Palladium-Catalyzed Defluorinative Coupling of Difluoroalkenes and Aryl Boronic Acids for Ketone Synthesis

The transition-metal-catalyzed carbonylation reaction is a useful approach for ketone synthesis. However, it is often problematic to use exogenous carbonyl reagents, such as gaseous carbon monoxide. In this manuscript, we report a novel palladium-catalyzed coupling reaction of gem-difluoroalkenes and aryl boronic acids that yields bioactive indane-type ketones with an all-carbon α-quaternary center. Characterization and stoichiometric reactions of the key intermediates RCF₂ Pd^II support a water-induced defluorination and cross-coupling cascade mechanism. The vinyl difluoromethylene motif serves as an in situ carbonyl precursor which is unprecedented in transition-metal-catalyzed coupling reactions. It is expected to raise broad research interest from the perspectives of ketone synthesis, fluoroalkene functionalization, and rational design of new synthetic protocols based on the unique reactivity of difluoroalkyl palladium(II) species.

Total Synthesis of (+)-Cochlearol B by an Approach Based on a Catellani Reaction and Visible-Light-Enabled [2+2] Cycloaddition

A 14-step synthesis of (+)-cochlearol B is reported. This renoprotective meroterpenoid features a unique core structure containing a densely substituted cyclobutane ring with three stereocenters. Our strategy employed an organocatalytic Kabbe condensation in route to the key chromenyl triflate. A subsequent Catellani reaction incorporated the remaining carbon atoms featured in the skeleton of cochlearol B. An ensuing visible-light-mediated [2+2] photocycloaddition closed the cyclobutane and formed the central bicyclo[3.2.0]heptane core. Notably, careful design and tuning of the Catellani and photocycloaddition reactions proved crucial in overcoming undesired reactivity, including cyclopropanation reactions and [4+2] cycloadditions.

Rapid Access to Multisubstituted Acrylamides from Cyclic Ketones via Palladium/Norbornene Cooperative Catalysis

A strategy to access diverse multisubstituted acrylamides from cyclic ketones is realized via palladium/norbornene-catalyzed α-carbamoylation and ipso-functionalization of the corresponding enol triflates. The development of bulky C2 secondary amide-substituted norbornene cocatalysts is the key to achieve the desired reactivity and selectivity. Readily available carbamoyl chlorides serve as effective electrophiles; various moieties including alkenyl, hydrogen, alkynyl, aryl, and alkyl groups can be installed at the ipso position. In addition, tandem α-carbamoylation/ipso-annulations are demonstrated to furnish lactam-containing polycyclic scaffolds. The utility of this method is exemplified in the streamlined preparation of a platelet-activating factor (PAF)-antagonist.

Chiral Indoline-2-carboxylic Acid Enables Highly Enantioselective Catellani-type Annulation with 4-(Bromomethyl)cyclohexanone

Chiral indoline-2-carboxylic acid has been identified to enable a highly enantioselective Catellani-type annulation of (hetero)aryl, alkenyl triflate and conjugated vinyl iodides with 4-(bromomethyl)cyclohexanone, directly assembling a diverse range of chiral all-carbon bridged ring systems. Control experiments and DFT calculations suggest that the coordinating orientation of the chiral amino acid to the arylpalladium(II) center allows for high levels of stereochemical control.

Alkyne Aminopalladation/Heck and Suzuki Cascades: An Approach to Tetrasubstituted Enamines

Alkyne aminopalladation reactions starting from tosylamides are reported. The emerging vinylic Pd species are converted either in an intramolecular Heck reaction with olefinic units or in an intermolecular Suzuki reaction by using boronic acids exhibiting broad functional group tolerance. Tetra(hetero)substituted tosylated enamines are obtained in a simple one-pot process.

Pseudo-Stereodivergent Synthesis of Enantioenriched Tetrasubstituted Alkenes by Cascade 1,3-Oxo-Allylation/Cope Rearrangement

The catalytic diastereodivergent construction of stereoisomers having two or more stereogenic centers has been extensively studied. In contrast, the switchable introduction of another stereogenic element, that is, Z/E configuration involving a polysubstituted alkene group, into the optically active stereoisomers, has not been recognized yet. Disclosed here is the pseudo-stereodivergent synthesis of highly enantioenriched tetrasubstituted alkene architectures from isatin-based Morita-Baylis-Hillman carbonates and allylic derivatives, under the cooperative catalysis of a tertiary amine and a chiral iridium complex. The success of the switchable construction of the tetrasubstituted alkene motif relies on the diastereodivergent 1,3-oxo-allylation reaction between N-allylic ylides and chiral π-allyliridium complex intermediates by ligand and substrate control, followed by the stereoselective concerted 3,3-Cope rearrangement process.

Alkylating Reagents Employed in Catellani-Type Reactions

The Catellani reaction is a powerful strategy that allows the expeditious synthesis of highly substituted arenes, which are not easily accessible through traditional transition-metal-catalyzed cross-coupling reactions. This reaction utilizes the synergistic interplay of palladium and norbornene catalysis to facilitate sequential ortho-C-H functionalization and ipso termination of aryl iodides in a single operation. Since pioneering work by the group of Catellani in 1997, and later by the group of Lautens, this chemistry has attracted considerable attention from the synthetic chemistry community. Dramatic progress has been made by a number of groups in the past two decades. In this Minireview, the alkylating reagents employed in this intriguing reaction and the corresponding applications in organic synthesis are summarized; thus complementing existing reviews to inspire future developments.

