Sesquicarbene Complexes:Bonding at the Interface Between M-C Single Bonds and M=C Double Bonds

Allylic dimetalated complexes [M2C(vinyl)]+ (M = Au(IPr) and Cr(CO)5 -) incorporate a new coordination mode of carbon. Digold complexes of this type have recently been detected experimentally. The intrinsic bond orbitals, partial charges, and structural parameters of the gold complexes and of chromium analogs were studied computationally and compared to those of the respective monometalated species and hydrocarbons. This showed that such digold complexes have a carbene character at both Au-C bonds comparable to typical carbene complexes of gold. Dichromium complexes with their stronger π-backdonation compete for interaction with carbon's π-orbital; each of the chromium atoms partakes in double bonding that is significant but weaker than that in the carbene analogs. Containing two M-C bonds on the interface between single and double bonds, these bridged complexes can be conceived as "sesquicarbene complexes". The π-system acted in a very adaptive manner and employed additional stabilization of the vinyl system only where needed. Significant carbene character is found simultaneously in both M-C bonds at the same carbon center. The discovery of these complexes with relatively strong double bond character between one carbon and two metal atoms could bring unusual single-carbon-centered organometallic cascade reactions to the horizon

Gold-Catalyzed Stereoselective Domino Cyclization/Alkynylation of N-Propargylcarboxamides with Benziodoxole Reagents for the Synthesis of Alkynyloxazolines

A concise and highly stereoselective synthesis of alkynyloxazolines via a gold-catalyzed domino cyclization-alkynylation cascade of N-propargylcarboxamides with benziodoxole reagents is reported. This new protocol, which represents an attractive alternative to two step sequences based on Sonagashira couplings, offers a broad substrate scope, excellent functional group tolerance, and perfect stereoselectivity. A comparison of the computed energies of the isomers of the product suggests kinetic control as the cause of the observed selectivity

Gold carbene complexes and beyond: new avenues in gold(I)-carbon coordination chemistry

The different coordination modes in carbon-gold(I) complexes are spotlighted with a focus on gold(I). The emergence of gold carbene complexes and their critical discussion in the community is presented, and an overview of their bonding fundamentals and synthetic procedures is given. Several intriguing and sometimes highly reactive gold-carbon coordination complexes that were recently synthesised and isolated are reviewed and a critical outlook into their potential applications is provided. This includes vinylidene and higher cumulenylidene complexes, free carbenes with gold substituents, carbodicarbene complexes, and non-Fischer/Schrock sesquicarbene complexes.11Nsciescopu

Tributyl(1-((dimethylamino)(dimethyliminio)methyl)-1,4-dihydropyridin-4-yl)phosphonium Ditrifluoromethanesulfonate

Site-selective functionalization of pyridines is a crucial tool for the synthesis of diverse pharmaceuticals and materials. We introduced diiminium pyridine adducts as highly convenient and potent Lewis acids. We report that tributylphosphine selectively adds to the 4-position of pyridine in tetramethyldiiminium pyridine ditrifluoromethanesulfonate, resulting in the formation of the title compound. This finding represents an advancement towards the utilization of diiminium units as organic reagents or catalysts for pyridine functionalization. We also employ computational models to determine fluoride and hydride ion affinities, Fukui function f+(r), molecular electrostatic potential, and pKa values, providing valuable insights for future investigations in this area

How to Enhance the Efficiency of Breslow Intermediates for SET Catalysis

Oxidative carbene organocatalysis, which proceeds via single electron transfer (SET) pathways, has been limited by the moderately reducing properties of deprotonated Breslow intermediates BI−s derived from thiazol-2-ylidene 1 and 1,2,4-triazolylidene 2. Using computational methods, we assess the redox potentials of BI−s based on ten different types of known stable carbenes and report our findings concerning the key parameters influencing the steps of the catalytic cycle. From the calculated values of the first oxidation potential of BI−s derived from carbenes 1 to 10, it appears that apart from the diamidocarbene 7, all the others are more reducing than thiazol-2-ylidene 1 and the 1,2,4-triazolylidene 2. We observed that while the reducing power of BI−s significantly decreases with increasing solvent polarity, the redox potential of the oxidant can increase at a greater rate, thus facilitating the reaction. The cation, associated with the base, also plays an important role when a non-polar solvent is used; large and weakly coordinating cations such as Cs+ are beneficial. The radical-radical coupling step is probably the most challenging step due to both electronic and steric constraints. Based on our results, we predict that mesoionic carbene 3 and abnormal NHC 4 are the most promising candidates for oxidative carbene organocatalysis

