Stereoselective addition reactions of ruthenium thioaldehyde complexes

Halfsandwich-ruthenium complexes of thiobenzaldehydes and thiocinnamaldehydes can be prepared either by hydride abstraction from benzylthiolate complexes or by condensation of a RuSH complex with the corresponding benzaldehyde or cinnamaldehyde. The former reaction is closely analogous to the key step of the biosynthesis of penicillin. The thiobenzaldehyde complexes add a variety of anionic carbon nucleophiles to give complexes of secondary thiolates. Furthermore, they undergo [2+4]-cycloadditions with 1,3-dienes forming complexes of 3,6-dihydro-2H-thiopyranes. The thiocinnamaldehyde complexes add electron-rich and -poor dienophiles to give complexes of 3,4-dihydro-2H-thiopyranes and 4H-thiopyranes, respectively. The reaction with diazomethane results in a formal [4+1]-cycloaddition giving metal-coordinated 2,3-dihydrothiophenes. These reactions are easily modified into diastereoselective addition reactions by the use of the enantiomerically pure complex fragment [CpRu{(S,S)-CHIRAPHOS}]+ as a chiral auxiliary. The organic part can be cleaved from the metal under mild conditions, and the metal complex is recovered in a form suitable to reenter the synthetic cycle.