for single-point energy corrections LACV3P**þþ augmented with two f-functions on Pd as (17) Crysalis CCD; Oxford Diffraction Ltd.: Abingdon, Oxfordshire, UK, 2005. (18) Crysalis RED; Oxford Diffraction Ltd.: Abingdon, Oxfordshire, UK, 2005. (19) Sheldrick, G. M. SHELXTL5.1, Program for Structure Solution and Least Squares Refinement; University of G€ottingen: G€ottingen, Germany, 1998. (20) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (21) Jaguar 7.5; Schr€ odinger LLC: Portland, OR. (22) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2006, 120, 215–241.

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suggested by Martin was used.23 All geometries were fully optimized using the Poisson-Boltzmann self-consistent reaction field (PBF)24 to simulate the effect of solvent implicitly. The dielectric constant was set to 2.284 and the probe radius to 2.60 to simulate benzene. All transition states were confirmed to be first-order saddle points by analytical frequency calculations. The enthalpies were calculated as the sum H = E(M06/ LACV3P**þþ(2f)) þ Gsolv þ ZPE þ H298.

Results Synthesis of the Methyl Complex. Ligand 1 was synthesized via a modified literature procedure in which sodium methoxide was used as a base instead of triethylamine.16 Using this method, the bromide salt obtained during the synthesis is more easily separated, and the rapid formation of a highly stable pincer palladium bromide as an impurity in later steps is avoided. Methylation of 2 to synthesize 3 by the previously published method with methyllithium6 sometimes proceeds with low yield likely due to competing benzylic deprotonation. The yield can be increased from 20% to 68% by instead using fresh (TMEDA)PdMe2 as described for platinum by Hughes and coworkers,25 generating methane and TMEDA as the only byproducts; see Scheme 2. Transmetalation using dimethyl zinc, on the other hand, appears to generate a highly unstable intermediate, which was not successfully isolated. A possibility is that it is a palladium-zinc adduct of the same type as has been observed in similar rhodium systems.26 Upon addition of 2,20 bipyridine, product formation is instantaneous, as indicated by immediate precipitation of the zinc bipyridine complex. Kinetics of Palladium Methyl Insertion. As previously communicated, complex 3 undergoes conversion to 4 in C6D6 at 80 °C in an atmosphere of dry CO2. In order to evaluate the kinetics of this insertion, we first set out to study the solubility of CO2 in C6D6 under the conditions used, since no literature data were available. Thus, a 13C NMR study was performed, similarly to the study on CO2 solubility in champagne reported by Autret et al.27 The concentration as a function of pressure at different temperatures is given in Figure 1; the full data set is given in the Supporting Information. The pressures reported are actual pressures after equilibration and were calculated on the basis of the amount of CO2 condensed into the J. Young tube and amount dissolved. The solubility of carbon dioxide in benzene in our conditions is in the range 14-47%. The data in Figure 1 were fitted to the linear equations given in Table 1. With this information the kinetics of the insertion was investigated using 1H NMR spectroscopy, keeping CO2 in large excess to ensure pseudo-first-order conditions. The reaction was followed for more than three half-lives and could be monitored by observing the decrease of the benzylic protons of 3 at 3.32 ppm and the concomitant increase of 4 at 2.93 ppm. The height of the peaks was plotted as a function of time, and the kinetic traces thus obtained were fitted to (23) (a) Martin, J. M. L. Chem. Phys. Lett. 1999, 310, 271–276. (b) Martin, J. M. L.; Sundermann, A. J. Chem. Phys. 2001, 114, 3408–3420. (24) (a) Tannor, D. J.; Marten, B.; Murphy, R.; Friesner, R. A.; Sitkoff, D.; Nicholls, A.; Rignalda, M.; Goddard, W. A., III; Honig, B. J. Am. Chem. Soc. 1994, 116, 11875–11882. (b) Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.; Rignalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100, 11775–11788. (25) Hughes, R. P.; Williamson, A.; Incarvito, C. D.; Rheingold, A. L. Organometallics 2001, 20, 4741–4744. (26) Kloek, S. M.; Heinekey, M.; Goldberg, K. I. Angew. Chem., Int. Ed. 2007, 46, 4736–4738. (27) Autret, G.; Liger-Belair, G.; Nuzillard, J. M.; Parmentier, M.; Dubois de Montreynaud, A.; Jeandet, P.; Doan, B. T.; Beloeil, J. C. Anal. Chim. Acta 2005, 535, 73–78.

