Introduction to New Trends in Cross-Coupling - New Trends in Cross-Coupling: Theory and Applications - page 8

Ar-I (64 kcal mol 1 ), 15 which highlights the diculty for an aryl chloride to
add oxidatively to a less electron-rich L n Pd(0) species. 16
Interestingly, in the transmetallation step, recent evidence suggests that
the trend is the opposite: chloride complexes are transmetallated faster than
those of bromides and iodides. 13,17 The size of the ligand also plays an
important role in the reductive elimination, 10 in addition to stabilizing the
coordinatively unsaturated L n Pd(0).
1.4 Brief Historical Notes on Cross-Coupling
Reactions and the Contents of This topic
The intent of this chapter is not to provide an exhaustive review of the his-
tory of cross-coupling reactions, 18 but to identify the most notable mile-
stones (Figure 1.6) and the genesis of some of the topics of the chapters
presented here.
Some argue that the history of the use of metals as catalysts to accomplish
organic transformations was initiated by Fittig, who recorded sodium-
mediated alkylations of halogenated arenes in 1862. 19 In the early 1900s,
Ullmann and Goldberg carried out extensive studies on copper-catalyzed
C-C, C-O and C-N bond-forming reactions. 20 Noteworthy is that the first
person to combine successfully organometallic reagents with catalysis, in
this case NiCl 2 , was the French chemist Andr´ Job. 21 He reported that
PhMgBr, in the presence of NiCl 2 , was able to absorb CO, NO, C 2 H 4 ,C 2 H 2
and H 2 . Since Job's underappreciated revolutionary discovery, nickel has
been overshadowed by palladium in similar transformations. Since this early
discovery, carbonylation has become an industrially important process and
its modern version, carbonylative cross-coupling, is reviewed in detail by
Xiao-Feng Wu and Christopher Barnard in Chapter 10.
Following Job's discoveries, the next notable milestone would be the
reports by Kharasch on the metal-catalyzed homo-couplings of organo-
magnesium reagents. 22 More specifically, he employed catalytic amounts of
CoCl 2 , MnCl 2 , FeCl 3 or NiCl 2 in the presence of Grignard reagents and
organic halides to affect this homo-coupling reaction (Scheme 1.2, top).
The use of vinyl bromide in place of bromobenzene, under the same
conditions, resulted not in the expected homo-coupling of the Grignard re-
agent, but in the first-ever reported catalytic cross-coupling reaction
(Scheme 1.2, bottom). 23 These findings, to some extent, make Kharasch
(Figure 1.7) the ''grandfather'' of cross-coupling reactions.
More than 20 years later came the next breakthrough in the Pd-catalyzed
cross-coupling area. Heck reported in 1968 that arylations of alkenes could
be achieved by using an organomercury arylating reagent and a palladium
catalyst (Scheme 1.3). 24
A modification of this Pd-catalyzed reaction was subsequently published
by Moritani and Fujiwara. They disclosed the direct coupling between arenes
and alkenes, first in the presence of stoichiometric amounts of palladium
compounds 25 and later using catalytic amounts (Scheme 1.4). 26 This finding