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Synlett 2006(6): 945-947  
DOI: 10.1055/s-2006-939038
LETTER
© Georg Thieme Verlag Stuttgart · New York

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

Qiang Xia,b, Weicheng Zhanga, 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;
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Publication History

Received 7 December 2005
Publication Date:
14 March 2006 (online)

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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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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).