Pd(0)-Catalyzed Intermolecular Dearomatizing [3 + 2] Spiroannulation of Phenol-Based Biaryls and Allenes

Organocatalytic Asymmetric Dearomative Spirocyclization/Oxa-Michael Addition Sequence: Synthesis of Polycyclic Tetralones

Herein, an organocatalytic asymmetric dearomative spirocyclization/oxa-Michael addition sequence with a newly designed substrate having two naphthol motifs has been developed. The reaction proceeds through in situ chiral vinylidene ortho-quinone methide (VQM) intermediate formation, dearomative spirocyclization of naphthol, and an oxa-Michael addition reaction. The densely functionalized tetralone products were formed in high yields with high diastereo- and enantioselectivities.

Palladium-Catalyzed Carbonylative Dearomatization of Indoles to Achieve Carbonyl-Containing Spirocyclic Indolenines Bearing an All-Carbon Quaternary Center

A Pd-catalyzed carbonylative dearomatization via an acyl Pd complex has been developed. Diversified carbonyl-containing spirocyclic indolenines with an all-carbon quaternary center were constructed in an efficient and straightforward way with good to excellent yields. The protocol features a simple catalytic system, operational simplicity, a broad substrate scope, easy scale-up, and versatile transformations. In addition, the asymmetric reaction was initially explored with moderate enantioselectivity.

Construction of Conjugated 1,3-Enynes via Pd-Catalyzed Cascade Alkynylation of Aryl Phenol-Tethered Alkynes with Alkynyl Bromides

An efficient Pd-catalyzed cascade alkynylation of aryl phenol-tethered alkynes with alkynyl bromides is described. This protocol could provide various conjugated 1,3-enynes possessing a polysubstituted spirocyclohexadienone, as well as an all-carbon tetrasubstituted alkene moiety. The products could also undergo ring-expansion and cyclization transformations under different conditions to convert to diverse fused cyclic scaffolds.

Dearomative cyclization of pyridines/isoquinolines with cyclopropenones: access to indolizinones and benzo-fused indolizinones

Dearomatization reactions provide a rapid approach to construct complicated molecules that are difficult to synthesize by traditional methods from simple aromatic compounds. Herein, we report an efficient dearomative [3+2] cycloaddition reaction of 2-alkynyl pyridines with diarylcyclopropenones, leading to the synthesis of densely functionalized indolizinones in moderate to good yields under metal-free conditions. In addition, this strategy can also be employed in dearomative cyclization of isoquinolines to access a variety of benzo-fused indolizinones. Density functional theory (DFT) calculations revealed that an appropriate substituent at the 2-position of pyridine is crucial to the dearomatization process.

Spirocarbocycle Synthesis from Chloromethylarenes via Transition-Metal-Catalyzed Allylative Dearomatization and Ring Closure Metathesis

A strategy for the synthesis of spirocarbocycles by using chloromethyl arenes as starting materials is described in this paper. The palladium-catalyzed allylative dearomatization and the subsequent ruthenium-catalyzed ring closure metathesis proceeded smoothly under mild conditions to produce the corresponding spirocarbocycle products with moderate to high yields. Benzene-ring-, naphthalene-ring-, and anthracene-ring-containing substrates can be easily transformed into spirocarbocycles by using the proposed method.

Electrooxidative palladium- and enantioselective rhodium-catalyzed [3 + 2] spiroannulations

Despite indisputable progress in the development of electrochemical transformations, electrocatalytic annulations for the synthesis of biologically relevant three-dimensional spirocyclic compounds has as of yet not been accomplished. In sharp contrast, herein, we describe the palladaelectro-catalyzed C-H activation/[3 + 2] spiroannulation of alkynes by 1-aryl-2-naphthols. Likewise, a cationic rhodium(iii) catalyst was shown to enable electrooxidative [3 + 2] spiroannulations via formal C(sp3)-H activations. The versatile spiroannulations featured a broad substrate scope, employing electricity as a green oxidant in lieu of stoichiometric chemical oxidants under mild conditions. An array of spirocyclic enones and diverse spiropyrazolones, bearing all-carbon quaternary stereogenic centers were thereby accessed in a user-friendly undivided cell setup, with molecular hydrogen as the sole byproduct.

Tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles: construction of vicinal quaternary carbon centers

A novel tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles has been developed, providing dispirooxindole–cyclopentane–cyclohexadienones with vicinal quaternary carbon centers.

Palladium-catalyzed dearomative 1,4-arylmethylenation of naphthalenes

An efficient palladium-catalyzed construction of E-exocyclic-double-bond-containing spirooxindoles through 1,4-arylmethylenation of naphthalenes has been developed. Aryl aldehyde-derived N‑tosylhydrazones were successfully applied as carbene precursors to capture the endocyclic π-allylpalladium intermediate, which...

