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Synlett 2007(9): 1365-1370  
DOI: 10.1055/s-2007-980371
LETTER
© Georg Thieme Verlag Stuttgart · New York

Synthesis of Novel Chiral Phosphine-Olefin Complexes and Their Evaluation as Ligands in the Rhodium-Catalyzed Asymmetric 1,4-Addition

René T. Stemmler, Carsten Bolm*
Institut für Organische Chemie der RWTH Aachen, Landoltweg 1, 52074 Aachen, Germany
Fax: +49(241)8092391; e-Mail: carsten.bolm@oc.rwth-aachen.de;
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Publication History

Received 2 March 2007
Publication Date:
23 May 2007 (online)

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Abstract

Novel chiral phosphine-olefin complexes containing ferrocene and cyrhetrene backbones have been synthesized and evaluated as ligands in the rhodium-catalyzed 1,4-addition of ­phenylboronic acid to cyclohexenone. The highest efficiency in this series was observed with planar-chiral ligands 5a and 20, which ­furnished the corresponding product with up to 71% ee.

Key words

asymmetric catalysis - conjugate addition - cyrhetrene - ferrocene - rhodium

14

Typical Procedure for the Synthesis of Compounds 14 and 15
Isopropylamine (418 µL, 302 mg, 2.98 mmol, 4.0 equiv) was dissolved in THF (2 mL) in a Schlenk tube. A solution of EtMgBr (1 M in THF, 2.98 mL, 2.98 mmol, 4.0 equiv) was added and the mixture was heated to reflux for 3 h. The reaction mixture was cooled to 0 °C and a solution of diphenylphosphinylferrocene (11, 575 mg, 1.49 mmol, 2.0 equiv) in THF (4 mL) was added dropwise. After stirring for 4 h at r.t., a solution of triflate 13a (172 mg, 0.745 mmol, 1.0 equiv) in THF (4 mL) and [Pd(dppf)Cl2] (27 mg, 37 µmol, 5 mol%) were added and the mixture was heated to 60 °C for 15 h. After cooling to r.t. the mixture was diluted with Et2O (10 mL) and H2O (10 mL) and the aqueous phase was extracted with Et2O (3 × 10 mL). The combined organic layers were dried over MgSO4 and concentrated. The crude product was purified by column chromatography (silica gel, pentane-EtOAc 1:1 → 1:2 → EtOAc) to give the 1,2-disubstituted ferrocene rac-14a as an orange oil (101 mg, 29%). Additionally, unreacted diphenylphosphinylferrocene was isolated (11, 446 mg, 77% based on employed ferrocene 11).
