www.asiachem.news December 2021 | 27 Cooperative synergistic catalysis To avoid this dilemma and promote catalytic cycles requiring distinct electronic properties in each step, the use of two different metal catalysts can be considered.1 One possible scenario is the use of a metal catalyst that activates electrophiles with another that preferentially reacts with nucleophiles. The thus-formed organometallic intermediates would react with each other to afford the products. This type of synergistic cooperative metal catalysis (Scheme 2a) has emerged as a powerful strategy to significantly expand the scope of cross-coupling reactions in recent years.2 The pioneering contribution of this type of synergistic cooperative metal/metal catalysis for cross-coupling was reported in 1975 by Japanese chemists Sonogashira, Tohda, and Hagihara (Scheme 2b).3 This reaction, which is now commonly called the Hagihara–Sonogashira coupling reaction, allows the coupling of organic halides and pseudo-halides with terminal alkynes via cooperative Pd/Cu catalysis. Pd(0) activates the organic halides through oxidative addition to give an electrophilic Pd(II) species, whereas Cu(I) reacts with terminal alkynes to generate Cu(I) acetylides as nucleophilic organometallic intermediates. These react with each other to give diorgano-Pd(II) intermediates that afford the product through C–C bond-forming reductive elimination. After 30 years, the scope of substrates for the Cu cycle was expanded to arenecarboxylic acids, which undergo decarboxylation to generate arylcopper nucleophiles. The decarboxylative coupling of carboxylic acids with organic electrophiles was established using cooperative Pd/Cu catalysis by Gooßen and coworkers (Scheme 2c).4 The Cu(I) cycle was then further developed to accommodate multi-step transformations to generate organocopper nucleophiles. We demonstrated the first example of such a system by integrating the functionalization of alkenes in the Cu(I) cycle to develop an arylboration reaction (Scheme 2d).5 This reaction can be useful to give highly functionalized organoboron compounds, which play versatile and important roles in modern organic synthesis. Combinations of other transition metals for cooperative synergistic catalysis have also been reported. A pioneering example was reported by Sawamura, Sudoh, and Ito, who showed that the combination of Pd and Rh, both bearing chiral ligands, effectively catalyzed the allylation of α-cyanoesters to construct quaternary carbon stereocenters (Scheme 3a).6 It is essential for both metal catalysts to be optically pure to obtain high enantioselectivity in the allylation reaction. Catalytic C–H functionalization is one of the most important and extensively studied organic transformations in modern organic synthesis.7 Cooperative synergistic catalysis involving C–H activation represents an ideal strategy to expand the scope of C–H functionalization, because metal complexes effective for C–H activation are not always competent for the subsequent functionalization events, in which the other metal catalyst could play a role in cooperative catalysis. Chang and coworkers reported a pioneering example using cooperative Pd/Ru catalysis (Scheme 3b).8 The Ru cycle is likely responsible for the C–H activation, and a cross-coupling-type reaction proceeds on Pd. Another challenging and useful transformation in current synthetic organic chemistry is the so-called cross-electrophile coupling reaction,9 in which two different electrophiles must be distinguished. Weix and coworkers demonstrated that cooperative synergistic Pd/Ni catalysis allowed the selective reductive cross-coupling of aryl halides and aryl triflates (Scheme 3c).10 Scheme 1: A simplified single-metal-based catalytic cycle for cross-coupling reactions.
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