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Issue 11, 2000
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Tricarbonylrhenium(I) halide complexes of 2-[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L1) and 2,6-bis[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L2): structure and solution stereodynamics

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Abstract

The non-racemic chiral ligands 2-[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L1) and 2,6-bis[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L2) reacted with the [ReX(CO)5] compounds (X = Cl, Br, or I) to form fac-[ReX(CO)3(L1)] and fac-[ReX(CO)3(L2)], respectively. The ligands co-ordinate to the metal moiety in a bidentate fashion via the N donor of the pyridine ring and one of the acetal O atoms. The bonding modes are confirmed by the molecular structures of [ReI(CO)3(L1)] and [ReBr(CO)3(L2)]. There are four possible diastereoisomers, depending on the configuration at the metal and the acetal carbon atom. In the solid state [ReI(CO)3(L1)] and [ReBr(CO)3(L2)] exist exclusively as the RR and SR diastereoisomers, respectively. In solutions of the complexes of L1 all four diastereoisomers are observed in varying concentrations, with the SR species being dominant. Only two of the four possible species are observed in solutions of the complexes of L2, with the SR diastereoisomer being present in ca. 96% diastereomeric excess over the RR diastereoisomer. Above ambient temperatures both series of complexes are fluxional. A reversible flip of the acetal ring in the complexes [ReX(CO)3(L1)] leads to the pair-wise exchange of diastereoisomers as a result of the formal inversion of configuration at the acetal carbon atom; the free energies of activation are in the range 78–82 kJ mol−1. The ring flip process appears to occur via a pseudo seven-co-ordinate transition state, in which both O atoms of the acetal ring are loosely bound to the metal. The pendant and co-ordinated acetal rings in the complexes of L2 are exchanged via the tick-tock mechanism; the free energies of activation are in the range 75–79 kJ mol−1.

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Article information


Submitted
14 Feb 2000
Accepted
04 Apr 2000
First published
11 May 2000

J. Chem. Soc., Dalton Trans., 2000, 1769-1776
Article type
Paper

Tricarbonylrhenium(I) halide complexes of 2-[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L1) and 2,6-bis[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L2): structure and solution stereodynamics

P. J. Heard, P. M. King and D. A. Tocher, J. Chem. Soc., Dalton Trans., 2000, 1769
DOI: 10.1039/B001206H

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