Compound 14a: orange oil. 1H NMR (400 MHz, CDCl3): δ = 1.34-1.41 (m, 2 H), 1.71-1.95 (m, 4 H), 1.97-2.07 (m, 1 H), 2.27-2.36 (m, 1 H), 3.77 (mc, 1 H, CpH), 4.28 (mc,1H, CpH), 4.35 (s, 5 H, Cp), 4.47 (mc, 1 H, CpH), 6.08 (m, 1 H, CH), 7.35-7.52 (m, 6 H, ArH), 7.57-7.65 (m, 2 H, ArH), 7.73-7.78 (m, 2 H, ArH). 13C NMR (75 MHz, CDCl3): δ = 21.8 (CH2), 22.8 (CH2), 25.5 (CH2), 30.1 (CH2), 69.2 (d, J = 11.5 Hz, CpCH), 70.3 (5 C, Cp), 71.3 (d, J = 9.9 Hz, CpCH), 71.7 (d, J = 112.8 Hz, CpC), 75.1 (d, J = 15.8 Hz, CpCH), 95.0 (d, J = 10.5 Hz, CpC), 127.9 (d, J = 12.0 Hz, 4 C, ArCH), 128.5 (CH), 131.0 (2 C, ArCH), 131.5 (d, J = 9.8 Hz, 4 C, ArCH), 131.7 (C), 133.8 (d, J = 15.2 Hz, ArC), 135.2 (d, J = 15.5 Hz, ArC). 31P NMR (162 MHz, CDCl3): δ = 28.78 (s). IR (CHCl3): ν = 3434, 2931, 2861, 2361, 2335, 1700, 1651, 1438, 1189, 1160, 1113, 823, 754, 699, 666, 588, 529 cm-1. MS (EI, 70 eV): m/z (%) = 466 (100) [M+], 398 (16), 273 (21), 265 (18) [M - Ph2PO]+. HRMS: m/z calcd for C28H27FeOP: 466.1149; found: 466.1149.
Compound 14b: orange oil; [α]D 22 +22.2 (c 0.30, CHCl3). 1H NMR (300 MHz, CDCl3): δ = 0.18 (d, J = 6.9 Hz, 3 H, CH3), 0.49 (d, J = 6.8 Hz, 3 H, CH3), 0.70-1.18 (m, 2 H), 1.08 (d, J = 7.1 Hz, 3 H, CH3), 1.42-1.17 (m, 2 H), 1.54-1.67 (m, 1 H), 1.79-1.94 (m, 1 H), 1.97-2.13 (m, 1 H), 4.13-4.20 (m, 6 H, CpH), 4.30-4.36 (m, 1 H, CpH), 4.36-4.41 (m, 1 H, CpH), 6.32 (s, 1 H, CH), 7.34-7.59 (m, 6 H, ArH), 7.76 (ddd, J = 11.7, 7.8, 1.3 Hz, 2 H, ArH), 7.85 (ddd, J = 11.6, 7.5, 1.7 Hz, 2 H, ArH). 13C NMR (75 MHz, CDCl3): δ = 15.6 (CH3), 19.8 (CH3), 21.2 (CH2), 22.1 (CH3), 28.1 (CH), 30.3 (CH2), 31.1 (CH), 43.0 (CH), 68.7 (d, J = 11.1 Hz, CpCH), 70.2 (d, J = 113.7 Hz, CpC), 70.7 (5 C, Cp), 72.9 (d, J = 14.9 Hz, CpCH), 75.5 (d, J = 10.4 Hz, CpCH), 97.7 (d, J = 10.8 Hz, CpC), 128.0 (d, J = 11.8 Hz, 2 C, ArCH), 128.1 (d, J = 11.9 Hz, 2 C, ArCH), 131.02 (d, J = 2.5 Hz, ArCH), 131.10 (d, J = 2.7 Hz, ArCH), 131.38 (d, J = 9.4 Hz, 2 C, ArCH), 131.47 (d, J = 9.2 Hz, 2 C, ArCH), 134.9 (d, J = 105.0 Hz, ArC), 135.0 (d, J = 104.9 Hz, ArC), 135.5 (C), 138.0 (CH). 31P NMR (121 MHz, CDCl3): δ = 26.48 (s). IR (CHCl3): ν = 2959, 2866, 2361, 2335, 1438, 1195, 1161, 1113, 822, 756, 700, 527 cm-1. MS (EI, 70 eV): m/z (%) = 522 (100) [M+], 479 (8), 462 (10), 412 (12), 322 (13), 321 (2) [M - Ph2PO]+. HRMS: m/z calcd for C32H35FeOP: 522.1775; found: 522.1775.
