(η5-Triphenylindenyl)Ru(CO)2Cl: A New Ruthenium Catalyst for the Highly Efficient Racemization of Chiral 1-Phenylethanol at Room Temperature

Qiang Xi; Weicheng Zhang; Xumu Zhang

doi:10.1055/s-2006-939038

LETTER

(η⁵-Triphenylindenyl)Ru(CO)₂Cl: A New Ruthenium Catalyst for the Highly Efficient Racemization of Chiral 1-Phenylethanol at Room Temperature

Qiang Xi^a,b, Weicheng Zhang^a, Xumu Zhang*^a
^a Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA
^b School of Chemical Engineering and Pharmacy, Wuhan Institute of Chemical Technology, Wuhan, P. R. of China
Fax: +1(814)8653932; e-Mail: xumu@chem.psu.edu;

Abstract

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Abstract

A new ruthenium complex, (η⁵-triphenylindenyl)Ru(CO)₂Cl, was synthesized and successfully applied in the racemization of (S)-1-phenylethanol at room temperature. Instead of potassium tert-butoxide, stronger bases such as sodium hydride and n-butyllithium were used to activate the precatalyst. In situ NMR experiments revealed the presence of ruthenium hydride as a key intermediate in the catalytic cycle.

Key words

organometallic reagents - ruthenium - complexes - catalysis - racemization

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References

References and Notes

1 For a recent review on DKR, see: Pamies O. Bäckvall J.-E. Chem. Rev. 2003, 103: 3247 ; and references cited therein

2 Dinh PM. Howarth JA. Hudnott AR. Williams JMJ. Harris W. Tetrahedron Lett. 1996, 37: 7623

3 Menasche N. Shvo Y. Organometallics 1991, 10: 3885

4a Larsson ALE. Persson BA. Bäckvall J.-E. Angew. Chem., Int. Ed. Engl. 1997, 36: 1211

4b Persson BA. Larsson ALE. Le Ray M. Bäckvall J.-E. J. Am. Chem. Soc. 1999, 121: 1645

5a Choi JH. Kim YH. Nam SH. Shin ST. Kim M.-J. Park J. Angew. Chem. Int. Ed. 2002, 41: 2373

5b Choi JH. Choi YK. Kim YH. Park ES. Kim EJ. Kim M.-J. Park J. J. Org. Chem. 2004, 69: 1972

6a Martin-Matute B. Edin M. Bogar K. Bäckvall J.-E. Angew. Chem. Int. Ed. 2004, 43: 6535

6b Martin-Matute B. Edin M. Bogar K. Kaynak FB. Bäckvall J.-E. J. Am. Chem. Soc. 2005, 127: 8817

7a Hart-Davis AJ. White C. Mawby RJ. Inorg. Chim. Acta 1970, 4: 441

7b Rerek ME. Ji L.-N. Basolo F. J. Chem. Soc., Chem. Commun. 1983, 1208

7c Rerek ME. Basolo F. J. Am. Chem. Soc. 1984, 106: 5908

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1,2,3-Triphenyl-1 H -indene (5) To a freshly prepared PhMgBr solution (5.68 mmol in 20 mL THF) was added dropwise a solution of 2,3-diphenyl-indenone (3.54 mmol, 1.0 g) in 10 mL THF at 0 °C. Then the reaction mixture was stirred at 50 °C for 12 h. When it was cooled to r.t., LiAlH₄ (26.3 mmol, 1.0 g) was added in portions and then stirred for 6 h. The reaction mixture was quenched with sat. aq NH₄Cl solution, filtered, and extracted with CH₂Cl₂. The organic phase was separated, washed with brine and then dried with Na₂SO₄. After evaporation of the solvent, flash chromatography (EtOAc-hexane = 1:15) on silica gel afforded the product as white powder (0.81 g, 66%). ¹H NMR (300 MHz, CD₂Cl₂): δ = 5.18 (s, 1 H), 7.09-7.28 (m, 14 H), 7.44-7.45 (m, 5 H).¹³C NMR (75 MHz, CD₂Cl₂): δ = 58.19, 120.82, 124.14, 126.04, 127.05, 127.11, 127.28, 127.88, 128.21, 128.46, 129.93, 129.07, 129.73, 129.86, 135.89, 135.93, 140.30, 141.23, 145.35, 146.08, 148.70. HRMS: m/z calcd: 344.1565; found: 344.1570.

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(η ⁵ -1,2,3-Triphenylindenyl)Ru(CO) ₂ Cl (4)
Toward the THF (10 mL) solution of 6 (440 mg, 1.28 mmol) was added n-BuLi (0.46 mL, 1.32 mmol) at -78 °C. Then it was allowed to warm to r.t. in 10 min. Then, [Ru(CO)₃Cl₂]₂ (327 mg, 0.64 mmol) was added into the solution, and stirred for 10 min. Afterwards, silica gel was added and the solvent was removed by evaporation. Flash chromatography (EtOAc-hexane, 1:10) on silica gel afforded the product as an air-stable yellow powder (150 mg, 44%). ¹H NMR (300 MHz, CD₂Cl₂): δ = 7.00-7.11 (m, 4 H), 7.17-7.20 (m, 1 H), 7.38-7.42 (m, 6 H), 7.49-7.57 (m, 6 H), 7.61-7.65 (m, 2 H). ¹³C NMR (75 MHz, CD₂Cl₂): δ = 92.01, 111.73, 116.78, 124.57, 128.16, 128.96, 129.03, 129.28, 130.94, 131.00, 131.71, 132.35, 197.19. HRMS [M - Cl^- + MeOH]: m/z calcd: 533.0691; found: 533.0690. FT-IR (KBr): 2042, 1986 cm^-1.

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Racemization Experiments
The racemization of (S)-7 was carried out in a 5-mL Schlenk tube at r.t. The two methods to activate 4 are described in this paper. When the racemization started, the reaction mixture was sampled regularly to determine the ee (chiral GC, β-dex 325 capillary column, 120 °C isothermal, t _R = 8.31 min, t _S = 8.50 min).

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Previously, only ¹³C NMR was used to study the formation of Ru tert-butoxide. Moreover, the long induction time observed in the attempt to racemize (S)-7 with ruthenium hydride and corresponding ketone gives no convincing evidence in support of the existence of ruthenium hydride during racemization (see ref. 6).

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Previous crossover experiments with 3 indicate that the ketone generated from β-hydride elimination stays within the coordination sphere of the Ru atom during racemization (see ref. 6b).