Visit our sponsor at www.chemglass.com

	Fischer Carbene Complexes

General Information

Carbene ligands possess a metal-carbon double bond and are closely related to alkylidenes. Carbene ligands (B) have a heteroatom substituent unlike alkylidenes (A) which usually have alkyl substituents on the alpha carbon atom:



These are sometimes called Fischer carbenes in honor of E.O. Fischer, who reported the first example in 1964 and later won a Nobel Prize for his pioneering work on ferrocene with Wilkinson.

Fischer carbenes are typically found on electron-rich, low oxidation state metal complexes (mid to late transition metals) containing pi-acceptor ligands. The presence of the heteroatom on the alpha carbon allows us to draw a resonance structure that is not possible for an unsubstituted (Schrock-type) alkylidene:



If we look at this from a molecular/atomic orbital perspective, one lone pair is donated from the singlet carbene to an empty d-orbital on the metal (red), and a lone pair is back-donated from a filled metal orbital into a vacant p_z orbital on carbon (blue). There is competition for this vacant orbital by the lone pair(s) on the heteroatom, consistent with our second resonance structure. Overall, the bonding closely resembles that of carbon monoxide. Therefore, carbene ligands are usually thought of as neutral species, unlike dianionic Schrock alkylidenes (which usually lack electrons for back-donation). But remember that electron counting is just a formalism!

The stabilization of a partial positive charge on the carbene ligand has predictable consequences that show up in the reactivity and spectroscopic features (see below).

Structural and Spectroscopic Features

Single crystal x-ray diffraction studies confirm that the second resonance form shown above plays a major role in describing the bonding in metal carbene complexes. The metal double carbon bonds in these complexes tend to be longer than typical M=C double bonds, but shorter than M-C single bonds. Likewise, the carbon-heteroatom bond length is somewhat shorter than a typical M-E bond. For example, in the case below, a "normal" C-N bond is 145 pm:



The stronger the pi-donor on the carbene carbon, the lower the M=C bond order and lower the barrier to rotation around the M-C bond.

Proton and carbon NMR data are similar to those observed for alkylidene complexes, however, Fischer carbenes do not typically display any unusual J_CH values that might indicate an agostic interaction between the carbene proton and the metal.

Fischer carbenes often contain carbonyl ligands which can provide very useful NMR and IR data.

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:

   

2. Activation of a neutral acyl complex:

   

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:

   

   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:

   

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

   

Reactivity

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):

   

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).

   

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

   

   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!

   

   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!):

   

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

[Index]   [Keyword Search]   [Books & Software]   [ILPI Home Page]

Please visit our sponsor to thank them for supporting this site!

This page was last updated Tuesday, March 31, 2015
This document and associated figures are copyright 1996-2024 by Rob Toreki or the contributing author (if any) noted above. Send comments, kudos and suggestions to us by email. All rights reserved.