Disulfide ligands

The dinuclear ion Mo2(S2) g (F - prepared from the reaction of molybdate and polysulfide solution (13) is a usehil starting material for the preparation of dinuclear sulfur complexes. These disulfide ligands are reactive toward replacement or reduction to give complexes containing the Mo2S " 4 core (Fig. 3f). 

Matsumoto et al. reported some reactions of diruthenium complexes containing a bridging disulfide ligand with unsaturated compounds such as olefins and ketones [135]. These diruthenium complexes show unique reactivities towards double-bond systems since the S-S bond has some double bond character from the contribution of the canonical structure B in Scheme 36. 

Another p-amino disulfide ligand was prepared by these authors starting from (5)-phenylglycine or (i )-styrene oxide and subsequently tested for similar reactions to those described above. The enantioselectivity (39-80% ee) was found to be lower than those observed for the disulfide depicted in Scheme 3.1,... 

Scheme 3.1 L-Proline-derived P-amino thiol and disulfide ligands for additions of ZnEt2 to aldehydes.

In the same context, these authors have also studied the catalytic activity of the p-amino thiol and its corresponding disulfide depicted in Scheme 3.8 for similar reactions.High levels of enantioselectivity were obtained for both these ligands but, however, in the case of the disulfide ligand the reaction rate was slower than that observed with the corresponding thiol, while the disulfide proved to be more chemically stable under the reaction conditions. 

Scheme 3.11 L-Valine-derived P-amino disulfide ligands for addition of ZnEt2 to benzaldehyde.

In 2001, Braga et al. reported the synthesis of new chiral C2-symmetric oxazolidine disulfide ligands from (R)-cysteine and successfully applied them as catalysts in the asymmetric addition of ZnEt2 to various aldehydes (Scheme 3.23). In the presence of 2mol% of ligand, excellent enantioselectivities of up to >99% ee were obtained even with aliphatic aldehydes such as n-decanal or n-hexanal. These authors proposed that the active catalyst did not maintain its C2-symmetry during the reaction. The disulfide bond was probably cleaved in situ by ZnEt2. 

Scheme 3.64 Oxazolidine disulfide ligand for alkynylations of aldehydes.

Organic disulfides can act as monodentate ligands. The simplest compound of this type, dimethyl disulfide, acts as a bridge in [Re2 Br2 (CO)6 (S2 Me2)], where each sulfur coordinates to one Re.18 Other acyclic disulfides have been shown to form complexes of Cu1 and Cu11.18-20 An example incorporating a monodentate cyclic disulfide ligand, the Ru11 complex ion (8), has been reported by Stein and Taube. 2... 

Based upon the above Michaelis—Menten type kinetic behavior, the visual observation of thiolate complex in a copper(I)-thiolate solution, and compatibility of soft thiolate and disulfide ligands with soft copper(I) (64—65), Lim et al. (66) postulated the following mechanism for the... 

Disulfide formation in the two-electron oxidation products is also supported by chemical oxidations of 1. The two-electron oxidation of each Ni center in the dimer by I2 leads to the formation of a five-coordinate mononuclear Ni complex with a disulfide ligand (78). The struc-... 

The structural chemistry associated with polysulfide molecules and materials reflects the stereochemical freedom of the sulfur atom as mentioned above, disulfide ligands can coordinate in a number of ways. Similarly, larger rings may adopt a variety of geometries (from X-ray studies) and the evidence that is available suggests that many of the rings are fluxional in solution. Muller has classified and discussed some aspects of this problem for molecular species, but the rationalization of the structures adopted in solid-state materials is not yet complete. ... 

Another factor in M-S bonding stability is the presence of S-S bonds. Calculations and fragmentary spectral studies indicate that the cr orbitals of S-S bonds are even lower in energy than those of S-H bonds (i.e., ct energies are lower in HjSj than in HjS). Thus disulfide ligands, and probably polysulfides in general, are better electron acceptors than sulfur itself. It is well known that transition metals form numerous compounds with polysulfides (Draganjac and Rauchfuss, 1985 Muller and Diemann, 1987). In many polysulfides (e.g., S/ ) there are also empty tt orbitals in addition to the empty ct orbitals, and these may also accept electron density from the metal. 

Fig. 28. Proposed mechanism for the sulfur-catalyzed and thermal isomerization of the coordinated vinyl disulfide ligand to 1,2-dithiolene.

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