The merger of photoredox and nickel
catalysis for the
decarboxylative
arylation of carboxylic acids has evolved into an effective strategy
to forge C–C bonds from readily available feedstock. Despite
its rapid industrial adoption, the mechanism of this dual-catalyzed
cross-coupling reaction has remained unclear and under-studied. Here,
we propose an alternative mechanism for the photoredox–Ni dual-catalyzed
decarboxylative arylation of α-amino acids based on control
experiments with NiIIArBr complexes, cyclic voltammetry
(CV), and computational studies. Our mechanistic studies revealed
that a Ni0–NiII–NiI–NiII–Ni0 cycle is feasible in
the dual-catalyzed Csp2
–Csp3
cross-coupling. Distinct from previous mechanism proposals,
we show with a series of CV studies and density functional theory
(DFT) calculations that a single electron transfer reduction of NiIIArBr to NiIAr by IrII is thermodynamically
favorable. Reductive elimination via a NiII-species rather
than via a NiIII-species is also supported by DFT calculations.
Those mechanistic insights allowed for the reaction scope to be extended
to encompass α-amino acids bearing pharmacophoric elements,
which were previously unexplored coupling partners. α-Amino
acids bearing broad functional groups, including heterocycles, successfully
underwent decarboxylative arylation with a diverse set of aryl bromides.
This strategy represents an advance in photoredox and Ni-catalysis
and broadens its industrial applicability as well as mechanistic understanding.
Active methylene compounds are regioselectively dialkylated by variety of alkyl halides using cesium carbonate in quantitative yield. The reaction yielded exclusively dialkylated products with no intermediate monoalkyaltion or mixture of products.
One-pot chemo-, regio-, and stereoselective synthesis of series of heterocyclic and spiroheterocyclic compounds was accomplished through mono- and bis[3 + 2]-cycloaddition reactions of (2E,4E)-ethyl 5-(phenylsulfonyl)penta-2,4-dienoate as a dipolarophile with azomethine ylides, nitrones, and nitrile oxides in good yields. The structures of the products were established by spectroscopic techniques as well as by single-crystal XRD study, and DFT calculations were performed to further understand the mechanism of this [3 + 2]-cycloaddition reaction.
A simple and efficient method for the synthesis of (E,E)-1-(arylsulfonyl)buta-1,3-dienes bearing electron-withdrawing substituents like cyano and ethoxycarbonyl at position 4, involving a one-pot alkylation of bis(phenylsulfonyl)methane with trans-ethyl 4-bromocrotonate/trans-4-bromocrotononitrile, and elimination of arylsulfinic acid, is described. These dienes undergo facile mono [3+2] cycloaddition with azomethine ylides chemoselectivity to furnish functionalized 1,3,4-trisubstituted pyrrolidines. Oxidation of these cycloadduct with MnO 2 ·SiO 2 under mild conditions provides 1,3,4-trisubstituted pyrroles.
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