A theoretical study of the essential role of DMSO as a solvent/ligand in the Pd(OAc)(2)/DMSO catalyst system for aerobic oxidation

• Published in: Organometallics 2005, Vol.24 (24), p.6019
• Main Authors: Zierkiewicz, W., Privalov, Timofei
• Format: Article
• Language: English
• Subjects: (-)-sparteine; chemicals; conversions; density; kinetic resolution; ligand; mechanistic investigations; molecular-orbital methods; secondary alcohols
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om0506217

Dimethyl sulfoxide (DMSO) has unique properties as an aprotic, polar solvent. The oxygen atom in DMSO can interact with positive charges and thus stabilize metal cation. The sulfur atom, although somewhat positively charged, does not interact with negative charges effectively. Also, two methyl groups surround the sulfur atom and influence binding properties of DMSO. These features of DMSO are addressed in the present computational study of the Pd(AcO)(2)/DMSO-catalyzed aerobic oxidation system. Mechanistic and computational details are provided. The step-by-step Gibbs energy of reaction was calculated using the electronic energy at the B3LYP density functional level with thermal functions calculated at the same level of theory. The solvent was modeled using the polarized medium (PCM) with additional DMSO molecules in the second coordination sphere of the complexes studied. The overall reaction pathway was divided into several steps in accord with available experimental data. All steps, including the first deprotonation and the beta-hydride elimination transition states, were elucidated in good detail. Coordination and reorganization of DMSO in Pd(II)(AcO)(2)/DMSO and Pd(0)/(DMSO)(n) complexes has been studied to provide realistic data about coordination of DMSO with hard (O) versus soft (S) ligand donor atoms. The P-hydride elimination transition state was identified computationally to give an estimation of the activation energy of the alcohol oxidation reaction. Therefore, we suggest that the rate-determining step is related to the alcohol part of the reaction cycle.