Page 139 - 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 487

FIGURE 70 Hydrocyanation of butadiene leading to adiponitrile.

or R 4 Sn, α-elimination occurs and a metal alkylidene is Many applications of these new catalysts to organic
formed, Cl 4 W=CR 2 , see elementary steps. Often oxy- chemistry are known as they can serve as a means to make
gen containing promoters including O 2 , EtOH, or PhOH large rings, for instance, after two allyl groups have been
are added. Turnover frequencies as high as 300,000 mol introduced to a nonlinear fragment; see Fig. 73. The re-
−1
(product) · mol (cat) · h −1 can be achieved, even at room sulting double bond can be further functionalized or hy-
temperature. The development of well-defined catalysts drogenated if needed.
based on molybdenum and tungsten is mainly due to
Schrock. Synthetically useful reactions, including acyclic
olefin metathesis, ring-opening metathesis polymerization L. Cyclopropanation
(ROMP), alkyne polymerization, acyclic diene metathe-
As in the previous section about metathesis, cyclopropa-
sis polymerization (ADMET), and ring-closing metathe-
nation is concerned with the transfer of carbene or alkyli-
sis (RCM) have been catalyzed by early-transition-metal
dene species from a metal to an organic molecule. The
alkylidenes. Porri introduced in situ catalysts based on
reaction involves the metal-catalyzed decomposition of a
ruthenium or iridium in polar media. The synthesis of
diazo compound to give a reactive metal-alkylidene com-
well-defined ruthenium alkylidene complexes of the type
plex followed by the transfer of the “carbene” to an alkene.
RuCl 2 (=CHPh)(PR 3 ) 2 was developed by Grubbs; see
The product of this very convenient reaction is a cyclo-
Fig. 71.
propane derivative in a single step (see Fig. 74). Catalysts
The peculiar property of the ruthenium (and also irid-
used for this reaction are based on copper, rhodium, and
ium catalysts) is that they are active in the presence of
ruthenium. A common, relatively stable diazo compound
polar substituents. The initial catalysts, generated from
is N 2 CHCO 2 Et. The example shown in Fig. 74 is carried
high-valence metal chlorides with alkylating agents were
out commercially using a copper complex containing a
not resistant to oxygen-containing compounds. Schrock
chiral imine dialkoxide ligand. After conversion of the
developed sterically hindered alkylidene complexes, such
as the one shown in Fig. 72. The in situ prepared catalysts
containing bulky phenols have also been reported. By re-
placing the two alkoxides by a chiral bulky bisnaphthol
asymmetric metathesis has been achieved.

FIGURE 72 Schrock’s catalyst for metathesis of functional
FIGURE 71 An example of Grubbs’ ruthenium catalysts. alkenes.