Enantioselective organic syntheses with chiral transition metal complexes,11 - Chiral ruthenium-sulfene complexes - Synthesis and C-C coupling reactions

Reaction of the chiral racemic complex [CpRu(mppe)(SO2)]PF6 (1, mppe = Me2P C2H4PPh2) with diazomethane or - ethane gave the sulfene complexes [CpRu(mp pe)(RHC=SO2)]PF6 (R = H, 2a; R = Me, 2b). Treatment of 2a with prochiral en amines or deprotonated beta-oxo esters yielded C-C coupling products with 3 2-60% de. An analog of 2a, [NmcpRu(mppe)(H2C=SO2)]PF6 (8, Nmcp = neomenthyl cyclopentadienyl) was prepared in a four-step synthesis starting from LiNmc p and [RuCl2(PPh3)(3)]. Repeated crystallization of the intermediate [NmcpR u(mppe)Cl] (6) provided diastereomerically pure 6' which added methylene st ereospecifically to give diastereomerically pure 8'. Compound 8 turned out to be much less reactive towards nucleophiles than 2a, but still added depr otonated ethyl 2-methyl-3-oxobutanoate with 44% de. The chiral, enantiomeri cally pure sulfur dioxide complex [CpRu(chir)(SO2)]PF6 [10, chir = (S,S) -P h2PCHMeCHMePPh2] was synthesized from [CpRu(chir)Cl] and SO2 and was charac terized by X-ray crystallography. Reaction of 10 with diazomethane gave the enantiomerically pure sulfene complex [CpRu(chir)(H2C=SO2)]PF6 (11). Addit ion reactions of 11 with N-(1-cyclopentenyl)morpholine, as well as with var ious enolates derived from beta-oxo esters or 1,3-diesters proceeded with h igh yields and 20-90% de. The structure of a diastereomerically pure additi on product, [CpRu(chir) (SO2CH2C(Me) {C(O)Me}{C(O)OtBu}] (13d'), was determ ined crystallographically and was shown to have (R) configuration at the qu aternary carbon atom. After alkylation of one of the S=O functions, the sul finate Ligand was cleaved from the metal center by Ligand substitution with acetonitrile, and the resulting acetonitrile complex 15 was converted back into 10 by treatment with SO2.