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Johnson et al. Scheme 2

Figure 1. Solubility of CO2 in C6D6 as a function of pressure. The solid lines denote the best fit to a straight line. (1=303 K, 3=313 K, 9=323 K, b=333 K, 4=343 K, 2=353 K, [=363 K, 0=373 K). Table 1. Equations Fitted to the Data in Figure 1, Showing the Linear Relationship between Carbon Dioxide Concentration and Partial Pressure at Various Temperatures T/K

fitted equation ([CO2]/M vs P)

R

303 313 323 333 343 353 363 373

-0.148 þ 0.171P -0.046 þ 0.122P -0.090 þ 0.115P -0.040 þ 0.085P -0.090 þ 0.083P -0.101 þ 0.075P -0.096 þ 0.065P -0.112 þ 0.056P

0.991 0.996 0.994 0.983 0.988 0.977 0.947 0.999

single exponentials, as shown in Figure 2. Variation of the initial palladium concentration caused no significant change in the observed rate constant, thus corroborating that the reaction is first-order in palladium. A second product, which was shown to be (PCP)PdOCO2H (5) by comparison with an authentic sample made from a separate experiment,28 was formed simultaneously with 4. The hydrocarbonate is likely to be formed either from carbonic acid obtained from residual water in the reaction mixture or by CO2 insertion (28) Johansson, R.; Wendt, O. F. Organometallics 2007, 26, 2426– 2430.

Figure 2. Typical kinetic trace of the reaction of 3 with carbon dioxide (entry 2 in Table 2). The solid lines denote the best fit to a single exponential.

into a (PCP)PdOH species obtained together with methane by hydrolysis of 3. Previously the carbonic acid route has been shown to be the most likely mechanism.29 Addition of water to a carbon dioxide saturated C6D6 solution of methyl complex 3 results in the predominant formation of carbonate 5. Although the carbon dioxide was subject to intense drying by both P2O5 and a -78 °C trap, the formation of the carbonate could never be completely avoided. However, there is a lag period before 5 starts to form, and the observed rate constants for the formation of 4 were used as good approximations for the actual insertion rate constants. From the exponential plots pseudo-first-order rate constants were obtained as a function of temperature and concentration of 3 and CO2 and are given in Table 2. Plotting the observed rate constants versus CO2 concentration at 353 K (entries 1, 4, 5, and 6) gives a straight line, as shown in Figure 3. Although there is a substantial intercept, the errors (see Figure 3) are such that we still interpret the data in the following rate law (see also Discussion):

Rate ¼ k obs ½3 ¼ k s ½3½CO2 

ð1Þ

(29) Pushkar, J.; Wendt, O. F. Inorg. Chim. Acta 2004, 1295–1298.

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13

where ks is 8.9 ( 0.8 M-1 s-1 at 353 K. The values of ks at three other temperatures are also given in Table 2; they were obtained by dividing the observed pseudo-first-order rate constant with the CO2 concentration. Fitting the Eyring equation to this data gives a linear plot (Figure S2, Supporting Information). Activation parameters are ΔHq = 73 ( 7 kJ/mol and ΔSq = -118 ( 19 J/K mol. Synthesis and Reactivity of Palladium Allyl Complexes. As previously reported,10 the carboxylation of allyl stannanes is catalyzed by a PCP pincer palladium complex. Since the carboxylation of the isolated pincer palladium allyl complex goes to completion within seconds at room temperature, kinetic studies are hard to perform, and valuable mechanistic insight can be gained by labeling the starting material and then characterizing the product. Deuterium labeling was difficult to do since scrambling is always a possibility in the synthesis of the allyl complex. Instead we chose to synthesize a γ-substituted allyl derivative to decide whether the reactivity is on the R- or γcarbon. To this end, the chloride precursor 6 was reacted with an 80:20 E and Z mixture of crotyl magnesium chloride30 to form crotyl complex 7. The presence of two crotyl complexes in a 65:35 ratio was observed as two 31P NMR peaks at 71.15 and 71.67 ppm. The E- and Z-isomers were assigned unambiguously on the basis of, for example, the olefinic 1H and terminal methyl

