Enantioselective Organic Syntheses With Chiral Transition Metal Complexes, 11 Chiral Ruthenium-Sulfene Complexes-Synthesis and C-C Coupling Reactions

摘要

Reaction of the chiral racemic complex [CpRu(mppe)(SO_2)]PF_6(1, mppe = Me_2Pc_2H_4PPh_2) with diazomethane or-ethane gave the sulfene complexes [CpRu(mppe)(RHC=SO_2)]PF_6 (R = H, 2a; R = Me, 2b). Treatment of 2a with prochiral enamines or deprotonated β-oxo esters yielded C-C coupling products with 32 - 60% de. An analog of 2a, [NmcpRu(mppe)(H_2C=SO_2)]PF_6 (8, Nmcp = neomenthyl-cyclopentadienyl) was prepared in a four-step synthesis starting from LiNmc and [RuCl_2(PPh_3)_3]. Repeated crystallization of the intermediate [NmcpRu(mppe)Cl] (6) provided diastereomerically pure 6' which added methylene stereospecifically to give diastereomerically pure 8'. Compound 8 turned out to be much less reactive towards nucleophiles than 2a, but still added deprotonated ethyl 2-methyl-3-oxobutanoate with 44% de. The chiral, enantiomerically pure sulfur dioxide complex [CpRu(chir)(SO_2)]PF_6 [10, chir = (S,S)-Ph_2PCHMeCHMePPh_2] was synthesized from [CpRu(chir)Cl] and SO_2 and was characterized by X-ray crystallography. Reaction of 10 with diazomethane gave the enantiomerically pure sulfene complex [CpRu(chir)(H_2C=SO_2)]PF_6 (11). Addition reactions of 11 with N-(1-cyclopentenyl) morpholine, as well as with various enolates derived from β-oxo esters or 1,3-diesters proceeded with high yields and 20-90% de. The structure of a diastereomerically pure addition product, [CpRu(chir)(SO_2CH_2C(Me){C(O)Me}{C(O)OtBu}] (13d'), was determined crystallographically and was shown to have (R) configuration at the quaternary carbon atom. After alkylation of one of the S=O functions, the sulfinate 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 SO_2.