Palladium-Catalyzed Carbene Coupling Reactions of Cyclobutanone N-Sulfonylhydrazones

The Carbene Chemistry of N-Sulfonyl Hydrazones: The Past, Present, and Future

N-Sulfonyl hydrazones have been extensively used as operationally safe carbene precursors in modern organic synthesis due to their ready availability, facile functionalization, and environmental benignity. Over the past two decades, there has been tremendous progress in the carbene chemistry of N-sulfonyl hydrazones in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Many carbene transfer reactions of N-sulfonyl hydrazones are unique and cannot be achieved by any alternative methods. The discovery of novel N-sulfonyl hydrazones and the development of highly enantioselective new reactions and skeletal editing reactions represent the notable recent achievements in the carbene chemistry of N-sulfonyl hydrazones. This review describes the overall progress made in the carbene chemistry of N-sulfonyl hydrazones, organized based on reaction types, spotlighting the current state-of-the-art and remaining challenges to be addressed in the future. Special emphasis is devoted to identifying, describing, and comparing the scope and limitations of current methodologies, key mechanistic scenarios, and potential applications in the synthesis of complex molecules.

Enantioselective Synthesis of Chiral Cyclobutenes Enabled by Brønsted Acid-Catalyzed Isomerization of BCBs

Chiral cyclobutene units are commonly found in natural products and biologically active molecules. Transition-metal-catalysis has been extensively used in asymmetric synthesis of such structures, while organocatalytic approaches remain elusive. In this study, bicyclo[1.1.0]butanes are involved in enantioselective transformation for the first time to offer a highly efficient route toward cyclobutenes with good regio- and enantiocontrol. The utilization of N-triflyl phosphoramide as a chiral Brønsted acid promoter enables this isomerization process to proceed under mild conditions with low catalyst loading as well as good functional group compatibility. The resulting chiral cyclobutenes could serve as platform molecules for downstream manipulations with excellent reservation of stereochemical integrity, demonstrating the synthetic practicality of the developed method. Control experiments have also been performed to verify the formation of a key carbocation intermediate at the benzylic position.

Pd-Catalyzed Regioselective (Markovnikov) Addition of Aryl Boronic Acids to Terminal Alkynes of 1,3-Dicarbonyl Compounds and Cyclization/Debenzoylation of Olefinic Dicarbonyl: Access to Arylated Pyran and (E)-4-Methylene-1,6-diphenylhex-5-en-1-one

This Letter outlines palladium-catalyzed regioselective (Markovnikov's) addition of aryl boronic acids to propargyl 1,3-dicarbonyl alkyne to accomplish olefinic/diene 1,3-dicarbonyl compounds without the need for water workup. This methodology showcases remarkable performance with wide-ranging substrate diversity, achieving high yields while employing merely 3 mol % [Pd] alongside a mild KOAc base. Moreover, the utility of dicarbonyl olefins is exemplified through their application in intramolecular cyclization and debenzoylation reactions to access valuable trisubstituted pyran building blocks and (E)-4-methylene-1,6-diphenylhex-5-en-1-one synthesis.

Palladium-Catalyzed Cyclization Coupling with Cyclobutanone-Derived N-Tosylhydrazones: Synthesis of Benzofuran-3-Cyclobutylidenes and Spirocyclobutanes

A palladium-catalyzed cyclization coupling of iodoarene-tethered alkynes with cyclobutanone-derived N-tosylhydrazones is reported, providing a convenient and efficient approach to benzofuran-3-cyclobutylidenes. On this basis, spirocyclobutanes can be generated smoothly in an efficient cascade manner by the addition of dienophiles. Good yields and scalability are demonstrated. Sequential intramolecular carbopalladation, palladium-carbene migratory insertion, δ-hydride elimination, and cycloaddition processes are involved.

Palladium-catalyzed intramolecular enantioselective C(sp3)-H insertion of donor/donor carbenes

Herein, the first palladium-catalyzed intramolecular enantioselective C(sp3)-H insertion reaction of donor-donor carbenes has been successfully achieved. This facile protocol enables the rapid construction of a collection of enantioenriched decorated indolines with two contiguous stereocenters in a single step. Both enynones and diazo compounds are efficient donor-donor carbene precursors for this reaction. By an adjustment of ligands and protecting groups of the substrates, the palladium-carbene intermediates from diazo compounds afford sparse trans-indolines with excellent enantioselectivities, while carbenes from enynones deliver cis-indolines exclusively. Based on the control reactions and Hammett analysis, a stepwise Mannich-type pathway through a short-lived and compact zwitterionic intermediate is proposed.

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.

N-Triftosylhydrazones: A New Chapter for Diazo-Based Carbene Chemistry

ConspectusOver recent decades, N-sulfonylhydrazones have attracted significant attention in academic and industrial contexts owing to their ease of preparation, versatile reactivity, high stability, and practicality. In particular, the use of N-sulfonylhydrazones as precursors for diazo compounds has paved the way for innovative and original organic reactions that are otherwise difficult to achieve. Three key developments are noteworthy in the history of N-sulfonylhydrazone chemistry: (1) Bamford and Stevens initially disclosed the application of N-tosylhydrazones as a diazo source in 1952; (2) Aggarwal and co-workers investigated N-tosylhydrazone salts as diazo precursors for sulfur ylide-mediated asymmetric epoxidation and aziridination in 2001; and (3) Barluenga, Valdés and co-workers first reported Pd-catalyzed cross-coupling reactions with N-tosylhydrazones in 2007, thus introducing the direct use of N-tosylhydrazones in carbene transfer reactions. In the past 2 decades, the synthetic exploration of N-sulfonylhydrazones in carbene chemistry has increased remarkably. N-Tosylhydrazones are the most commonly used N-sulfonylhydrazones, but they are not easy to decompose and normally need relatively high temperatures (e.g., 90-110 °C). Temperature, as a key reaction parameter, has a significant influence on the selectivity and scope of organic reactions, especially the enantioselectivity. Aggarwal and co-workers have addressed this issue by using N-tosylhydrazone salts and achieved a limited number of asymmetric organic reactions, but the method is greatly limited because the salts must be freshly prepared or stored in the dark at -20 °C prior to use. Hence, easily decomposable N-sulfonylhydrazones, especially those capable of decomposing at low temperature, should open up new opportunities for the development of N-sulfonylhydrazone chemistry. Since 2014, our group has worked toward this goal and eventually identified N-2-(trifluoromethyl)benzenesulfonylhydrazone (i.e., N-triftosylhydrazone) as an efficient diazo surrogate that can decompose at temperatures as low as -40 °C. This allowed us to carry out a range of challenging synthetic transformations and to broaden the applications of some known reactions of great relevance.In this Account, we report our achievements in the application of N-triftosylhydrazones in carbene chemistry. On the basis of the reaction types, such applications can be categorized as (i) C(sp³)-H insertion reactions, (ii) defluorinative reactions of fluoroalkyl N-triftosylhydrazones, (iii) cycloaddition reactions with alkenes and alkynes, and (iv) asymmetric reactions. Additional applications in Doyle-Kirmse rearrangements and cross-coupling with isocyanides (ours) and benzyl chlorides (from the group of Xia) are also summarized in this Account concerning miscellaneous reactions. In terms of reaction efficiency, selectivity, and functional group tolerance, N-triftosylhydrazones are generally superior to traditional N-tosylhydrazones because of their easy decomposition. Mechanistic investigations by theoretical calculations provide insights into both the reaction mechanisms and the origin of selectivity. We hope that this Account will inspire broad interest and promote new progress in the synthetic exploration of easily decomposable N-sulfonylhydrazones.