Reactions of t-BuOK and dibenzyl, benzyl allyl, and allyl beta-keto sulfide complexes of the chiral rhenium Lewis acid [(eta(5)-C5H5)Re(NO)(PPh(3))](+): Highly diastereoselective or chemoselective [2,3]-sigmatropic rearrangements
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abstract
Reaction of the dibenzyl sulfide complex [(5-C5H5)Re(NO)(PPh3(S(CH 2Ph)2)]+TfO- (4a+TfQ-) and t-BuOK (THF, -80C) gives the thiolate (5-C5H5)Re(NO)(PPh 3)(SCH(o-C6H4CH3)Ph) (5a; 99%) as a 96:4 mixture of SR,RS/SS,RR Re:C diastereomers. This transformation involves initial deprotonation of a benzyl group to give an ylide, followed by [2,3]- and [1,3]-rearrangements. A crystal structure of (SR,RS)-5a establishes the configuration. Similar reactions of the benzyl allyl sulfide complexes [(5-C5H5)Re(NO)(PPh3)(S(CH 2CR=CH2)CH2-Ph)]+TfO- (R = H, CH3) give mainly the thiolates (SS,RR)-(5-C5H5)Re(NO)PPh 3(SCH(CH2-CR=CH2)Ph), derived from benzyl group deprotonation. Some thiolates derived from allyl group deprotonation also form. Similar reactions of the allyl -keto sulfide complexes [(5-C5R5)Re(NO)(PPh3)(S(CH 2CR=CR2)CH2COPh)]+X- (R/R/R = H/H/H, H/H/CH3, H/CH3/ H, CH3/H/H) give the thiolates (5-C5R5)Re(NO)(PPh 3)(SCH(CR2CR=CH2)COPh), derived from CH2COPh group deprotonation, as >96: <4 to 68:32 diastereomer mixtures. Reactions of 4a+TfO- with MeLi give mainly the [1,2]-rearrangement product (5-C5H5)Re(NO)(PPh 3)-(SCH(CH2Ph)Ph) ((35-70):(65-30) diastereomer mixtures), which has been independently synthesized. The thiolate ligands are easily converted to free methyl sulfides. Mechanisms of diastereoselection, and similar reactions of organic sulfonium salts, are discussed.
