Synthesis, structure, and interconversion of chiral rhenium oxygen- and sulfur-bound sulfoxide complexes of formula [(eta(5)-C5H5)Re(NO)(PPh3)(OS(Me)R)](+) X-; diastereoselective oxidations of coordinated methyl alkyl sulfides
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abstract
Reactions of the chiral chlorobenzene complex [(5-C5H5)Re(NO)(PPh3) (ClC6H5)]+BF4- (1+ BF4-) and alkyl methyl sulfoxides O=S(Me)R (R = a, Me; b, Et; c, i-Pr or d, t-Bu), at -15C gave the oxygen-bound sulfoxide complexes [(5-C5H5)Re(NO)(PPh)3 (O=S(Me)R)]+ BF4- (2a-2d+BF4-; 94-56%). Above 0C, 2a-2d+BF4- rearrange to sulfur-bound linkage isomers [(5-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 (5-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 [(5-C5H5)Re(NO)(PPh3) (S(Me)R)]+ X- and dimethyldioxirane react (acetone, -20C) to give 3a-3d+X-. Diastereoselectivities (3b-3d+X-) are fair to good. However, some overoxidation to Ph3P=O occurs, lowering yields.
