Mechanistic Details of the Nickel-Mediated Formation of Acrylates from

May 28, 2013 - Methyl iodide induces the stoichiometric cleavage of nickelalactones, which are key intermediates in the nickel-mediated reaction of CO...
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Mechanistic Details of the Nickel-Mediated Formation of Acrylates from CO2, Ethylene and Methyl Iodide Philipp N. Plessow,†,‡ Laura Weigel,§ Ronald Lindner,‡ Ansgar Schaf̈ er,† Frank Rominger,§ Michael Limbach,*,‡,∥ and Peter Hofmann*,‡,§ †

BASF SE, Quantum Chemistry, GVM/M-B009, Carl-Bosch-Strasse 38, D-67056 Ludwigshafen, Germany CaRLa (Catalysis Research Laboratory), Im Neuenheimer Feld 584, D-69120 Heidelberg, Germany § Ruprecht-Karls-Universität Heidelberg, Organisch-Chemisches Institut, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany ∥ BASF SE, Synthesis & Homogeneous Catalysis, GCS/C-M313, Carl-Bosch-Strasse 38, D-67056 Ludwigshafen, Germany ‡

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ABSTRACT: Methyl iodide induces the stoichiometric cleavage of nickelalactones, which are key intermediates in the nickelmediated reaction of CO2 and alkenes to acrylates. Herein, we propose a modified and extended mechanism for this reaction on the basis of theoretical and experimental investigations for the bidentate P ligand 1,2-bis(di-tert-butylphosphino)ethane (dtbpe). The calculated elementary steps agree well with experimental findings: reaction barriers are reasonable and explain the facile liberation of acrylate from a nickelalactone by methyl iodide. We were able to isolate reactive intermediates and to verify the existence of proposed reaction pathways. Additionally, we have identified unproductive pathways leading to byproducts (e.g., propionates and catalytically inactive organometallic species). Although those side reactions can be suppressed to a certain extent, the strong binding of acrylate to nickel prevents a catalytic reaction, at least for the chosen ligand.

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INTRODUCTION Carbon dioxide is an abundantly available C1 building block and could become a cheap raw material1−3 for a limited number of bulk chemicals, such as formates and acrylates. Whereas the technology for formates is already mature,4 the development of acrylates has been in its infancy since the seminal work of Hoberg5 and Yamamoto6 in the 1980s. The endergonic nature of the overall reaction and the high activation barrier for the proposed β-hydride elimination from a nickelalactone (ΔG = 164 kJ/mol)7,8 makes the direct reaction of CO2 and ethylene to give acrylic acid a difficult task to study. We have recently overcome both limitations and assembled the first catalytic reaction of CO2 and ethylene to give acrylates by targeting Na acrylate as a commercially attractive salt of acrylic acid and by cleaving the intermediate lactones by strong bases bearing Lewis acidic cations.9 Both experimental and theoretical data point to the fact that the nickelalactone is deprotonated by a base instead of undergoing β-H elimination. Already Yamamoto, Hoberg, and Walther et al. had found10 that certain electrophiles such as alkyl halides are capable of cleaving nickelalactones, as did Puddephatt and Chatami et al. for the less explored zircona-11 and platinalactones.12 More recently, Kü h n and Rieger et al.13,14 have shown in stoichiometric reactions that an excess of methyl iodide (MeI) cleaves nickelalactones to give an equimolar mixture of © XXXX American Chemical Society

methyl acrylate and methyl propionate. The fate of the organometallic species remained unclear. The equimolar formation of unsaturated and saturated organic products is in accord with a first mechanistic hypothesis (Scheme 1),14,15 as 1 equiv of H+ (as HI) is formed and may protonate nickelalactone A or related organometallic species. The transition state TSA‑C for the transformation of A to C has been computed as a four-membered cyclic arrangement with an activation barrier of ΔG⧧ = 240 kJ/mol,14 which seems unreasonably high. As this barrier is even higher than that computed for a β-H elimination without MeI (ΔG⧧ = 164 kJ/ mol),7 to which it is compared by Kühn et al., one has to expect the latter reaction to be clearly preferred over methylation. Furthermore, such a high activation barrier is rather inconsistent with experimental observations by these authors: i.e., substantial product formation within hours at room temperature. As derived from transition state theory, realistic barriers are ca. 100 kJ/mol, and the explanation of Kühn et al. invoking concentration effects14,15 certainly does not seem reasonable for a barrier more than twice as large. Due to this apparent inconsistency, we became interested in revisiting the proposed mechanistic pathway and, to clarify the central Received: March 28, 2013

