Palladium-Catalyzed Cross Coupling Reaction of Benzyl Bromides with Diazoesters for Stereoselective Synthesis of (E)-α,β-Diarylacrylates

Diazo compounds and palladium-aryl complexes: trapping the elusive carbene migratory insertion organometallic products

The reactions of Pd-aryl complexes with diazo compounds N₂CH-CHCHPh and N₂CHPh allowed us to isolate the organometallic products formed right after the migratory insertion of a non-stabilized CHR carbene into the Pd-aryl bond. η³-Allylic and η³-benzylic palladium complexes were formed respectively. This is compelling experimental evidence for the key step in the palladium-catalyzed cascade transformations of diazo derivatives leading to multiple C-C or C-X bond formation.

Olefination of 3-Diazoindolin-2-imines with Hydrazones: An Approach toward Stereoselective Synthesis of (E)-3-Arylideneindolin-2-imines

Stereoselective synthesis of (E)-3-arylideneindolin-2-imines from 3-diazoindolin-2-imines and hydrazones was achieved using copper bromide as the catalyst. The olefination reaction proceeded via copper-catalyzed aerobic oxidation of hydrazones to diazo compounds and copper-catalyzed cross coupling of two different diazo components in a tandem manner.

Convergent Synthesis of 1,4-Dicarbonyl Z-Alkenes through Three-Component Coupling of Alkynes, α-Diazo Sulfonium Triflate, and Water

We report a general protocol for the convergent synthesis of 1,4-dicarbonyl Z-alkenes form alkynes using α-diazo sulfonium triflate and water. The C═O, C═C, and C-H bonds are formed under mild conditions with a wide range of functional groups tolerated. The reaction exhibits excellent Z-selectivity and complete regioselectivity. The resulting 1,4-dicarbonyl Z-alkenes can smoothly undergo follow-up conversion to a variety of heteroaromatic scaffolds. Moreover, the reaction also provides a facile access to the corresponding deuterated Z-alkenes and deuterated heteroarenes with a high level of deuterium incorporation (90-97% D-inc.) by directly using D₂O, thus rendering the method highly valuable. The comprehensive mechanistic studies indicate that a free carbyne radical intermediate is formed via the photocatalytic single electron transfer process, and KH₂PO₄ plays a crucial role in significant improvements on yield and selectivity based on density-functional theory calculations, providing a new direction for radical coupling reactions of diazo compounds.

Palladium-Catalyzed Carbene Migratory Insertion/Carbonylation Cascade Reaction: Synthesis of 2-Indolones with a C3 All-Carbon Quaternary Center

An attractive palladium-catalyzed three-component reaction of ortho-amino aryl diazo esters, allyl carboxylates, and carbon monoxide (CO) has been developed. This catalytic system rendered domino carbene migratory insertion and carbonylation. Remarkably, 2-indolones 3 with a C3 all-carbon quaternary center can be selectively obtained in good to excellent yields via one-pot synthesis, in which two different C-C bonds and one C-N bond were formed in a straightforward manner.

Investigating the mechanism and origins of selectivity in palladium-catalysed carbene insertion cross-coupling reactions

The mechanism of palladium-catalysed carbene insertion cross-coupling reactions has been experimentally and computationally studied, with carbene insertion identified as the likely selectivity determining step.

Copper-Catalyzed Regioselective Coupling of Tosylhydrazones and 2-Pyridones: A Strategy for the Production of N-Alkylated Compounds

The highly regioselective N-alkylation reaction of 2-pyridones was achieved through hydrazone chemistry, especially for substrates with bulky secondary alkyl groups. Described herein is a copper-catalyzed coupling reaction of pyridone derivatives with tosylhydrazones.

Blue Light-promoted Carbene Transfer Reactions of Tosylhydrazones

Metal-free photochemical carbene-transfer reactions of tosylhydrazones were developed under blue light irradiation at room temperature. This reaction constructs C-X (X=C, N, O, S) bonds and cyclopropanes from readily available and stable starting materials.