C chemical shifts, and it was also clear that the E-isomer was the major component. However, the compound is very sensitive to air and moisture, and thus we could not fully characterize it using mass spectroscopy or elemental analysis. Attempts to synthesize the corresponding prenyl complex using its organolithium, organomagnesium, and organostannane derivatives were unsuccessful, likely due to higher steric requirements compared to the simpler allylic and crotylic counterparts. After pressurizing an NMR tube of 7 in benzene-d6 carboxylate 8 is obtained within minutes, as confirmed by multinuclear NMR and an independent synthesis through a substitution reaction between compound 6 and the silver carboxylate, cf. Scheme 3. The compound was isolated, but it always contains traces of solvent, thus giving an analysis that is slightly off. The presence of three different allylic hydrogens in 8, as observed in the 1H NMR spectrum, in combination with the fact that only a single product formed from the two isomers of 7, clearly rules out a direct 1,2-insertion mechanism; it is only compatible with reactivity on the γ-carbon. Such reactivity is commonly observed in the so-called metallo-ene mechanism, in which the π-electrons would attack the electrophilic carbon dioxide carbon to generate a six-membered cyclic transition state; see Scheme 4. This is obviously compatible with the observations in Scheme 3, and as previously mentioned, this type of transition state is commonly encountered in, for example, the Lewis acid catalyzed electrophilic substitution of allylstannanes with various electrophiles.11 Neither 7 nor 8 provided X-ray quality crystals, but in the process of synthesizing a prenyl complex for insertion reactions the expected product was synthesized from 6 and the silver carboxylate to give 9, which could be isolated as crystals suitable for X-ray diffraction. The molecular structure is given in Figure 4 together with selected bond distances and angles. The coordination geometry is distorted square planar with a bent P-Pd-P angle of 165.78°, typical of PCP metal complexes. The Pd-O distance is 2.123 A˚, which is similar to other examples of acetates trans to aryl groups,31 and the structure shows the proposed carboxylate product of CO2 insertion into an η1-coordinated allyl bond. Computational Results. To gain a better understanding of the reactivity and stereochemical outcome of the insertion reactions of palladium methyl and allyl bonds, we decided to model the reactions using density functional theory (M06/ B3LYP) combined with an implicit solvation model (PBF). One can envision four different modes of reaction between a palladium allyl bond and CO2 (Scheme 5 and 6): (A) direct 1,2-insertion into the Pd-C1 bond; (B) an SE2-type reaction with stereochemical inversion at C1; (C) a cyclic transition state where C3 reacts with CO2 and the CO2 is pointing toward the palladium center; (D) a similar transition state to C but where the CO2 is reacting from the opposite side of the π-bond. The fact that the branched allyl carboxylate 8 is formed from the crotyl complex shows that only reaction paths C and D could be operating. For the much less reactive methyl complex, on the other hand, only reactions of types A and B are possible, and we wanted to understand what makes these paths less favored. From the kinetic experiments these two cannot be distinguished since they are both bimolecular, as predicted by the rate law and activation parameters. For all four different reaction paths transition states were located for the allyl case. Reaction at C1 can occur via two

(30) The mixture was reported by the manufacturer to be 80:20, but we did not check the composition.

(31) Canty, A. J.; Minchin, N. J.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1987, 6, 1477–1483.