Ambient Moisture Accelerates Hydroamination Reactions of Vinylarenes with Alkali‐Metal Amides under Air

A straightforward alkali-metal-mediated hydroamination of styrenes using biorenewable 2-methyltetrahydrofuran as a solvent is reported. Refuting the conventional wisdom of the incompatibility of organolithium reagents with air and moisture, shown here is that the presence of moisture is key in favoring formation of the target phenethylamines over competing olefin polymerization products. The method is also compatible with sodium amides, with the latter showing excellent promise as highly efficient catalysts under inert atmosphere conditions

How To Enhance the Efficiency of Breslow Intermediates for SET Catalysis

Oxidative carbene organocatalysis, which proceeds via single electron transfer (SET) pathways, has been limited by the moderately reducing properties of deprotonated Breslow intermediates BI–s derived from thiazol-2-ylidene 1 and 1,2,4-triazolylidene 2. Using computational methods, we assess the redox potentials of BI–s based on ten different types of known stable carbenes and report our findings concerning the key parameters influencing the steps of the catalytic cycle. From the calculated values of the first oxidation potential of BI–s derived from carbenes 1 to 10, it appears that, apart from the diamidocarbene 7, all the others are more reducing than thiazol-2-ylidene 1 and the 1,2,4-triazolylidene 2. We observed that while the reducing power of BI–s significantly decreases with increasing solvent polarity, the redox potential of the oxidant can increase at a greater rate, thus facilitating the reaction. The cation, associated with the base, also plays an important role when a nonpolar solvent is used; large and weakly coordinating cations such as Cs+ are beneficial. The radical–radical coupling step is probably the most challenging step due to both electronic and steric constraints. Based on our results, we predict that mesoionic carbene 3 and abnormal NHC 4 are the most promising candidates for oxidative carbene organocatalysis

Diiminium Nucleophile Adducts are Stable and Convenient Lewis Superacids

Strong Lewis acids are essential tools for a manifold of chemical procedures that aim to react weakly basic centres but their scalable deployment is severely limited by their costs and safety concerns. We report that dicationic relatives of guanidinium can be conveniently synthesised in a two-step one-pot procedure from tetramethylurea. Triflic anhydride is used to generate an isouronium intermediate. Substitution of the bound triflate with pyridines yield the dicationic tetramethyldiiminium bis(triflate) nucleophile adducts (TMDINu). Their proposed diiminium character is demonstrated by substituting pyridine from the corresponding adduct with other nucleophiles. The observation of a chelation effect in the 2,2’-bipyridine adducts supports Lewis acidic character of the diiminium π-system and flexibility towards accepting another bond. High fluoride, hydride, and oxide affinities are demonstrated, leading to their classification as soft and hard Lewis superacids. An example reaction is reported which shows that the tetramethyldiiminium bis(triflate) pyridine complex (TMDIPy) is more effective than conventional reagents in the activation of electron-poor amines for amide couplings

Entropy-Induced Selectivity Switch in Gold Catalysis: Fast Access to Indolo[1,2-a]quinolines

New N-heterocyclic compounds for organic functional materials and their efficient syntheses are highly demanded. A surprising entropy-induced selectivity switch in the gold-catalyzed intramolecular hydroarylation of 2-ethynyl N-aryl indoles was found and its exploitation led to straightforward syntheses of indolo[1,2-a]quinolines. Experimental and computational mechanistic investigations gave insight into this uncommon selectivity phenomenon and into the special reactivity of the indolo[1,2-a]quinolines. The high functional group tolerance of this methodology enabled access to a diverse scope with high yields. In addition, bidirectional approaches, post-functionalization reactions, and pi-extension of the core structure were feasible. An in-depth study of the photophysical properties explored the structure-effect relationship for different derivatives and revealed a high potential of these compounds for future applications as functional materials.11Nsciescopu