Experimental and Computational Study of the Bonding Properties of Mixed Bis−Ylides of Phosphorus and Sulfur

Abstract

The reactivity of the known ylide-sulfonium salt [Ph(3)P=CHC(O)CH(2)SMe(2)]Br 1 and the new ylide-sulfide [Ph(3)P=CHC-(O)CH(2)SMe] 2 toward Pd(II) complexes has been studied. Compound 1 reacts with PdCl(2)(NCMe)(2) and NEt(3) to give cis-[PdCl(2)[Ph(3)PCHC(O)CHSMe(2)-kappa-C,C]] 3, which is obtained selectively as the meso diastereoisomer (RS/SR). The reactivity of 3 has been studied, and shows the stability of the bis-ylide unit. However, reflux in NCMe of [PdCl(PPh(3))-[Ph(3)PCHC(O)CHSMe(2)-kappa-C,C]]ClO(4) 6 promotes orthopalladation and affords [PdCl(PPh(3))(C(6)H(4)-2-PPh(2)CHC(O)CH(2)SMe(2)-kappa-C,C)]ClO(4) 12, which is characterized by X-ray methods. Density functional theory (DFT) and Bader's Atoms in Molecules (AIM) studies on S-ylides, mixed P-S bis-ylides, and the corresponding Pd complexes have been performed. Free S-ylides show strong conformational preferences, which lies with the establishment of a set of cooperative intramolecular interactions of weak strength: the 1,4 S...O interactions and the 1,6-C-H...O H-bonds between the protons of the methyl substituents and the carbonyl oxygen was fully characterized for the first time. For free mixed P-S bis-ylides, an additional 1,4-P...O intramolecular interaction of moderate strength was characterized. These interactions play a key role in determining the preferred conformations, which then are transferred to the complexes, explaining the observed diastereoselectivity in complex 3. The ylide-sulfide 2 reacts with PdCl(2)(NCMe)(2) and NEt(3) affording [Pd(Cl)[Ph(3)PCHC(O)CHSMe]](2) 9, which in turn reacts with PPh(3) giving [Pd(Cl)(PPh(3))[Ph(3)PCHC(O)CHSMe-kappa-C,C]] 10. The X-ray structure of 10 shows the anion [Ph(3)PCHC(O)CHSMe](-) acting as a C,C-chelate. The bonding in 10 is produced with complete diastereoselectivity but, instead of the expected meso form, the d,l pair (RR/SS) is formed. This inversion is observed for the first time.