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Fischer Carbene Complexes

General Information

Structural and Spectroscopic Features

Synthesis

There are many synthetic methods for the synthesis of carbene complexes. The four most common ones are shown here.

  1. From metal carbonyls. This is the most common method:

     a synthesis

  2. Activation of a neutral acyl complex:

    another synthesis

  3. Rearrangement of coordinated ligands. The tautomerization of terminal alkyne complexes to acetylides is well known. Transfer of the hydride to the beta-carbon affords a vinylidene:

    another synthesis

    Acetylides can be easily synthesized from Li or SiMe3 acetylides. These species are quite reactive towards electrophiles at the beta carbon, i.e. they are easily protonated or alkylated:

    attack at beta carbon

  4. From activated olefins (very strained or electron-rich). This methodology was used by Grubbs to synthesize olefin metathesis catalysts:

    strained olefin reactions

Reactivity

Fischer carbenes exhibit such a wide manifold of reactivity, that an entire text could be devoted to them. Some of the more common reaction pathways are shown here.
  1. Heteroatom substitution. The electrophilic nature of the C atom in Fischer carbenes means the heteroatom can often be exchanged by simple nucleophilic displacement (Casey, C. P.; Shusterman, A. J. J. Mol. Catal. 1980, 8, 1):

    a reaction

  2. Olefin metathesis. While some Fischer carbenes are excellent metathesis catalysts, they are rarely energetically favorable. More commonly, reaction with olefins results in cyclopropanation (next item).

  3. Cyclopropanation. The intermediate metallacyclobutane that is formed from the [2 + 2] cycloaddition of an olefin to a carbene can decompose in several different ways. A retro [2 + 2] can lead to olefin metathesis. However, the metallacycle can also decompose with the formation of a cyclopropane (Casey et. al. J. Am Chem. Soc. 1976, 98, 608).

    a reaction

    These reactions go in good yield and are stereospecific. A second example is by Helquist et al. J. Am. Chem. Soc. 1982, 104, 1869:

    a reaction

    Substituting a phosphine for one CO in the above reaction leads to good enantiomeric excess. One should note that there is well-developed cyclopropanation chemistry using diazoalkanes (explosive!) as the carbene source; organometallic reagents are much safer.

  4. The Dötz Reaction. A phenyl-substituted carbene, alkyne and carbon monoxide are converted into substituted naphthols!

    a reaction

    The metal can be easily cleaved from the naphthol. Dötz-type chemistry is well-developed and has been used to prepare Vitamin E as well as some antibiotics. What is the mechanism of this "termolecular" reaction? Experiments show that the rate is inhibited by excess CO or phosphine, so the first step must be dissociation of CO. Coordination of the alkyne, formation of a metallacyclobutene, insertion of CO into the ring, rearrangement to a pi-complex, ring closure and a proton shift account for our observed product (whew!):

    a reaction

  5. Many more! Fischer carbene complexes undergo many other kinds of annulation reactions, Diels-Alder reactions etc.

dividing line

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This page was last updated Thursday, November 20, 2003
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.