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1 Efficient Hydrogenation of Organic Carbonates, Carbamates and Formates Indicates Alternative Routes to Methanol Based on CO 2 and CO Ekambaram Balaraman, Chidambaram Gunanathan, Jing Zhang, Linda J. W. Shimon, and David Milstein * Department of Organic Chemistry and Department of Chemical Research Support, Weizmann Institute of Science, Rehovot, 76100, Israel. General Experimental: All experiments with metal complexes and phosphine ligands were carried out under an atmosphere of purified nitrogen in a Vacuum Atmospheres glove box equipped with a MO 40-2 inert gas purifier or using standard Schlenk techniques. All solvents were reagent grade or better. All non-deuterated solvents were refluxed over sodium/benzophenone ketyl and distilled under argon atmosphere. Deuterated solvents were used as received. All solvents were degassed with argon and kept in the glove box over 4Å molecular sieves. Most of the chemicals used in the catalytic reactions (organic carbonates and formates) were purified according to standard procedure. 1 The carbamates were synthesized according to literature procedures and purified by sublimation/distillation under vacuum. 2, 3 Thin layer chromatography (TLC) was performed using Merck aluminum sheets precoated with silica gel 60 F254 and the spots were visualized with UV light at 254 nm or under iodine vapor. Column chromatography purifications were performed by flash chromatography using Merck silica gel 60 ( mm). NATURE CHEMISTRY 1

2 1 H, 13 C and 31 P NMR spectra were recorded using a Bruker AMX-300 NMR and AMX-500 spectrometers. 1 H and 13 C{ 1 H} and 13 C-DEPTQ 4 NMR chemical shifts are reported in ppm downfield from tetramethylsilane. 31 P NMR chemical shifts are reported in ppm downfield from H 3 PO 4 and referenced to an external 85% solution of phosphoric acid in D 2 O. Abbreviations used in the NMR follow-up experiments: br, broad; s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; v, virtual. IR spectra were recorded on a Nicolet FT-IR spectrophotometer. Mass spectra were recorded on Micromass Platform LCZ 4000, using Electro Spray Ionization (ESI) mode. Elemental analyses were performed on Thermo Finnigan Italia S.p.A-FlashEA 1112 CHN Elemental Analyzer. GC MS was carried out on HP 6890 (flame ionization detector and thermal conductivity detector) and HP 5973 (MS detector) instruments equipped with a 30 m column (Restek 5MS, 0.32 mm internal diameter) with a 5% phenylmethylsilicone coating (0.25 mm) and helium as carrier gas. The gaseous CO was analyzed by GC TCD with a Carbon plot capillary column (J&W Scientific). GC analysis were carried out using a Carboxen 1000 column on a HP 690 series GC system or HP-5 cross linked 5% phenylmethylsilicone column (30m 0.32mm 0.25 µm film thickness, FID) on a HP 6890 series GC system using m-xylene (1 mmol) as an internal standard. Synthesis of complexes 1, 2 and 4: The dearomatized ruthenium-pincer complexes 1 5, 2 6 and 4 6 were prepared according to previously reported procedures from our group. Complex 4 was stable in air for at least two days in solution. In the solid state, it was stable for a month without decomposition. NATURE CHEMISTRY 2

3 Synthesis of complex 3: The trans-dihydride 3 was previously reported by our research group. 5 To a screw-capped NMR tube was added a toluene-d8 (~0.8 ml) solution of complex 1 (13.5 mg, 0.03 mmol) under N 2 atmosphere and pure hydrogen gas (99.999%) was bubbled slowly through the dark solution for about ~15 min with occasional shaking until the color of the solution changed to yellow. The solution was concentrated to ~0.5 ml and 31 P{ 1 H} NMR was measured, indicating 100% conversion to the transdihydride complex P{ 1 H}NMR (toluene-d8, 122 MHz): (s); 1 H NMR (toluene-d8, 122 MHz): (d, J PH = 17.1 Hz, 2H, Ru-H), 1.13 (t, J HH = 6.5 Hz, 6H, N(CH 2 CH 3 ) 2 ), 1.48 (d, J PH = 5.6 Hz, 18H, P(C(CH 3 ) 3 ) 2 ), 3.05 (m, 2H, N(CH a2 CH 3 ) 2 ), 3.13 (d, J PH = 8.3 Hz, 2H, CH 2 P), 3.17 (m, 2H, N(CH b2 CH 3 ) 2 ), 3.80 (s, 2H, Py-CH 2 N Arm ), 6.40 (d, J HH = 8.0 Hz, 1H, Py-H3), 6.59 (d, J HH = 8.0 Hz, 1H, Py-H5), 6.63 (t, J HH = 8.0 Hz, 1H, Py-H4). Single crystals suitable for X-ray were prepared by slow diffusion of a hydrogensaturated pentane solution to the hydrogen-saturated toluene solution of the dearomatized ruthenium-pincer complex 1 at room temperature. Yellow crystals were obtained after 2 days. X-ray crystal structure determination of 3: A crystal was mounted in a loop and flash frozen in a nitrogen stream at K. Data were collected on a Nonius Kappa CCD diffractometer mounted on a FR590 generator equipped with a sealed tube with MoKα radiation (λ = Å) and a graphite monochromator. The structure was solved using direct methods with SHELXS-97 7 based on F 2. NATURE CHEMISTRY 3

