α-Diazo Sulfonium Triflates: Synthesis, Structure, and Application to the Synthesis of 1-(Dialkylamino)-1,2,3-triazoles

The one-pot synthesis of a series of sulfonium salts containing transferable diazomethyl groups is described, and the structure of these compounds is elucidated by X-ray crystallography. Under photochemical conditions, reaction of these salts with N,N-dialkyl hydrazones affords 1-(dialkylamino)-1,2,3-triazoles via diazomethyl radical addition to the azomethine carbon followed by intramolecular ring closure. The straightforward transformation of the structures thus obtained into mesoionic carbene-metal complexes is also reported and the donor properties of these new ligands characterized.

1,2,3-Triazolium-Derived Mesoionic Carbene Ligands Bearing Chiral Sulfur-Based Moieties: Synthesis, Catalytic Properties, and Their Role in Chirality Transfer

1,2,3-Triazole-derived mesoionic carbenes (MICs) having a chiral sulfur functional group at the C5 position are easily available through a CuAAC between chiral alkynyl sulfoxides and different azides. The MICs form complexes with several metals (Au, Ag, Ir, Rh, and Ru) that are enantiomerically pure. Moreover, enantiomerically pure MIC sulfinilimines are obtained from the corresponding sulfoxide retaining the chirality. Through this article, the participation of sulfoxide moieties in different catalytic and chirality transfer processes, as well as in discovering mechanistically new processes will be shown. The role of the sulfur chiral moiety in catalytic cycloisomerization and cycloisomerization-dimerization processes using Au-MIC catalysts is dual. The sulfur functional group either stabilizes intermediates in the catalytic cycle, allowing for the reaction to occur or significantly increases the selectivity of the cyclization processes. 1,2,3-Triazole MICs having chiral sulfoxides at C5 are extremely efficient in preparing chiral at the metal complexes by C-H insertion processes. The chiral at the metal half-sandwich complexes, having the enantiopure sulfur chiral group unaltered, experiences different reactions with complete retention of the configuration. Finally, mechanistically new processes, like the desulfinilation of 1,2,3-triazolium salts in Ag-MIC complexes have been uncovered. These still-nascent classes of compounds will offer opportunities for the discovery of novel catalytic applications and to study new mechanistically sound processes.

Unprecedented E-stereoselectivity on the sigmatropic Hurd-Claisen rearrangement of Morita-Baylis-Hillman adducts: a joint experimental-theoretical study

Herein we report the first systematic investigation of the tandem mercury(ii) catalysed transvinylation/Hurd-Claisen rearrangement of MBH adducts derived from alkyl acrylates. This is the first report of E-selectivity for MBH adducts with alkyl side chains and is complementary to the previously reported Johnson-Claisen and Eschenmoser-Claisen rearrangements. The rearrangement products were obtained in good yields and could be readily converted to 2-alkenyl δ-valerolactones. Combined DFT and F-SAPT studies demonstrate that reaction rates are primarily governed by non-covalent interactions dictating the relative stability of the transition states. Our F-SAPT calculations revealed that the hyperconjugative effects are not so significant, but that electrostatic interactions, instead, are the driving forces for the relative E : Z stereoselectivity.

Hydroxyl-functionalized triazolylidene-based PEPPSI complexes: metallacycle formation effect on the Suzuki coupling reaction

We report the preparation and full characterization of a series of hydroxyl functionalized 1,2,3-triazolylidene-based PEPPSI complexes 2a-c and their catalytic application in the Suzuki cross coupling reaction of aryl chlorides/amides with boronic acids. Under basic reaction conditions, complexes 2a-c show a notable increase in their catalytic efficiency compared with two ether-wingtip functionalized PEPPSI analogues (3 and 4) and a commercially available NHC-Pd complex (5). The catalytic results suggest that deprotonation of the hydroxyl group in complexes 2a-c plays an important role in the overall process. Deprotonation of the alcohol moiety of complexes 2a-b with sodium tert-butoxide allows for the isolation of metallacycles 6a-b, which are proposed as the active species of cross coupling reactions.

4-Halo-1,2,3-triazolylidenes: stable carbenes featuring halogen bonding

Synthesis and coordination of 4-halo-1,2,3-triazolylidenes have been developed. These novel ligands featured the character with σ-donation at carbon and a σ-hole at the halogen. Halogen bonding was observed by single crystal X-ray diffraction in their coinage metal complexes. The electronic properties of 4-iodo-1,2,3-triazolylidene were studied by both Ir-CO frequencies of the Tolman electronic parameter (TEP) and Huynh's electronic parameter (HEP) method, which suggested similar electronic properties to those of imidazolylidenes. During HEP tests, an interesting tunability was observed when different electron donors were employed.