Size-Dependent Catalytic Activity of PVA-Stabilized Palladium Nanoparticles in p-Nitrophenol Reduction: Using a Thermoresponsive Nanoreactor

Palladium nanoparticles (Pd NPs) of various average global diameters (2.1-7.1 nm) encapsulated with hydrophilic polymer polyvinyl alcohol (PVA) have been synthesized and used as catalysts for sodium borohydride assisted reduction of p-nitrophenol to p-aminophenol. The synthesized catalysts exhibit excellent and typical size-dependent catalytic activity in the green protocol. UV-visible absorption spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were employed to characterize the prepared Pd NPs. The kinetics of this reaction was easily monitored by a UV-visible absorption spectrophotometer. The mechanism of the reaction is explained by the Langmuir-Hinshelwood model. The catalytic performance increases with decreasing size of the synthesized nanoparticles. The apparent rate constants (k app × 103/s-1) of the catalytic reduction in the presence of Pd NPs of average diameters of 2.1, 3.35, 6.2, and 7.1 nm are determined as 8.57, 7.67, 6.16, and 5.04, respectively, at 298 K by using 2.91 mol % palladium nanocatalyst in each case. Moreover, the estimated activation energy of 22.2 kJ mol-1 obtained for Pd NPs with the smallest average diameter of 2.1 nm is very low as reported in the literature for the reduction. The influences of catalyst dose and concentration of p-nitrophenol on catalytic reduction are fully investigated. The catalyst with the largest diameter shows a temperature-sensitive property that might be due to the presence of a very low amount of rapped PVA used as stabilizer during the fabrication process. Thus, the synthetic protocol provides a unique fabrication process of a catalytically active thermoresponsive nanoreactor consisting of Pd NPs encapsulated into a PVA stabilizing agent.

TFA-Catalyzed [3+2] Spiroannulation of Cyclobutanols: A Route to Spiro[cyclobuta[a]indene-7,1'-cyclobutane] Skeletons

A straightforward method for the synthesis of spiro[cyclobuta[a]indene-7,1'-cyclobutane] derivatives from cyclobutanols has been developed via one-pot [3+2] spiroannulation. A series of new spiro[cyclobuta[a]indene-7,1'-cyclobutane] derivatives are facilely synthesized in good yields under mild reaction conditions.

Atypical Dearomative Spirocyclization of β-Naphthols with Diazoacetamides Using a Silver Catalyst

A chemoselective dearomatization of the less reactive benzenoid unit in β-naphthol was developed. Spirocyclization with a reductant constructs a pivotal structure for drug candidates. One-pot oxidative conversion enabled the tandem dearomatization of β-naphthol, producing conjugated tetraenone variants. The potential utility of the product as an F^--selective anion sensor was also demonstrated. Theoretical studies revealed the intermediacy of silver-carbenoid species leading to chemoselective spirocyclization over arene cyclopropanation.

Pd-Catalyzed Oxidative Annulation of Aryl Ethers with Alkynes: Synthesis of Functionalized Spirocycles and Naphthalenes

Palladium-catalyzed dearomative [2+2+1] annulations of aryl ethers with alkynes are reported via para-selective C-H functionalization, providing highly functionalized spirocyclohexadienones in moderate to excellent yields under mild reaction conditions. Importantly, mechanistic investigation indicated an unusual C-O bond cleavage was involved. Moreover, polyarylated naphthalenes could be obtained via oxidative [2+2+2] annulation by tuning aryl ethers from monomethoxybenzenes to polymethoxybenzenes under an identical catalytic system.

Thio-Michael addition of thioamides and allenes for the selective construction of polysubstituted 2-arylthiophenes via TBAI/H2O2 promoted tandem oxidative annulation and 1,2-sulfur migration

A novel TBAI-catalyzed tandem thio-Michael addition/oxidative annulation of allenes and thioamides for the construction of polysubstituted 2-arylthiophenes under a sulfur migration transformation protocol has been developed. The transition-metal-free protocol achieves the oxidative cyclization reaction of thioamides containing electron-rich substituents with allenes to construct polysubstituted thiophenes selectively by controlling oxidation conditions.

Rh-Catalyzed aminative dearomatization of 2-naphthols

β-Naphthalenones bearing a naked α-amino group could be rapidly constructed through a Rh-catalyzed aminative dearomatization reaction of 2-naphthols with O-(2,4-dinitrophenyl)hydroxylamine.

A facile approach to synthesize azaindoline functionalized spirocarbocyclic scaffolds via a Pd-catalyzed cascade cyclization/dearomatization process

Azaindoline functionalized spirocarbocyclic scaffolds bearing two all-carbon quaternary centers are synthesized.

Pd(ii)-Catalyzed [3 + 2] spiroannulation of α-aryl-β-naphthols with alkynes via a C–H activation/dearomatization approach

A Pd(ii)-catalyzed [3 + 2] spiroannulation of α-aryl-β-naphthols with internal alkynes has been developed by relying on a C–H activation/arene dearomatization approach.