Cycloaddition reactions between oxyallyl
cations and alkenes are
important transformations for the construction of ring systems. Although
(4 + 3) cycloaddition reactions of oxyallyl cations are well-developed,
(3 + 2) cycloadditions remain rare, and an asymmetric version has
not yet been developed. Moreover, because oxyallyl cations are highly
electrophilic, only electron-rich olefins can be used as cycloaddition
partners. We herein report a method for enantioselective (3 + 2) cycloaddition
reactions between palladium-oxyallyl species and electron-deficient
nitroalkenes. This transformation was enabled by a rationally designed
hydrogen-bond-donating ligand (FeUrPhos) and proceeded via an inverse
electron demand pathway. Using this method, we could assemble cyclopentanones
with up to three contiguous stereocenters with high enantioselectivity
and good to excellent diastereoselectivity.
For nearly 30 years, considerable research effort has been focused on the development of methods for catalytic (3 + 2) cycloaddition reactions of palladium-oxyallyl species with alkenes. However, because C−O bond formation is kinetically favored, the (3 + 2) cycloadditions achieved to date have involved C−O reductive elimination. We herein report a method of lithium triflate-promoted (3 + 2) cycloaddition reactions of palladiumoxyallyl species with 1,3-dienes that proceed via a pathway terminated with C−C bond formation to give a five-membered carbocycle. Coordination of the lithium ion with the alkoxide moiety disrupts the C−O reductive elimination and forms a metalenolate tethered π-allyl-Pd. The π-allyl-Pd moiety then accepts intramolecular allylic attack from the enolate moiety to form carbocyclic products. Furthermore, by tuning the steric properties of the palladium ligand, we could also accomplish the competing (4 + 3) cycloadditions, and thus this method provides regiodivergent access to both cyclopentanones and cycloheptanones. The reaction mechanism was investigated by DFT calculation and the origins of the regioselectivities of the cycloaddition were rationalized.
In
this work, a rhodium-catalyzed oxidative cycloaromatization
of dienynes, which provides a highly straightforward and efficient
way to access polysubstituted naphthols and phenols under mild conditions,
is described. Challenged electron-withdrawing groups are well tolerated
in this protocol, and late-stage phenyl ring formation is demonstrated.
A rhenium-catalyzed carboalkoxylation and carboamination of alkyne is reported. This reaction provides an efficient route to synthesize de novo C3-substituted benzofurans and indoles under mild conditions in moderate to good yields. Mechanistic studies revealed that the rhenium played the role of a π acid catalyst to activate the alkynes, followed by a charge-accelerated [3,3]sigmatropic rearrangement.
This work reports ad ual gold-catalyzed tetradehydro-Diels-Alderr eaction for the synthesis of nitrogencontaining aromatic heterocycles. Under the catalytic system (IPrAuNTf 2 /DIPEA), indolinesa nd carbazoles as well as other N-containinga romatic heterocyclesw ere prepared in high yields with good functional group tolerance. Unlike the traditional thermalt etradehydro-Diels-Alder reactions, diluted reaction concentration and radical prohibitors are not required for this protocol. Experimental data support am echanism involving gold vinylidene species, which undergoes a6 p electrocyclization, followed with 1,2-hydrogen shift.Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.
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