Table 2. Observed Rate Constants As a Function of Temperature and Palladium and Carbon Dioxide Concentrationsa entry

[Pd]/mM

[CO2]/mM

T/K

105 kobs/s-1

1 2 3 4 5 6 7 8 9

9.71 19.7 14.8 9.71 9.71 9.71 9.71 9.71 9.71

178 178 178 96 294 359 308 250 215

353 353 353 353 353 353 342 362 373

1.86 1.97 1.98 0.97 2.25 2.65 1.01 4.20 6.62

105 ks/M-1 s-1 10.4 10.1 7.6 7.4 3.3 ( 0.7 16.8 ( 3.5 31 ( 6

a The rate constant at 353 K reported in the text is obtianed from the slope of Figure 3.

Figure 3. Observed rate constant as a function of carbon dioxide concentration. T = 353 K. [Pd] = 9.71 mM.

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Johnson et al. Scheme 3

Scheme 4

transition states, where path A is calculated to have a high enthalpic barrier Hq of 122.7 kJ mol-1, and path B is calculated to have a lower barrier of 90.7 kJ mol-1. It is interesting that the SE2 path is lower than the 1,2-insertion, which is likely due to the steric repulsion of the tert-butyl groups, which essentially protect the palladium from other interactions outside the square-planar coordination sphere. Moreover it has been found before that electron-rich alkyl groups can react preferentially via inversion of the carbon.33 Since it is not clear that transition state B leads directly to the product, we performed an intrinsic reaction coordinate (IRC) scan. We found that after the C-C bond has been formed the carboxylate rearranges to the butenoate via an initially very flat potential energy surface. When one of the oxygen atoms is close enough to the palladium center, the interaction increases and the Pd-O bond is formed. In agreement with the experimental results, the reactions at terminal carbon C3 have substantially lower barriers than reactions at C1. Somewhat surprising is that we calculate the C3-SE2 path to be the lowest one, which again seems to be due to steric repulsion from the tert-butyl groups. The energy barrier for path C is 61.6 kJ mol-1, and path D is calculated to have an energy barrier of only 44.9 kJ mol-1. However, when we perform a scan of the intrinsic reaction coordinate, (32) Brandenburg, K. DIAMOND, Program for Molecular Graphics; Crystal Impact: Bonn, Germany, 2000. (33) Tenn, W. J., III; Conley, B. L.; H€ ovelmann, C. H.; Ahlquist, M.; Nielsen, R. J.; Ess, D. H.; Oxgaard, J.; Bischof, S. M.; Goddard, W. A., III; Periana, R. A. J. Am. Chem. Soc. 2009, 131, 2466–2468.

we find that transition state D does not directly connect to the product complex, but instead to a complex where the carbon-carbon π-bond coordinates to palladium. The transition state for rearrangement from the π-complex to the carboxylate is calculated to be 59.6 kJ mol-1 above the reactants, making it the highest point on the path to the palladium butenoate. The overall barrier for the C3 SE2 reaction is thus only slightly lower than the metallo-ene transition state, cf. Scheme 7. The methyl complex 3 can also react with CO2 to form the acetate complex 4. For 3 paths A and B are the two possibilities for insertion. Again, we find that the SE2 path B is more favorable than the 1,2-insertion. We calculate the barriers for paths A and B to be 102.9 and 79.9 kJ mol-1, respectively. Interestingly, when the tert-butyl groups on the phosphorus are replaced with hydrogen atoms, and hence the steric bulk is removed, the 1,2-insertion and SE2 transition states are close to isoelectronic. This implies that the steric bulk of the tBu groups blocks the palladium and disfavors the 1,2-insertion, cf. Figure 5. To understand why the reaction at C3 is much more favorable than reaction at C1, the geometries of Bts and Dts were examined (Figures S2 and S3). It appears that in Bts there is significantly more rearrangement of the allyl group; that is, the Pd-C distance in Bts is elongated to 2.52 A˚, while in Dts it is only slightly elongated to 2.28 A˚, from 2.21 A˚ in 10. By removing the CO2 from the geometries and just performing a single-point energy calculation we get a measurement of the cost of rearranging the palladium-allyl moiety.

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Scheme 6. Activation Enthalpies of the C-C Bond-Forming Step in kJ mol-1a

a Path D has the lowest barrier, but it is not directly connected to the product complex. See Scheme 7 for the complete mechanism.

