Abstract
Transforming C(sp3)–H bonds efficiently and selectively into C(sp3)–C(sp3) or C(sp3)–X bonds is a highly relevant task. The direct arylation of allylic C(sp3)–H bonds provides an elegant method for the formation of unconjugated aryl-substituted olefins. Although both ionic- and radical-based transition metal catalysis has been applied to achieve this transformation, numerous challenges remain. The requirement for persistent radical coupling partners, moderate selectivity and the need for tri- or tetrasubstituted olefins have limited the generality of existing methods. Now we report a ternary catalytic method that combines organic photoredox, hydrogen atom transfer and nickel catalysis, and can directly arylate allylic C(sp3)–H bonds of readily available olefins. This process operates under mild conditions and exhibits a remarkable reaction scope in both aryl halide and olefin coupling partners. Mechanistic experiments, coupled with density functional theory calculations of Ni-oxidation states and reaction energetics allowed the elucidation of a ternary catalytic cycle and the origin of regioselectivity. A ternary catalytic method combining organic photoredox, hydrogen atom transfer and nickel catalysis is reported. This combination can directly arylate the allylic C(sp3)–H bonds of a broad range of readily available olefins. Mechanistic experiments, coupled with density functional theory calculations aid the elucidation of the ternary catalytic cycle and the origin of regioselectivity.
Highlights
Transforming C(sp3)–H bonds efficiently and selectively into C(sp3)–C(sp3) or C(sp3)–X bonds is a highly relevant task
The simple feedstocks 1-hexene 1a and aryl bromide 2a were chosen for the initial optimization of the direct allylic C(sp3)–H arylation (DAA; Fig. 2a)
We have shown that a ternary catalytic system that combines organic photoredox, hydrogen atom transfer (HAT) and nickel catalysis can be successfully applied to a direct allylic C(sp3)–H arylation to yield unconjugated alkenes, which are highly challenging to obtain under mild and redox neutral conditions
Summary
Transforming C(sp3)–H bonds efficiently and selectively into C(sp3)–C(sp3) or C(sp3)–X bonds is a highly relevant task. Building on the pioneering work by Arnold and co-workers in the reaction between dicyanobenzene with olefins, Inoue, MacMillan, Kanai and their co-workers developed elegant methods to achieve allylic C(sp3)–H arylation through radical–radical couplings by merging hydrogen atom transfer (HAT) catalysis with photocatalysis (Fig. 1b). These methods are plagued by two main limitations: [1] only persistent radicals (for example, from cyanoarenes) can be used as the coupling partners, which limits the reaction scope, and [2] only a moderate selectivity is usually achieved, owing to the uncontrollable radical– radical coupling. Owing to the interplay between several catalytic cycles, this ternary catalytic system has to face major obstacles regarding mutual compatibility, selectivity and efficiency
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