4 Complex 3: C 20 H 37 N 2 OPRu, Yellow, plates, mm 3, monoclinic, P2 1 /n, a = (3) Å, b = (3)Å, c = (3)Å, β = (3)deg., V = (9)Å 3, Z = 4, fw = , D c = Mg/m 3, µ = mm -1. Full matrix least-squares of refinement based on F 2 gave an agreement factor R = for data with I >2σ(I) and R = for all data (3936 reflections) with a goodness-of-fit of Idealized hydrogen atoms were placed and refined in the riding mode, with the exception of the Ru hydrides. The hydride atom H1A, was located in the difference map and refined independently to a H1A-Ru1 distance of 1.71(4)Å. H1B-Ru distance was fixed at 1.61Å and not refined. CCDC Deposition: Summary of Data CCDC Authors: Ekambaram Balaraman, Chidambaram Gunanathan, Jing Zhang, Linda J. W. Shimon, David Milstein Journal: Nature Chemistry (1441) Formula: C20 H37 N2 O1 P1 Ru1 Unit cell parameters: a (3) b (3) c (3) beta (3) space group P21/n Reaction of 3 with dimethyl carbonate: To complex 3 (9.0 mg, 0.02 mmol in 0.5 ml toluene-d8) in a screw-capped NMR tube was added dimethyl carbonate (4 µl, 0.05 mmol, 2.5 eq) in a nitrogen glove box. The tube was taken out of the box and heated at 50 ºC for 5 min. NATURE CHEMISTRY 4

5 Free methyl formate and methanol were observed in the crude reaction mixture. The formate proton (HCO 2 Me) appeared as singlet at 7.50 ppm ( 1 H NMR). The 31 P NMR showed formation of complex 5. Reaction of complex 1 with dimethyl carbonate. Formation of complex 5: To a solution of complex 1 (13.5 mg, 0.03 mmol) in benzene (1 ml) was added dimethyl carbonate (DMC) (8 µl, 0.09 mmol, 3 eq) and the solution was stirred at room temperature in a septum capped vial. After 4 hrs, the liquid as well as gaseous sample was analyzed by GC and GC-MS. Formation of methanol was observed by GC-MS. The solvent was removed under vacuum and the residue was dissolved in pentane (2 ml) and kept at 32 ºC overnight. The yellow solid thus obtained was isolated by filtration, washed with cold pentane (2 0.5 ml) and dried under vacuum to yield 9.5 mg (66 %) of complex 5. IR (KBr): 1908, 1952, 2017 cm -1 ; 31 P{ 1 H}NMR (C 6 D 6, 122 MHz): (s); 1 H (C 6 D 6, 300 MHz): δ (d, 1H, J PH = 24.0 Hz, Ru-H), 0.37 (t, 3H, J HH = 8.0 Hz, N(CH 2 CH a3 ) 2 ), 0.84 (t, 3H, J HH = 8.0 Hz, N(CH 2 CH b3 ) 2 ), 1.32 (d, 9H, J PH = 12.0 Hz, P(C(CH a3 ) 3 ) 2 ), 1.42 (d, 9 H, J PH = 16.0 Hz, P(C(CH b3 ) 3 ) 2 ), 2.04 (m, 1H, N(CH a2 CH 3 ) 2 ), 2.57 (m, 1H, N(CH a'2 CH 3 ) 2 ), 2.75 (m, 1H, N(CH b2 CH 3 ) 2 ), 2.92 (m, 1H, N(CH b'2 CH 3 ) 2 ), 3.00 (dd, 1H, J HH = 16.0 Hz, J PH = 4.0 Hz, Py-CH a H b ), 3.38 (br d, 1H, J HH = 16 Hz, Py-CH a H b ), 3.56 (d, 1H, J PH = 4.0 Hz, =PCH), 5.43 (d, 1H, J HH = NATURE CHEMISTRY 5

