Introduction to catalysis Textbook H: Chapter 14 Textbook A: Part IV – Introduction

Introduction to homogeneous catalysis 1850 Berzelius 1895 Ostwald: a catalyst is a substance that changes the rate of a chemical reaction without itself appearing into the products. Definition: a catalyst is a substance that increases the rate at which a chemical reaction approaches equilibrium without becoming itself permanently involved. Catalysis is a kinetic phenomenon. is favorable 2

Homogeneous catalysis Homogeneous catalysts:  Are soluble metal complexes, usually mononuclear.  TON (turnover number): moles of product/moles of catalyst  TOF (turnover frequency): TON/time  Resting state: the (pre)-catalytic species present in the highest concentration  Turnover-limiting (rate-determining) step: smallest rate constant  Can be characterized spectroscopically, but are more difficult to separate than heterogeneous catalysts.  Operate under moderate conditions. Homogeneous processes without a heterogeneous counterpart:  Pd-catalyzed oxidation of ethylene to acetaldehyde (Wacker process)  Ni-catalyzed hydrocyanation of 1,3-butadiene to adiponitrile (DuPont)  Rh- and Ru-catalyzed reductive coupling of CO to ethylene glycol  An increasing number of enantioselective hydrogenation, isomerization, and oxidation reactions.

Production of commodity chemicals Operation Scale (million tonnes per year) Terephthalic acid and poly(ethylene terephthalate) Acetic acid and acetyl chemicals Aldehydes and alcohols via hydroformylation Adiponitrile Detergent-range alkenes via SHOP Total fine chemicals manufacture Olefin polymerization (60% uses Ziegler-Natta) < 1 60

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12 Principles of green chemistry 1.Prevent waste 2.Increase atom economy 3.Use and generate no/less toxic chemicals 4.Minimize product toxicity during function 5.Use safe solvents and auxiliaries 6.Carry out processes with energy economy (ambient temperature and pressure) 7.Use renewable feedstocks 8.Reduce derivatives and steps 9.Use catalytic instead of stoichiometric processes 10.Keep in mind product life time (degradation vs. biodegradation processes) 11.Perform real-time analysis for pollution prevention 12.Use safe chemistry for accident prevention Topics in Organometallic Chemistry 2005, vol. 16, Springer, Berlin

Guiding principles The catalytically active species must have a vacant coordination site (total valence electrons = 16 or 14) to allow the substrate to coordinate. Noble metals (2 nd and 3 rd period of groups 8-10) are privileged catalysts (form 16 e species easily). In general, the total electron count alternates between 14/16 and 16/18. Ancillary ligands insure stability and a good stereoelectronic balance. One of the catalytic steps in the catalytic cycle is rate-determining.

General observations about catalysis Kinetic competence: catalysis is a kinetic phenomenon, so the activity of a system may rely on a minor component of a catalyst (if an intermediate can be observed does not mean is a true intermediate in the catalytic cycle). Homogeneous vs. heterogeneous: liquid Hg selectively poisons any heterogeneous platinum group (Ru, Os, Rh, Ir, Pd, Pt) metal catalyst. Reversibility: if a catalytic cycle is formed only of reversible steps then a thermodynamic product ratio is obtained. Chiral poisoning: an enatiomerically pure compound selectively binds to and poisons one enantiomer of a racemic catalyst. Chiral amplification: the product of the catalytic reaction has a higher ee than expected because the major enantiomer of the catalyst acts as a poison for the minor enantiomer.

Asymmetric catalysis: introduction In order to discriminate between enantiotopic atoms or faces of an achiral molecule, the energy difference of the two intermediates has to exceed 2.4 kcal/mol. Chiral complexes as catalysts have the advantage of synthesizing, in principle, both enantiomers; enzymes produce only one enantiomer. In order to pass the enantiomeric character on to the substrate, the ligand has to adopt C 2 and not C s symmetry.

Phosphine ligands in asymmetric hydrogenation