Mechanistic investigation inspired "on water" reaction for hydrobromic acid-catalyzed Friedel-Crafts-type reaction of β-naphthol and formaldehyde

Exploring the Effects of Water on the Mechanism of the Catalyst-Free Reaction between Isatin and 3-Methyl-2-pyrazolin-5-one from the Mixed Implicit/Explicit Multiple Types of Water Clusters

Density functional theory calculations with implicit/explicit water cluster models were conducted to pursue deeper understandings about the mechanism and the water effects in the reaction of isatin with 3-methyl-2-pyrazolin-5-one. The proposed preferential mechanistic scenario here undergoes three major steps: first, 3-methyl-2-pyrazolin-5-one converts to its enol form and then, the aldol addition reaction takes place between isatin and enol to generate the intermediate INT2, followed finally by the tautomerization of INT2 to become the product 3-pyrazolone. The computed results indicate that the direct aldol reaction without the water auxiliary is feasible in the second step and the remaining tautomerization steps (steps 1, 3, and 4) assisted by tri-, tri-, and six-water cluster models, respectively, are the most favorable cases. It is further noted that more hydrogen bonding interactions in the tri-water auxiliary reaction are essential for the reduction of the free energy barrier ΔG^⧧ in the proton transfer largely than those assisted by the other types of water cluster models. The origin of the more stable transition state in the rate-determining step of the tri-water cluster model is ascribed to smaller cyclic strain and more global electron density transfer associated to its structure than the other types of water cluster models.

Mechanistic insights into N-Bromosuccinimide-promoted synthesis of imidazo[1,2-a]pyridine in water: Reactivity mediated by substrates and solvent

The mechanism of N-Bromosuccinimide (NBS) promoted synthesis of imidazo[1,2-a]pyridine in water as well as the effective activation modes of NBS was investigated by Density Functional Theory (DFT) calculations. Two main mechanisms that differ in the reaction sequence of substrate were explored: styrene with NBS then followed by 2-aminopyridine (M1) or simultaneously with NBS and 2-aminopyridine (M2), and water-assisted M2 is the more favored one. We found that the adding sequence of 2-aminopyridine affects profoundly on the title reaction. Moreover, upon the assistance of water and NBS, the preferential mechanistic scenario involves three major processes: nucleophilic addition, stepwise H-shift and intramolecular cyclization, three-step deprotonation, rather than a classical bromonium ion species. Specifically, the cooperative interaction of NBS and water plays a critical role in the title reaction. Water acts as solvent, reactant, anchoring, stabilizer, and catalyst. NBS promotes the above three processes by the effective forms of Br⁺ /Br^- , succinimide, and its ethanol isomer. Furthermore, noncovalent interactions between catalysts and substrates are responsible for the different reactive activities of M1 and M2. Our results indicate that simultaneous adding of all reactants is recommended toward economical synthesis. © 2018 Wiley Periodicals, Inc.