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

The reactivity of the known ylide-sulfonium salt [Ph3PCHC(O)CH2SMe2]Br 1 and the new ylide-sulfide [Ph3PCHC-(O)CH2SMe] 2 toward PdII complexes has been studied. Compound 1 reacts with PdCl2(NCMe)2 and NEt3 to give cis-[PdCl2[Ph3PCHC(O)CHSMe2-κ-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(PPh3)-[Ph3PCHC(O)CHSMe2-κ-C,C]]ClO4 6 promotes orthopalladation and affords [PdCl(PPh3)(C6H4-2-PPh2CHC(O)CH2SMe2-κ-C,C)]ClO4 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 PdCl2(NCMe)2 and NEt3 affording [Pd(Cl)[Ph3PCHC(O)CHSMe]]2 9, which in turn reacts with PPh3 giving [Pd(Cl)(PPh3)[Ph3PCHC(O)CHSMe-κ-C,C]] 10. The X-ray structure of 10 shows the anion [Ph3PCHC(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.