Industrial Importance of Alkenes
In organic chemistry, methods to form carbon-carbon bonds are very important. Available methods often require drastic reaction conditions (e.g. deprotonation with strong acids to obtain carbanions) and/or special functional groups in at least one of the reactants (e.g. Grignard reaction, Wittig reaction, and radical reactions). Olefin metathesis has been used on an industrial scale for several decades to react non-functionalized alkenes. Because of the development of new custom-made catalysts in the nineties, olefin metathesis became a standard method in preparative organic chemistry.,
- In organic chemistry, metathesis (Greek for rearrangement) refers to the interchange of the alkylidene groups (R1R2C=) of different alkene molecules (or alkyne molecules). The reaction is carried out with special organometallic catalysts.
The reaction proceeds under mild conditions and complements the very large number of methods for selectively introducing double bonds into organic molecules. The development of new catalysts is the deciding factor for progress in olefin metathesis. Stable ruthenium, molybdenum, tungsten, and rhenium catalysts directly interact with the multiple bond of the alkenes. Frequently used and commercially available metathesis catalysts are the so-called Grubbs and Schrock catalysts 1 or 2 and 3, respectively. An important advantage of the Grubbs catalyst is its pronounced tolerance towards many functional groups.
Some metathesis examples:
- Ring closure metathesis (RCM): An acyclic diene is converted into a cyclic and an acyclic alkene with the help of an olefin metathesis catalyst. Similarly, eneynes are also cyclized into rings of different size.
- Ring opening metathesis (ROM): ROM formally is an inversion of RCM.
- Crossed metathesis (CM): The metathesis of two different acyclic alkenes is called crossed metathesis. Homodimerization, in this case the undesirable dimerization of either or both of the alkenes, is a fundamental problem in olefin metathesis.
- Domino metathesis: Domino reactions considerably can shorten complex syntheses by forming and/or cleaving several bonds in one reaction sequence. Ring closure - ring opening - ring closure metathesis sequences are known.