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halide.28 All Gibbs free energies in solution are given at standard conditions in kJ/mol and relative to the nickelalactone A1. Open shell species have been calculated using the unrestricted Kohn−Sham formalism (UKS). The connectivity of transition states has been confirmed by a displacement along the transition vector followed by steepest descent optimization. Activation energies for dissociation of ligands from pseudotetrahedral endergonic Ni intermediates for different functionals and basis sets were in general found to be only slightly above those intermediates (typically 2σ(I)), μ = 0.90 mm−1, Tmin = 0.89, Tmax = 0.92, 256 parameters refined, the three hydrogen atoms at the coordinated double bond were refined isotropically, goodness of fit 1.03 for observed reflections, final residual values R1(F) = 0.037, wR2(F2) = 0.081 for observed reflections, residual electron density −0.27 to 0.40 e Å−3. [(dtbpe)Ni(CH(CH3)COOCH3)BArF] (B6-BArF). A solution of H(Et2O)2BArF (437 mg, 431.6 μmol) in CH2Cl2 (4 mL) was added at −78 °C to a solution of D (200 mg, 431.7 μmol) in CH2Cl2 (10 mL). The reaction mixture was stirred at this temperature for 15 min and then warmed to room temperature over 3 h. The solvent was evaporated, and the crude product of the reaction was crystallized at −60 °C from a mixture of CH2Cl2 and pentane to yield B6-BArF (500 mg, 87%) as an orange solid. Anal. Calcd for C54H59BF24O2P2Ni: C, 48.86; H, 4.48; P, 4.67. Found: C, 48.57; H, 4.28; P, 4.64. 1H NMR (300.51 MHz, CD2Cl2): δ 1.28 (d, JH,H = 19.0 Hz, 3H, CH3), 1.32 (dd, JH,H = 5.1 Hz, JP,H = 13.5 Hz, 18H, C(CH3)3), 1.44 (dd, 18 H, JH,H = 5.1 Hz, JP,H = 13.5 Hz, 18H, C(CH3)3), 1.83−2.13 (m, 4H, PCH2CH2P), 3.22−3.25 (m, 1H, NiCH), 3.78 (s, 3H, OCH3), 7.56 (s, 4H, p-H), 7.72 (s, 8H, o-H). 13C{1H} NMR (150.93 MHz, CD2Cl2): δ 15.1 (s, CH3), 20.6 (dd, JP,C = 6.0 Hz, JP,C = 22.6 Hz, PCH2), 26.8 (dd, JP,C = 13.6 Hz, JP,C = 25.7 Hz, PCH2), 30.1−30.8 (m, C(CH3)3), 37.2 (dd, JP,C = 15.1 Hz, JP,C = 27.2 Hz, C(CH3)3), 38.0 (dd, JP,C = 1.5 Hz, JP,C = 19.6 Hz, C(CH3)3), 41.4 (dd, JP,C = 9.1 Hz, JP,C = 33.2 Hz, Ni-CH), 53.9 (s, OCH3), 118.1 (s, p-Ar), 125.2 (q, JF,C = 273.18 Hz, CF3), 129.5 (q, JF,C = 30.19 Hz, m-Ar), 135.4 (s, o-Ar), 162.4 (q, JB,C = 49.8 Hz, ipso-CAr), 167.7 (s, CO). 31P{1H} NMR (121.65 MHz, CD2Cl2): δ 94.6 (d, JP,P = 6.1 Hz), 95.5 (d, JP,P = 6.1 Hz). IR (KBr): ν 2969, 2872, 1611, 1495, 898, 744, 682, 498 cm−1. MS (LIFDI) m/z: 463.1 [M − BArF]. Single crystals were obtained from a mixture of CH2Cl2 and pentane at −60 °C. Crystal data: yellow crystal (needle), dimensions 0.15 × 0.09 × 0.07 mm3, crystal system orthorhombic, space group Pbca, Z = 8, a = 18.8623(15) Å, b = 24.459(2) Å, c = 25.783(2) Å, α = 90°, β = 90°, γ = 90°, V = 11895.4(18) Å3, ρ = 1.482 g/cm3, θmax = 22.72°, mean redundancy 11.76 and completeness 100.0% for a resolution of 0.92 Å, 97690 reflections measured, 8004 unique reflections (R(int) = 0.0801), 5536 observed reflections (I > 2σ(I)), μ = 0.50 mm−1, Tmin = 0.93, Tmax = 0.97, 852 parameters refined, H2 at C2 was refined isotropically, goodness of fit 1.02 for observed reflections, final residual values R1(F) = 0.051, wR2(F2) = 0.115 for observed reflections, residual electron density −0.38 to 0.48 e Å−3. [(dtbpe)Ni(CH2CH2COOCH3)BArF] (B1-BArF). Lactone A1 (200 mg, 446 μmol) was dissolved in CH2Cl2 (8 mL), and MeOTf (50.5 μL, 446 μmol) was added to the solution, which was immediately cooled to −78 °C and stirred for 5 min. This mixture was added to a cooled solution (−78 °C) of NaBArF (396 mg, 447 μmol) in CH2Cl2 (30 mL). The yellow solution was stirred for 10 min while a yellow solid precipitated. The supernatant was removed via filter cannula, and the solvent was evaporated; during these procedures the temperature was not allowed to reach more than −40 °C to avoid isomerization. The crude product of the reaction was crystallized at −60 °C from a mixture of CH2Cl2 and pentane to yield B1-BArF (385 mg, 64%) as a yellow solid. Anal. Calcd for C54H59BF24O2P2Ni: C, 48.86; H, 4.48; P, 4.67. Found: C, 48.76; H, 4.33; P, 4.61. 1H NMR (300.51 MHz, CD2Cl2): δ 1.31 (bs, 2H, NiCH2), 1.39 (d, 18H, JP,H = 15.0 Hz, C(CH3)3), 1.41 (d, JP,H = 12.0 Hz, 18H, C(CH3)3), 1.59−1.70 (m, 2H, PCH2CH2P), 1.87−2.02 (m, 2H, PCH2CH2P), 2.45−2.53 (m, 2H, CH2COO), 3.88 (s, 3H, OCH3), 7.56 (s, 4H, p-H), 7.72 (s, 8H, o-H). 13 C{1H} NMR (150.93 MHz, CD2Cl2): δ 9.8 (q, JP,C = 33.2 Hz, NiCH2), 18.7 (dd, JP,C = 6.0 Hz, JP,C = 19.6 Hz, PCH2), 27.1 (dd, JP,C =

