Cobalt-Catalyzed (3 + 2) Cycloaddition of Cyclopropene-Tethered Alkynes: Versatile Access to Bicyclic Cyclopentadienyl Systems and Their CpM Complexes

Low-valent cobalt complexes can promote intramolecular (3 + 2) cycloadditions of alkyne-tethered cyclopropenes to provide bicyclic systems containing highly substituted cyclopentadienyl moieties with electronically diverse functional groups. The adducts can be easily transformed into new types of CpRh(III) and CpIr(III) complexes, which show catalytic activity in several relevant transformations. Preliminary computational (DFT) and experimental studies provide relevant information on the mechanistic peculiarities of the cobalt-catalyzed process and allow us to rationalize its advantages over the homologous rhodium-promoted reaction.

Robust Cobalt Catalyst for Nitrile/Alkyne [2+2+2] Cycloaddition: Synthesis of Polyarylpyridines and Their Mechanochemical Cyclodehydrogenation to Nitrogen-Containing Polyaromatics*

The transition-metal-catalyzed [2+2+2] cycloaddition of nitriles and alkynes is an established synthetic approach to pyridines; however, these cycloadditions often rely on the use of tethered diynes or cyanoalkynes as one of the reactants. Thus, examples of efficient, fully intermolecular catalytic [2+2+2] pyridine synthesis, especially those employing unactivated nitriles and internal alkynes leading to pentasubstituted pyridines, remain scarce. Herein, we report on simple and inexpensive catalytic systems based on cobalt(II) iodide, 1,3-bis(diphenylphosphino)propane, and Zn that promote [2+2+2] cycloaddition of various nitriles and diarylacetylenes for the synthesis of a broad range of polyarylated pyridines. DFT studies support a reaction pathway involving oxidative coupling of two alkynes, insertion of the nitrile into a cobaltacyclopentadiene, and C-N reductive elimination. The resulting tetra- and pentaarylpyridines serve as precursors to hitherto unprecedented nitrogen-containing polycyclic aromatic hydrocarbons via mechanochemically assisted multifold reductive cyclodehydrogenation.

Air-Stable CpCoI-Phosphite-Fumarate Precatalyst in Cyclization Reactions: Comparing Different Methods of Energy Supply

The robust CoI precatalyst [CpCo(P{OEt}3)(trans-MeO2CHC=CHCO2Me)] was investigated in cyclotrimerizations, furnishing benzenes and pyridines from triynes, diynes and nitriles, comparing the influence of different ways of energy supply; namely, irradiation and conventional (thermal) or microwave heating. The precatalyst was found to work under all conditions, including the possibility to catalyze cyclotrimerizations at room temperature under photochemical conditions at longer reaction times. Performance of the reactions in a microwave reactor proved to be the most time-efficient way to rapidly assemble the expected reaction products; however, careful selection of reaction conditions can be required. The synthesis of pyridines and isoquinolines successfully involved the utilization of versatile functionalized nitriles, affording structurally interesting reaction products. Comparison with the known and often applied precatalyst CpCo(CO)2 demonstrated the significantly higher reactivity of the CpCoI-phosphite-olefin precatalyst.

Microwave effects on NiMoS and CoMoS single-sheet catalysts

Single-sheet nanoclusters of MoS₂, NiMoS or CoMoS are widely used in hydrodesulfurization (HDS) catalysis in the petroleum industry. In HDS reactions under microwave irradiation, experiments indirectly pointed out that for pristine MoS₂ reaction rates are accelerated because hot spots are generated on the catalyst bed. In this work, we investigated NiMoS and CoMoS isolated single-sheet substituted catalysts before and after thiophene adsorption focusing on quantifying the effect of microwave irradiation. For that purpose, density functional theory (DFT) molecular charge densities of each system were decomposed according to the distributed multipole analysis (DMA) of Stone. Site dipole values of each system were directly associated with a larger or smaller interaction with the microwave field according to a proposed general approach. We showed that microwave enhancement of HDS reaction rates can occur more efficiently in the CoMoS and NiMoS promoted clusters compared to pristine MoS₂ in the following order: CoMoS > NiMoS > MoS₂. The atomic origin of the catalyst hot spots induced by microwaves was clearly established in the promoted clusters.

Understanding MAOS through computational chemistry

Computational chemistry may explain and rationalize the impact of microwave irradiation in organic synthesis.

Transition metal-catalyzed [2 + 2 + 2] cycloaddition of nitrogen-linked 1,6-diynes: a straightforward route to fused pyrrolidine systems

Transition metal-catalyzed [2 + 2 + 2] cycloadditions of nitrogen-linked 1,6-diynes with unsaturated motifs have recently attracted more attention from synthetic organic chemists because of their high efficiency in the construction of numerous pyrrolidine based systems.

Microwave catalytic effect: a new exact reason for microwave-driven heterogeneous gas-phase catalytic reactions

The microwave catalytic effect (lowering of activation energy) under microwave irradiation results in NO conversion exceeding that realized through conventional heating.

Recent application of calculations of metal complexes based on density functional theory

Recent application of density functional theory (DFT) for metal complexes is reviewed to show the achievements of DFT and the challenges for it, as well as the methods for selecting proper functionals.

New page to access pyridine derivatives: synthesis from N-propargylamines

The pyridine is one of the simplest, yet most important N-heterocycles.

Influence of Polarity and Activation Energy in Microwave-Assisted Organic Synthesis (MAOS)

The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20-30 kcal mol(-1) and a polarity (μ) between 7-20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation.

The issue of 'molecular radiators' in microwave-assisted reactions. Computational calculations on ring closing metathesis (RCM)

A DFT computational mechanistic study of the ring closing metathesis (RCM) reaction of diallyl ether or N,N-diallyl-p-toluenesulfonamide catalyzed by a second generation Grubbs-type ruthenium carbene complex has been carried out. This study was performed at the PCM(CH2Cl2)-B3LYP/6-311+G(2d,p)//B3LYP/SDD theory level. The aim of this work was to shed light on the influence that microwave irradiation has on these reactions and to gain insight into the so-called 'molecular radiator' effect. The outcomes obtained indicate that thermal effects induced by microwave irradiation decrease the catalytic induction period. The presence of a polar reagent and/or polar species in the reaction that increases the polarity of the medium may enhance this thermal effect.