Venkataramana Imandi's Talk

Jul 30, 2015 - 16:00

Jul 30, 2015 - 17:00

Room 130 Chemical Engg.

Seminars

Venkataramana Imandi PhD Student IIT Kanpur

Abstract: The Wacker oxidation of olefines to carbonyl compounds by an oxidizing agent under aqueous acidic conditions in the presence of PdCl2/CuCl2 catalyst mixture is considered as an example of dualhomogeneous catalysis and catalyst regeneration until continuous supply of olefines CuCl2 and oxidizing agent. Moreover this reaction is the starting point for the development of catalytic palladium compounds. The elementary steps of this catalytic process involve ligand substitution nucleophilic addition b-hydrogen elimination and insertion which are common to several other organometallic reactions. General mechanism of the Wacker oxidation is well known today however some of the elementary steps are under debate for more than four decades. The major controversy is associated with the nucleophilic addition (hydroxypalladation) of a water molecule on alkene whether it proceeds by an inner sphere or by an outer sphere mechanism. Two different reaction routes have been proposed based on the modes of hydroxypalladation. The inner sphere reaction route involves a water molecule coordinated cis to the alkene involved in the nucleophilic attack to form the hydroxyethylpalladium intermediate. On the other hand the outer sphere reaction pathway is based on an outer sphere water molecule attacking the alkene. Although there are several experimental results that could be interpreted in the support of tale of two hydroxypalladations (inner sphere as well as outer sphere pathways)[1 2].One of the noted experiment along the oxidation of [D2]allylic1 1alcohol (1) to [D2]allylic3 3alcohol (2) was studied and reported that isotopic scrambling of 1 to 2 does not take place during reaction[3 4]. This finding hints that hydroxypalladation proceeds through inner sphere mechanism. For understanding the genesis of parallel routes of hydroxypalladation proposed for the Wacker oxidation We employed state of the art periodic density functional theory (DFT) based Car-Parrinello (CP) ab initio molecular dynamics simulation. Our simulations considered finite solvent water molecules along with catalyst and olefines (ethene and allyl alcohol) for predicting mechanism and kinetics. Herein free-energy barriers were computed by using accelerated molecular dynamics technique (metadynamics) within the framework of Car-Parrinello Molecular Dynamics (CPMD). The crucial experimental results are revisited in this work and gives a picture on the mechanism and kinetics. In short our results showed that hydroxypalladation occurs via outer sphere mechanism and is in fact equilibrium nature. We proposed that rate determining step involves isomerization of Cl ligand in the hydroxypalladate complex occurring prior to the bhydrogen transfer thus explaining the absence of primary kinetic isotope effects in the hydrogen transfer steps. Moreover we investigated isotopic scrambling of 1 to 2 does not occur within outer sphere mechanism of hydroxypalladation in our observed allyl alcohol mechanism by doing micro-kinetics.Biodata: Venkataramana Imandi graduated with M.Sc. in Physical Chemistry from Andhra University during 2007-2009. He joined Ph.D. programme under the guidance of Dr. Nisanth N. Nair on July 2010. His area of research was understanding the mechanism and kinetics of the Wacker Oxidation using Ab Initio Molecular Dynamics. He has submitted his Ph.D. thesis and is a postdoctoral candidate in the Chemical Engineering department.