Here we report that palladium(0)
complexes can coordinate in a
η2 fashion to 1,3-dienes and significantly raise
the energy of their highest occupied molecular orbital (HOMO) by donating
the electrons from the d-orbitals to the empty antibonding molecular
orbitals of double bonds (π*) via back-bonding. Thus, the uncoordinated
double bond, as a more reactive partner on the basis of the principle
of vinylogy, can directly attack imines, furnishing a formal hydrodienylation
reaction enantioselectively. A chemoselective cascade vinylogous addition/allylic
alkylation difunctionalization process between 1,3-dienes and imines
with a nucleophilic group is also compatible, by trapping in situ formed π-allylpalladium species after initial
ene addition. This π-Lewis base catalytic mode, featuring simple
η2coordination, vinylogous activation, and compatibility
with both conjugated neutral polyenes and electron-deficient polyenes,
is elucidated by control experiments and density functional theory
(DFT) calculations.
The vinyl group tethered to furfurals could be LUMO-lowered by forming formal vinylogous iminium ion intermediates catalyzed by a chiral secondary amine and underwent asymmetric [3 + 2] cycloaddition reactions with N-trifluoroethylsubstituted isatin imines, furnishing a variety of spirooxindoles incorporating a 3,2′-pyrrolidine motif with excellent stereoselectivity. In addition, this strategy has been successfully expanded to a number of vinyl-substituted electron-rich heteroaryl aldehydes and even some specific aryl aldehydes.
The
cinnamates having an ortho-formyl group can
potentially form vinylogous iminium ion species under the catalysis
of chiral amines, which facilitates the Diels–Alder cycloaddition
reaction with the concurrently generated trienamines between dienals
and amine catalysts in a regioselectivity umpolung manner. A cascade
intramolecular aldol reaction was followed, finally furnishing polyhydrophenanthrene
frameworks with excellent diastereo- and enantioselectivity.
An efficient approach to construct chiral 1,1-disubstituted ethane derivatives is presented. This strategy relies on the formation of the key dearomatizative vinylogous iminium ion species through protonation of the formal trienamine intermediates between 2-(3-vinylbenzofuran-2-yl)ethan-1-ones and a chiral primary amine. An array of nucleophiles, including 4-hydroxycoumarins, indoles, etc., have been effectively assembled at the benzylic site, delivering the expected 1,1-disubstituted ethane products in moderate to excellent enantioselectivity.
Numerous protocols have been developed for the functionalization of aromatic substances. Among them, the strategy by which aromatic substrates are activated in situ to generate dearomatized intermediates is highly efficient but challenging, especially in the field of asymmetric catalysis. In this Concept article, the application of some well-established chiral Lewis base catalysis, including primary/secondary amines and N-heterocyclic carbenes, that can covalently form catalyst-tethered dearomatized ortho/para-quinodimethane species with diverse heteroaryl and aryl carbonyl substrates is summarized in a number of asymmetric cycloaddition and addition reactions with diverse reagents generally having electrophilic properties. As a result, a variety of enantioenriched aromatic products with higher molecular complexity are constructed effectively through a rearomatization process.
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