Lecture 39 Organometallic reactions and catalysis1) Olefin metathesis. Ziegler-Natta olefin polymerization • Olefin metathesis and catalytic olefin polymerization are used in industry and in the lab. They allow for easy build up of carbon skeleton of organic molecules. • Metathesis. In the olefin metathesis reactionsC=C bond in olefins are cut and then rearranged in a statistical fashion: • Because the reaction is reversible, any factor, volatility like in the case of ethene, enhanced stability of 5 or 6–membered rings etc., can help shift the position of the equilibrium and bring the reaction to completion. • The most efficient catalysts for this reaction are arylimidocarbene complexes of molybdenum (Shrock) and Grubbs ruthenium carbene complexes (TOF > 10000 h-1): • Shrock catalysts are moisture and air sensitive though tolerate a number of functional groups while some of the Grubbs catalysts can be used in the presence of water and air.
2) Applications of the olefin metathesis • Regular metathesis allows for easy preparation of olefins which are difficult to make by conventional methods of organic chemistry from readily available precursors. In the example below gaseous ethylene can be removed from the reaction mixture so helping shift the equilibrium to the right: • Acyclic diene metathesis (ADMET) allows to make polymers from a,w-unsaturated dienes. Ethylene gas which is another reaction product can be removed by purging the mixture with N2 so shifting the equilibrium: • Ring-opening metathesispolymerization (ROMP) allows to make polymers from strained cycloolefins such as norbornene. The reaction driving force is the relief of the strain: • Ring-closing metathesis(RCM) allows for easy preparation of cyclic molecules especially if the product has the enhanced stability (five or six-membered rings):
3) The mechanism of the olefin metathesis • The mechanism of olefin metathesis reactions includes transition metal carbene complexes A and C as the key intermediates (see scheme below). • The unusual feature of the catalysts is that the metal carbenes react readily with substrates with C=C bond to form metallacyclobutanes BandD(2+2 cycloaddition). • Such cyclization reactions are symmetry-forbiddenfor olefins themselves at ambient temperatures (you cannot prepare cyclobutane from two ethylene molecules). • For metal carbenes these cycloadditions are symmetry-allowed. • Because cyclobutanes B and D are highly strained they appear as unstable intermediates only. • Formation of B and D is facilitated when the metal is low coordinate and readily accessible for olefin. • A reasonable number of bulkyligands attached to the metalhelps decrease the metalcoordination number (look at the structures of Shrock and Grubbs catalysts).
4) Ziegler – Natta olefin polymerization • The original Ziegler catalysts for ethylene polymerization at atmospheric pressure were discovered in early 50-s. They were formed by TiCl3 (solid) and AlEt3. • Natta extended the method to other olefins. Further generations of Ziegler-Natta catalysts were based on halides of Ti, Zr, Cr or V “activated” by AlEt3. • The commonly accepted mechanism of the polymerization includes olefin insertion into M-C bond which occurs over and over again. This can be represented by the scheme below showing an example of homogeneous polymerization catalyst: • Reactive intermediates such as A or B, alkyl olefin complexes, should be unsaturated in order for the catalysts to be highly active (check it, electron-count A or B!). • If the catalyst is chiral (see below), stereoregular olefin polymerization becomes possible: