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Posts Tagged ‘alkyls

Epic Ligand Survey: Metal Alkyls (Part 3)

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In this last post on alkyl ligands, we’ll explore the major modes of reactivity of metal alkyls. We’ve discussed β-hydride elimination in detail, but other fates of metal alkyls include reductive elimination, transmetallation, and  migratory insertion into the M–C bond. In a similar manner to our studies of other ligands, we’d like to relate the steric and electronic properties of the metal alkyl complex to its propensity to undergo these reactions. This kind of thinking is particularly important when we’re interested in controlling the relative rates and/or extents of two different, competing reaction pathways.

Reactions of Metal Alkyl Complexes

Recall that β-hydride elimination is an extremely common—and sometimes problematic—transformation of metal alkyls. Then again, there are reactions for which β-hydride elimination is desirable, such as the Heck reaction. Structural modifications that strengthen the M–H bond relative to the M–C bond encourage β-hydride elimination; the step can also be driven by trapping of the metal hydride product with a base (the Heck reaction uses this idea).

During the Heck reaction, beta-hydride elimination is driven by a base.

During the Heck reaction, beta-hydride elimination is driven by a base.

On the flip side, stabilization of the M–C bond discourages elimination and encourages its reverse: migratory insertion of olefins into M–H. Previously we saw the example of perfluoroalkyl ligands, which possess exceptionally stable M–C bonds. The fundamental idea here—that electron-withdrawing groups on the alkyl ligand stabilize the M–C bond—is quite general. Hartwig describes an increase in the “ionic character” of the M–C bond upon the addition of electron-withdrawing groups to the alkyl ligand (thereby strengthening the M–C bond, since ionic bonds are stronger than covalent bonds). Bond energies from organic chemistry bear out this idea to an extent; for instance, see the relative BDEs of Me–Me, Me–Ph, and Me–CCH in this reference. I still find this explanation a little “hand-wavy,” but it serves our purpose, I suppose. Read the rest of this entry »

Epic Ligand Survey: Metal Alkyls (Part 2)

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In this post, we’ll explore the most common synthetic methods for the synthesis of alkyl complexes. In addition to enumerating the reactions that produce alkyl complexes, this post will also describe strategies for getting around β-hydride elimination when isolable alkyl complexes are the goal. Here we go!

Properties of Stable Alkyl Complexes

Stable alkyl complexes must be resistant to β-hydride elimination. In the last post we identified four key conditions necessary for elimination to occur:

1. The β-carbon must bear a hydrogen.
2. The M–C and C–H bonds must be able to achieve a syn coplanar orientation (pointing in the same direction in parallel planes).
3. The metal must bear 16 total electrons or fewer and possess an open coordination site.
4. The metal must be at least d2.

Stable alkyl complexes must violate at least one of these conditions. For example, titanium(IV) complexes lacking d electrons β-eliminate very slowly. The complex below likely also benefits from chelation (see below).

Without d electrons, elimination becomes difficult.

No d electrons here!

Read the rest of this entry »

Written by Michael Evans

March 5, 2012 at 9:35 am

Epic Ligand Survey: Metal Alkyls (Part 1)

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Epic Ligand Survey: Metal Alkyls (Part 1)With this post we finally reach the defining ligands of organometallic chemistry, alkyls. Metal alkyls feature a metal-carbon σ bond and are usually actor ligands, although some alkyl ligands behave as spectators. Our aim will be to understand the general dependence of the behavior of alkyl ligands on the metal center and the ligand’s substituents. Using this knowledge, we can make meaningful comparisons between related metal alkyl complexes and educated predictions about their likely behavior. Because alkyl ligands are central to organometallic chemistry, I’ve decided to spread this discussion across multiple posts. We’ll deal first with the general properties of metal alkyls.

General Properties

In the Simplifying the Organometallic Complex series, we decomposed the M–C bond into a positively charged metal and negatively charged carbon. This deconstruction procedure is consistent with the relative electronegativities of carbon and the transition metals. It can be very useful for us to imagine metal alkyls essentially as stabilized carbanions—but it’s also important to understand that M–C bonds run the gamut from extremely ionic and salt-like (NaCH3) to essentially covalent ([HgCH3]+). The reactivity of the alkyl ligand is inversely related to the electronegativity of the metal center.

Reactivity decreases as the metal's electronegativity increases.

Reactivity decreases as the metal's electronegativity increases. Values given are Pauling electronegativities.

The hybridization of the carbon atom is also important, and the trend here follows the trend in nucleophilicity as a function of hybridization in organic chemistry. sp-Hybridized ligands are the least nucleophilic, followed by sp2 and sp3 ligands respectively. Read the rest of this entry »

Written by Michael Evans

March 1, 2012 at 12:45 pm