Synthesis and Application of Complexes of a Novel Chiral Diphenylphosphino-Functionalized N-Heterocyclic Carbene

Abstract

Complexes of the ligand (S,S)-1-(2-diphenylphosphanyl-naphtalen-1-yl)-3-(2-isopropyl-phenyl)-4,5-diphenyl-imidazolin-2-ylidene, a representative of a new class of chiral ligands, were obtained via a short route starting from (S,S)-1,2-diphenyl-ethylen-1,2-diamine; a Rh-complex was found to promote catalytic hydrogenation of α,β unsaturated esters with up to 99% ee.

asymmetric catalysis - chiral N-heterocyclic carbenes - chiral P ligands - rhodium - enantioselective hydrogenation

References

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15

Physical data of (S,S)-4: mp 195-197 °C; [α]_D ²⁴ -297 (c 1.13, CHCl₃). ¹H NMR (300 MHz, CDCl₃) for the major isomer of (S,S)-4: δ = 0.93 (d, J = 6.6 Hz, 3 H, CH₃), 1.02 (d, J = 6.6 Hz, 3 H, CH₃), 3.45 [sept, J = 7.0 Hz, 1 H, CH(CH₃)₂], (dd, J = 13.6 Hz, J _PH = 5.2 Hz, 1 H, CHN), 6.57 (d, J = 13.6 Hz, 1 H, CHN), 6.92-7.79 (m, 27 H, CH_Ar), 7.82 (d, J = 8.5 Hz, 1 H, CH_Naph), 7.99 (d, J = 7.7 Hz 1 H, CH_Naph), 8.11 (br s, 1 H, NCHN), 8.54 (d, J = 8.8 Hz, 1 H, CH_Naph). ¹³C NMR (125.8 MHz, CDCl₃) for the major isomer of (S,S)-4: δ = 23.60, 24.96, 27.70, 74.97, 79.00 (d, J = 14.1 Hz), 124.36, 126.18, 126.88, 127.26, 127.54, 128.54, 128.87, 128.94, 129.11, 129.13, 129.24 (d, J = 6.6 Hz), 129.39 (d, J = 7.5 Hz), 129.56, 129.72, 129.77, 129.98, 130.08, 130.15, 130.26, 130.66 (d, J = 4.7 Hz), 130.96, 131.42, 132.23, 132.38 (d, J = 14.1 Hz), 132.68 (d, J = 18.8 Hz), 133.30 (d, J = 19.8 Hz), 133.76 (d, J = 18.8 Hz), 134.05 (d, J = 6.6 Hz), 134.30, 134.73 (d, J = 5.7 Hz), 136.41 (d, J = 25.4 Hz), 144.35, 157.38. ³¹P NMR (121.5 MHz, CDCl₃): δ = -19.52 (s, minor isomer, 13%), -21.80 (s, major isomer, 87%). HRMS (FAB+, direct insert): m/z calcd for C₄₆H₄₀N₂P [M - BF₄]⁺: 651.2929. Found: 651.2936.

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18

X-ray single crystal structure determination of compound 5: colorless crystal (polyhedron), dimensions 0.12 × 0.17 × 0.2 mm³, crystal system orthorhombic, space group P2₁2₁2, Z = 2, a = 16.730 (2) Å, b = 22.902 (3) Å, c = 11.2472 (15) Å, V = 4309.3 (10) ų, ρ = 1.303 g/cm³, T = 200(2) K, θ _max = 23.91 deg, radiation MoK_α, λ = 0.71073 Å, 0.3 deg ω-scans with CCD area detector, covering a whole sphere in reciprocal space, 30565 reflections measured, 6649 unique [R(int) = 0.0893], 5558 observed [I>2σ(I)], intensities were corrected for Lorentz and polarization effects, an empirical absorption correction was applied using SADABS (program SADABS V2.03 for absorption correction; G. M. Sheldrick, Bruker Analytical X-ray-Division, Madison, Wisconsin 2001) based on the Laue symmetry of the reciprocal space, µ = 0.56mm^-1, structure solved by direct methods and refined against F ² with a Full-matrix least-squares algorithm using the SHELXTL-PLUS (5.10) software package (software package SHELXTL V5.10 for structure solution and refinement, G. M. Sheldrick, Bruker Analytical X-ray-Division, Madison, Wisconsin 1997), 525 parameters refined, hydrogen atoms were treated using appropriate riding models, Flack absolute structure parameter 0.07 (6), goodness of fit 1.21 for observed reflections, final residual values R1 (F) = 0.077, wR2 (F²) = 0.167 for observed reflections, residual electron density -1.14 to 1.10 eÅ^-3. CCDC 236937 contains the supplementary crystallographic data for this structure. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html [or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: [phone]; e-mail: [e-mail]].

