Dichlorooxo(2,2,2-trifluoroethanolato-0) vanadium

 · (CF3CH2O)VOCl2  · (MW 236.84)

(reagent used as a versatile one-electron oxidant for effecting coupling reactions1)

Physical Data: bp 135-138 °C.

Solubility: soluble in most common noncoordinating organic solvents (CH2Cl2, toluene, hexane, etc.). Due to its hydrolytic instability and Lewis acidity, protic solvents and coordinating solvents are unsuitable.

Preparative Methods: readily prepared in 82% isolated yield by the treatment of VOCl3 with CF3CH2OH in hexane.1 A 250 mL round-bottomed flask equipped with a magnetic stirring bar, reflux condenser and septum was flamed under a stream of nitrogen. The flask was then charged with hexane (120 mL) and vanadium oxychloride (9.42 mL, 0.1 mol). The solution was stirred at 23 °C and 2,2,2-trifluoroethanol (7.28 mL, 0.1 mol) was added dropwise via syringe with a constant gentle flow of nitrogen being passed through the apparatus to remove the gaseous HCl. Upon completion of the addition, the reaction mixture was heated at reflux for 60 min and then the solvent was removed by distillation at atmospheric pressure. The residue was then distilled at atmospheric pressure to provide (CF3CH2O)VOCl2 (19.4 g, 82%, bp 135-138 °C) as a light yellow liquid. 1H NMR (C6D6, 300 mHz) d 4.4 (br s); 13C NMR (C6D6, 75 mHz) d 127.7 (d, J = 925 Hz), 84.1-83.2 (m); 51V NMR (C6D6, 250 mHz) d -281.6 (s) from 51VOCl3.

Purification: fractional distillation at atmospheric pressure.

Handling, Storage, and Precautions: the pure substance is extremely sensitive to atmospheric moisture and, although not sensitive to oxygen, should be stored in tightly sealed bottles under dry nitrogen. All samples are corrosive and are vigorously hydrolyzed to HCl, V2O5 and trifluoroethanol.

Oxidative Methylenecyclopentannulations

The new vanadium(V) ester, (CF3CH2O)VOCl2 1a,1 which can be economically prepared on large scale in 82% yield by treatment of VOCl3 with 2,2,2-trifluoroethanol (1.0 equiv), promotes remarkably efficient cyclizations of representative 3-[2-(trimethylsilylmethyl)-2-propenyl] substituted silyl enol ethers 5 in < 30 min at -78 °C (eqs 1 and 2).

General Procedure for Oxovanadium(V) Induced Cyclizations

Preparation of (2H)-hexahydro-5-methylene-1-pentalenone (6a). A 100 mL round-bottomed flask equipped with a magnetic stirring bar and septum was flame dried under a stream of argon, charged with dichloromethane (40 mL), (CF3CH2O)VOCl2 (1.95 mL, 12 mmol) and cooled to -78 °C. A solution of silyl enol ether, 5a (1.69 g, 6.00 mmol) in CH2Cl2 (5 mL) was then added via syringe pump over 30 min. The reaction mixture was stirred for a further 10 min at -75 °C and poured into a separatory funnel containing aqueous HCl (1.5 M, 5 mL) and diethyl ether (30 mL). The layers were mixed, separated and the aqueous layer was extracted with ether (2 × 20 mL). The combined organic extracts were washed with saturated aqueous sodium bicarbonate (10 mL), brine (10 mL), dried (MgSO4), filtered and concentrated in vacuo. The oily residue was purified by chromatography on silica gel (2.5% Et2O/pentane with 0.01% Et3N for elution) to give pentalenone (6a) (660 mg, 82%) as a colorless oil. Spectral data were consistent with those reported in the literature.2

The enhanced reactivity of 1a relative to (EtO)VOCl2 (1b),3 previously regarded as the reagent of choice for the oxidative coupling of silyl enol ethers with allylsilanes,3b is consistent with an increase in the electrophilicity of the active vanadium center present in this ester. The 3-[2-(trimethylsilylmethyl)-2-propenyl] substituted silyl enol ethers 5 used in this study were readily prepared by the silylative 1,4-addition of 2-(trimethylsilyl)-2-propenyl copper 44,5 to the enone of interest. It should be noted that the sensitive silyl enol ethers 5 are typically subjected to oxidative cyclization with 1a without rigorous pre-purification (2). The generality of this new reagent for effecting oxidative methylenecyclopentannulation was demonstrated for a range of a-unsaturated carbonyl compounds. Some notable advantages of this procedure are as follows: Enones bearing a substituent (methyl) at the 2-position are viable substrates for cyclopentenannulation, in contrast with the previously reported procedure involving Pd(0) catalysis.2 When applied to carvone (3d) annulation proceeds with excellent stereoselectivity as a consequence of clean trans 1,4-addition of the organocopper reagent. The cyclic lactone coumarin (3e) can be readily modified by this procedure. As is evident from the examples assembled in 1, the use of 1a for oxidative carbon-carbon bond formation constitutes one of the most efficient methods for methylenecyclopentannulation.

As an example of an intramolecular cyclization involving a simple alkene terminator, exposure of 7 to 1a under the usual set of reaction conditions led to a smooth 5-exo-trig cyclization resulting in the production of the bicyclo[3.3.0]octanones 8a and 8b (8a/8b 8:1) in 65% yield (3).6

Oxidative Coupling of Silyl Enol Ethers to 1,4-diones

The use of one-electron oxidants for the synthesis of unsymmetrical 1,4-diones from equimolar quantities of the corresponding enolate derivatives has remained an endeavor in which only modest success has been achieved. Among the various reagents that have been utilized for the oxidative coupling of silyl enol ethers, (EtO)VOCl2 (1b), would appear the most effective, despite several noteworthy limitations.3a The diminished ability of 1b to promote unsymmetrical couplings involving moderately reactive partners (e.g. 9a), even when used in excess (9a/10 2:1), constitutes a significant preparative deficiency. Accordingly, (CF3CH2O)VOCl2 1a, was examined for efficacy in a short series of oxidative couplings that were previously reported as the worst-case scenarios for 1b (4). A summary of this comparative study appears in 2.

As is evident from the results presented above, 1a is a superb reagent for the unsymmetrical oxidative coupling of silyl enol ethers and is capable of effecting the union of equimolar quantities of reactant molecules at -78 °C in < 30 min7

1. Ryter, K.; Livinghouse, T., J. Am. Chem. Soc. 1998, 120, 2658.
2. Trost, B. M.; Chan, D. M. T., J. Am. Chem. Soc. 1983, 105, 2315 and references therein.
3. (a) Fujii, T.; Hirao, T.; Ohshiro, Y., Tetrahedron Lett. 1992, 33, 5823. (b) Hirao, T.; Fujii, T.; Ohshiro, Y., Tetrahedron 1994, 50, 10207.
4. Ryter, K.; Livinghouse, T., J. Org. Chem. 1997, 62, 4842.
5. Johnson, C. R.; Marren, T. J., Tetrahedron Lett. 1987, 28, 27.
6. Sha, C.-K.; Jean, T.-S.; Wang, D.-C., Tetrahedron Lett. 1990, 31, 3745.
7. Hirao, T., Chem. Rev. 1997, 97, 2707.

Kendal Ryter & Tom Livinghouse

Montana State University, Bozeman, Montana, USA

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