Synthesis and characterization of {[(COD)Rh(bis-(2R,3R)-2,5-diethylphospholanobenzene)]+BARF−} for use in homogeneous catalysis in supercritical carbon dioxide

doi:10.1016/S0020-1693(01)00601-6

Inorganica Chimica Acta

Volume 325, Issues 1–2, 3 December 2001, Pages 45-50

https://doi.org/10.1016/S0020-1693(01)00601-6 Get rights and content

Abstract

Reaction of [(COD)₂Cl₂Rh] (COD: cyclo-octadiene) with sodium tetrakis((3,5-trifluoromethyl)phenyl)borate (NaBARF) in the presence of an excess of COD yields [(COD)₂Rh]⁺BARF⁻. The COD ligands are readily displaced by the bidentate ligand 1,2-bis((2R,5R)-2,5-diethylphosphalono)benzene (Et-DuPHOS) to form [(COD)Rh(Et-DuPHOS)]BARF, the structure of which has been determined by X-ray crystallography. BARF was selected as the counterion in order to achieve solubility in supercritical carbon dioxide for use in asymmetric hydrogenation and hydroformylation reactions. Density-functional theory calculations were used to study the intermediates in asymmetric hydroformylation of styrene. The energies of the two-enantiomer models differ by 11.3 kcal mol⁻¹.

1,2-Bis((2R,5R)-2,5-diethylphosphalono)benzene was co-ordinated to [Rh(I)(COD)₂]BARF. The resulting catalyst was characterized structurally. Density-functional theory calculations have been used to study potential intermediates in asymmetric hydroformylation of styrene.

Introduction

Chiral rhodium phospholane complexes have found important applications as catalysts in hydroformylation and hydrogenation reactions. Hydroformylation is one of the most versatile methods for the functionalization of CC bonds [1]. Numerous chiral phospholanes and phosphites have been synthesized to be used as ligands for transition-metal-catalyzed homogeneous asymmetric synthesis [2], [2](a), [2](b), [3], [4], [4](a), [4](b), [4](c), [5].

In recent years, there has been increasing interest in using supercritical carbon dioxide (scCO₂) as the reaction medium for organic synthesis [6], [6](a), [6](b), [6](c), [7], [7](a), [7](b), [7](c), [7](d), [7](e). Use of a supercritical reaction medium, in addition to being an environmentally benign solvent, has other advantages. Supercritical fluids have density tunable physicochemical properties affecting reaction rates and selectivities. The mass transfer characteristics are superior in comparison to liquid reaction media due to high diffusion coefficients and low viscosities. Finally, scCO₂ is inert to most reactions, it is non-toxic, non-flammable, readily available, inexpensive and has rather mild critical properties. Most homogeneous catalysts, however, are not soluble in scCO₂ without modification. It is well known that fluorine groups attached to ligands increase their solubility in scCO₂ [8]. Burk et al. [9] used Et-DuPHOS as a chiral bidentate ligand and prepared the [(COD)Rh(Et-DuPHOS)]⁺ (COD: cyclo-octadiene) complex, with the counterion being either trifluoromethyl sulfonate or tetrakis(3,5-bis(trifloromethyl)phenyl)borate (BARF). They reported that the solubility in scCO₂ of the complex with the BARF counterion at 40 °C and 5000 psia was 0.030 mM.

In this study, we report the synthesis and structural characterization of [(COD)Rh(Et-DuPHOS)]BARF. The synthesis follows a different synthetic route than that reported by Burk et al.

The BARF counterion renders the complex soluble in scCO₂, which makes the catalysis suitable for homogeneous catalysis in scCO₂. The structure of [(COD)Rh(Et-DuPHOS)]BARF is determined by single crystal X-ray crystallography for the first time. Density-functional theory (DFT) calculations are used to assess the use of the complex as an enantioselective catalyst for hydroformylation reactions.

Section snippets

Results and discussion

The general method of synthesis is given in Scheme 1. Analytical and spectroscopic data can be found in detail in Section 4.

Density-functional theory calculations

Calculations were performed for the molecular models shown in Fig. 4. The calculations were at the DFT level with a B3LYP formalism, as described in Section 6. The purpose of these calculations is to investigate enantioselectivity from the use of 3 as a catalyst for the hydroformylation of styrene, a reaction we have been pursuing experimentally. Enantioselectivity in hydroformylation reactions results from hydride insertion to yield two possible enantiomers, if the reaction follows textbook

General

All synthetic procedures were carried out under a nitrogen atmosphere using standard Schlenk and glove box techniques, and using flame-dried glassware. Diethyl ether (Et₂O), hexane, and tetrahydrofuran (THF) were distilled from sodium benzophenone ketyl under nitrogen. Methylene chloride (CH₂Cl₂) was distilled from CaH₂. Preparation of the sodium salt of BARF was carried out (Scheme 1) as described by Brookhart et al. [16] and Nishida et al. [17] and the product was used without

X-ray structure determination of 3

Suitable crystals of 3 were obtained by slow evaporation of a methylene chloride/hexane (1:2) solution at 25 °C. The structure was determined using direct methods (shelxs 86 (Sheldrick, 1985)). Details of the crystal data, parameters for data collection, the solution and refinement of the structure are given in Table 2.

Computational details

All calculations were performed with the Gaussian 98 suite of programs [20] for the molecular models in Fig. 4. The calculations were at the DFT level [21] using the Becke three parameter [22] hybrid exchange functional and the Lee et al. [23], [23](a), [23](b) correlation functional, B3LYP. A Huzinaga/Dunning basis set [24] of a double-zeta quality was used for carbon and hydrogen atoms. A double-zeta basis set plus one polarization function on the P atoms was used, in order to properly

Supplementary material

A CIF file of the crystallographic data for the structure of 3.

Acknowledgements

Support from the Robert A. Welch Foundation and the Texas A&M Office of the Vice President for Research are gratefully acknowledged.

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Synthesis and characterization of {[(COD)Rh(bis-(2R,3R)-2,5-diethylphospholanobenzene)]+BARF−} for use in homogeneous catalysis in supercritical carbon dioxide

Abstract

Introduction

Section snippets

Results and discussion

Density-functional theory calculations

General

X-ray structure determination of 3

Computational details

Supplementary material

Acknowledgements

J. Am. Chem. Soc.

Organometallics

J. Am. Chem. Soc.

Acc. Chem. Res.

Chem. Rev.

Tetrahedron Lett.

J. Am. Chem. Soc.

Catal. Surv. Jpn.

Chem. Rev.

Chem. Rev.

Science

Angew. Chem., Int. Ed. Engl.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Synthesis and characterization of {[(COD)Rh(bis-(2R,3R)-2,5-diethylphospholanobenzene)]⁺BARF⁻} for use in homogeneous catalysis in supercritical carbon dioxide