Asymmetric Aza-Wacker-Type Cyclization of N-Ts Hydrazine-Tethered Tetrasubstituted Olefins: Synthesis of Pyrazolines Bearing One Quaternary or Two Vicinal Stereocenters

We have developed an asymmetric aza-Wacker-type cyclization of N-Ts hydrazine-tethered tetrasubstituted olefins, affording optically active pyrazolines bearing chiral tetrasubstituted carbon stereocenters. This reaction is tolerant to a broad range of substrates under mild reaction conditions, giving the desired chiral products with high enantioselectivities. Generation of two vicinal stereocenters on the C═C double bonds was also achieved with high selectivities, a process which has been rarely studied for Wacker-type reactions. A mechanistic study revealed that this aza-Wacker-type cyclization undergoes a syn-aminopalladation process. It was also found that for substrates bearing two linear alkyl substituents on the outer carbon atom of the olefin, both of which are larger than a methyl group, the alkyl substituent that is cis to the intranucleophilic group participates more readily in β-hydride elimination. When one of the two alkyl substituents on the outer carbon atom of the olefin is a methyl group, β-hydride elimination proceeds selectively at the methylene side, thus both diastereomers can be prepared via switching the configuration of the olefin. Furthermore, the product can be converted to a pharmaceutical compound in high yields over three steps.

Zirconocene-Mediated Selective C-C Bond Cleavage of Strained Carbocycles: Scope and Mechanism

Several approaches using organozirconocene species for the remote cleavage of strained three-membered ring carbocycles are described. ω-Ene polysubstituted cyclopropanes, alkylidenecyclopropanes, ω-ene spiro[2.2]pentanes, and ω-ene cyclopropyl methyl ethers were successfully transformed into stereodefined organometallic intermediates, allowing an easy access to highly stereoenriched acyclic scaffolds in good yields and, in most cases, excellent selectivities. DFT calculations and isotopic labeling experiments were performed to delineate the origin of the obtained chemo- and stereoselectivities, demonstrating the importance of microreversibility.

Exploring the mechanism of the Pd-catalyzed spirocyclization reaction: a combined DFT and experimental study

The mechanism of the palladium-catalyzed spirocyclization of acrylamides has been investigated by density functional theory and experimental studies. The results support a mechanistic pathway that proceeds via oxidative addition, intramolecular carbopalladation, C-H bond activation, and migratory insertion sequence. The M06L/def2-TZVPP//BP86/6-31G(d,p)/LANL2DZ level of theory used and the inclusion of solvent effects provide results in good agreement with the experimental data. The C-H bond activation step proceeds via a concerted outer-sphere metallation deprotonation mechanism that explains the absence of a measurable kinetic isotopic effect. The subsequent intermolecular migratory insertion of arynes is significantly faster than the insertion of internal alkynes. Furthermore, the regioselectivities calculated in the case of unsymmetrical reactants are remarkably close to the experimental values. Evaluation of the potential energy surfaces for specific substrates provides an explanation for the lack of product formation observed experimentally. Finally, the computational and experimental analyses of potential side reactions are also presented and support the initially proposed mechanism.

Direct access to α-sulfenylated amides/esters via sequential oxidative sulfenylation and C–C bond cleavage of 3-oxobutyric amides/esters

An efficient, environmentally benign and unprecedented synthesis of various α-sulfenylated amides/esters has been developed under oxygen atmosphere.

A unique Pd-catalysed Heck arylation as a remote trigger for cyclopropane selective ring-opening

Combining functionalization at a distant position from a reactive site with the creation of several consecutive stereogenic centres, including the formation of a quaternary carbon stereocentre, in acyclic system represents a pinnacle in organic synthesis. Here we report the regioselective Heck arylation of terminal olefins as a distant trigger for the ring-opening of cyclopropanes. This Pd-catalysed unfolding of the strained cycle, driving force of the chain-walking process, remarkably proved its efficiency and versatility, as the reaction proceeded regardless of the molecular distance between the initiation (double bond) and termination (alcohol) sites. Moreover, employing stereodefined polysubstituted cyclopropane vaults allowed to access sophisticated stereoenriched acyclic scaffolds in good yields. Conceptually, we demonstrated that merging catalytically a chain walking process with a selective C-C bond cleavage represents a powerful approach to construct linear skeleton possessing two stereogenic centres.

Zirconocene-assisted remote cleavage of C-C and C-O bonds: application to acyclic stereodefined metalated hydrocarbons

The molding of molecules through remote functionalisation has increasingly become popular as it provides original and flexible synthetic alternatives to classical retrosynthetic analysis. In this Perspective article, we summarise more than a decade of studies in the specific field of remote activation of inert C-C and C-O bonds using the unique abilities of organozirconocene species mainly from our own research group. By demonstrating that these reactions represent novel and powerful entries towards acyclic stereodefined reactive organometallic species, we aim to show the vast opportunities this concept-driven methodology discovery offers.