Compound 15b: orange oil; [α]D 22 +119 (c 0.13, CHCl3). 1H NMR (300 MHz, CDCl3): δ = 0.54 (d, J = 6.8 Hz, 3 H, CH3), 0.82 (d, J = 7.0 Hz, 3 H, CH3), 0.98 (d, J = 7.0 Hz, 3 H, CH3), 1.01-1.09 (m, 1 H), 1.29-1.45 (m, 1 H), 1.60 (dtd, J = 13.4, 6.7, 3.4 Hz, 1 H), 1.68-1.81 (m, 1 H), 1.83-1.96 (m, 1 H), 1.96-2.06 (m, 1 H), 2.07-2.18 (m, 1 H), 4.16 (m, 1 H, CpH), 4.19-4.35 (m, 7 H, CpH), 5.91 (s, 1 H, CH), 7.35-7.55 (m, 6 H, ArH), 7.60-7.76 (m, 4 H, ArH). 13C NMR (75 MHz, CDCl3): δ = 16.7 (CH3), 20.9 (CH3), 21.2 (CH2), 22.5 (CH3), 29.1 (CH), 29.7 (CH2), 30.3 (CH), 41.3 (CH), 68.0 (CpCH), 68.8 (CpCH), 69.2 (2 C, CpCH), 72.6 (d, J = 117.5 Hz, CpC), 73.0 (d, J = 12.8 Hz, CpCH), 73.8 (d, J = 12.8 Hz, CpCH), 74.4 (d, J = 10.2 Hz, CpCH), 74.5 (d, J = 10.2 Hz, CpCH), 89.5 (CpC), 128.1 (d, J = 11.9 Hz, 4 C, ArCH), 131.33 (2 C, ArCH), superimposed by 131.34 (d, J = 9.4 Hz, 2 C, ArCH), 131.40 (d, J = 9.7 Hz, 2 C, ArCH), 133.4 (CH), 134.4 (d, J = 106.1 Hz, ArC), 134.6 (d, J = 106.1 Hz, ArC), 136.0 (C). 31P NMR (121 MHz, CDCl3): δ = 26.86 (s). IR (CHCl3): ν = 3853, 3745, 3058, 2956, 2867, 2361, 1558, 1439, 1200, 1169, 1117, 1030, 827, 752, 722, 700, 668, 568, 533 cm-1. MS (EI, 70 eV): m/z (%) = 522 (100) [M+], 479 (8), 321 (55) [M - Ph2PO]+, 278 (9), 197 (8), 141 (7). HRMS: m/z calcd for C32H35FeOP: 522.1775; found: 522.1774.
Compound 15c: orange oil; [α]D 22 -26.7 (c 0.33, CHCl3). 1H NMR (300 MHz, CDCl3): δ = 0.75 (s, 3 H, CH3), 0.79 (s, 3 H, CH3), 0.84-0.98 (m, 1 H), 1.01-1.08 (m, 1 H), 1.10 (s, 3 H, CH3), 1.51 (ddd, J = 11.9, 8.8, 3.4 Hz, 1 H), 1.83 (tdd, J = 12.3, 8.8, 3.6 Hz, 1 H), 2.23 (t, J = 3.5 Hz, 1 H), 4.25-4.34 (m, 6 H, CpH), 4.39-4.43 (m, 1 H, CpH), 5.94 (d, J = 3.3 Hz, 1 H), 7.36-7.60 (m, 6 H, ArH), 7.62-7.76 (m, 4 H, ArH). 13C NMR (75 MHz, CDCl3): δ = 13.1 (CH3), 19.6 (CH3), 19.9 (CH3), 25.6 (CH2), 31.9 (CH2), 51.5 (CH), 54.9 (C), 56.7 (C), 67.0 (CpCH), 68.1 (CpCH), 69.7 (CpCH), 69.9 (CpCH), 72.6 (d, J = 108.5 Hz, CpC), 72.8 (d, J = 12.9 Hz, CpCH), 73.0 (d, J = 12.9 Hz, CpCH), 73.3 (d, J = 3.0 Hz, CpCH), 73.4 (d, J = 2.8 Hz, CpCH), 84.4 (C), 128.1 (d, J = 12.0 Hz, 4 C, ArCH), 131.0 (CH), 131.39 (d, J = 9.7 Hz, 4 C, ArCH), superimposed by 131.40 (d, J = 2.7 Hz, 2 C, ArCH), 134.47 (d, J = 106.2, ArC), 134.52 (d, J = 106.0 Hz, ArC), 144.3 (C). 31P NMR (121 MHz, CDCl3): δ = 28.70 (s). IR (KBr): ν = 3436, 3075, 2948, 2870, 1619, 1437, 1379, 1196, 1161, 1111, 1069, 1002, 818, 722, 698, 561, 496 cm-1. MS (EI, 70 eV): m/z (%) = 520 (100) [M+], 492 (15)
[M - C2H4 +], 427 (7), 411 (7), 319 (4) [M - Ph2PO]+. HRMS: m/z calcd for C32H33FeOP: 520.1618; found: 520.1617.