Sophalines E-I, Five Quinolizidine-Based Alkaloids with Antiviral Activities against the Hepatitis B Virus from the Seeds of Sophora alopecuroides

Five novel quinolizidine-based alkaloids (1-5) were obtained from the seeds of Sophora alopecuroides L. Compounds 1 and 2 are the first examples of sparteine-indolizine and matrine-indolizine alkaloids, respectively, whereas 3 and 4 are epimeric normatrine-julolidine alkaloids with unusual skeletons. The structures of these compounds were determined by spectroscopic methods, single-crystal X-ray diffraction, and electronic circular dichroism analyses. Hypothetical biosynthetic pathways of 1-5 were proposed. Compound 2 exhibited significant antiviral activity against the hepatitis B virus.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Phosphinines versus mesoionic carbenes: a comparison of structurally related ligands in Au(i)-catalysis

Gold(i) complexes based on a 2,4,6-triarylphosphinine and a mesoionic carbene derivative have been prepared and characterized crystallographically. Although structurally related, both heterocycles differ significantly in their donor/acceptor properties. These opposed electronic characteristics have been exploited in Au(i)-catalyzed cycloisomerization reactions. For the conversion of the standard substrate dimethyl 2-(3-methylbut-2-enyl)-2-(prop-2-ynyl)malonate the results obtained for both Au-catalysts were found to be very similar and comparable to the ones reported in the literature for other carbene- or phosphorus(iii)-based Au(i)-complexes. In contrast, a clear difference between the catalytic systems was found for the cycloisomerization of the more challenging substrate N-2-propyn-1-ylbenzamide. A combination of the phosphinine-based complex and [AgSbF₆] or [Cu(OTf)₂] leads to a catalytic species, which is more active than the mesoionic carbene-based coordination compound. We attribute these differences to the stronger π-accepting ability of phosphinines in comparison to mesoionic carbenes. The here presented results show for the first time that phosphinines can be used efficiently as π-accepting ligands in Au(i)-catalyzed cycloisomerization reactions.

Synthesis and characterization of a gold(i) bis(triazolylidene) complex featuring a large [(TpMe2)2K] anion

We report the synthesis, charaterization and catalytic performance of a unique bis(triazolylidene) gold(i) complex featuring a large [(Tp^Me2)₂K] anion.

Chrysin functionalized NHC–Au(i) complexes: synthesis, characterization and effects on the nematode Caenorhabditis elegans

The synthesis and biological effects on the nematodeCaenorhabditis elegansof chrysin functionalized Au(i)–NHC complexes are reported.

Palladium(ii)-Acetylacetonato Complexes with Mesoionic Carbenes: Synthesis, Structures and Their Application in the Suzuki-Miyaura Cross Coupling Reaction

A series of novel palladium(ii) acetylacetonato complexes bearing mesoionic carbenes (MICs) have been synthesized and characterized. The synthesis of the complexes of type (MIC)Pd(acac)I (MIC = 1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (1), 1,4-(2,4,6-methyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (2), 1,4-(2,6-diisopropyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (3); acac = acetylacetonato) via direct metalation starting from the corresponding triazolium iodides and palladium(ii) acetylacetonate is described herein. All complexes were characterized by ¹H- and ¹³C-NMR spectroscopy and high resolution mass spectrometry. Additionally, two of the complexes were characterized by single crystal X-ray crystallography confirming a square-planar coordination geometry of the palladium(ii) center. A delocalized bonding situation was observed within the triazolylidene rings as well as for the acac ligand respectively. Complex 2 was found to be an efficient pre-catalyst for the Suzuki-Miyaura cross coupling reaction between aryl-bromides or -chlorides with phenylboronic acid.

Synthesis and catalytic applications of 1,2,3-triazolylidene gold(i) complexes in silver-free oxazoline syntheses and C-H bond activation

A series of novel 1,2,3-triazolylidene gold(i) chloride complexes have been synthesised and fully characterised. Silver-free methodologies for chloride ion abstraction of these complexes were evaluated for their potential as Au-based catalyst precursors. Using simple potassium salts or MeOTf as chloride scavengers produced metal complexes that catalyse both the regioselective synthesis of oxazolines and the C-H activation of benzene or styrene for carbene transfer from ethyl diazoacetate. These results indicate that Ag-free activation of 1,2,3-triazolylidene gold(i) chloride complexes is feasible for the generation of catalytically active Au triazolylidene species. However, silver-mediated activation imparts substantially higher catalytic activity in oxazoline synthesis.

Morita–Baylis–Hillman reaction in eutectic solvent under aqueous medium

Aqueous solvent-catalyst system formed by DES (1ChCl/2Gly) and DABCO is more useful and practical to the Morita–Baylis–Hillman reaction.

Biosourced 1,2,3-triazolium ionic liquids derived from isosorbide

The drastic influence of 1,4-3,6-dianhydrohexitols stereochemistry on physical, thermal and ion conducting properties.