Palladium-catalyzed cross-coupling reactions between benzyl, aryl, or allyl bromides and conjugated ene-yne-ketones lead to the formation of 2-alkenyl-substituted furans. This novel coupling reaction involves oxidative addition, alkyne activation-cyclization, palladium carbene migratory insertion, β-hydride elimination, and catalyst regeneration. Palladium (2-furyl)carbene is proposed as the key intermediate, which is supported by DFT calculations. The palladium carbene character of the key intermediate is validated by three aspects, including bond lengths, Wiberg bond order indices, and molecular orbitals, by comparison to those reported for stable palladium carbene species. Computational studies also revealed that the rate-limiting step is ene-yne-ketone cyclization, which leads to the formation of the palladium (2-furyl)carbene, while the subsequent carbene migratory insertion is a facile process with a low energy barrier (<5 kcal/mol).
Ahead of the PAC: Polycyclic aromatic compounds (PACs) can be easily accessed by the combination of Suzuki–Miyaura cross‐coupling and a [Rh2(OAc)4]‐catalyzed carbene reaction using easily available bis(N‐tosylhydrazone)s as intermediates (see scheme; Ts=4‐toluenesulfonyl).
This contribution describes an advanced
compartmentalized micellar
nanoreactor that possesses a reversible photoresponsive feature and its application toward
photoregulating reaction pathways for incompatible tandem catalysis
under aqueous conditions. The smart nanoreactor is based on multifunctional
amphiphilic poly(2-oxazoline)s and covalently cross-linked with spiropyran
upon micelle formation in water. It responds to light irradiation
in a wavelength-selective manner switching its morphology as confirmed
by dynamic light scattering and cryo-transition electron microscopy.
The compartmental structure renders distinct nanoconfinements for
two incompatible enantioselective transformations: a rhodium–diene
complex-catalyzed asymmetric 1,4-addition occurs in the hydrophilic
corona, while a Rh-TsDPEN-catalyzed asymmetric transfer hydrogenation
proceeds in the hydrophobic core. Control experiments and kinetic
studies showed that the gated behavior induced by the phototriggered
reversible spiropyran to merocyanine transition in the cross-linking
layer is key to discriminate among substrates/reagents during the
catalysis. The smart nanoreactor realized photoregulation to direct
the reaction pathway to give a multichiral product with high conversions
and perfect enantioselectivities in aqueous media. Our SCM catalytic
system, on a basic level, mimics the concepts of compartmentalization
and responsiveness Nature uses to coordinate thousands of incompatible
chemical transformations into streamlined metabolic processes.
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