6 8.0 Hz, Py-H5), 6.40 (d, 1H, J HH = 8.0 Hz, Py-H3), 6.59 (vt, 1H, J HH = 8.0 Hz, Py- 4H); 13 C{ 1 H}NMR (C 6 D 6, 75 MHz): δ 8.4 (s, N(CH 2 C a H 3 ) 2 ), 11.7 (s, N(CH 2 C b H 3 ) 2 ), 29.9 (d, J PC = 6.0 Hz, P(C(C a H 3 ) 3 ) 2 ), 31.3 (d, J PC = 4.0 Hz, P(C(C b H 3 ) 3 ) 2 ), 35.8 (d, J PC = 33.2 Hz, P(C a (CH 3 ) 3 ) 2 ), 38.2 (d, J PC = 20.0 Hz, P(C b (CH 3 ) 3 ) 2 ), 54.2 (s, N(C a H 2 CH 3 ) 2 ), 55.7 (s, N(C b H 2 CH 3 ) 2 ), 64.5 (d, J PC = 58.3 Hz, =PCH), 68.9 (s, Py-CH 2 ), 98.7 (s, Py- C5), (d, J PC = 18.0 Hz, Py-C3), (s, Py-C4), (d, J PC = 2.0 Hz, Py- C6), (d, J PC = 14.0 Hz, Py-C2), (d, J PC = 5.0 Hz, Ru-C a O), (d, J PC = 11.0 Hz, Ru-C b O). Assignment of signals was confirmed by DEPT 135, COSY, and HSQC; Anal. For C 21 H 35 N 2 O 2 PRu: Calcd.: C, 52.59; H, 7.36; Found: C, 52.47; H, Reaction of 3 with methyl formate: To complex 3 (9.0 mg, 0.02 mmol) in 0.5 ml toluene-d8 in a screw-capped NMR tube was added methyl formate (3 µl, 0.05 mmol, 2.5 eq) in a nitrogen glove box. Formation of methanol and a complex tetatively assigned as the formaldehyde Ru(II) complex 6 were observed after 10 min. The formyl proton of 6 appeared as a singlet at 9.21 ppm (~0.5 ppm downfield relative to free formaldehyde which appears as singlet at 8.73 ppm in toluene-d8). Upon prolonged standing at room temperature (~6 hrs), the signal of complex 6 slowly diminished (at least three new signals appeared in 31 P NMR) along with formation of the methanol adduct complex 7. O-H activation of methanol by complex 1. Synthesis of the aromatic complex 7: NATURE CHEMISTRY 6

7 To a solution of complex 1 (9.0 mg, 0.02 mmol) in toluene-d8 (~0.6 ml) was added excess of methanol (4 µl, 0.1 mmol, 5 eq) in a nitrogen glove box. An immediate color change from dark brown to yellow was observed and the resulting clear solution was stirred at room temperature for about 4 hrs. The 31 P{ 1 H} NMR spectrum indicated 100% conversion to complex 7. The complex was precipitated by addition of pentane. Upon drying at room temperature, decomposition started to take place (to give mostly complex 1 and methanol), hence the solvent was not evaporated to dryness. IR (thin film): 2047, 1912 cm P{ 1 H} NMR (toluene-d8, 202 MHz): (s); 1 H NMR (toluene-d8, 500 MHz): (d, J PH = 28.5 Hz, 1H, Ru-H), 0.95 (t, J HH = 7.0 Hz, 3H, N(CH 2 CH a3 ) 2 ), 1.06 (t, J HH = 7.0 Hz, 3H, N(CH 2 CH b3 ) 2 ), 1.11 (d, J PH = 13.5 Hz, 9H, P(C(CH a3 ) 3 ) 2 ), 1.27 (d, J PH = 13.0 Hz, 9H, P(C(CH b3 ) 3 ) 2 ), (m, 1H, N(CH a HCH 3 ) 2 ), (m, 1H, N(CHH b CH 3 ) 2 ), 2.81 (dd, J HH = 15.5 Hz, J PH = 9.0 Hz, 1H, CH a HP Arm ), 3.25 (br s, 4H, 3H for OCH 3, 1H for CHH b P Arm ; these protons are merged with excess MeOH), (m, 2H, N(CH a H b Me) 2 ), 3.45 (br s, 1H, CH a HN Arm ), 4.60 (d, J HH = 15.0 Hz, 1H, CHH b N Arm ), 6.75 (d, J HH = 8.0 Hz, 1H, Py-H5), 6.89 (d, J HH = 7.5 Hz, 1H, Py-H3), 7.14 (vt, J HH = 8.0 Hz, 1H, Py-H4); 13 C{ 1 H}NMR (toluene-d8, 125 MHz): 8.16 (s, N(CH 2 C a H 3 ) 2 ), (s, N(CH 2 C b H 3 ) 2 ), (d, J PC = 5.0 Hz, P(C(C a H 3 ) 3 ) 2 ), (d, J PC = 3.8 Hz, P(C(C b H 3 ) 3 ) 2 ), (d, J PC = 25.0 Hz, P(C a (CH 3 ) 3 ) 2 ), (d, J PC = 11.3 Hz, P(C b (CH 3 ) 3 ) 2 ), (d, J PC = 20.0 Hz, PCH 2 ), (s, N(C a H 2 CH 3 ) 2 ), (br s, NATURE CHEMISTRY 7

