Page 138 - 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

486 Catalysis, Homogeneous

FIGURE 68 Simplified scheme for p-xylene oxidation.

J. Adiponitrile by Hydrocyanation the kinetics of the reactions; the reaction 2 to 4 reaches
equilibrium, but the reaction 2 to 3 does not. Note that the
The transition metal-catalyzed addition of HCN to alkenes
nickel complex not only is responsible for the addition of
is potentially a very useful reaction in organic synthesis,
HCN but that it is also capable of catalyzing selectively
and it certainly would have been widely applied if its at-
the isomerization. The final step is the addition of HCN
traction was not largely offset by the toxicity of HCN.
to 4 to give 5, adiponitrile.
Industrially the difficulties can sometimes be minimized
to an acceptable level, and Du Pont has commercialized
the addition of HCN to butadiene for the production of K. Alkene Metathesis
adiponitrile (NC(CH 2 ) 4 CN), a precursor to 6,6-nylon.
Metathesis of alkenes has been known for quite some time.
The catalyst precursor is a nickel(0) tetrakis (phosphite)
The initial catalysts were based on tungsten, molybdenum,
complex which is protonated to form a nickel(II) hydride.
and rhenium. Both homogeneous and heterogeneous cat-
Alternatively, we could write an oxidative addition of
HCN to nickel zero. Subsequently coordination and inser- alysts find application. Well-known, older homogeneous
catalysts can be formed from WCl 6 and an alkylating
tion of the diene takes place followed by reductive elim-
ination of a pentenenitrile with concurrent regeneration reagent. After dialkylation with reagents such as EtAlCl 2
of the nickel zero complex. Two isomerization reactions
must occur in order to achieve a high selectivity, and then
the HCN addition cycle is performed for the second time
on the much less reactive 4-pentenenitrile. In Fig. 69 the
hydrocyanation mechanism in a simplified form is given
for ethene; the basic steps are the same as for butadiene
but the complications due to isomer formation are lacking.
Hydrocyanation of butadiene is more complicated than
that of ethene; it requires two hydrocyanation steps and
several isomers can be observed. The isomers obtained in
the first step of the HCN addition to butadiene are shown
in Fig. 70. The addition first leads to compounds 1 and 2,
which equilibrate perhaps via the retroreaction. In order
to obtain adiponitrile, 2 should isomerize to 4, and not to
the thermodynamically more stable 3 (conjugation). The
thermodynamic ratio is 2:3:4 = 20:78:1.6. The isomer-
ization of 2 to 4 happens to be favorably controlled by FIGURE 69 Hydrocyanation of ethene.