Page 133 - 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 481

bromide. The third reaction that occurs is the reductive
elimination giving the organic cross-coupling product and
the palladium(0) catalyst in its initial state.
Less reactive organometallics derived from tin and
boron can also be used. These reagents do not react with
water, but they are still able to alkylate the palladium
bromide intermediate. As mentioned above, their forma-
tion does involve one more step because they are also
FIGURE 57 One of Trost’s asymmetric ligands for “AAA,” asym-
made via Grignard type reagents. The coupling using tin
metric allylic alkylation.
organometallics is referred to as “Stille” couplings. The
second reagent based on boron was introduced by Suzuki.
The boronic ester derivative is made from trimethyl borate
metals Mg and Li. They are both highly reactive materials,
and an aryl anion reagent followed by hydrolysis of the
for instance, with respect to water.
two remaining methyl ester groups. This phenylboronic
The reactions of these carbon centered anions with po-
acid is soluble in water and the coupling reaction can even
lar compounds such as esters, ketones, and metal chlo-
be carried out in water. See Fig. 59 for a scheme of the
rides are indeed very specific and give high yields. The
Suzuki coupling.
reaction of Grignard reagents and the like with alkyl or
The cross-coupling reaction is applied industrially in
aryl halogenides, however, is extremely slow giving many
the synthesis of alkyl-aryl compounds that are used in liq-
side-products, if anything happens at all.
uid crystals, aryl-aryl compounds in agrochemicals, phar-
The “cross coupling” reaction has found wide applica-
maceuticals, etc.
tion both in organic synthesis in the lab and in industrial
environment. The transition metal catalysts are usually
nickelandpalladium.Inadditiontoorganomagnesiumand
3. Heck Reaction
organolithium a great variety of organometallic precursors
can be used. Also, many precursors can serve as starting A reaction related to the cross coupling is the Heck reac-
materials for the carbocation. Last but not least, the ligand tion. The reaction involves the coupling of an aryl halide
on the transition metal plays an important role in deter- (or pseudo halide) with an alkene in the presence of a base.
mining the rate and selectivity of the reaction. Here we Owing to the latter, as in the cross-coupling reaction an
will present only the main scheme and take palladium as equivalent of base is formed. In the base case, the Heck
the catalyst example. Figure 58 gives the general scheme reaction will produce substituted styrenes. These products
of the palladium-catalyzed cross-coupling reaction. can also be made in the cross-coupling reaction discussed
We can start with palladium(II) or palladium(0), but for above. It is attractive that the Heck reaction does not in-
the present explanation the latter is more convenient. Ox- volve Grignard-type reagents and thus it allows the pres-
idative addition of an aryl halide (PhBr in Fig. 58) to palla- ence of groups reactive toward Grignard reagents such as
dium zero takes place and a square planar Pd(II) complex esters, acids, ketones, etc.
is formed. Subsequently the inorganic bromide reacts with The reaction starts again with the oxidative addition of
the Grignard or lithium alkyl reagent (here 2-BuMgBr) an aryl halide (Br or I) to palladium zero. The next step
giving a diorganopalladium complex and magnesium di- is the insertion of an alkene into the palladium carbon

FIGURE 58 Palladium-catalyzed cross coupling.