Transition-Metal-Catalyzed Cross-Couplings through Carbene Migratory Insertion

Transition-metal-catalyzed cross-coupling reactions have been well-established as indispensable tools in modern organic synthesis. One of the major research goals in cross-coupling area is expanding the scope of the coupling partners. In the past decade, diazo compounds (or their precursors N-tosylhydrazones) have emerged as nucleophilic cross-coupling partners in C-C single bond or C═C double bond formations in transition-metal-catalyzed reactions. This type of coupling reaction involves the following general steps. First, the organometallic species is generated by various processes, including oxidative addition, transmetalation, cyclization, C-C bond cleavage, and C-H bond activation. Subsequently, the organometallic species reacts with the diazo substrate to generate metal carbene intermediate, which undergoes rapid migratory insertion to form a C-C bond. The new organometallic species generated from migratory insertion may undergo various transformations. This type of carbene-based coupling has proven to be general: various transition metals including Pd, Cu, Rh, Ni, Co, and Ir are effective catalysts; the scope of the reaction has also been extended to substrates other than diazo compounds; and various cascade processes have also been devised based on the carbene migratory insertion. This review will summarize the achievements made in this field since 2001.

Copper-catalyzed N-alkylation of indoles by N-tosylhydrazones

An efficient method for direct N-alkylation of indoles via copper-catalyzed reductive cross coupling between N-tosylhydrazones and indoles has been developed.

Novel Tacrine-Benzofuran Hybrids as Potent Multitarget-Directed Ligands for the Treatment of Alzheimer's Disease: Design, Synthesis, Biological Evaluation, and X-ray Crystallography

Twenty-six new tacrine-benzofuran hybrids were designed, synthesized, and evaluated in vitro on key molecular targets for Alzheimer's disease. Most hybrids exhibited good inhibitory activities on cholinesterases and β-amyloid self-aggregation. Selected compounds displayed significant inhibition of human β-secretase-1 (hBACE-1). Among the 26 hybrids, 2e showed the most interesting profile as a subnanomolar selective inhibitor of human acetylcholinesterase (hAChE) (IC50 = 0.86 nM) and a good inhibitor of both β-amyloid aggregation (hAChE- and self-induced, 61.3% and 58.4%, respectively) and hBACE-1 activity (IC50 = 1.35 μM). Kinetic studies showed that 2e acted as a slow, tight-binding, mixed-type inhibitor, while X-ray crystallographic studies highlighted the ability of 2e to induce large-scale structural changes in the active-site gorge of Torpedo californica AChE (TcAChE), with significant implications for structure-based drug design. In vivo studies confirmed that 2e significantly ameliorates performances of scopolamine-treated ICR mice. Finally, 2e administration did not exhibit significant hepatotoxicity.

P(NMe2)3-Mediated Umpolung Alkylation and Nonylidic Olefination of α-Keto Esters

A commercial phosphorus-based reagent (P(NMe2)3) mediates umpolung alkylation of methyl aroylformates with benzylic and allylic bromides, leading to either Barbier-type addition or ylide-free olefination products upon workup. The reaction sequence is initiated by a two-electron redox addition of the tricoordinate phosphorus reagent with an α-keto ester compound (Kukhtin-Ramirez addition). A mechanistic rationale is offered for the chemoselectivity upon which the success of this nonmetal mediated C-C bond forming strategy is based.

Palladium-catalyzed C-H functionalization of acyldiazomethane and tandem cross-coupling reactions

Palladium-catalyzed C-H functionalization of acyldiazomethanes with aryl iodides has been developed. This reaction is featured by the retention of the diazo functionality in the transformation, thus constituting a novel method for the introduction of diazo functionality to organic molecules. Consistent with the experimental results, the density functional theory (DFT) calculation indicates that the formation of Pd-carbene species in the catalytic cycle through dinitrogen extrusion from the palladium ethyl diazoacetate (Pd-EDA) complex is less favorable. The reaction instead proceeds through Ag2CO3 assisted deprotonation and subsequently reductive elimination to afford the products with diazo functionality remained. This C-H functionalization transformation can be further combined with the recently evolved palladium-catalyzed cross-coupling reaction of diazo compounds with aryl iodides to develop a tandem coupling process for the synthesis of α,α-diaryl esters. DFT calculation supports the involvement of Pd-carbene as reactive intermediate in the catalytic cycle, which goes through facile carbene migratory insertion with a low energy barrier (3.8 kcal/mol).