Homogeneous and Gas-Liquid Catellani-Type Reaction Enabled by Continuous-Flow Chemistry

A practical homogeneous and gas-liquid palladium-catalyzed Catellani-type reaction using a continuous-flow platform is described. The implementation of continuous-flow technology allowed the acceleration of the transformation and, for the first time, expansion of the chemical space to gaseous olefins (i.e., ethylene, propylene and 3,3,3-trifluoropropene), thus providing a safe and practical approach to sterically hindered ortho-disubstituted styrenes and vinyl arenes. The complete control over the stoichiometry of gaseous reagents through flow technology proved essential for directing the selectivity of the Catellani reaction to the desired products.

The Discovery of a Palladium(II)-Initiated Borono-Catellani Reaction

Reported is a novel palladium(II)-initiated Catellani-type reaction that utilizes widely accessible aryl boronic acids as the substrates instead of aryl halides, thereby greatly expanding the existing scope of this powerful transformation. This borono-Catellani reaction was promoted by cooperative catalysis between Pd(OAc)₂ and the inexpensive 5-norbornene-2-carbonitrile. Practicality is the striking feature of the reaction: it is run open to air at ambient temperature and no phosphine ligand is needed. This mild, chemoselective, and scalable protocol is compatible with a large range of readily available functionalized aryl boronic acids and bromides, as well as terminating olefins (50 examples, 39-97 % yields). Moreover, the orthogonal reactivity between the borono-Catellani and classical Catellani reaction was demonstrated. This work is expected to open new avenues for developing novel Catellani-type reactions.

Palladium-Catalyzed 4-Fold Domino Reaction for the Synthesis of a Polymeric Double Switch

A palladium-catalyzed 4-fold domino reaction consisting of two carbopalladation reactions and two C-H activation reactions, followed by the introduction of an acrylate moiety, led to the tetra-substituted helical alkene A2, using the dialkyne A3 as a substrate. The alkene was copolymerized with butyl acrylate by using the reversible addition-fragmentation chain transfer polymerization (RAFT) to give the desired polymeric switch A1.

Palladium/Norbornene-Catalyzed C-H Alkylation/Alkyne Insertion/Indole Dearomatization Domino Reaction: Assembly of Spiroindolenine-Containing Pentacyclic Frameworks

Reported is a highly chemoselective intermolecular annulation of indole-based biaryls with bromoalkyl alkynes by using palladium/norbornene (Pd/NBE) cooperative catalysis. This reaction is realized through a sequence of Catellani-type C-H alkylation, alkyne insertion, and indole dearomatization, by forming two C(sp² )-C(sp³ ) and one C(sp² )-C(sp² ) bonds in a single chemical operation, thus providing a diverse range of pentacyclic molecules, containing a spiroindolenine fragment, in good yields with excellent functional-group tolerance. Preliminary mechanistic studies reveal that C-H bond cleavage is likely involved in the rate-determining step, and the indole dearomatization might take place through an olefin coordination/insertion and β-hydride elimination Heck-type pathway.

Direct Annulation between Aryl Iodides and Epoxides through Palladium/Norbornene Cooperative Catalysis

Herein we report a direct annulation between aryl iodides and epoxides through palladium/norbornene (Pd/NBE) cooperative catalysis. An iso-propyl ester substituted NBE was found to be most efficient to suppress the formation of multiple-NBE-insertion byproducts and affords the desired 2,3-dihydrobenzofuran derivatives in 44-99 % yields. The reaction is scalable and tolerates a range of functional groups. Asymmetric synthesis is realized using an enantiopure epoxide. Application of this method into a concise synthesis of insecticide fufenozide is demonstrated.

Palladium nanoparticles supported on organofunctionalized kaolin as an efficient heterogeneous catalyst for directed C–H functionalization of arylpyrazoles

A heterogeneous catalyst system based on the immobilization of Pd⁰ nanoparticles onto organofunctionalized kaolin is reported with a view to introducing new synthetic routes of directed C–H functionalization of arylpyrazoles.

Palladium-catalyzed cascade C-H trifluoroethylation of aryl iodides and Heck reaction: efficient synthesis of ortho-trifluoroethylstyrenes

A palladium-catalyzed selective C-H bond trifluoroethylation of aryl iodides has been explored. The reaction allows for the efficient synthesis of a variety of ortho-trifluoroethyl-substituted styrenes. Preliminary mechanistic studies indicate that the reaction might involve a key Pd(IV) intermediate, which is generated through the rate-determining oxidative addition of CF3CH2I to a palladacycle; the bulky nature of CF3CH2I influences the reactivity. Reductive elimination from the Pd(IV) complex then leads to the formation of the aryl-CH2CF3 bond.

Palladium/norbornene-mediated tandem C-H amination/C-I alkenylation reaction of aryl iodides with secondary cyclic O-benzoyl hydroxylamines and activated terminal olefins

A novel palladium-catalyzed norbornene-mediated three-component reaction for the construction of ortho-alkenyl aromatic tertiary amines has been achieved, which represents a useful extension of the Catellani-type tandem ortho-selective CH amination transformations.