Page 132 - 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

480 Catalysis, Homogeneous

FIGURE 55 Regioselectivity in allylic substitution.

the synthesis of many complex organic molecules. During rically 1,3-diphenyl substituted allylpalladium complex
the reaction a new carbon-carbon bond is formed and the shown in Fig. 56. The ligand used is a C 2 chiral diphos-
resulting molecule still contains a double bond that might phine ligand, but also C 1 asymmetric ligands have been
be used for further derivatization. successfully applied. The orientation of the two phenyl
The reaction starts with an oxidative addition of an al- substituents at carbons 1 and 3 of the allyl fragment is
lylic compound to palladium zero. Allyl halides, carboxy- different under the influence of the chiral C 2 ligand. With-
lates, etc., can be used. At first we will consider triph- out the chiral C 2 ligand carbons 1 and 3 are mirror im-
enylphosphine as the ligand, but often ***large ligand ages; palladium is attached to the “local” re- and si-face.
influences have been detected. A π-allyl-palladium com- The chiral C 2 ligand makes carbons 1 and 3 diastero-
plex forms. Formally, the allyl group is an anion in this topic, i.e., they are chemically different. As a result one
complex, but owing to the high electrophilicity of palla- specific carbon atom (carbon 1) undergoes selective at-
dium, the allyl group undergoes attack by nucleophilic tack by the nucleophile. This way a chiral compound is
reagents, especially soft nucleophiles. After this attack, obtained.
palladium(0) “leaves” the allyl group and the product is Agreatvarietyofligandshavebeenused.Trosthasbeen
obtained. Palladium zero can reenter the cycle and hence especially successful using a chiral bidentate phosphine
the reaction is catalytic in palladium. As a side-product with a very large bite angle (110 ); see Fig. 57.
◦
a salt is formed. For small-scale industrial applications it This ligand “embraces” the metal and thus exerts its in-
is not a problem to make salts in stoichiometric amounts. fluence on the allyl group. Even substrates carrying small
The mechanism and a few reagents are shown in Fig. 54. substituents can now be asymmetrically substituted.
Regioselectivity is high in this reaction and depends
on the ligand used. Ligand effects can ensure substitution
2. Cross Coupling
at the allylic carbon carrying most carbon substituents,
or just the reverse. When a diphosphine ligand is used The making of carbon-to-carbon bonds from carbo-
a hexenyl group instead of the allyl group substitution cations and carbo-anions is a straightforward and sim-
occurs mainly at the terminal carbon, see Fig. 55. ple reaction. Easily accessible carbo-anions are Grignard
An asymmetric application of this reaction has been de- reagents RMgBr and lithium reagents RLi. They can be
veloped. The model substrate studied most is the symmet- conveniently obtained from the halides RBr or RCl and the

FIGURE 56 Asymmetric allylic substitution.