18.1 Hz, JP,C = 22.6 Hz, PCH2), 30.4 (d, JP,C = 4.5 Hz, C(CH3)3), 30.8 (d, JP,C = 3.0 Hz, C(CH3)3), 35.5 (d, JP,C = 10.6 Hz, C(CH3)3), 37.0 (dd, JP,C = 1.5 Hz, JP,C = 4.5 Hz, CH2COO), 38.3 (d, JP,C = 19.6 Hz, C(CH3)3), 56.6 (s, OCH3), 118.0 (s, p-Ar), 125.2 (q, JF,C = 271.7 Hz, CF3), 129.1−129.8 (m, m-Ar), 135.4 (s, o-Ar), 162.3 (q, JB,C = 49.8 Hz, ipso-CAr), 195.4 (d, JP,C = 12.1 Hz, CO). 31P{1H} NMR (121.65 MHz, CD2Cl2): δ 78.9 (d, JP,P = 3.6 Hz), 82.4 (d, JP,P = 3.6 Hz). IR (KBr): ν 2963, 1615, 1469, 1379, 1355, 1278, 1132, 900, 682 cm−1. MS (LIFDI) m/z: 463.1 [M − BArF]. Single crystals were obtained from a mixture of CH2Cl2 and pentane at −60 °C. Crystal data: yellow crystal (polyhedron), dimensions 0.07 × 0.07 × 0.06 mm3, crystal system triclinic, space group P1̅, Z = 4, a = 12.9017(14) Å, b = 20.347(2) Å, c = 25.577(3) Å, a = 70.864(3)°, β = 86.826(3)°, γ = 74.241(3)°, V = 6100.5(11) Å3, ρ = 1.492 g/cm3, θmax = 21.04°, mean redundancy 4.12 and completeness 100.0% for a resolution of 0.99 Å, 54200 reflections measured, 13150 unique reflections (R(int) = 0.1264), 6687 observed reflections (I > 2σ(I)), μ = 0.53 mm−1, Tmin = 0.96, Tmax = 0.97, 1790 parameters refined, goodness of fit 1.02 for observed reflections, final residual values R1(F) = 0.075, wR2(F2) = 0.182 for observed reflections, residual electron density −0.52 to 0.64 e Å−3. Reaction of [(dtbpe)Ni(CH(CH3)COOCH3)BArF] (B6-BArF) and NEt3. To a solution of B6-BArF (10 mg, 21.6 μmol) in CD2Cl2 (0.6 mL) was added NEt3 (2.1 μL, 21.6 μmol). The product in solution was characterized by 31P NMR spectroscopy. Reaction of [(dtbpe)Ni(CH2CH2COO)] (A1) with [(dtbpe)Ni(CH(CH3)COOCH3)BArF] (B6-BArF). B6-BArF (20 mg, 15.1 μmol) and A1 (7 mg, 15.1 μmol) were dissolved in THF-d8 (0.6 mL), and the products were characterized by 31P NMR spectroscopy. Reaction of [(dtbpe)Ni(η2-methyl acrylate)] (D) with MeI. To a solution of D (20 mg, 43.2 μmol) in benzene-d6 (0.6 mL) was added MeI (27.0 μL, 432 μmol). The solution was stirred for several days at ambient temperature, and the products were characterized by 31P NMR spectroscopy. Reaction of [(dtbpe)NiI2] (H) with [(dtbpe)Ni(η2-methyl acrylate)] (D). H (20 mg, 31.8 μmol) and D (14.5 mg, 31.8 μmol) were dissolved in CD2Cl2 (0.6 mL). The solution was stirred for 30 min and then subjected to characterization by 1H and 31P NMR spectroscopy. [(dtbpe)Ni(CH3CH2COO)BArF] (E3). Nickel propionate (30 mg, 145.1 μmol) and dtbpe (47 mg, 145.1 μmol) were dissolved in CH2Cl2 (10 mL), and the mixture was stirred for 10 min. NaBArF (129 mg, 145.1 μmol) was added to the mixture. The red solution was stirred for 10 min, and a white solid precipitated. The supernatant was removed via filter cannula. Afterward the solvent was evaporated and the precipitate was washed with pentane (2 × 5 mL). The resulting solid was dried under vacuum to afford 145 mg (76%) of E3 as an orange powder. Anal. Calcd for C54H59BF24O2P2Ni: C, 48.47; H, 4.37; P, 4.72. Found: C, 48.44; H, 4.35; P, 4.76. 1H NMR (300.51 MHz, CD2Cl2): δ 1.01 (t, 3H, JH,H = 9.0 Hz, CH3), 1.51 (d, 36H, JP,H = 12.0 Hz, C(CH3)3), 1.82 (d, 4H, JP,H = 9.0 Hz, PCH2CH2P), 2.16 (q, 2H, JH,H = 9.0 Hz, CH2COO), 7.55 (s, 4H, p-H), 7.71 (s, 8H, o-H). 13C{1H} NMR (150.93 MHz, CD2Cl2): δ 7.9 (s, CH3), 22.9 (dd, JP,C = 16.6 Hz, JP,C = 19.6 Hz, PCH2), 30.0 (s, C(CH3)3), 31.1 (s, CH2), 37.9 (dd, JP,C = 7.6 Hz, JP,C = 9.0 Hz, C(CH3)3), 118.0 (s, p-Ar), 125.2 (q, JF,C = 271.7 Hz, CF3), 129.1−129.8 (m, m-Ar), 135.4 (s, o-Ar), 162.3 (q, JB,C = 48.3 Hz, ipso-CAr), 201.0 (s, COO). 31P{1H} NMR (121.65 MHz, CD2Cl2): δ 101.6 (s). IR (KBr): ν 2972, 1612, 1567, 1485, 1356, 1279, 1131 cm−1. MS (LIFDI) m/z: 449.2 [M − BArF]. Single crystals could be obtained from a mixture of CH2Cl2 and pentane at −35 °C. Crystal data: orange crystal (polyhedron), dimensions 0.10 × 0.08 × 0.07 mm3, crystal system triclinic, space group P1̅, Z = 4, a = 12.6924(5) Å, b = 18.3585(7) Å, c = 25.7549(10) Å, α = 86.316(1)°, β = 88.748(1)°, γ = 88.911(1)°, V = 5986.5(4) Å3, ρ = 1.457 g/cm3, θmax= 22.72°, mean redundancy 2.95 and completeness 100.0% for a resolution of 0.92 Å, 48015 reflections measured, 16082 unique reflections (R(int) = 0.0648), 9177 observed reflections (I >2σ(I)), μ = 0.49 mm−1, Tmin = 0.95, Tmax = 0.97, 1691 parameters refined, goodness of fit 1.02 for observed reflections, final J

