from first principles PP-I-1

OP-III-30Mechanistic and Kinetic Studies of Aromatic Carbon-Carbon Coupling(Suzuki Reaction) with Heterogeneous CatalystsJacquemin M., Hauwaert D., Gaigneaux E.M.Institute of Condensed Matter and Nanosciences – IMCN,Division “MOlecules, Solids and reactiviTy - MOST, Université catholique de Louvain (UCL),Croix du Sud 2/17, B1348, Louvain-la-Neuve, Belgiummarc.jacquemin@uclouvain.beSuzuki coupling is often a crucial step in the synthesis of many molecules likepharmaceuticals, liquid crystals, organic conductive materials and semi-conductors [1].Indeed, this reaction allows the combination of aryl halides with aryl boronic acids to formbiaryls in the presence of a Pd-based catalyst and a base. The knowledge about the Suzukicoupling mechanism with heterogeneous catalysts is relatively limited. Actually, currentresearch leads to contradictory results which bring to a blurred situation [2]. Is it a realheterogeneous catalysis or does the active metal (Pd) have to be leached from the supports inorder to perform the reaction? Furthermore, the nature of the active Pd oxidation state (Pd 2+and/or Pd 0 ) has not yet been well elucidated and investigations on this subject lead toconflicting conclusions [2]. In this context, better understanding of the Suzuki coupling wouldallow to improve the catalytic performances of this important and useful reaction for theconstruction of carbon-carbon bonds.A methodical study of different parameters of the reaction and the catalyst, namely palladiumsupported on carbon black, has permitted to determine that the Pd is not leached from thecarbon support. So, it is a real heterogeneous catalysis. Furthermore, the oxidation state of theactive Pd species is +II. The reduced form is not active.An experimental reaction rate expression has then been determined by varying theconcentration of the reactants (4-bromotoluene, phenylboronic acid and the base (K 2 CO 3 )) andthe reaction temperature. A theoretical rate expression has been also determined by makingassumptions of the reaction pathway with a heterogeneous catalyst. The catalytic cycle isdivided into 7 elementary steps. By comparing the experimental and theoretical expressions, wewere able to identify that the rate limiting step of the Suzuki coupling is the transmetallation ofthe phenylboronic acid on an intermediate species formed by the adsorption of 4-bromotolueneonto the Pd. So, for improving the catalytic performances of the Suzuki reaction withheterogeneous catalysts, the rate of the transmetallation step has to be increased.[1] F.X. Felpin, T. Ayad, S. Mitra, Eur. J. Org. Chem. 12 (2006) 2679.[2] N.T.S. Phan, M. Van Der Sluys, C.W. Jones, Adv. Synth. Catal. 348 (2006) 609.74