Stereospecific Syntheses
and Structures of Planar Chiral Bidentate η<sup>5</sup>:κ<i>S</i><b>-</b>Indenyl<b>-</b>Sulfanyl and <b>-</b>Sulfinyl Complexes of Rhodium(III)
- Publication date
- Publisher
Abstract
Axially chiral <i>rac</i>-1-(2-methyl-1<i>H</i>-inden-3-yl)-2-(methylsulfanyl)naphthalene (<i>rac</i>-<b>3</b>) was synthesized from methyl 2-(methylsulfanyl)-1-naphthoate
through reaction with the di-Grignard reagent derived from 1-bromo-2-(2-bromopropyl)benzene,
followed by acid-catalyzed dehydration of the intermediate indanol.
Oxidation of <i>rac</i>-<b>3</b> with <i>m</i>-CPBA gave the diastereomeric sulfoxides (a<i>R</i>*,<i>R</i><sub>S</sub>*)-<b>5</b> and (a<i>R</i>*,<i>S</i><sub>S</sub>*)-<b>6</b>, with the relative configuration
of <b>5</b> established using single-crystal X-ray diffraction.
The dichloro[η<sup>5</sup>:κ<i>S</i>-indenyl-sulfanyl
and -sulfinyl]rhodium complexes <i>rac</i>-<b>4</b>, (p<i>R</i>*,<i>S</i><sub>S</sub>*)-<b>7</b>, and (p<i>R</i>*,<i>R</i><sub>S</sub>*)-<b>8</b> were synthesized through reaction of the ligands <i>rac</i>-<b>3</b>, (a<i>R</i>*,<i>R</i><sub>S</sub>*)-<b>5</b>, and (a<i>R</i>*,<i>S</i><sub>S</sub>*)-<b>6</b>, respectively, with rhodium
trichloride in 9:1 methanol/water solution heated under reflux. The
use of water as a cosolvent was found to be critical for obtaining
good yields in the complexation reactions. Solid-state structures
for the racemic rhodium complexes were determined through single-crystal
X-ray diffraction. The enantiomers of the ligands <b>3</b>, <b>5</b>, and <b>6</b> were obtained in high enantiopurity
through subjecting <i>rac</i>-<b>3</b> to a series
of Kagan asymmetric sulfoxidation, deoxygenation, and resulfoxidation
reactions. The enantiomeric relationship of the rhodium complexes
derived from the enantio-enriched ligands was confirmed by CD spectroscopy,
and the high enantiopurity of the complexes established by <sup>1</sup>H NMR analysis using the chiral shift reagent Eu(hfc)<sub>3</sub>. The absolute configurations of the nonracemic ligands and rhodium
complexes were established by a single-crystal X-ray diffraction determination
of the solid-state structure of (p<i>S</i>,<i>S</i><sub>S</sub>)-<b>8</b>, with the Flack parameter refining to
0.00(2)
