www.asiachem.news December 2021 | 29 Combinations of Lewis-acidic metal catalysts and late transition metal catalysts have been established for cooperative double activation catalysis. Epoxides are highly versatile organic building blocks that are frequently used in organic synthesis. They have a Lewis-basic functionality that undergoes ring-opening reactions, often catalyzed by Lewis acids. Thus, the combination of Lewis acids and low-valent transition metal complexes can be envisaged for the development of novel ring-opening reactions of epoxides catalyzed uniquely via cooperative catalysis. The development of cooperative double activation catalysis for epoxides was pioneered by Coates and coworkers using an anionic cobalt catalyst bearing cationic aluminum as a Lewis-acidic counterion for the reaction of epoxides with carbon monoxide to give b-lactone products (Scheme 6b).19 The epoxides form Lewis pairs with the cationic Al center and react with the nucleophilic anionic cobalt complex to give a metallacycle intermediate, in which CO insertion into the C–Co bond takes place. The lactone products are ejected to regenerate the cooperative catalyst system. Alkynes and alkenes can also serve as Lewis bases via their π-electrons. Some metal complexes are known to act as Lewis acids to form Lewis pairs with alkynes and alkenes, which in turn can act as electrophiles to react with electron-rich low-valent transition metal complexes. In an early example, the regioselective dimerization of styrene was reported by Tsuchimoto, Shirakawa, and coworkers using Pd/In as a cooperative double activation catalyst (Scheme 6c).20 Styrene is likely activated by an In Lewis acid to act as an electrophile for the Pd(0) species; the subsequent carbopalladation reaction across another styrene gives an alkylpalladium intermediate, which undergoes b-hydride elimination to afford the dimerization products. Cooperative double activation catalysis has proved to be effective for catalytic C–H functionalization. As an early example, we reported that cooperative Ni/Zn catalysis allowed C2-alkenylation of pyridine (Scheme 7a).21 Pyridine coordinates to the Lewis-acidic Zn catalyst, and is then activated by the electron-rich Ni(0). Cooperative Ni/Al catalysis with bulky ligands was shown to be useful not only for the acceleration of the C–H functionalization, but also for controlling the site-selectivity. The steric repulsion between bulky Ni and Al catalysts likely induces the high C-4 selectivity of the alkylation reaction (Scheme 7b).22 The Ni/Al catalysis was then applied to site-selective C–H functionalization of substituted benzenes. Para-selective alkylation of benzamides with alkenes was achieved using a similar bulky Ni/Al catalysis system (Scheme 7c).23 Coordination of the Lewis-basic aminocarbonyl functionality to the bulky Al Lewis acid accelerates the rate and controls the para-selectivity of C–H activation by the bulky Ni catalyst. This is in stark contrast to Scheme 6: The concept behind cooperative double activation catalysis, and pioneering examples. Scheme 5: Cooperative synergistic Ni/Ir photoredox catalysis. Scheme 7: Cooperative double activation Ni/Al catalysis for siteselective C–H alkenylation and alkylation.
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