19 Beck J. Reitz M. Z. Naturforsch  1997,  52b:  604 

20 Chojnacki J. Becker B. Konitz A. Potrzebowski MJ. Woljnowski W. J. Chem. Soc., Dalton Trans.  1999,  3063 

21Physical data of (S,S)-6: mp 209-211 °C; [α]_D ²⁴ +10.6 (c 0.30, CHCl₃). ¹H NMR (300 MHz, CDCl₃, signals of major and minor isomer are distinguished by indices a and i, respectively): δ = 0.52 (d, J = 6.4 Hz, 3 H_a, CH₃), 1.15 (d, J = 6.6 Hz, 3 H_a, CH₃), 1.11-1.23 (m, COD-CH₂), 1.29 (d, J = 6.8 Hz, 3 H_i, CH₃), 1.41 (d, J = 6.6 Hz, 3 H_i, CH₃), 1.34-1.77 (m, COD-CH₂), 2.00-2.34 (m, COD-CH₂), 2.37 (br s, 1 H_i, COD-CH₂), 2.86 [sept, J = 6.7 Hz, 1 H_a, CH(CH₃)₂], 3.18 [sept, J = 6.8 Hz, 1 H_i, CH(CH₃)₂], 3.62-3.78 (m, 1 H_a + 1 H_i, COD-CH), 3.91-4.05 (m, 1 H_a + 1 H_i, COD-CH), 4.59 (d, J = 10.7 Hz, 1 H_i, CHN), 4.96 (d, J = 5.1 Hz, 1 H_a, CHN), 5.36-5.48 (m, 1 H_a + 1 H_i, COD-CH), 5.50-5.60 (m, 1 H_i, COD-CH), 5.79-5.90 (m, 2 H_a + 1 H_i, 2 CHN, COD-CH), 6.16 (d, J = 7.4 Hz, 1 H_a, CH_Ar), 6.23 (d, J = 7.4 Hz, 1 H_i, CH_Ar), 6.67 (t, J = 7.8 Hz, 1 H_a, CH_Ar), 6.84 (t, J = 7.6 Hz, 1 H_i, CH_Ar), 6.89-7.97 (m, 28 H_a + 28 H_i, CH_Ar). ¹³C NMR (125.67 MHz, CDCl₃, 253 K, major and minor isomer): δ = 23.58, 23.83, 25.30, 25.32, 25.75, 25.95, 26.04, 28.42, 29.03 (2 C), 34.19, 34.26, 35.77, 36.57, 75.24, 76.70, 78.69, 81.60, 88.31 (dd, J _CP = 15.5 Hz, J _CRh = 6.2 Hz), 88.57 (dd, J _CP = 14.5 Hz, J _CRh = 6.4 Hz), 93.03 (m), 94.39 (dd, J _CP = 10.3 Hz, J _CRh = 4.7 Hz), 100.46 (d, J _CP = 6.0 Hz), 101.05 (d, J _CP = 6.4 Hz), 102.81 (d, J _CP = 5.2 Hz), 104.06 (d, J _CP = 6.0 Hz), 121.31 (d, J _CP = 49.9 Hz), 122.86 (d, J _CP = 49.0 Hz), 122.86, 123.95, 124.64 (m), 125.17, 125.41, 125.77, 126.15, 126.49, 126.54, 126.74 (d, J _CP = 4.3 Hz), 127.10 (d, J _CP = 6.6 Hz), 127.35, 127.43, 127.69, 127.74, 127.86, 128.33-128.63 (several signals are overlapping in this range), 128.78, 128.88 (d, J _CP = 34.9 Hz), 129.11, 129.27, 129.44, 129.66, 130.16, 130.26, 130.56, 130.63, 130.75 (d, J _CP = 37.2 Hz), 131.38, 131.53, 132.03, 132.53 (d, J _CP = 11.3 Hz), 132.78, 133.52 (d, J _CP = 44.26 Hz), 133.75, 133.91, 134.46, 134.95, 135.53, 137.21, 139.14 (d, J _CP = 2.1 Hz), 142.32 (d, J _CP = 13.2 Hz), 143.70, 144.32 (d, J _CP = 12.3 Hz), 144.78, 207.67 (d, J _RhC = 44.4 Hz, only visible in ¹³C{³¹P}), 209.87 (d, J _RhC = 43.5 Hz, only visible in ¹³C{³¹P}). ³¹P NMR (202.47 MHz, CDCl₃, 253 K): δ = 17.89 (d, J _RhP = 168.1 Hz, major isomer, 66%), 19.90 (d, J _RhP = 162.0 Hz, minor isomer, 33%). HRMS (FAB+, direct insert): m/z calcd for C₅₄H₅₁N₂PRh [M - BF₄]⁺: 861.2844. Found: 861.2820.

21

22

With PhCF₃, n-hexane, MeOH, and THF enantioselectivities were lower.

23

Representative Procedure for Catalytic Hydrogenation: Preparation of (R)-8: An autoclave was charged with dimethyl itaconate (158 mg, 1.00 mmol), (S,S)-6 (0.9 mg, 1.0 µmol) and CH₂Cl₂ (15 mL). The autoclave was then sealed and pressurized to 30 bar of H₂. The reaction mixture was stirred for 20 h at r.t. The solution was passed over a short plug of silica gel with CH₂Cl₂ as eluent. After evaporation of the solvent, 2-methyl-succinic acid dimethyl ester was obtained in quantitative yield. The ee value was determined to be 98.2% ee by GC on a chiral phase (analytical data cf. Table [1] ).

24a

The rhodium complex (S,S)-6 was also tested in the catalytic asymmetric addition of phenylboronic acid to enones. Using 3 mol% of the complex (S,S)-6 enantioselectivities up to 94% could be achieved

24b

J.-M. Becht and E. Bappert, unpublished results.

25 Ostermeier M. Brunner B. Korff C. Helmchen G. Eur. J. Org. Chem.  2003,  17:  3453