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Allyl carboxylates are useful synthetic intermediates in a variety of organic transformations, including catalytic nucleophilic/electrophilic allylic substitution reactions and 1,2-difunctionalization reactions. However, the catalytic 1,3-difunctionalization of allyl carboxylates remains elusive. Herein, we report the first photoinduced, phosphine-catalyzed 1,3-carbobromination of allyl carboxylates, affording a range of valuable substituted isopropyl carboxylates (sIPC). The transformation has broad functional group tolerance, is amenable to the late-stage modification of complex molecules and gram-scale synthesis, and expands the reaction profiles of allyl carboxylates and phosphine catalysis. Preliminary experimental and computational studies suggest a non-chain-radical mechanism involving the formation of an electron donor–acceptor complex, 1,2-radical migration (RaM), and Br-atom transfer processes. We anticipate that the 1,2-RaM reactivity of allyl carboxylates and the phosphine-catalyzed radical reaction will both serve as a platform for the development of new transformations in organic synthesis. 

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 S_N2 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.