Palladium complexes sulfur-phosphorus ligands

In 2004, Molander et al. developed another type of chiral sulfur-containing ligands for the intermolecular Heck reaction. Thus, their corresponding novel cyclopropane-based phosphorus/sulfur palladium complexes proved to be active as catalysts for the reaction between phenyltriflate and dihydrofuran, providing at high temperature a mixture of the expected product and its iso-merised analogue (Scheme 7.7). The major isomer C was obtained with a maximum enantioseleetivity of 63% ee. 

The most recent development concerns the heterocyclic (amino)(ylide)carbenes AYC. Such compounds have been known for some years [203] but so far had little impact compared to their diamino stabilized relatives. Both phosphorus ylide (86) and sulfur ylide (87) stabilized AYC ligands have been generated in situ and were stabilized at suitable metal centers (Fig. 27) [204, 205]. The palladium complex 88 with an anionic (amino) [bis(ylide)]carbene is also known [206]. 

Sulfur-phosphorus bidentate ligands such as o-C6H4(PPh2)SMe fail to form 2 1 complexes with palladium(II) halides, but o-CgH AsPh SMe forms [PdX2(L—L)2] (X = Cl, Br) from aqueous acetone. 5 The crystal structure of the diiodide complex has been determined and shows a pseudohexacoordinate palladium(II) centre with long trans axial Pd S interactions (3.84 A). 

Palladium-complex-promoted asymmetric Diels-Alder reaction stereoselective synthesis of a new sulfinyl-substituted phosphine ligand containing three carbon, one phosphorus and one sulfur stereogenic centres <95CC1747>. 

Kawashima and co-workers attempted to prepare the phosphorus ylide-sta-bilized AYC ligand 129 which could, however, not be isolated because of an intramolecular rearrangement giving 130. The intermediate formation of 129 was confirmed by its reaction with sulfur to give 131 and by the formation of the rhodium complexes 132 and 133 (Scheme 1.17). Infrared spectroscopy of the carbonyl complex 133 confirmed the superb donor properties of AYC 129. A palladium complex bearing the phosphorus ylide-stabilized NHC was also prepared. Fiirstner et al. synthesized the analogous... 

Reaction of this lO-S-3 [279] tetraazapentalene derivative with [Pd(PPhj) ] or [Rh(PPh3)3)Cl] results in the formal substitution of sulfur by the transition metal accompanied by a redox reaction (see Figure 4.93) [280], The endocyclic sulfur atom is transferred to a PPhj ligand (oxidation of phosphorus to PhjP=S). At the same time the transition metal is oxidis (palladium from 0 to +11 rhodium from +1 to +III), which leaves sulfur to be reduced by four electrons (it is -II in Ph I S and thus must have been +II in the tr-sulfurane starting material). It follows from this electron transfer analysis that the rt-sulfurane is indeed better desaibed as the sulfur complex of a doubly amide functionahsed NHC ligand. 

D.ii. Palladium(II) Complexes with Chiral Phosphorus, Nitrogen, or Sulfur Ligands... 

---