Figure 4. DIAMOND32 drawing of 9. For clarity hydrogen atoms have been omitted. Selected bond distances (A˚) and angles (deg) with estimated standard deviations: Pd1-P1 = 2.312(1); Pd1-P2 = 2.309(1); Pd1-C1 = 2.019(2); Pd1-O1 = 2.123(1); O1-C25 = 1.273(3); O2-C25 = 1.217(3); C29-C30 = 1.289(4); P1-Pd1-P2 = 165.78 (2); C1-Pd1-O1 = 173.60(7); P1-Pd1C1 = 83.83(6); P1-Pd1-O1 = 96.75(4); P2-Pd1-C1 = 83.25(6); P2-Pd1-O1 = 95.43(4); O2-C25-O1 = 125.3(2).

Scheme 7. Lowest Enthalpy Path for the Reaction of X with CO2a

Scheme 5

a

Rearrangement to the geometry of Bts costs 99.6 kJ mol-1, while rearrangement to the geometry of Dts costs only 33.9 kJ mol-1, and hence it is clear that reaction at the C3 carbon requires much less rearrangement of the complex to react. However, to get a complete picture, we also need to calculate the energy for rearranging CO2 to the geometry of the respective transition states and the interaction energy between the fragments.34 Rearrangement of the CO2 frag(34) The decomposition of the activation energy in strain energy and interaction energy has been proposed as a method of understanding differences in reactivity. (a) Mitchell, D. J.; Schlegel, H. B.; Shaik, S. S.; Wolfe, S. Can. J. Chem. 1985, 63, 1642–1648. (b) Fernandez, I.; Bickelhaupt, F. M.; Cossio, F. P. Chem.;Eur. J. 2009, 15, 13022–13032. (c) Ess, D. H.; Houk, K. N. J. Am. Chem. Soc. 2008, 130, 10187–10198.

Enthalpies in italics in kJ mol-1.

ment was found to require 107.9 and 91.6 kJ mol-1 for reaction at C3 and C1, respectively. Hence, the energy required for rearrangement of the CO2 fragment is larger for reacting at C3 and reduces the difference in reactivity slightly. The interaction energy between the fragments was calculated to be 77.8 and 86.0 kJ mol-1 for transition states Dts and Bts, respectively. Also the interaction energy is larger for reaction at C1, and we conclude that a large part of the difference in reactivity is due to the facile rearrangement of the allyl fragment in Dts relative to Bts. In Figure 6 the HOMO of 10 is shown, and it has significant density at both C1 and C3 and is not likely to be the selectivitydetermining factor. The NBO charge on C1 was calculated to be -0.75 and on C3 -0.51, which means that if the partial

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Figure 5. Space-filling model of 10. It is clear that the space available perpendicular to the coordination plane of palladium is limited due to the large steric bulk of the tert-butyl groups.

Figure 6. The HOMO of the allyl complex 10 (tert-butyl groups removed for clarity) has large density at both C1 and C3. At C1 the density is larger toward the palladium; however this side is blocked by the tert-butyl groups, and hence the backside SE2 reaction is favored over the 1,2-insertion.

charge was the more important selectivity-determining factor, the opposite selectivity should be observed. Hence, we conclude that the much higher reactivity of C3 is due to the fact that only minor rearrangement of the complex is required to reach the transition state.

Discussion On the basis of our previously reported reactivity patterns, we reasoned that there must be some difference in the mechanism of carboxylation of 3 and 10. The kinetic results for the carboxylation of 3 include a second-order rate law, a negative entropy of activation, and a fairly high (given the negative entropy) activation enthalpy and are best interpreted in an associative mechanism with substantial bond breaking in the transition state. The errors in the kinetics are quite large, but still one could argue that the intercept is substantial. A possible explanation for this would be a reversible reaction, but this can be ruled out since the acetate 4 is completely stable toward decarboxylation once it is formed. Another possibility that we considered is the formation of an encounter complex in a fast pre-equilibrium; this could give saturation kinetics. However, fitting the data to a pre-equilibrium gives equilibrium constants that are unreasonably large for the association of two neutral species such as carbon dioxide

