Thioethers, rhodium complexes

Numerous studies have been directed toward expanding the chemistry of the donor/ac-ceptor-substituted carbenoids to reactions that form new carbon-heteroatom bonds. It is well established that traditional carbenoids will react with heteroatoms to form ylide intermediates [5]. Similar reactions are possible in the rhodium-catalyzed reactions of methyl phenyldiazoacetate (Scheme 14.20). Several examples of O-H insertions to form ethers 158 [109, 110] and S-H insertions to form thioethers 159 [111] have been reported, while reactions with aldehydes and imines lead to the stereoselective formation of epoxides 160 [112, 113] and aziridines 161 [113]. The use of chiral catalysts and pantolactone as a chiral auxiliary has been explored in many of these reactions but overall the results have been rather moderate. Presumably after ylide formation, the rhodium complex disengages before product formation, causing degradation of any initial asymmetric induction. 

Efforts to optimize rhodium-based systems for methanol carbonylation led to the development of new supporting ligands containing phosphorus and sulfur donor atoms, both thiolates and thioethers, such as those used in the preparation of complexes (20) and (21). Ligands such as 2-diphenylphosphinothiolate have been shown to give rise to complexes that exhibit higher activities, up to four times faster, for the carbonylation of methanol compared to [Rh(CO)2l2] . ... 

A Cr(VI) sulfoxide complex has been postulated after interaction of [CrOjtClj] with MejSO (385), but the complex was uncharacterized as it was excessively unstable. It was observed that hydrolysis of the product led to the formation of dimethyl sulfone. The action of hydrogen peroxide on mesityl ferrocencyl sulfide in basic media yields both mesityl ferrocenyl sulfoxide (21%) and the corresponding sulfone (62%) via a reaction similar to the Smiles rearrangement (165). Catalytic air oxidation of sulfoxides by rhodium and iridium complexes has been observed. Rhodium(III) and iridium(III) chlorides are catalyst percursors for this reaction, but ruthenium(III), osmium(III), and palladium(II) chlorides are not (273). The metal complex and sulfoxide are dissolved in hot propan-2-ol/water (9 1) and the solution purged with air to achieve oxidation. The metal is recovered as a noncrystalline, but still catalytically active, material after reaction (272). The most active precursor was [IrHClj(S-Me2SO)3], and it was observed that alkyl sulfoxides oxidize more readily than aryl sulfoxides, while thioethers are not oxidized as complex formation occurs. 

T he expectation that, by analogy to phosphines, thioethers should function as tt acceptor ligands and thereby stabilize low oxidation state compounds, led several investigators to try to synthesize thioether complexes of rhodium (I). Walton (I) treated [Rh(DTH)Cl2]Cl (DTH = CH3SCH2CH2SCH3) with ethanolic potassium hydroxide, a reducing system developed by Chatt and Shaw (2), but he failed to obtain a complex of the expected type. Attempts to obtain rhodium(I) derivatives by reducing [Rh(DTH)2Cl.]Cl with sodium borohydride or by electrochemical methods were equally unsuccessful. 

The formation of the stable adduct with the Lewis acid BF3 established the enhanced basicity of the Rh(I) in Rh(TTP) over that of the previously known Rh(l)-phosphine complexes. Although [Ir(PPh3)-(CO)Cl] adds BF3, the rhodium analog does not (17). A stronger Lewis acid, e.g. BBrs or BCI3, is required for an observable interaction with [Rh(PPh3)(CO)Cl] (18). Indeed, the only other Rh(I) complex known to form a stable BF3 adduct is chloro-bis(3-diphenylphosphino-propyl)phenylphosphine rhodium(I) (19). The enhanced nucleophilicity of the rhodium in [Rh(TTP)] is considered as evidence of the poor TT-acceptor qualities of the sulfur atoms in the thioether ligand as compared with those of the phosphorus atoms in their similar complexes. 

At lower temperatures, these cis and trans complexes exhibit inversion at the coordinated sulfur atoms, as do the complexes [MX2 MeS(CH2) CH=CH2 ] where M = Pt or Pd, and n = 2 or Inversion at sulfur in thioether complexes of platinum(II) has been documented, and an example of such inversion at rhodium(III) was mentioned in Section The whole area of fluxional... 

This group has been fully dominated by the synthesis of dithiolate complexes, most of them prepared with the aim of obtaining efficient catalysts for hydroformylation. A large number of thioether ligands were prepared for the design of chiral rhodium catalysts." ... 

This simple rule is broadly applicable to the chemistry of palladium(II) and platinum(II) and also to isomerization reactions of square-planar iridium(I), rhodium(I) and gold(III) complexes. However, there are exceptions. In complexes of the type [MX2L2] (M = Pd, Pt X = halide L = phosphine, thioether) the favored direction of isomerization is highly dependent upon the nature of the phosphine or thioether. The favored direction of isomerization reactions in solution is, of course, dependent upon solvent polarity in cases where a pair of isomers exhibit different dipole moments. 

Quaterthiophene, a representative of the oligomers of thiophene [97JPC(A)4437] reacts with the labile 16-electron species [(triphos)RhH] to yield 252 (970M1517). The sulfur atom bonded to rhodium has expressed nucleophilic properties and attack by methyl iodide followed by NaBPlu gives the thioether 253. The latter reacts with [(triphos)RhH] to give the binuclear complex 254 that can be further methylated to give 255. 

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