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residual values R1(F) = 0.062, wR2(F2) = 0.130 for observed reflections, residual electron density −0.44 to 0.96 e Å−3. Reaction of [(dtbpe)Ni(CH2CH2COO)] (A1) with H(Et2O)2BArF. A1 (20 mg, 44.6 μmol) and H(Et2O)2BArF (45 mg, 44.6 μmol) were dissolved in CD2Cl2 (0.6 mL). The solution was characterized by 31P NMR spectroscopy after 10 min, 1 h, and 1 day. Reaction of [(dtbpe)Ni(CH2CH2COO)] (A1) with MeI. (a) To a solution of A1 (20 mg, 44.5 μmol) in THF-d8 (1 mL) were added mesitylene (10.0 μL, 72.1 μmol, internal standard) and MeI (27.8 μL, 445 μmol). The solution was stirred for 24 h at ambient temperature and characterized by 1H and 31P NMR spectroscopy. (b) To a solution of A1 (67.4 mg, 150 μmol) in THF-d8 (1 mL) was added MeI (93 μL, 1.5 mmol). The solution was stirred for 12 h at ambient temperature, evaporated to 0.2 mL, and precipitated with Et2O (2 mL). I precipitated as a pale red solid (25 mg, 33%). Anal. Calcd: C, 42.89, H, 8.00. Found: C, 42.07, H, 7.94%. MS (FAB+) measured: 503.1 [C18H40P2NiI]. Reaction of [(dtbpe)Ni(CH2CH2COO)] (A1) with MeI and Cs2CO3. To a suspension of A1 (20 mg, 44.5 μmol) and Cs2CO3 (58 mg, 178 μmol) in THF-d8 (1 mL) were added mesitylene (10.0 μL, 72.1 μmol) and MeI (27.8 μL, 445 μmol). The solution was stirred for 2 h, and the resulting compounds were characterized by 1H and 31P NMR spectroscopy. CCDC 927636 (B1), 927637 (B6), 927638 (D), and 927639 (E3) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving additional experimental details, characterization data, details of the calculations, and crystallographic data for B1, B6, D, and E3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.H.); michael.limbach@ basf.com (M.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.N.P., R.L., M.L., and P.H. work at CaRLa of Heidelberg University, being cofinanced by Heidelberg University, the state of Baden-Württemberg, and BASF SE. The support of these institutions is gratefully acknowledged. We are indebted to Professor Peter Kündig, CaRLa Fellow 2013, for careful proofreading of the final manuscript.

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REFERENCES

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(37) Software package SHELXTL 2008/4 for structure solution and refinement: Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122. (38) Reger, D. L.; Wright, T. D.; Little, C. A.; Lamba, J. J. S.; Smith, M. D. Inorg. Chem. 2001, 40, 3810−3814. (39) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11, 3920−3922. (40) Bickley, R. I.; Edwards, H. G. M.; Gustar, R.; Rose, S. J. J. Mol. Struct. 1991, 248, 237−250.

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