Enantioselective organic syntheses using chiral transition metal complexes, 7 - Synthesis of chiral rhenium complexes containing functionalized thiolate ligands

Chiral racemic rhenium thiolate complexes [CpRe(NO)(PPh3)(SR)] were obtaine d under either acidic or basic conditions. Thus, when [CpRe(NO)[PPh3) (CH3) ] (1) was treated with etheral HBF4 and HSR the thiolate complexes [CpRe(NO ) (PPh3)(SR)] [SR = SCH2(2-furyl) (2), SCH2C-(O)OEt (3)] were obtained afte r chromatographic workup, Ligand exchange reactions between [CpRe(NO)(PPh3) (OC4H8)]BF4 (4) and sodium thiolates yielded analogous complexes with SR = SH (5), SCH2CH2Ph (6), SCH2CH=CH2 (7). SR groups which tolerate strongly al kaline conditions may be introduced by treatment of 4 with HSR in the prese nce of sodium ethoxide as demonstrated by the high-yield synthesis of 2 as well as of complexes with SR = SCH2CH2NHAc (8), SCH2CH2C(O)OH (9). A milder synthesis using hydrated sodium carbonate as a base provided 8 and compoun ds with SR = SCH2CH2C(O)OMe (10), SCH2CH2C(O)NHCH2Ph (11) in high yields. U sing similar methods, thiolate complexes of (R)-N-acetylcysteine (13), its methyl ester (14), (R)-N-phthaloylcysteine (16), and N-[(S)-3-mercapto-2-me thylpropionyl]-S-proline (Captopril) (17) were obtained as diastereomeric p airs. The formation of 13 was preceded by the O-bonded isomer 12 which slow ly rearranges in solution. 13 can be converted under acidic conditions into its methyl (14) or ethyl (15) esters. The diastereomers of 16 were separat ed by crystallization, and the structure of the (R,R)-isomer 16a determined .