15

Typical Procedure for the Reduction of Phosphine Oxides to 16 and 17
In a Schlenk tube, Et3N (0.12 mL, 86 mg, 0.85 mmol, 7.5 equiv) and HSiCl3 (57 µL, 77 mg, 0.57 mmol, 5.0 equiv) were added to a solution of phosphine oxide rac-14a (53 mg, 0.12 mmol, 1.0 equiv) in toluene (2 mL), and the reaction mixture was heated to 90 °C for 40 h. After cooling, the reaction was quenched with a sat. aq solution of NaHCO3 (10 mL). The aqueous layer was extracted with EtOAc (3 × 10 mL) and the combined organic layers were dried over MgSO4 and concentrated. The crude product was purified by column chromatography (silica gel, pentane-EtOAc 4:1 → 1:1) to furnish the phosphine rac-16a as orange oil (27 mg, 53%) in addition to unreacted starting material rac-14a (14 mg, 26%).
Compound 16a: orange oil. 1H NMR (400 MHz, CDCl3): δ = 1.48-1.77 (m, 5 H), 1.85-1.96 (m, 1 H), 1.97-2.09 (m, 1 H), 2.29-2.39 (m, 1 H), 2.47-2.57 (m, 1 H), 3.64-3.67 (m, 1 H), 4.05 (s, 5 H, CpH), 4.25 (t, J = 2.5 Hz, 1 H, CpH), 4.45-4.49 (m, 1 H, CpH), 5.80-5.85 (m, 1 H, CH), 7.16-7.27 (m, 5 H, ArH), 7.34-7.40 (m, 3 H, ArH), 7.54 (m, 2 H, ArH). 13C NMR (100 MHz, CDCl3): δ = 22.4 (CH2), 23.2 (CH2), 25.8 (CH2), 29.8 (d, J = 4.7 Hz, CH2), 69.0 (CpCH), 69.7 (d, J = 3.5 Hz, CpCH), 69.8 (5 C, Cp), 70.7 (d, J = 18.4 Hz, CpC), 71.9 (d, J = 3.8 Hz, CpCH), 73.1 (d, J = 9.7 Hz, C), 95.3 (d, J = 20.4 Hz, CpC), 125.5 (d, J = 9.6 Hz, ArCH), 127.6 (s, ArCH), 127.9 (d, J = 7.3 Hz, 2 C, ArCH), 128.0 (d, J = 5.7 Hz, 2 C, ArCH), 128.9 (CH), 132.2 (d, J = 17.9 Hz, 2 C, ArCH), 135.3 (d, J = 21.4 Hz, 2 C, ArCH), 138.0 (d, J = 11.1 Hz, ArC), 140.1 (d, J = 10.7 Hz, ArC). 31P NMR (162 MHz, CDCl3): δ = -19.30 (s). IR (CHCl3): ν = 3746, 3673, 2929, 2855, 2362, 2336, 1699, 1652, 1559, 1541, 1458, 1217, 820, 758, 698, 668 cm-1. MS (EI, 70 eV):
m/z (%) = 450 (100) [M+], 421 (14), 385 (13) [M - Cp]+, 226 (7). HRMS: m/z calcd for C28H27FeP: 450.1200; found: 450.1200.
Compound 16b: orange oil. [α]D 22 -137 (c 0.40, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 0.21 (d, J = 6.89 Hz, 3 H, CH3), 0.53 (d, J = 6.77 Hz, 3 H, CH3), 1.04-1.22 (m, 2 H), 1.15 (d, J = 7.00 Hz, 3 H, CH3), 1.33-1.49 (m, 2 H), 1.66-1.77 (m, 1 H), 1.84 (br s, 1 H), 2.06-2.20 (m, 1 H), 3.93 (s, 1 H, CpH), 3.96 (s, 5 H, Cp), 4.27 (s, 1 H, CpH), 4.31 (s, 1 H, CpH), 6.36 (s, 1 H, CH), 7.22-7.25 (m, 3 H, ArH), 7.28-7.40 (m, 5 H, ArH), 7.58-7.62 (m, 2 H, ArH). 13C NMR (100 MHz, CDCl3): δ = 15.7 (CH3), 20.0 (CH3), 21.5 (CH2), 22.5 (CH3), 28.0 (CH), 31.1 (CH2), 31.3 (CH), 44.7 (d, J = 7.2 Hz, CH), 68.0 (CpCH), 68.1 (d, J = 6.6 Hz, CpC), 69.9 (5 C, Cp), 70.7 (d, J = 4.1 Hz, CpCH), 73.6 (d, J = 4.3 Hz, CpCH), 73.8 (d, J = 12.9 Hz, CpC), 127.7 (d, J = 6.0 Hz, 2 C, ArCH), 127.9 (d, J = 6.2 Hz, 2 C, ArCH), 128.7 (2 C, ArCH), 132.6 (d, J = 19.2 Hz, 2 C, ArCH), 135.0 (d, J = 21.8 Hz, 2 C, ArCH), 136.0 (C), 137.9 (CH), 138.4 (d, J = 10.8 Hz, ArC), 140.1 (d, J = 10.5 Hz, ArC). 31P NMR (162 MHz, CDCl3): δ = -24.24 (s). IR (CHCl3): ν = 3746, 3673, 2361, 2336, 1699, 1652, 1559, 1507, 1457, 1216, 757, 669 cm-1. MS (EI, 70 eV): m/z (%) = 506 (36)[M+], 463 (100) [M - i-Pr]+. HRMS: m/z calcd for C32H35FeP: 506.1826; found: 506.1825.
