Sulfur ligands trans effect

Thiocarbamate (tc, RHNCSO-) is a monodentate ambidentate ligand, and both oxygen- and sulfur-bonded forms are known for the simple pentaamminecobalt(III) complexes. These undergo redox reactions with chromium(II) ion in water via attack at the remote O or S atom of the S- and O-bound isomers respectively, with a structural trans effect suggested to direct the facile electron transfer in the former.1045 A cobalt-promoted synthesis utilizing the residual nucleophilicity of the coordinated hydroxide in [Co(NH3)5(OH)]2+ in reaction with MeNCS in (MeO)3PO solvent leads to the O-bonded monothiocarbamate, which isomerizes by an intramolecular mechanism to the S-bound isomer in water.1046... 

The syntheses of iron isonitrile complexes and the reactions of these complexes are reviewed. Nucleophilic reagents polymerize iron isonitrile complexes, displace the isonitrile ligand from the complex, or are alkylated by the complexes. Nitration, sulfonation, alkylation, and bromina-tion of the aromatic rings in a benzyl isonitrile complex are very rapid and the substituent is introduced mainly in the para position. The cyano group in cyanopentakis(benzyl isonitrile)-iron(ll) bromide exhibits a weak "trans" effect-With formaldehyde in sulfuric acid, benzyl isonitrile complexes yield polymeric compositions. One such composition contains an ethane linkage, suggesting dimerization of the transitory benzyl radicals. Measurements of the conductivities of benzyl isonitrile iron complexes indicate a wide range of A f (1.26 e.v.) and o-o (1023 ohm-1 cm.—1) but no definite relationship between the reactivities of these complexes and their conductivities. 

The reaction of trans-Pt with two equivalents of GSH results in a monomeric complex in which two GS units are coordinated via the cys-S atom [Eq. (10)] (76,131,189). The unstable intermediate species, [trans-Pt(NH3)2(GS)Cl] and [frans- Pt(NH3)2Cl 2GS], could also be detected by Berners-Price (189). The reaction of [Pt(NH3)3Cl]Cl with two equivalents of GS- initially yields [Pt(NH3)3GS], which reacts further, eventually forming [irans-Pt(NH3)2(GS)2] and free NH3 [Eq. (11) (131). This is rationalized by the large trans effect of the coordinated sulfur and the presence of a second nucleophilic sulfur. Therefore, the amine ligand is easily substituted by a second GS" unit. 

The inner-sphere reductions of [Co(NH3)5(SCONHR)] and [Co(NH3)5 (OCSNHR)] by Gr involve attack at the remote oxygen and sulfur atoms, respectively, with a subsequent isomerization of the 0-bonded ehromium(III) product in the former reaction. The unusually rapid reactions of the S-bonded cobalt(III) complexes are attributed to a structural tran -effect on the Co—N bond length, reducing the reorganization energy needed to form the transition state. A kinetic study of the Cr reduction of [Co(NH3)5(pyruvate)] reveals that the rate of reduction is dependent on the nature of pyruvate ligand, with the keto form about 400 times as reactive as the hydrated form. An inner-sphere mechanism has be postulated for the Cr reduction of [Co(NH3)5(pyridine N-oxide)] on the basis of the rate and activation parameters. The outer-sphere Cr reduction of [Co(sepulchrate)] is catalyzed by halide ions, with the ion-pair formation constants for [Co(sep), estimated to be 5.5, 2.3, and 1.7 M" for Cl", Br", and I", respectively. ... 

The thiophene ligand as an electron-rich 7r-system acts as a donor and, as a result of the trans effect, should transfer electron density to the aUyUc carbon atom trans to the sulfur atom. The carbon atom should less willingly undergo nucleophilic attack because of the increased electron density. Thus, the favorable attack occurs by route A to give (5)-2[2.3I (Scheme 4). 

Williams has shown that the use of non C2-synimetric ligands is equally effective. The regiochemical approach of the nucleophile is influenced by the relative trans effect of the two coordinating atoms in the optically pure ligand (i.e. approach trans to sulfur and phosphorus is favoured over nitrogen). Protocol 6, which involves the reaction of epoxycyclopentadiene 33 with dimethyl propargylmalonate 38, demonstrates the excellent stereochemical control that can be obtained using this reaction. 

Simpler chiral pyrrolidine thioethers, reported in 2004 by Skarzewski et al., proved to be effective ligands in the test reaction. The sense of the stereoinduction was in agreement with the nucleophilic attack directed at the allylic carbon located trans to the sulfur atom in the intermediate complex (Scheme 1.40). 

---