Synthesis of Benzopyran-Phenylpropanoid Hybrids via Matsuda-Heck-Arylation and Allylic Oxidation

The synthesis of coumarin- and flavonoid-chalcone hybrids via Pd-catalyzed Heck-type coupling of arene diazonium salts and 8-allylcoumarins and -flavonoids is reported. The β-hydride elimination step proceeds with high regioselectivity if an OMOM-substituent is present at the position C7, adjacent to the allyl group. A selective allylic oxidation of the coupling products was accomplished using DDQ in the presence of silica to furnish the chalcones.

Palladium-Catalyzed Suzuki-Miyaura Reactions with Triazenyl-Tethered Aryl Bromides: Exploiting the Orthogonal Coupling Sites under Different Conditions

Transition-metal-catalyzed cross-coupling of arenes bearing two or more potential coupling sites is often challenging because of the chemoselectivity issue. If orthogonal cross-couplings were applicable, one can develop a synthetically useful approach for consecutive functionalization of the starting arenes compounds. We herein reported a Suzuki-Miyaura coupling of triazenyl-substituted aryl bromides catalyzed by PdCl2(PCy3)2/PPh3 under basic conditions. The resultant polyfunctionalized aryl triazenes could undergo Suzuki-Miyaura couplings under acidic conditions or be converted to many other functionalized arenes. This orthogonal coupling strategy allows for a sequential functionalization of arenes with same type of nucleophilic reagents toward the synthesis of diverse biaryls and teraryls.

Dihydro-10H-indeno[1,2-b]benzofurans and Tetrahydroindeno[1,2-c]isochromenes via Stereoselective Intramolecular Carbocation Cascade Cyclization

Nazarov cyclization of the (E)-(2-stilbenyl)methanols under the catalysis of p-TsOH immobilized on silica (PTS-Si) proceeded to give the corresponding indanyl cation with the exclusive trans relationship at the two newly formed adjacent stereogenic centers. The ensuing intramolecular nucleophilic addition by the MOM-protected phenol (m = 0) or benzyl alcohol (m = 1) furnished the Indane-fused benzofuran [5/5] or isochroman [5/6] system, respectively, with the exclusive cis stereocontrol at the two-carbon ring junction. Thus, in a single step, from nonchiral starting materials, the intramolecular cascade carbocation cyclization (CCC) furnished the [5/5] or [5/6] oxygen-containing Indane fused-ring systems in moderate to good yields with excellent stereoselectivity on all three contiguous stereogenic centers.

Synthesis and Fluorescent Properties of Alkynyl- and Alkenyl-Fused Benzotriazole-Derived α-Amino Acids

Fluorescent unnatural α-amino acids are widely used as probes in chemical biology and medicinal chemistry. While a variety of structural classes have been developed, there is still a requirement for new environmentally sensitive analogues that can closely mimic proteinogenic α-amino acids. Here, we report the synthesis and fluorescent properties of highly conjugated, benzotriazole-derived α-amino acids designed to mimic l-tryptophan. Alkynyl-substituted analogues were prepared using three key steps, nucleophilic aromatic substitution with a 3-aminoalanine derivative, benzotriazole formation via a one-pot diazotization and cyclization process, and a Sonogashira cross-coupling reaction. E-Alkenyl-substituted benzotriazoles were accessed by stereoselective partial hydrogenation of the alkynes using zinc iodide and palladium catalysis. The alkynyl analogues were found to possess higher quantum yields and stronger brightness and, a solvatochromic study with the most fluorogenic α-amino acids demonstrated sensitivity to polarity.

Pd-Catalyzed Cross-Couplings: On the Importance of the Catalyst Quantity Descriptors, mol % and ppm

This Review examines parts per million (ppm) palladium concentrations in catalytic cross-coupling reactions and their relationship with mole percentage (mol %). Most studies in catalytic cross-coupling chemistry have historically focused on the concentration ratio between (pre)catalyst and the limiting reagent (substrate), expressed as mol %. Several recent papers have outlined the use of “ppm level” palladium as an alternative means of describing catalytic cross-coupling reaction systems. This led us to delve deeper into the literature to assess whether “ppm level” palladium is a practically useful descriptor of catalyst quantities in palladium-catalyzed cross-coupling reactions. Indeed, we conjectured that many reactions could, unknowingly, have employed low “ppm levels” of palladium (pre)catalyst, and generally, what would the spread of ppm palladium look like across a selection of studies reported across the vast array of the cross-coupling chemistry literature. In a few selected examples, we have examined other metal catalyst systems for comparison with palladium.