Compound 17b: orange oil; [α]D 22 +290 (c 0.25, CHCl3). 1H NMR (300 MHz, CDCl3): δ = 0.60 (d, J = 6.8 Hz, 3 H, CH3), 0.87 (d, J = 7.0 Hz, 3 H, CH3), 0.95-1.17 (m, 1 H), 1.02 (d, J = 7.0 Hz, 3 H, CH3), 1.34-1.48 (m, 1 H), 1.66 (dtd, J = 13.4, 6.8, 3.3 Hz, 1 H), 1.72-1.85 (m, 1 H), 1.89-2.12 (m, 2 H), 2.14-2.25 (m, 1 H), 3.89-3.97 (m, 2 H, CpH), 4.02 (t, J = 1.9 Hz, 2 H, CpH), 4.11-4.15 (m, 1 H, CpH), 4.24 (t, J = 1.8 Hz, 2 H, CpH), 4.25-4.29 (m, 1 H, CpH), 5.95 (br s, 1 H, CH), 7.28-7.45 (m, 10 H, ArH). 13C NMR (75 MHz, CDCl3): δ = 16.8 (CH3), 20.9 (CH3), 21.3 (CH2), 22.6 (CH3), 29.2 (CH), 29.8 (CH2), 30.3 (CH), 41.4 (CH), 67.6 (CpCH), 68.5 (CpCH), 68.6 (2 C, CpCH), 73.3 (d, J = 3.9 Hz, CpCH), 73.4 (d, J = 4.0 Hz, CpCH), 74.2 (d, J = 14.7 Hz, CpCH), 74.4 (d, J = 14.4 Hz, CpCH), 75.8 (d, J = 6.0 Hz, CpC), 88.8 (CpC), 128.0 (d, J = 5.4 Hz, 4 C, ArCH), 128.4 (2 C, ArCH), 133.0 (CH), 133.4 (d, J = 19.3 Hz, 2 C, ArCH), 133.5 (d, J = 19.4 Hz, 2 C, ArCH), 136.4 (C), 139.3 (d, J = 10.2 Hz, ArC), 139.4 (d, J = 10.2 Hz, ArC). 31P NMR (121 MHz, CDCl3): δ = -16.70 (s). IR (CHCl3): ν = 3746, 2956, 2865, 2362, 2336, 1434, 1217, 1161, 1027, 825, 756, 698, 496 cm-1. MS (EI, 70 eV): m/z (%) = 506 (100) [M+], 463
[M - i-Pr]+, 278 (24), 276 (11), 171 (7). HRMS: m/z calcd for C32H35FeP: 506.1826; found: 506.1826.
Compound 17c: orange oil; [α]D 22 -23.6 (c 0.92, CHCl3). 1H NMR (300 MHz, CDCl3): δ = 0.77 (s, 3 H, CH3), 0.83 (s, 3 H, CH3), 0.89-1.01 (m, 1 H), 1.05-1.17 (m, 1 H), 1.15 (s, 3 H, CH3), 1.44-1.59 (m, 2 H), 1.76-1.91 (m, 2 H), 2.24 (t, J = 3.4 Hz, 1 H), 3.98-4.08 (m, 4 H, CpH), 4.16-4.21 (m, 1 H, CpH), 4.21-4.25 (m, 1 H, CpH), 4.29-4.35 (m, 2 H, CpH), 5.92 (d, J = 3.3 Hz, 1 H, CH), 7.30-7.37 (m, 10 H, ArH). 13C NMR (75 MHz, CDCl3): δ = 13.2 (CH3), 19.6 (CH3), 20.0 (CH3), 25.7 (CH2), 31.9 (CH2), 51.5 (CH), 54.9 (C), 66.6 (CpCH), 67.6 (CpCH), 69.1 (CpCH), 69.3 (CpCH), 72.4 (d, J = 3.6 Hz, 2 C, CpCH), 73.5 (d, J = 10.7 Hz, CpCH), 73.7 (d, J = 10.2 Hz, CpCH), 75.8 (d, J = 7.1 Hz, CpCH), 83.6 (CpC), 128.1 (d, J = 6.7 Hz, 4 C, ArCH), 128.4 (2 C, ArCH), 130.2 (CH), 133.5 (d, J = 19.3 Hz, 4 C, ArCH), 139.2 (d, J = 10.9 Hz, 2 C, ArC), 144.8 (C). 31P NMR (121 MHz, CDCl3): δ = -16.84 (s). IR (CHCl3): ν = 3054, 2952, 2871, 1475, 1435, 1384, 1194, 1162, 1100, 1068, 1028, 821, 749, 698, 499 cm-1. MS (EI, 70 eV): m/z (%) = 504 (100) [M+], 489 (16), 476 [M - C2H4]+, 461 (16), 435 (61), 276 (22). HRMS: m/z calcd for C32H33FeP: 504.1669; found: 504.1668.

