Page 730 - Advanced Organic Chemistry Part B - Reactions & Synthesis

706 epoxide ring-opening reactions are available for the synthesis of alcohols. The reactants
for conjugate addition include -unsaturated ketones, esters, amides, and nitriles,
CHAPTER 8
and these reactions can be combined with tandem alkylation. These synthetic transfor-
Reactions Involving mations are summarized below.
Transition Metals
R R′ RCu(Z) + R′ X R R′ RCu(Z) + R′
R = alkyl, R′= alkyl, R = alkyl, X
alkenyl, aryl alkenyl, aryl alkenyl, aryl
coupling allylic coupling

OH OH
O O
R RCu(Z) + R RCu(Z) +
R′ R′ R′ R′
R = alkyl, R = alkyl,
alkenyl, aryl alkenyl, aryl
epoxide ring - opening vinylogous epoxide ring-opening
R′
R RCu(Z) + R RCu(Z) +
Y Y Y Y + R′ X
R = alkyl, R = alkyl,
alkenyl, aryl Y = CR, CO 2 R, CN alkenyl, aryl Y = CR, CO 2 R, CN
O O
conjugate addition conjugate additon with tandem alkylation

8.2. Reactions Involving Organopalladium Intermediates

Organopalladium intermediates are very important in synthetic organic chemistry.
Usually, organic reactions involving palladium do not involve the preparation
of stoichiometric organopalladium reagents. Rather, organopalladium species are
generated in situ during the course of the reaction. In the most useful processes only
a catalytic amount of palladium is used. The overall reaction mechanisms typically
involve several steps in which organopalladium species are formed, react with other
reagents, give product, and are regenerated in a catalytically active form. Catalytic
processes have both economic and environmental advantages. Since, in principle,
the catalyst is not consumed, it can be used to make product without generating
by-products. Some processes use solid phase catalysts, which further improve the
economic and environmental advantages of catalyst recovery. Reactions that involve
chiral catalysts can generate enantiomerically enriched or pure materials from achiral
starting materials. In this section we focus on carbon-carbon bond formation, but in
Chapter 11 we will see that palladium can also catalyze aromatic substitution reactions.
Several types of organopalladium intermediates are of primary importance in the
reactions that have found synthetic applications. Alkenes react with Pd(II) to give
complexes that are subject to nucleophilic attack. These reactions are closely related
to the solvomercuration reactions discussed in Section 4.1.3. The products that are
formed from the resulting intermediates depend upon specific reaction conditions. The
palladium can be replaced by hydrogen under reductive conditions (path a). In the
absence of a reducing agent, elimination of Pd(0) and a proton occurs, leading to net
substitution of a vinyl hydrogen by the nucleophile (path b). We return to specific
examples of these reactions shortly.