Introduction to New Trends in Cross-Coupling - New Trends in Cross-Coupling: Theory and Applications - page 13

Figure 1.9 Prof. Sonogashira (Pd/Cu-catalyzed sp-sp 2 coupling).

H

0.005 mol % Pd(PPh 3 ) 2 Cl 2
0.01 mol % CuI

I

+

90 %

HNEt 2 ,23°C

Sonogashira 1975

Scheme 1.7 Cu-mediated Pd-catalyzed coupling of alkenes with aryl iodides.

respectively. In addition, Sonogashira's studies showed that the presence of
catalytic amounts of CuI enabled the reaction to proceed eciently under
significantly milder reaction conditions (Scheme 1.7). Recent work from a
few laboratories, including ours, demonstrated that Cu is not required; in
fact, it is detrimental for alkynylation reactions of aryl chlorides and un-
activated aryl bromides where electron-rich Pd catalysts are required. 37 An
insight into this phenomenon has been proposed. 37a
Until 1976, the development of cross-coupling reactions had focused on
the use of magnesium-based Grignard reagents as nucleophiles, with the
exceptions of the discoveries of Mizoroki, Heck and Sonogashira. However,
in 1976, Negishi showed that other organometallic reagents could be used as
coupling partners. 38 First, Ni- or Pd-catalyzed cross-coupling reactions with
organoaluminium reagents as nucleophiles were disclosed, 39 followed by the
use of arylzinc reagents (Negishi reaction) (Scheme 1.8). 40