Synthesis of (R)-sulforaphane using [CpRu((R,R)-CHIRAPHOS)](+) as chiral auxiliary

A new enantioselective (80% ee) synthesis of (R)-sulforaphane and its epimer (S)-sulforaphane is described, which makes use of the pseudotetrahedral complex fragment [CpRu-(CHIRAPHOS)](+) as a chiral auxiliary. Reaction of the chloride complexes [CpRu(L-L)Cl] [L-L = 1,2-bis(diphenylphosphino)ethane (dppe), (2S,3S)- and (2R,3R)-bis(diphenylphosphino)butane ((S,S)- and (R,R)-CHIRAPHOS, respectively)] with phthalimidobutyl methyl sulfide gives the thioether complexes [CpRu(L-L)(MeSC(4)H(8)NPhth)]PF6. Oxygen transfer from dimethyldioxirane (DMD) produces the corresponding sulfoxide complexes in high yield and high diastereoselectivity. Cleavage of the phthaloyl group with aqueous hydrazine and subsequent reaction with thiophosgene yields the sulforaphane complexes [CpRu(L-L)(MeS(O)C4H8NCS)]PF6. Treatment of these with sodium iodide finally liberates the sulforaphane without noticeable racemization.