8 OCH 3 ), (s, N(C b H 2 CH 3 ) 2 ), (s, Py-CH 2 N Arm ), (s, Py-C5), (d, J PC = 10.0 Hz Py-C3), (s, Py-C4), (s, Py-C6), (d, J PC = 3.8 Hz, Py-C2), (d, J PC = 16.3 Hz, Ru-CO). Assignment of signals was confirmed by DEPT 135, COSY, and HSQC. Catalytic hydrogenation of organic carbonates: (a) A 100 ml Fischer-Porter tube was charged under nitrogen with catalyst 1 or 2 (0.01 mmol), organic carbonate (10.0 mmol), and THF (2 ml). The Fischer-Porter tube was purged by three successive cycles of pressurization/venting with H 2 (30 psi), then pressurized with H 2 (10 atm). The solution was heated at 110 ºC (bath temperature) with stirring for 48 hrs. After cooling to ~5 ο C (ice/water), the H 2 was vented carefully and the products were determined by GC. (b) Hydrogenation under solvent-free (neat) conditions: Catalyst 2 (4.5 mg, 0.01 mmol) and dimethyl carbonate (0.90 g, 10 mmol) were added to a 100 ml Fischer- Porter tube in a nitrogen glove box. The catalyst was completely soluble in dimethyl carbonate and afforded a homogeneous solution. The Fischer-Porter tube was purged by three successive cycles of pressurization/venting with H 2 (30 psi), then pressurized with H 2 (10 atm). The solution was heated at 100 C (bath temperature) with stirring for 8 hrs. After cooling to room temperature, excess H 2 was vented carefully at 0 ºC and the products were analyzed by GC. NATURE CHEMISTRY 8

9 (c) Substrate/catalyst 5000/1: Complex 2 (2.2 mg, mmol) and THF (2 ml) were placed in a stainless-steel 30 ml autoclave equipped with a magnetic stirring bar under glove box (nitrogen atm). A solution of dimethyl carbonate (1.50 g, 25 mmol) in THF (3 ml) was added, and the autoclave was purged by three successive cycles of pressurization/venting with H 2 (4 atm), then pressurized with H 2 (50 atm) and closed. The solution was heated at 110 ºC (bath temperature) with stirring. After the specified period, the autoclave was cooled in an ice/water bath and the excess H 2 was vented carefully and the products were determined by GC. (d) Complex 1 (4.5 mg, 0.01 mmol) and 1,4-dioxane (5 ml) were placed in a stainless-steel 100 ml Parr Instrument under glove box (nitrogen atm). A solution of dimethyl carbonate (1.50 g, 25 mmol) in 1,4-dioxane (15 ml) was added, and the Parr Apparatus was purged by three successive cycles of pressurization/venting with H 2 (4 atm), then pressurized with H 2 (40 or 60 atm), closed and placed in a heating pan with a thermostat at 145 ο C with stirring. After the specified time, the reaction vessel was cooled in an ice/water bath and the excess H 2 was vented carefully and the products were determined by GC. Catalytic hydrogenation of organic carbamates: NATURE CHEMISTRY 9

