Enantioselective Organic Syntheses Using Chiral Transition Metal Complexes, 9 Atropo-Diastereoselective Ring Opening of Biaryl Thionolactones Using [CpRu{(S,S)-CHIRAPHOS}]+ as a Chiral Auxiliary

Abstract

The substitution-labile thiophene complex [CpRu{(S,S)-CHIRAPHOS}(SC4H4)]BF4 (2) [(S,S)-CHIRAPHOS = (2S,3S)bis(diphenylphosphanyl)butane], prepared from [CpRu{(S,S)-CHIRAPHOS}Cl] (1), thiophene, and AgBF4, reacted with the biaryl-thionolactones 3a–f to give the corresponding S-coordinated complexes 4a–f in high yields. The structure of 4c, which crystallized as the pure (S,S,P) diastereoisomer, was determined by X-ray crystallography. Coordination of the ruthenium fragment caused an elongation of the C=S bond, a contraction of the C–O bond within the lactone ring and a flattening of that ring. Single hydride transfer with LiBEt3H converted 4a–f into the thiolactolate complexes 5a–f in good yields and diastereoselectivities. An X-ray structure determination of the major isomer of 5a revealed it to be the (S,S,S,P) diastereoisomer. Protonation with NH4PF6 converted 5a–f into the corresponding ring-opened thioaldehyde complexes 6a–f. Alkylation of 5a with methyl iodide resulted in Ru–S cleavage to give the oxothioacetal 7a and [CpRu{(S,S)-CHIRAPHOS}I] (8). Full reduction of 4a–f with LiAlH4 produced the thiolate complexes 9a–f in high yields and 6–74% de. Methylation at sulfur converted 9a–c into the corresponding thioether complexes 10a–c, which were cleaved to 8 and the free methyl thioethers 11a–c without isomerization of the biaryl axis.