Palladium-catalyzed ortho-halogen-induced deoxygenative approach of alkyl aryl ketones to 2-vinylbenzoic acids

The 2-vinylbenzoic acids have wide applications in the field of polymer chemistry and are key precursors for the synthesis of important bioactive molecules. Herein, an ortho-halogen-induced deoxygenative approach for the generation of 2-vinylbenzoic acids from alkyl aryl ketones by palladium catalysis is discovered and explored. This approach requires no base or stoichiometric additives and can be carried out through a simple one-step process. Furthermore, the present reaction is scalable up to one-gram scale. The commercially available palladium on carbon (5 wt%) was used as a heterogeneous catalyst and showed excellent recyclability (<5 times) without significant loss in catalytic activity. Pleasingly, under our optimized conditions, the alpha alkyl substituted 2-iodoacetophenones exhibit good diastereoselectivity and predominantly (E)-2-vinylbenzoic acids were obtained with good to excellent yields.

Palygorskite-anchored Pd complexes catalyze the coupling reactions of pyrimidin-2-yl sulfonates

In this work, an anchored Pd complex (PGS–APTES–Pd(OAc)₂) was prepared via simple and green steps from the natural clay mineral palygorskite and was well characterized by XPS, XRD, IR, SEM, and EDX. This complex was further utilized as a fine catalyst for the C–C/C–N coupling reactions of pyrimidin-2-yl sulfonates. Subsequently, the cyclic utilization test indicated the high stability and sustainability of this PGS–APTES–Pd(OAc)₂ catalyst, and no activation was required in the recycling process, providing an applicable and reusable catalyst in organic synthesis.

PGS–APTES–Pd(OAc)₂ was prepared through simple and green steps from the natural clay mineral palygorskite. Obviously, the stability and reusability of PGS–APTES–Pd(OAc)₂ were superior to those of the PGS–Pd catalyst (prepared by the impregnation method) in recycling test.

Efficient Suzuki⁻Miyaura C-C Cross-Couplings Induced by Novel Heterodinuclear Pd-bpydc-Ln Scaffolds

An easy access to a series of previously unreported heterodinuclear Pd-Ln compounds, Pd-bpydc-La, Pd-bpydc-Ce and Pd-bpydc-Nd (bpydc = 2,2'-bipyridine-5,5'-dicarboxylate) has been developed. The Pd-Ln hybrid networks were effectively applied as catalysts in Suzuki⁻Miyaura C-C cross-coupling reactions of 4-bromoanisole and 4-bromobenzonitrile with phenylboronic acid, under mild conditions. A systematic investigation revealed Pd-bpydc-Nd as the most active catalyst. In all cases, reaction yields varied with the base, catalyst loading and substantially augmented with temperature (from 30 to 60 °C). Substituent effects were operative when changing from 4-bromoanisole to 4-bromobenzonitrile. The key role played by the lanthanides, aromatic substrate and base, in modulating the Pd-catalytic cycle has been highlighted. Importantly, the new catalysts proved to be stable in air and vs. functionalities and are quite efficient in Suzuki⁻Miyaura carbon-carbon bond formation conducted in protic solvents.

The Effect of Functional Group Polarity in Palladium Immobilized Multiwalled Carbon Nanotube Catalysis: Application in Carbon-Carbon Coupling Reaction

Carbon nanotubes (CNTs) are effective supports for nanometals and together they represent hybrids that combine the unique properties of both. A microwave-induced reaction was used to deposit nanopalladium on carboxylated and octadecylamine functionalized multiwall CNTs, which were used to carry out C-C coupling reactions in dimethylformamide (DMF) and toluene. These hybrids showed excellent catalytic activity with yield as high as 99.8%, while its enhancement with respect to commercially available Pd/C catalyst reached as high as 109%, and the reaction times were significantly lower. The polarity of the functionalized form was found to be a significant factor with the polar carboxylated CNT showing better activity in DMF while the relatively nonpolar octadecyl amine was better in toluene. The results suggest the possibility of tailor making functionalized CNTs when used as catalyst supports.