10 A 100 ml Fischer-Porter tube was charged under nitrogen with catalyst 2 (4.5 mg, 0.01 mmol), carbamate (1.0 mmol), and THF (2 ml). The Fischer-Porter tube was purged by three successive cycles of pressurizing/venting with H 2 (30 psi) and pressurized with H 2 (10 atm). The solution was heated at 110 ºC (bath temperature) with stirring for 48 hrs. After cooling ~5 ο C (ice/water), the H 2 was vented carefully and the products were analyzed by GC-MS and the yields were determined by GC. In the case of benzyl N-benzylcarbamate (E) using the procedure described above, 53% of benzyl alcohol, 55% of benzylamine and 53% of methanol were obtained. Catalytic Hydrogenation of Methyl Formate: (a) Using catalysts 1, 2 or 3: A 100 ml Fischer-Porter tube was charged under nitrogen with the catalyst 1, 2 or 3 (0.01 mmol), methyl formate (15.0 mmol), and THF (2 ml). The nitrogen present in the tube was replaced by H 2 (twice with 30 psi) at room temperature, then it was filled with H 2 (10 atm). The solution was heated at NATURE CHEMISTRY 10

11 110 ºC (bath temperature) with stirring for 48 hrs. After cooling to room temperature, the H 2 was vented carefully and the products were determined by GC. (b) Using complex 4 + base (generation of catalyst 2 in situ) and charging under air: In open air, a solution of methyl formate (15.0 mmol) in THF (1.0 ml) was transferred via syringe into a Fischer-Porter tube (100 ml) containing 0.01 mmol of catalyst 4. Then, KO t Bu (0.01 mmol) in THF (1.0 ml) was added and the air present in the Fischer-Porter tube was flushed with hydrogen (thrice with 30 psi) and pressurized with H 2 (10 atm). The solution was heated at 110 ºC (bath temperature) with stirring for 48 hrs. After cooling to room temperature, the H 2 was vented carefully and the products were determined by GC. (c) Substrate/Catalyst 5000/1: Complex 2 (2.2 mg, mmol) and THF (2 ml) were placed in a stainless-steel 30 ml autoclave equipped with a magnetic stirring bar under N 2. A solution of methyl formate (1.50 g, 25 mmol) in THF (3 ml) was added, and the autoclave was purged by three successive cycles of pressurization/venting with H 2 (4 atm), then pressurized with H 2 (40 atm) and closed nicely. The solution was heated at 110 ºC (bath temperature) with stirring. After 14 hrs the autoclave was cooled in an ice/water bath and excess H 2 was vented carefully and the products were determined by GC. (d) Hydrogenation of methyl formate under solvent-free (neat) conditions: A 100 ml Fischer-Porter tube was charged under nitrogen with catalyst 2 (0.01 mmol), and methyl formate (10.0 mmol). The catalyst was completely soluble in methyl formate. After the nitrogen in the Fischer-Porter tube was replaced by H 2 (twice with 30 psi) at NATURE CHEMISTRY 11

12 0 ºC, the tube was pressurized with H 2 (10 atm). The homogeneous reaction mixture was heated at 80 C (bath temperature) with stirring. After 8 hrs the Fischer-Porter tube was cooled in an ice/water bath and the excess H 2 was vented carefully and the products were determined by GC. Hydrogenation of other alkyl formates: A 100 ml Fischer-Porter tube was charged under nitrogen with the catalyst 2 (0.01 mmol), a formate (15.0 mmol), and THF (2 ml). The nitrogen present in the Fischer- Porter was replaced by H 2 (twice with 30 psi) at room temperature, then it was filled with H 2 (10 atm). The solution was heated at 110 ºC (bath temperature) with stirring for 48 hrs. After cooling to ~5 ο C (ice/water), the H 2 was vented carefully and the products were analyzed by GC-MS and the yields were determined by GC. References: 1. Armarego, W. L. F. & Perrin, D. D. Purification of Laboratory Chemicals (Pergamon Press, Oxford, 1988) ed Kim. J. G. & Jang, D. O. Tetrahedron Lett. 50, (2009). 3. Yadav, V. K. & Babu, K. G. J. Org. Chem. 69, (2004). 4. Bigler, P., Kümmerle, R. & Bermel, W. Magn. Reson. Chem. 45, (2007). 5. Zhang, J., Leitus, G., Ben-David, Y. & Milstein. D. Angew. Chem. Int. Ed. 45, (2006). NATURE CHEMISTRY 12

13 6. Balaraman, E., Gnanaprakasam, B., Shimon, L. J. W. & Milstein, D. J. Am. Chem. Soc. 132, (2010). 7. (a) Sheldrick, G. M. SHELXS-97, Programm for Crystal StructureSolution; University of Göttingen, Göttingen, Germany, 1997; (b) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refinement;University of Göttingen, Göttingen, Germany, NATURE CHEMISTRY 13