18

Compound 20: In a Schlenk tube, 1-chloroethyl chloroformate (18 µL, 23 mg, 0.16 mmol, 2.1 equiv) was added to a solution of cyrhetrene 19 (46 mg, 78 µmol, 1.0 equiv) in THF (2.5 mL) at 0 °C and the reaction was stirred for 2 h at 0 °C. The solvent was removed in vacuo, and the residue was dissolved in Et2O (10 mL), passed through a short plug of neutral alumina and concentrated. The residue was dissolved in THF (1.5 mL), KOt-Bu (53 mg, 0.47 mmol, 6.0 equiv) was added and the reaction mixture was stirred at r.t. for 2 h. The reaction mixture was diluted with Et2O (8 mL), filtered and concentrated. The crude product was purified by column chromatography (silica gel, pentane-Et2O = 3:1) to furnish the phosphine-olefin 20 (20 mg, 46%) as a pale red solid; mp 78-80 °C; [α]D 21 -17.3 (c 0.39, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 4.91 (dd, J = 2.7, 1.8 Hz, 1 H, CpH), 5.14 (d, J = 11.0 Hz, 1 H, CH cis ), 5.26 (t, J = 2.8 Hz, 1 H, CpH), 5.51 (d, J = 17.5 Hz, 1 H, CH trans ), 5.75-5.80 (m, 1 H, CpH), 6.60 (ddd, J = 17.4, 10.9, 1.8 Hz, 1 H, CH), 7.24-7.32 (m, 2 H, ArH), 7.32-7.40 (m, 6 H, ArH), 7.40-7.47 (m, 2 H, ArH). 13C NMR (100 MHz, CDCl3): δ = 82.8 (CpCH), 83.6 (d, J = 1.6 Hz, CpCH), 91.1 (d, J = 4.2 Hz, CpCH), 95.5 (d, J = 22.2 Hz, CpC), 111.5 (d, J = 19.8 Hz, CpC), 117.7 (d, J = 2.7 Hz, =CH2), 126.8 (d, J = 9.6 Hz, =CH), 128.5 (d, J = 6.1 Hz, 2 C, ArCH), 128.6 (d, J = 6.8 Hz, ArCH), 129.6 (2 C, ArCH), 132.2 (d, J = 18.7 Hz, 2 C, ArCH), 134.6 (d, J = 20.8 Hz, 2 C, ArCH), 137.5 (d, J = 10.7 Hz, 2 C, ArC), 193.3 (3 C, CO). 31P NMR (121 MHz, CDCl3): δ = -23.48 (s). IR (CHCl3): ν = 2926, 2857, 2023, 1927, 1438, 912, 737, 698, 603, 505 cm-1. MS (EI, 70 eV): m/z (%) = 546 (47) [M+], 544 (26) [M - 2]+, 518 (57) [M - CO]+, 516 (33) [M - 2 - CO]+, 462 (100) [M - 3CO]+, 460 (75) [M - 2 - 3CO]+, 384 (32), 382 (34), 356 (19), 230 (35). Anal. Calcd for C22H16O3PRe: C, 48.44; H, 2.96. Found: C, 48.68; H, 3.15.

19

PPEF (21) was prepared by hydrogenation of phosphine-olefin 20 (50 bar H2, benzene). Alternatively, this compound can be prepared by a procedure reported by Hayashi, see ref. 10.