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Abstract The search for more effective and highly selective C–H bond oxidation of accessible hydrocarbons and biomolecules is a greatly attractive research mission. The elucidating of mechanism and controlling factors will, undoubtedly, help to broaden scope of these synthetic protocols, and enable discovery of more efficient, environmentally benign, and highly practical new C–H oxidation reactions. Here, we reveal the stepwise intramolecular SN2 nucleophilic substitution mechanism with the rate-limiting C–O bond formation step for the Pd(II)-catalyzed C(sp3)–H lactonization in aromatic 2,6-dimethylbenzoic acid. We show that for this reaction, the direct C–O reductive elimination from both Pd(II) and Pd(IV) (oxidized by O2oxidant) intermediates is unfavorable. Critical factors controlling the outcome of this reaction are the presence of the η3-(π-benzylic)–Pd and K+–O(carboxylic) interactions. The controlling factors of the benzylic vs ortho site-selectivity of this reaction are the: (a) difference in the strains of the generated lactone rings; (b) difference in the strengths of the η3-(π-benzylic)–Pd and η2-(π-phenyl)–Pd interactions, and (c) more pronounced electrostatic interaction between the nucleophilic oxygen and K+cation in the ortho-C–H activation transition state. The presented data indicate the utmost importance of base, substrate, and ligand in the selective C(sp3)–H bond lactonization in the presence of C(sp2)–H.
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Allyl‐Allyl Coupling Promoted by Catalyst Systems with two Palladium Atoms – A Plethora of Potentially Pericyclic Processes
Abstract Recently, Huang and co‐workers reported a catalytic reaction that utilizes H2as the sole reductant for a C−C coupling of allyl groups with yields up to 96 %. Here we use computational quantum chemistry to identify several key features of this reaction that provide clarity on how it proceeds. We propose the involvement of a Pd−Pd bound dimer precatalyst, demonstrate the importance of ligand π‐π interactions and counterions, and identify a new, energetically viable, mechanism involving two dimerized, outer‐sphere reductive elimination transition structures that determine both the rate and selectivity. Although we rule out the previously proposed transmetalation step on energetic grounds, we show it to have an unusual aromatic transition structure in which two Pd atoms support rearranging electrons. The prevalence of potential metal‐supported pericyclic reactions in this system suggests that one should consider such processes regularly, but the results of our calculations also indicate that one should do so with caution.
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- Award ID(s):
- 2154083
- PAR ID:
- 10517324
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Angewandte Chemie International Edition
- ISSN:
- 1433-7851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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