Elsevier

Tetrahedron: Asymmetry

Volume 10, Issue 2, 29 January 1999, Pages 347-354
Tetrahedron: Asymmetry

New planar chiral P,N-ligands containing tricarbonyl(arene)chromium for enantioselective asymmetric hydroboration of styrenes

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Abstract

Optically active (1,2-disubstituted arene) chromium tricarbonyl complexes 2A2D with a pyridyl and a phosphorus group in the two ortho benzylic positions have been stereoselectively synthesized from a commercially available (+)-(4,6-O-benzylidene)methyl-α-d-glucopyranoside. These chromium complexes have been used as chiral ligands in the preparation of rhodium catalysts for the hydroboration of styrene derivatives. High enantioselectivities were observed in the hydroboration of vinylarenes.

Introduction

We recently reported a new and easy method for the asymmetric synthesis of chiral benzaldehyde complexes modified by a sugar moiety.[1] Thus, starting from chiral (benzaldehyde) chromium tricarbonyl compound, optically active (1,2-disubstituted arene) chromium tricarbonyl complexes with a diamine and a phosphorus group in the two ortho benzylic positions have been stereoselectively synthesized and used as chiral P,N-ligands in the hydroboration of styrenes.[2] The new chiral ligands have been shown to afford moderate enantioselectivities (19–81%) at −15°C and to exhibit pronounced effects on the enantioselectivity depending on the electronic substituent on the styrene. The P,N-ligand represents a class of chiral auxiliaries that are easily tuned by steric and electronic modification.[3] As an extension of our recent work, we have prepared new planar chiral P,N-arene chromium compounds having chromium carbonyls, pyridine, and phosphorus and investigated their role as chiral ligands in the rhodium-catalyzed asymmetric hydroboration of styrenes. Herein we report the results of our investigation.

Section snippets

Synthesis of planar chiral (arene)Cr(CO)3 complexes

The planar chiral compounds 2A2D were synthesized from 1 (Eq. (1)). Compound 1 was previously prepared by us.[2]Treatment of 1 with 2-pyridine lithium at −78°C led to a diastereomeric mixture (de 31%) in 64% yield. The diastereomeric mixture was easily separated by flash column chromatography. The major (42% yield) diastereomer shown in Eq. (1)was named 2A and used to make compounds 2B2D. Compound 2B was prepared in 73% yield by deprotonation of 2A in THF followed by addition of MeI. In the

Conclusion

We have demonstrated that planar chiral P,N-ligands 2A2D can be synthesized from the chiral benzaldehyde chromium complex 1 and have been used as chiral P,N-ligands in the preparation of a rhodium catalyst for the hydroboration of styrene derivatives. Good enantioselectivities were observed.

Experimental

All solvents were purified by standard methods, and all synthetic procedures were performed under a nitrogen atmosphere. Reagent grade chemicals were used without further purification.

Elemental analyses were carried out at the Chemical Analytic Center, College of Engineering, Seoul National University or the Chemical Analytic Center. 1H NMR spectra were obtained with a Bruker 300 or a Bruker AMX-500 instrument. Infrared spectra were recorded on a Shimadzu IR-470 spectrometer. Compound 1 was

Acknowledgements

This work was supported by the Ministry of Education (BSRI 1998-015-D00165) and KOSEF through the Center for Molecular Catalysis.

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