Page 113 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: FPP Revised Pages
Encyclopedia of Physical Science and Technology EN002C-85 May 17, 2001 20:35

Catalysis, Homogeneous 461

FIGURE 8 β-hydride elimination.

into an octahedral 18-electron complex. In Fig. 9 we have
depicted the oxidative addition of methyl iodide to Vaska’s
complex (L = phosphine).
FIGURE 6 Hydride migration to ethene.
The oxidative addition of acids to zerovalent metals
such as nickel is also an instructive example. It resem-
(Fig. 8). The migration reaction diminishes the total elec- bles the reactions with alkyl halides and may result in an
tron count of the complex by two, and creates formally “amphoteric” hydride:
a vacant site at the metal; β-elimination does the oppo- L 4 Ni + H → L 4 HNi +
+
site. β-Elimination requires a vacant site at the complex
(neglecting solvent coordination) and during the process The starting material is an 18-electron nickel zero complex
the electron count of the complex increases by two elec- which is protonated forming a divalent nickel hydride.
trons. The reaction resembles the β-elimination occurring This can react further with alkenes to give alkyl groups,
in many organic reactions, but the difference lies in the but it also reacts as an acid with hard bases to regenerate
intramolecular nature of the present process, as the elim- the nickel zero complex.
inated alkene may be retained in the complex. In organic The oxidative addition of alkyl halides can proceed in
chemistry the reaction may well be a two-step process, different ways, although the result is usually a trans addi-
e.g.,protoneliminationwithabasefollowedbytheleaving tion independent of the mechanism. In certain cases the
of the anion. In transition metal chemistry the availability reaction proceeds as an S N 2 reaction as in organic chem-
of d-orbitals facilitates a concerted cis β-elimination. istry. That is to say that the electron-rich metal nucleophile
Instead of β-elimination one will also find the terms de- attacks the carbon atom of the alkyl halide, the halide be-
insertion and extrusion, especially for CO. The process is ing the leaving group. This process leads to inversion of
completely analogous because the stereochemistry of the carbon atom (only when the
carbon atom is asymmetric this can be observed). There
1. a vacant site is needed for the reaction to occur; are also examples in which racemization occurs. This has
2. the electron count of the metal increases with two been explained on the basis of a radical chain mechanism.
during the de-insertion (provided that the carbon The reaction sequence for the radical chain process reads
monoxide remains coordinated to the metal and as follows:
solvent coordination is neglected).
L n M + R•→ L n RM•
L n RM•+ RX → L n RMX + R•
D. Oxidative Addition
OxidativeadditionsinvolvingC-Hbondbreakingisatopic
In an oxidative addition reaction a compound XY adds of interest, usually referred to as C-H activation; the idea
to a metal complex during which the XY bond is broken is that the M-H and M–hydrocarbyl bonds formed will
and two new bonds are formed, MX and MY. X and Y are be much more prone to functionalization than the unre-
reduced and will at least formally have a minus one charge active C-H bond. Intramolecular oxidative additions of
and hence the formal valency of the metal is raised by two. C-H bonds have been known for quite some time, see
The coordination number of the metal increases by two. Fig. 10. This process is named orthometalation. It occurs
The electron count around the metal complex increases frequently in metal complexes, and is not restricted to “or-
by two, but the d-electron count of the metal decreases by tho” protons.
two. The 16-electron square planar complex is converted Oxidative addition reactions involving carbon-to-
oxygen bonds are of relevance to the catalysis with

FIGURE 7 Syn additin of metal hydride to alkene. FIGURE 9 Oxidative addition of methyl iodide to Ir(I).