Diversity-oriented synthesis as a tool for identifying new modulators of mitosis

The synthesis of diverse three-dimensional libraries has become of paramount importance for obtaining better leads for drug discovery. Such libraries are predicted to fare better than traditional compound collections in phenotypic screens and against difficult targets. Herein we report the diversity-oriented synthesis of a compound library using rhodium carbenoid chemistry to access structurally diverse three-dimensional molecules and show that they access biologically relevant areas of chemical space using cheminformatic analysis. High-content screening of this library for antimitotic activity followed by chemical modification identified 'Dosabulin', which causes mitotic arrest and cancer cell death by apoptosis. Its mechanism of action is determined to be microtubule depolymerization, and the compound is shown to not significantly affect vinblastine binding to tubulin; however, experiments suggest binding to a site vicinal or allosteric to Colchicine. This work validates the combination of diversity-oriented synthesis and phenotypic screening as a strategy for the discovery of biologically relevant chemical entities.

Efficient construction of C–S and C–N bonds via metal-free reductive coupling of N-tosylhydrazones with benzo[d]thiazole-2-thiol

An efficient procedure for the synthesis of diverse benzo[d]thiazole derivatives has been developed via metal-free reductive coupling of N-tosylhydrazones with benzo[d]thiazole-2-thiols.

Palladium catalyzed N–H bond insertion and intramolecular cyclization cascade: the divergent synthesis of heterocyclics

A palladium-catalyzed one-pot procedure of N–H insertion and Heck cyclization/base promoted cyclization has been developed.

Diazo compounds and N-tosylhydrazones: novel cross-coupling partners in transition-metal-catalyzed reactions

Transition-metal-catalyzed carbene transformations and cross-couplings represent two major reaction types in organometallic chemistry and organic synthesis. However, for a long period of time, these two important areas have evolved separately, with essentially no overlap or integration. Thus, an intriguing question has emerged: can cross-coupling and metal carbene transformations be merged into a single reaction cycle? Such a combination could facilitate the development of novel carbon-carbon bond-forming methodologies. Although this concept was first explored about 10 years ago, rapid developments inthis area have been achieved recently. Palladium catalysts can be used to couple diazo compounds with a wide variety of organic halides. Under oxidative coupling conditions, diazo compounds can also react with arylboronic acids and terminal alkynes. Both of these coupling reactions form carbon-carbon double bonds. As the key step in these catalytic processes, Pd carbene migratory insertion plays a vital role in merging the elementary steps of Pd intermediates, leading to novel carbon-carbon bond formations. Because the diazo substrates can be generated in situ from N-tosylhydrazones in the presence of base, the N-tosylhydrazones can be used as reaction partners, making this type of cross-coupling reaction practical in organic synthesis. N-Tosylhydrazones are easily derived from the corresponding aldehydes or ketones. The Pd-catalyzed cross-coupling of N-tosylhydrazones is considered a complementary reaction to the classic Shapiro reaction for converting carbonyl functionalities into carbon-carbon double bonds. It can also serve as an alternative approach for the Pd-catalyzed cross-coupling of carbonyl compounds, which is usually achieved via triflates. The combination of carbene formation and cross-coupling in a single catalytic cycle is not limited to Pd-catalyzed reactions. Recent studies of Cu-, Rh-, Ni-, and Co-catalyzed cross-coupling reactions with diazo compounds or N-tosylhydrazones show that these transformations also work with other transition metals, demonstrating the generality of the diazo compounds as new cross-coupling partners in transition-metal-catalyzed coupling reactions.