Chemo- and regioselective oxygenation of C(sp3)-H bonds in aliphatic alcohols using a covalently bound directing activator and atmospheric oxygen

Aerobic, site-selective C(sp³)–H oxygenation using a novel N-oxyl radical directing activator (chemically reactive directing group) is described.

Chemically reactive directing groups (directing activators) represent a promising strategy for mild and regioselective C(sp³)–H functionalization. The use of a radical N-oxyl directing activator promoted the aerobic oxygenation of benzylic, propargylic, tertiary, and unactivated acyclic methylene C(sp³)–H bonds in aliphatic alcohols with γ- (or δ-) selectivity under mild conditions (room temperature to 50 °C). The reaction was unaffected by the presence of various oxidation-sensitive functional groups, which proved to be problematic in previously reported studies on the oxidation of C(sp³)–H bonds. Structural modifications on the directing activator altered the regioselectivity, and thus provided an ultra-remote aerobic C(sp³)–H oxygenation. The observed reactivity and regioselectivity could be rationalized in terms of the intramolecular conformational accessibility of the N-oxyl radical and the electronic characteristics of C(sp³)–H bonds.

Coordinating activation strategy for C(sp(3))-H/C(sp(3))-H cross-coupling to access β-aromatic α-amino acids

The past decade has witnessed significant advances in C–H bond functionalizations with the discovery of new mechanisms. Non-precious transition-metal-catalysed radical oxidative coupling for C(sp³)–H bond transformations is an appealing strategy for C–C bond formations. The radical oxidative C(sp³)–H/C(sp³)–H cross-coupling reactions of α-C(sp³)–H bonds of amines with free radicals represent a conceptual and practical challenge. We herein develop the coordinating activation strategy to illustrate the nickel-catalysed radical oxidative cross-coupling between C(sp³)–H bonds and (hetero)arylmethyl free radicals. The protocol can tolerate a rich variety of α-amino acids and (hetero)arylmethanes as well as arylmethylenes and arylmethines, affording a large library of α-tertiary and α-quaternary β-aromatic α-amino acids. This process also features low-cost metal catalyst, readily handled and easily removable coordinating group, synthetic simplicity and gram-scale production, which would enable the potential for economical production at commercial scale in the future.

Oxidative coupling of two C(sp3)-H groups is an attractive approach for the formation of carbon-carbon bonds, building structural complexity while avoiding the formation of waste salts. Here, the authors report such a transformation, using a nickel catalyst to functionalise amino acids with benzylic radicals.

Single-Flask Multicomponent Palladium-Catalyzed α,γ-Coupling of Ketone Enolates: Facile Preparation of Complex Carbon Scaffolds

A three-component palladium-catalyzed reaction sequence has been developed in which γ-substituted α,β-unsaturated products are obtained in a single flask by an α-alkenylation with either a subsequent γ-alkenylation or γ-arylation of a ketone enolate. Coupling of a variety of electronically and structurally different components was achieved in the presence of a Pd/Q-Phos catalyst (2 mol %), usually at 22 °C with yields of up to 85 %. Most importantly, access to these products is obtained in one simple operation in place of employing multiple reactions.

Remote functionalization of hydrocarbons with reversibility enhanced stereocontrol

Remote functionalization of hydrocarbons could be achieved through successive zirconocene-mediated allylic C-H bond activations followed by a selective C-C bond cleavage. Determination of the reaction mechanism by density functional theory (DFT) calculations shows that the high stereocontrol observed in this process results from a large number of energetically accessible equilibria feeding a preferred reactive channel that leads to the major product. A distinctive consequence of this pattern is that stereoselectivity is enhanced upon heating.

Asymmetric Remote C–H Functionalization: Use of Internal Olefins in Tandem Hydrometallation–Isomerization–Asymmetric Conjugate Addition Sequences

We describe catalytic asymmetric C–C formation using terminal alkyl-metal nucleophiles generated from internal olefins through a ‘chain-walking’ isomerization mechanism. Hydrometallation of internal olefins with the Schwartz reagent gives the least hindered alkyl-zirconocene after thermal (60°C in THF) isomerization. After switching the solvent from THF to dichloromethane, the alkyl-zirconocenes can be used in copper-catalyzed asymmetric conjugate additions. Addition to a variety of cyclic α,β-unsaturated species were achieved in modest (22–50 %) yield with high (84–92 % ee) enantioselectivity. This work demonstrates that remote C–H functionalization coupled with asymmetric C–C bond formation is possible, but the present procedures are limited in terms of yield and olefin scope.