Palladium-catalyzed allene synthesis enabled by β-hydrogen elimination from sp2-carbon

Palladium-catalyzed asymmetric carbene coupling en route to inherently chiral heptagon-containing polyarenes

Developing facile and direct synthesis routes for enantioselective construction of cyclic π-conjugated molecules is crucial. However, originate chirality from the distorted structure around heptagon-containing polyarenes is largely overlooked, the enantioselective construction of all-carbon heptagon-containing polyarenes remains a challenge. Herein, we present a highly enantioselective synthesis route for fabricating all carbon heptagon-containing polyarenes via palladium-catalyzed carbene-based cross-coupling of benzyl bromides and N-arylsulfonylhydrazones. A wide range of nonplanar, saddle-shaped tribenzocycloheptene derivatives are efficiently prepared in high yields with excellent enantioselectivities using this approach. In addition, stereochemical stability experiments show that these saddle-shaped tribenzocycloheptene derivatives have high inversion barriers.

Asymmetric Allenylation of Alkynes mediated by Chiral Organoselenated Reagents under Oxidative Conditions

The reactivity of novel chiral lactamide-substituted diselenide-based reagents under oxidative conditions was exploited to develop a metal-free method for the preparation of enantioenriched allenylamides from simple alkynes in good yields, and with enantiomeric excesses up to 99 %. The key of the success in this method is attributed to the hydrogen-bonded lactamide appendages that ensure configurational stability of chiral vinyl selenoxide intermediates for an optimal enantiotopic β-syn-elimination step.

Pd/Xu-Phos-catalyzed asymmetric elimination of fully substituted enol triflates into axially chiral trisubstituted allenes

The β-H elimination, as one of the most important elementary reactions in transition metal chemistry, is a key step in quenching the carbon-palladium bond for the Heck reaction. However, the β-H elimination of the alkenyl palladium species leading to allene is an energetically unfavored process, and therefore, it has been a long-standing challenge in control of this process via enantioselective manner. We developed a concise and efficient methodology to construct trisubstituted chiral allenes from stereodefined fully substituted enol triflates by the enantioselective β-H elimination of the alkenyl palladium species under mild conditions. The identified Xu-Phos play a crucial role in the chemoselectivity and enantioselectivity. Multiple linear regression analysis shows the important steric effect on enantioselectivity. DFT computation results allow us to propose an intramolecular base (^-OAc)-assisted deprotonation mechanism for this progress. Distortion-interaction and energy decomposition analysis indicate that the difference in electrostatic energy (E_elec) of the two intramolecular base-assisted deprotonation transition states dominates the stereoselectivity.

Functionalized Chromans from ortho-Quinone Methides and Arylallenes

ortho-Quinone methides (o-QMs) underwent formal [4 + 2]-cycloaddition reactions with arylallenes regioselectively at the styrenyl olefin to furnish the corresponding 3-methylene-2-arylchromans in moderate to good yields (up to 88%). When R ≠ H, the reactions also proceeded with moderate stereoselectivity (up to 5:1) which was governed by the nature of the R group. The 3-methylene-2-arylchromans could serve as common intermediates for further functionalization including epoxidation, oxidative cleavage/Baeyer-Villiger oxidation, Riley oxidation, acid-catalyzed rearrangement, and Pd-catalyzed cross-coupling reactions to furnish the corresponding derivatives in moderate to good yields.

Palladium/GF-Phos-catalyzed asymmetric carbenylative amination to access chiral pyrrolidines and piperidines

The cross-coupling of N-tosylhydrazones has emerged as a powerful method for the construction of structurally diverse molecules, but the development of catalytic enantioselective versions still poses considerable challenges and only very limited examples have been reported. We herein report an asymmetric palladium/GF-Phos-catalyzed carbenylative amination reaction of N-tosylhydrazones and (E)-vinyl iodides pendent with amine, which allows facile access to a range of chiral pyrrolidines and piperidines in good yields (45-93%) with up to 96.5 : 3.5 er. Moreover, mild conditions, general substrate scope, scaled-up preparation, as well as the efficient synthesis of natural product (-)-norruspoline are practical features of this method.

Tunable Construction of Multisubstituted 1,3-Dienes and Allenes via a 1,4-Palladium Migration/Carbene Insertion Cascade

Efficient palladium-catalyzed vinylic C-H alkenylation and allenylation of gem-disubstituted ethylenes with N-tosylhydrazones of aryl alkyl and diaryl ketones were achieved to access trisubstituted 1,3-dienes and tetrasubstituted allenes, respectively. An aryl to vinyl 1,4-palladium migration/carbene insertion/β-hydride elimination sequence proceeded to switch the chemo- and regioselectivities to give structurally diverse products. Use of 2-FC₆H₄OH additive enables enhancement of the reaction efficiency through accelerating the key 1,4-palladium migration process.

Synthesis of tetrasubstituted allenes via a 1,4-palladium migration/carbene insertion/β-H elimination sequence

A palladium-catalyzed synthesis of tetrasubstituted allenes from aryl bromides and aryl diazoacetates is developed. This transformation proceeded via an aryl to alkenyl 1,4-palladium migration/carbene insertion/β-H elimination sequence under mild reaction conditions.

Synthesis of Tetrasubstituted Allenes via Visible-Light-Promoted Radical 1,3-Difunctionalization of Alkynyl Diazo Compounds

Herein, we described an unprecedented process for generating allenyl radicals through radical addition to alkynyl diazo compounds followed by a 1,2-radical shift with the loss of nitrogen. Using this protocol, radical 1,3-difunctionalization of alkynyl diazo compounds for the synthesis of tetrasubstituted allenes with RSO₂X (X = SeR', SR', and I) as the radical sources was developed. The reactions were promoted by visible light without photocatalyst and any additives.

Pd-Catalyzed Coupling of N-Tosylhydrazones with Benzylic Phosphates: Toward the Synthesis of Di- or Tri-Substituted Alkenes

This study shows that various di- and tri-substituted alkenes with high chemoselectivity were obtained in good to high yields by coupling N-tosylhydrazones (NTHs) with benzylic phosphates as electrophilic partners. The obtained new catalytic system consisted of PdCl₂(CH₃CN)₂/dppp, LiOtBu as a base, and cyclopentyl methyl ether as a green solvent. In addition, we performed a gram-scale transformation using NTH derivatives and benzylic phosphates having a C sp²-Cl bond. The latter was used as a starting point for further postfunctionalization of the key intermediates.

Cu(i)- and Pd(ii)-catalyzed decarboxylative cross-couplings of alkynyl carboxylic acids with N-tosylhydrazones: access to trisubstituted allenes and conjugated enynes

This paper reports the copper- and palladium-catalyzed decarboxylative cross-coupling reactions of alkynyl carboxylic acids and N-tosylhydrazones, affording trisubstituted allenes and conjugated enynes, respectively.