Page 114 - 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

462 Catalysis, Homogeneous

FIGURE 10 Oxidative addition of a carbon-hydrogen bond. FIGURE 12 Palladium-catalyzed cross coupling.

palladium complexes. The most reactive carbon-oxygen F. α-Elimination Reactions
bond is that between allylic fragments and carboxylates.
α-Elimination reactions have been the subject of much
The reaction starts with a palladium zero complex and the
study since the mid-1970s mainly due to the pioneering
product is a π-allylic palladium(II) carboxylate (Fig. 11).
work of Schrock. The Early Transition Metals are most
The point of interest is the “amphoteric” character of the
prone to α-elimination, but the number of examples of
allyl anion in this complex. On the one hand it may react
the later elements is growing. A classic example is shown
as an anion, but on the other hand it is susceptible to nu-
in Fig. 13. The sequence of elementary steps, the nature
cleophilic attack by, for example, carbon centered anions.
of the reagents, and the reaction conditions are pertinent
This has found widespread use in organic synthesis. The
to the success of such reactions, but these details do not
reaction with the anion releases a zerovalent palladium
concern us here. Dimethyl complexes of many metals lead
complex and in this manner palladium can be employed
as a catalyst. to formation of methane via an α-elimination process, but
often the putative metal-alkylidene species is too reactive
to be isolated.
E. Reductive Elimination
Metal alkylidene complexes find application in the
Reductive elimination is simply the reverse reaction of metathesis of alkenes, the cyclopropanation of alkenes,
oxidative addition; the formal valence state of the metal Wittig-type reactions, and the McMurry reaction. In suit-
is reduced by two (or one, in a bimetallic reaction) and able complexes α-elimination can occur twice yielding
the total electron count of the complex is reduced by two. alkylidyne complexes. Fig. 14 shows a schematic exam-
While oxidative addition can also be observed for main ple for tungsten. Alkylidyne complexes can be used as
group elements, this reaction is more typical of the transi- catalysts for the metathesis of alkynes.
tion elements in particular the electronegative, noble met-
als. In a catalytic cycle the two reactions occur pairwise.
G. Cyclometallation
At one stage the oxidative addition occurs, followed by,
e.g., insertion reactions, and then the cycle is completed Cyclometallation refers to a process of unsaturated moi-
by a reductive elimination of the product. eties forming a metallacyclic compound. Examples of
Reductive elimination of molecules with carbon-carbon the process are presented in Fig. 15. Metal complexes
bonds has no counterpart in oxidative addition reactions. revealing these reactions comprise: M = L 2 Ni for reac-
First, because the metal-carbon bonds energies may not tion a,M = Cp 2 Ti for reactions b and c,M = Ta for d,
always be large enough to compensate for the energy and M = (RO) 3 W for e. The latter examples involving
of the carbon-carbon bond, and secondly the carbon- metal-to-carbon multiple bonds have only been observed
carbon bond is much less reactive than a carbon-hydrogen for early transition metal complexes, the same ones men-
bond or a dihydrogen bond due to repulsive interactions. tioned under α-elimination, Section F.
In organic synthesis the palladium- or nickel-catalyzed The reverse reaction of a cyclometallation is of impor-
cross-coupling presents a very common example involv- tance for the construction of catalytic cycles. The retro-
ing oxidative addition/reductive elimination. A simplified cyclometallations of reactions a and b are not productive,
scheme is shown in Fig. 12. The third step shows the re- unless the structures were obtained via another route. For
ductive elimination. c–e the following retro reactions can be envisaged leading
to new products; see Figs. 16 and 17.

FIGURE 11 Oxidative addition of allyl acetate to Pd(0). FIGURE 13 α-elimination leading to a metal alkylidne complex.