Johnson et al.

and 3, and we conclude that the rate law in eq 1 is the best interpretation of the data.35 Previous work on insertion of carbon dioxide in late transition metal-methyl bonds4b,12b has suggested associative mechanisms where there is a metal-oxygen interaction in the transition state (TS type A in Scheme 4), and this was also our working hypothesis for the present palladium-methyl complex. The calculations, however, support an SE2 mechanism, where the methyl group acts as a nucleophile on a completely noncoordinated carbon dioxide. The calculated enthalpy of activation was 79.9 kJ mol-1, in excellent agreement with the experimentally determined 73 kJ mol-1. This type of transition state is also compatible with the experimental results, and we believe that the SE2 mechanism is favored over the 1,2-insertion due to the steric shielding of the palladium center by the bulky tert-butyl groups, which prevent the interaction between the oxygen of CO2 and palladium. The carboxylation of the allyl palladium complex is a much faster reaction, which seems to indicate a different mechanism compared to the methyl case. The use of a crotyl complex in the carboxylation gives a nonlinear product, showing that the C-C bond is formed with the γ-carbon (C3). This is also in agreement with the results of Iwasawa, where they found that carboxylation takes place on the carbon that was proposed to be γ to palladium in the catalytic cycle,3c and with the regioselectivity of pincer-catalyzed allylation of aldehydes.9 The computational results support an SE2 mechanism at C3, but the cyclic metallo-ene reaction path was calculated to have only a slightly higher barrier, and it cannot be ruled out as an alternative. It can be noted that a similar transition state has been proposed by Aydin et al. based on calculations for the pincer palladium-catalyzed allylation of sulfonimines.36 Interestingly the preferred reactivity on the γ-carbon seems to be mainly based on sterics since we calculate a higher partial charge on the R-carbon. We also note that solvation favors Dts over Cts by 11.7 kJ mol-1, likely since the oxygen atoms of the CO2 fragment at which negative charge is built up are more exposed to the solvent. Comparing the methyl reaction and the allyl reaction, the computational results showed a 20.0 kJ mol-1 lower barrier for the allyl complex, which was concluded to be due to the much lower energy required for rearranging the allyl moiety in the transition state compared to the methyl group. However, reaction at the C1 carbon was found to have a higher barrier at the allyl complex compared to the methyl complex. It could be that the nucleophilicity of the methyl group is higher since the electron density of the allyl group is partly delocalized to the C2 and C3 carbon atoms.

Conclusions We have studied the reactions between carbon dioxide and pincer palladium methyl and allyl complexes and found that the different complexes had completely different reactivities. In both cases they react via SE2 mechanisms where (35) Fitting the data to the equation kobs = kKeq[CO2]/(1 þ Keq[CO2]) gives a value of Keq = 3.6 ( 1.5 M-1, which is around 1 order of magnitude larger than the constant calculated using the Eigen-Fuoss equation: Fuoss, R. M. J. Am. Chem. Soc. 1958, 80, 5059. Eigen, M. Z. Elektrochem. 1960, 64, 115. (36) Aydin, J.; Kumar, K. S.; Sayah, M. J.; Wallner, O. A.; Szabo, K. J. J. Org. Chem. 2007, 72, 4689–4697.

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there is no palladium-oxygen interaction during the formation of the C-C bond. The reactivity difference is explained by the fact that the allyl group can utilize the nucleophilicity of the γ-carbon, thereby avoiding the steric strain closer to the metal center imposed by the bulky tert-butyl groups.

Acknowledgment. Financial support from the Swedish Research Council, the Crafoord Foundation, the Knut and Alice Wallenberg Foundation, the Royal Physiographic

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Society in Lund and the Swedish Links programme (cosponsored by National Science Foundation, South Africa and SIDA) is gratefully acknowledged. Supporting Information Available: Eyring plot for the kinetic data of the insertion (Figure S1). Details of all calculated transition states (Figures S2, S3, Tables S1, S3). Solubility data for carbon dioxide in benzene (Table S2). Full crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.