Lecture 37 Organometallic reactions and catalysis 1) Catalytic olefin hydrogenation

1. Lecture 37 Organometallic reactions and catalysis1) Catalytic olefin hydrogenation • A thermodynamically favorable reaction may be slow at modest temperatures and thus of little value for synthesis. Increased temperatures may help making it faster, but competing reactions may be accelerated as well. • A more attractive approach to increase the rate of a reaction is to use catalysis. • We will discuss the most well studied and/or useful examples of homogeneous catalysis: olefin hydrogenation (addition of hydrogen across C=C bond) olefin hydroformylation (addition of hydrogen and formyl HC(O) across C=C bond)olefin metathesisolefin polymerizationcarbonylation of methanol (Monsanto acetic acid process) • Olefin hydrogenation. The following reaction is favorable but does not proceed at all at room temperature: • In the beginning of the 20th century Paul Sabatier discovered that hydrogenation of olefins can be accelerated with finely dispersed Ni, Pt or Pd (heterogeneous catalysis). • If hydrogenation of substrates with many functional groups in them is needed, heterogeneous catalysis is not a good choice because of the lack of selectivity:

2. 2) Catalysts for homogeneous olefin hydrogenation • The first effective and selective homogeneous catalyst for olefin hydrogenation is RhCl(PPh3)3 known as Wilkinson’s complex. • It allows for fast selective hydrogenation of variety of olefins at room temperature and 1 atm of H2 gas: • Wilkinson’s complex is unique in the sense that its analogues derived from more electron-rich and less bulky phosphines such as PEt3, RhCl(PEt3)3, or iridium, IrCl(PPh3)3, which forms stronger bonds with ligands, are not active in hydrogenation. • More active cationic complexes were found later such as [Rh(PPh3)2(cod)]+PF6-, [Ir(cod)(PCy3)(py)]+PF6- etc (cod = 1,5-cyclooctadiene). Metal atoms here are more electron deficient and bind olefins more strongly which is an important step of the catalysis. • Homogeneous catalysts are used mostly in the lab and in pharmaceutical industry when selectivity is important and if separation of homogeneous catalyst is not a big problem. • At times, proving the real nature of a catalyst(heterogeneous/homogeneous) may be not a simple task. Polymer-bond olefin hydrogenation test can be used:

3. 3) Mechanism of olefin hydrogenation by Wilkinson catalyst • Wilkinson complex is not only highly active, it is also tolerant of a number of functional groups which may be present in the olefin substrate: C=O, OAc, COOH, OH, NO2, C≡N, C(OR)2, OR etc. • Its selectivity with respect to the type of C=C bond to be hydrogenated: • Detailed kinetic studies allowed to establish the mechanism of olefin hydrogenation by RhClL3. The Wilkinson complex A is actually a pre-catalyst which form catalytically active 14 electron species B (K = 10-5 M). Bulkiness of L helps accelerate this step. Then B reacts fast with H2 to produce dihydride C. • C is capable of coordinating olefin to form D. Bulky olefins will be less prone to form D, consistent with the activity of olefins (see above). • The rate limiting step of the reaction is D E. • Fast reductive elimination of alkane to regenerate B is the last and fast step of the reaction catalytic cycle. The catalytic cycleof the reaction includes intermediates B-E. • Empty coordination sites in the intermediatesB, C, E may be occupied by solvent molecules(benzene – ethanol). • Ethylene cannot be hydrogenated by RhClL3since if forms very strong complex with Rh. • Alkynes are even more reactive than alkenes because of the stronger binding to Rh. Theycan be selectively hydrogenated to olefins.

4. 4) Similar catalytic reaction of olefins • Very important type of catalytic hydrogenation reactions is asymmetric hydrogenation. Here is an example of such hydrogenation leading to enantiomerically pure L-Dopa, (Parkinson’s disease drug). All you need is to use chiral ligands attached to Rh atom: There is a number of catalytic addition reactions which proceed via mechanisms similar to those for olefin hydrogenation. • Hydrosilylation. One of the simplest and efficient (pre)catalysts is H2PtCl6 or PdL4 (L = phosphine). • Hydrocyanation. The most known catalysts for synthesis of adiponitrile are NiL4 complexes with L = P(OAr)3.