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Olefin Metathesis

General Information

Mechanism

Catalysts

    There have been roughly four distinct generations of olefin metathesis catalysts:

  1. "Black Box" heterogeneous catalysts consisting of a high valent transition metal halide, oxide or oxo-halide with an alkylating co-catalyst such as an alkyl zinc or alkyl aluminum. Some of these catalyst systems are placed on an alumina or silica support. Classic examples, some of which are still used today, include WCl6/SnMe4 and Re2O7Al2O3.
  2. While these catalysts are exceedingly active, they have an exceedingly low tolerance for functional groups because of their Lewis acidic nature. Likewise, less than one percent of the material is an active catalyst, and nothing is known about the nature of the actual catalytic species in these systems. One commercial application still using these catalysts is the ROMP of dicyclopentadiene to produce tough plastics for use in golf carts, snow mobile hoods etc.

  3. Titanocene-based catalysts. Reaction of Cp2TiCl2 with two equivalents of AlMe3 to yield Cp2Ti(µ-Cl)(µ-CH2)AlMe2, commonly called Tebbe's Reagent. In the presence of a strong base such as pyridine, the reagent is functionally equivalent to "Cp2Ti=CH2".
  4. These Ti-based catalysts are not nearly as active or tolerant of carbonyl functionalities as the later catalysts, but Grubbs has shown that these Ti complexes undergo stoichiometric Wittig-like reactions with ketones, aldehydes and other carbonyls to form the corresponding methylene derivatives. The mechanism of this reaction is identical to that of the olefin metathesis reaction except that the final step is not reversible.

    Tebbe's reagent in action

  5. Schrock W, Mo and Re Catalysts R.R. Schrock (MIT) has invented a variety of catalysts, but the most important of these are arylimido complexes of Mo with the general formula (Ar'N)(RO)2Mo=CHR' where Ar' is typically 2,6-diisopropylphenyl, R' can be virtually anything and R is neopentyl or neophyl (CMe2Ph). These catalysts are exceedingly active, metathesizing over 1,000 equivalents of cis-2-pentene to equilibrium in less than one minute for R = CMe(CF3)2. The reactivity of these catalysts can be tuned very easily by changing the nature of the alkoxide ligands. For example when R = tert-butyl, the complex reacts only with strained cyclic olefins, making it an ideal ROMP catalyst.
  6. Schrock catalysts

    These catalysts have a high tolerance for functionality, although they are air and water-sensitive. Two important features of these catalysts are that they are 100% active and have been fully characterized by NMR and X-ray crystallography. The success of these catalysts stems from their coordinative and electronic unsaturation (making them electrophilic) and their bulky ligands (prevents bimolecular decomposition).

  7. Grubbs Ru Catalysts In the early 1990's Bob Grubbs (CalTech) developed a series of Ru catalysts that differ from the previous generations in several distinct ways. First, the metal is not in its highest oxidation state and is supported by phosphine ligands. Second, these catalysts are so tolerant of functionality that some of them can operate in water on the benchtop! Such functional group tolerance comes at the expense of lower metathesis rates than the Schrock catalysts, but these systems are extremely promising. Stay tuned for further developments!
  8. Ru catalysts

Other Types of Olefin Metathesis

Further Reading

dividing line

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This page was last updated Monday, November 16, 2009
This document and associated figures are copyright 1996-2014 by Rob Toreki or the contributing author (if any) noted above. Send comments, kudos and suggestions to us by email. All rights reserved.