Synthesis, structure, and interconversion of chiral rhenium oxygen- and sulfur-bound sulfoxide complexes of formula [(η5-C5H5)Re(NO)(PPh3)(OS(Me)R)]+ X−; diastereoselective oxidations of coordinated methyl alkyl sulfides †

Abstract

Reactions of the chiral chlorobenzene complex [(η5-C5H5)Re(NO)(PPh3)(ClC6H5)]+BF4− (1+ BF4−) and alkyl methyl sulfoxides OS(Me)R (R = a, Me; b, Et; c, i-Pr or d, t-Bu), at −15 °C gave the oxygen-bound sulfoxide complexes [(η5-C5H5)Re(NO)(PPh3)(OS(Me)R)]+ BF4− (2a–2d+BF4−; 94–56%). Above 0 °C, 2a–2d+BF4− rearrange to sulfur-bound linkage isomers [(η5-C5H5)Re(NO)(PPh3)(S(O)(Me)R)]+BF4− (3a–3d+BF4−; 96–20%). The triflate salts 2c,2d+TfO− and 3b,3c+TfO− are analogously prepared from (η5-C5H5)Re(NO)(PPh3)(OTf). Complexes 2b–2d+X− and 3b–3d+X− can exist as two Re, S configurational diastereomers. Stereochemistry is assigned from reactions of (S)- or (R)-1+BF4− with enantiomerically enriched sulfoxides (R)-b–d, and a crystal structure of (SReRS,RReSS)-2d+TfO−·0.5CH2Cl2. Relative diastereomer stabilities, and the basis for the divergent kinetic and thermodynamic oxygen/sulfur binding selectivities, are analysed. The alkyl methyl sulfide complexes [(η5-C5H5)Re(NO)(PPh3)(S(Me)R)]+ X− and dimethyldioxirane react (acetone, −20 °C) to give 3a–3d+X−. Diastereoselectivities (3b–3d+X−) are fair to good. However, some overoxidation to Ph3PO occurs, lowering yields.