Diradical generation followed by
radical–radical cross-coupling
is a powerful synthetic tool, but its detailed mechanism has yet to
be established. Herein, we proposed and confirmed a new model named
relayed proton-coupled electron transfer (relayed-PCET) for diradical
generation, which could open a door for new radical–radical
cross-coupling reactions. Quantum mechanics calculations were performed
on a selected carbene-mediated diradical cross-coupling reaction model
and a designed model, and the exact electronic structural changes
during the radical processes have been observed for the first time.
Amides are among the most fundamental functional groups and essential structural units, widely used in chemistry, biochemistry and material science. Amide synthesis and transformations is a topic of continuous interest in organic chemistry. However, direct catalytic asymmetric activation of amide C-N bonds still remains a long-standing challenge due to high stability of amide linkages. Herein, we describe an organocatalytic asymmetric amide C-N bonds cleavage of N-sulfonyl biaryl lactams under mild conditions, developing a general and practical method for atroposelective construction of axially chiral biaryl amino acids. A structurally diverse set of axially chiral biaryl amino acids are obtained in high yields with excellent enantioselectivities. Moreover, a variety of axially chiral unsymmetrical biaryl organocatalysts are efficiently constructed from the resulting axially chiral biaryl amino acids by our present strategy, and show competitive outcomes in asymmetric reactions.
Contrary to the traditional implementation as a difluoromethyl group and recently disclosed role of C1 synthons in synthetic organic chemistry, difluorocarbene (:CF 2 ) is reported herein to proceed unprecedented atom recombination as both C1 synthon and F1 reagent simultaneously to render valuable 3-fluorinated oxindoles from 2aminoarylketones. The reaction does not require any catalyst and features a broad range of substrate scope with good functional group compatibility and ease of execution. This transformation could be employed to the quickconstructions of certain bioactive molecule derivatives. The mechanistic experiments and DFT calculations indicate that this atom recombination reaction of difluorocarbene for the synthesis of 3-fluorinated oxindoles may involve a rearrangement process of epoxide intermediates.
Although
breakthroughs in the N-heterocyclic carbene (NHC)-catalyzed
inert C–C bond activation strategy have been achieved, understanding
the role of the catalyst as well as the origin of its chemo- and stereoselectivities
is still one of the most challenging questions in the field of organocatalysis.
Herein, we propose an NHC and NHC·H+ cooperative catalytic
model for these kinds of reactions and perform density functional
theory calculations in the case of an NHC-catalyzed [4 + 2] annulation
reaction of conjugated dienal and α-aryl ketone. The calculated
results indicate that the organocatalyst either works as a Lewis base
to prevent the bad frontier molecular orbital overlap mode, promoting
[2 + 2] cycloaddition, or as a noncovalent organocatalyst to provide
a hydrogen bonding network to facilitate the release of CO2. The latter is remarkably different from its well-known role as
a Lewis base. In addition, we devised an atomic electrophilicity index
to correctly predict the site of the stereoselective C–C bond
formation involved in [4 + 2] cycloaddition. Further analysis results
show that hydrogen bonds significantly contribute to the favorable
stereoselective pathway, which was associated with axial chirality
of the main final product in an experiment. The obtained insights
should be valuable for the prediction and rational design of organocatalytic
inert C–C activations with special chemoselectivity and high
stereoselectivity.
By performing density functional theory (DFT) calculations, we investigated and identified the fundamental pathway for N-heterocyclic carbene (NHC)-catalyzed synthesis of axially chiral benzothiophene-fused biaryl using enal and 2-benzyl-benzothiophene-3-carbaldehyde, which includes...
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