Synthesis of Chiral Rhenium Complexes Containing Functionalized Thiolate Ligands (original) (raw)

1998, European Journal of Inorganic Chemistry

Chiral racemic rhenium thiolate complexes [CpRe-SCH 2 CH 2 NHAc (8), SCH 2 CH 2 C(O)OH (9). A milder synthesis using hydrated sodium carbonate as a base (NO)(PPh 3)(SR)] were obtained under either acidic or basic conditions. Thus, when [CpRe(NO)(PPh 3)(CH 3)] (1) was provided 8 and compounds with SR = SCH 2 CH 2 C(O)OMe (10), SCH 2 CH 2 C(O)NHCH 2 Ph (11) in high yields. Using treated with etheral HBF 4 and HSR the thiolate complexes [CpRe(NO)(PPh 3)(SR)] [SR = SCH 2 (2-furyl) (2), SCH 2 C-similar methods, thiolate complexes of (R)-N-acetylcysteine (13), its methyl ester (14), (R)-N-phthaloylcysteine (16), and (O)OEt (3)] were obtained after chromatographic workup. Ligand exchange reactions between [CpRe(NO)(PPh 3)-N-[(S)-3-mercapto-2-methylpropionyl]-S-proline (Captopril) (17) were obtained as diastereomeric pairs. The formation of (OC 4 H 8)]BF 4 (4) and sodium thiolates yielded analogous complexes with SR = SH (5), SCH 2 CH 2 Ph (6), SCH 2 CH=CH 2 13 was preceded by the O-bonded isomer 12 which slowly rearranges in solution. 13 can be converted under acidic (7). SR groups which tolerate strongly alkaline conditions may be introduced by treatment of 4 with HSR in the conditions into its methyl (14) or ethyl (15) esters. The diastereomers of 16 were separated by crystallization, and presence of sodium ethoxide as demonstrated by the highyield synthesis of 2 as well as of complexes with SR = the structure of the (R,R)-isomer 16a determined. Thiolate ligands have some remarkable properties which conditions. Treatment of the racemic methylrhenium complex 1 [13] with a twofold excess of etheral HBF 4 and 2-(mer-are exploited by nature in a number of important metalloenzymes. [2] SR groups being soft donors bind strongly to captomethyl)furan or ethyl mercaptoacetate followed by chromatography over silica gave the corresponding rhenium most of the transition metal ions. Due to their high polarizability and π-donor capacity, [3] they are able to stabilize thiolate complexes 2 and 3 in fair to good yields (Eq. 1). various oxidation states of the metal and to promote the formation of clusters [2] [4] which can function as electron reservoirs for redox processes. Enzymatic reactions which lead to the transformation of coordinated thiolate ligands seem to be comparatively rare. One of the few prominent examples is the penicillin biosynthesis [5] whose first step involves the oxidative dehydrogenation of an iron-coordinated, cysteine-containing tripeptide to a thioaldehyde intermediate. [6] We have recently described a similar oxidative route for the synthesis of stable thioaldehyde complexes of ruthenium [7] [8] and rhenium. [9] In order to further exploit the characteristic reactivity of coordinated thioaldehydes in Since the strongly acidic conditions of this route may not nucleophilic additions and cycloadditions [8] [10] [11] [12] we debe compatible with a number of functional groups, a syncided to investigate thiolate complexes bearing various thesis based on a nucleophilic substitution at rhenium was functional groups on the SR ligand. sought. The tetrahydrofuran complex [CpRe(NO)(P-Results Ph 3)(OC 4 H 8)]BF 4 (4) which is easily obtained through acid cleavage of 1 [14] seemed to be an appropriate starting mate-Achiral Thiolates rial. Although 4 was noted to be labile [14] it has found only Two synthetic strategies were chosen in which the rhesporadic use in ligand exchange reactions. [15] When 4 was nium-sulfur bond is formed under either acidic [8] or basic treated with isolated sodium thiolates in THF/ethanol, the