Green chemistry, The Wittig Reaction

An example of a green Wittig Reaction

Reference: Solvent-Free Wittig Reaction: A Green Organic Chemistry Laboratory Experiment. Sam H. Leung and Stephen A. Angel. J. Chem. Educ., 2004, 81 (10), p 1492. DOI: 10.1021/ed081p1492.

http://pubs.acs.org/doi/abs/10.1021/ed081p1492

In this experiment (E)- and (Z)-1-(4-bromophenyl)-2-phenylethene are synthesized by a solvent-free Wittig reaction. The reaction is effected by grinding the reactants in a mortar with a pestle. Both the E and Z isomers of the product are produced as evidenced by thin-layer chromatography and ¹H NMR analysis. The E isomer is isolated by crystallization with ethanol in this experiment. In addition to learning about the Wittig reaction, students are also introduced to the ideas of mechanochemistry and green chemistry. This experiment can be extended to include ¹H NMR analysis of the products. Students can observe the difference in the coupling constants of the alkenyl protons between the E and Z isomers of 1-(4-bromophenyl)-2-phenylethene.

The Wittig reaction or Wittig Olefination is a chemical reaction of an aldehyde or ketonewith a triphenyl phosphonium ylide (often called a Wittig reagent) to give an alkene andtriphenylphosphine oxide.^[1][2]

The Wittig reaction was discovered in 1954 by Georg Wittig, for which he was awarded theNobel Prize in Chemistry in 1979. It is widely used in organic synthesis for the preparation of alkenes.^[3][4][5] It should not be confused with the Wittig rearrangement.

Wittig reactions are most commonly used to couple aldehydes and ketones to singly substituted phosphine ylides. With simple ylides this results in almost exclusively the Z-alkene product. In order to obtain the E-alkene, the Schlosser modification of the Wittig reaction can be performed.

1.  Georg Wittig, Ulrich Schöllkopf (1954). “Über Triphenyl-phosphin-methylene als olefinbildende Reagenzien I”.Chemische Berichte 87 (9): 1318.doi:10.1002/cber.19540870919.
2. Georg Wittig, Werner Haag (1955). “Über Triphenyl-phosphin-methylene als olefinbildende Reagenzien II”. Chemische Berichte 88 (11): 1654–1666.doi:10.1002/cber.19550881110.
3. Maercker, A. Org. React. 1965, 14, 270–490. (Review)
4. W. Carruthers, Some Modern Methods of Organic Synthesis, Cambridge University Press, Cambridge, UK, 1971, pp81–90. (ISBN 0-521-31117-9)
5. R. W. Hoffmann (2001). “Wittig and His Accomplishments: Still Relevant Beyond His 100th Birthday”. Angewandte Chemie International Edition 40 (8): 1411–1416. doi:10.1002/1521-3773(20010417)40:8<1411::AID-ANIE1411>3.0.CO;2-U.PMID 11317288.
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