Page 115 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002C-85 May 17, 2001 20:35

Catalysis, Homogeneous 463

FIGURE 14 α-elimination leading to a metal alkylidyne complex.

H. Activation of a Substrate Toward FIGURE 16 Retrocyclometallation reactions.
Nucleophilic Attack
1. Alkenes
with water to metal hydrides and carbon dioxide (“shift
Coordination of an alkene to an electronegative metal ac- reaction”); see Fig. 19.
tivates the metal toward attack of nucleophiles. After the
nucleophilic attack the alkene complex has been converted I. σ-Bond Metathesis
into a σ-bonded alkyl complex with the nucleophile at the
β-position. With respect to the alkene (in the “organic” A reaction that is relatively new and not mentioned in
terminology) the alkene has undergone anti-addition of older textbooks is the so-called σ-bond metathesis. It is
M and the nucleophile Nu; see Fig. 18. a concerted 2 + 2 reaction immediately followed by its
As indicated under section B the overall result is the retrograde reaction giving metathesis. Both late and early
same as that of an insertion reaction, the difference being transition metal alkyls are prone to this reaction, but its
that insertion gives rise to a syn-addition and nucleophilic occurrence had to be particularly invoked in the case of
attack to an anti-addition. In Fig. 18 the depicted nucle- the early transition metals; the latter often lack d-electrons
ophile is anionic, but Nu may also be a neutral nucleophile needed for other mechanisms such as oxidative addition.
0
such as an amine. The addition reaction of this type is the This alternative does not exist for d complexes such as
keystep in the Wacker-type processes catalyzed by palla- Sc(III), Ti(IV), Ta(V), W(VI), etc., and in such cases
dium. σ-bond metathesis is the most plausible mechanism. In
Fig. 20 the reaction has been depicted.

2. Carbon Monoxide
J. Dihydrogen Activation
Coordinated carbon monoxide is subject to activation to-
ward nucleophilic attack. Through σ-donation and π-back In the above reactions we have not explicitly touched upon
∗
donation into the antibonding CO π orbitals the carbon the reactions of dihydrogen and transition metal com-
atom has obtained a positive character. This makes the car- plexes. Here the reactions that involve the activation of
bon atom not only more susceptible for a migrating anion dihydrogen will be summarized, because they are very
at the metal center, but also for a nucleophile attacking common in homogeneous catalysis and because a compar-
from outside the coordination sphere. In this instance it is ison of the various mechanisms involved may be useful.
more difficult to differentiate between the two pathways. Four reactions are usually distinguished for hydrogen:
The nucleophilic attack by alkoxides, amines, and water
is of great interest to homogeneous catalysis. A dominant 1. oxidative addition (see Fig. 9),
reaction in syn-gas systems is the conversion of carbonyls 2. bimetallic oxidative addition,
3. heterolytic cleavage, and
4. σ-bond metathesis (see Fig. 20).

1. Oxidative Addition
Oxidative addition of dihydrogen commonly involves
8
transformation of a d square planar metal complex into
6
ad octahedral metal complex, or similar transformations

FIGURE 15 Schematic overview of cyclometallation reactions. FIGURE 17 Cyclization through reductive elimination.