Mass spectrometry in organic and bio-organic catalysis: Using thermochemical properties to lend insight into mechanism

In this review, we discuss gas phase experimentation centered on the measurement of acidity and proton affinity of substrates that are useful for understanding catalytic mechanisms. The review is divided into two parts. The first covers examples of organocatalysis, while the second focuses on biological catalysis. The utility of gas phase acidity and basicity values for lending insight into mechanisms of catalysis is highlighted.

Spectroscopic and Crystallographic Characterization of the R3 N+ -C-H⋅⋅⋅X Interaction

As appreciation for nonclassical hydrogen bonds has progressively increased, so have efforts to characterize these interesting interactions. Whereas several kinds of C-H hydrogen bonds have been well-studied, much less is known about the R₃ N⁺ -C-H⋅⋅⋅X variety. Herein, we present crystallographic and spectroscopic evidence for the existence of these interactions, with special relevance to Selectfluor chemistry. Of particular note is the propensity for Lewis bases to engage in nonclassical hydrogen bonding over halogen bonding with the electrophilic F atom of Selectfluor. Further, the first examples of ¹ H NMR experiments detailing R₃ N⁺ -C-H⋅⋅⋅X (X=O, N) hydrogen bonds are described.

Perfluorinated Trialkoxysilanol with Dramatically Increased Brønsted Acidity

The Brønsted acidity of the perfluorinated trialkoxysilanol {(F₃C)₃CO}₃SiOH is more than 13 orders of magnitude higher than that of orthosilicic acid, Si(OH)₄, and even more for most previously known silanols. It is easily deprotonated by simple amines and pyridines to give the conjugate silanolates [OSi{OC(CF₃)₃}₃]⁻, which possess extremely short Si−O bonds, comparable to those of silanones.

Synergetic effect of ZnCo2O4/inorganic salt as a sustainable catalyst system for CO2 utilization

Currently, it is essential to consider the rapidly increasing emission of CO₂ into the atmosphere, causing major environmental issues such as climate change and global warming. In this work, we have developed the binary catalyst system (ZnCo₂O₄/inorganic salt) for chemical fixation of CO₂ with epoxides into cyclic carbonates without solvent, and all reactions were performed on a large scale using a 100 ml batch reactor. Two mesoporous catalysts of ZnCo₂O₄ with different architecture, such as flakes (ZnCo-F) and spheres (ZnCo-S) were synthesized and utilized as a heterogeneous catalyst for cycloaddition reaction. The bifunctional property of catalysts is mainly attributed to strong acidic and basic properties confirmed by TPD (NH₃ & CO₂) analysis. The ZnCo-F catalyst exhibited excellent conversion of propylene oxide (99.9%) with good corresponding selectivity of propylene carbonate (≥99%) in the presence of inorganic salt (KI) at 120 °C, 2 MPa, 3 h. In addition, ZnCo-F catalyst demonstrated good catalytic applicability towards the various substrates scope of the epoxide. Furthermore, the catalytic properties were examined by evaluating the reaction parameter such as catalyst loading, pressure, temperature and time. The proposed catalyst exhibited good reusability for cycloaddition reaction without significant change in its catalytic activity and proposed a possible reaction mechanism for chemical fixation of CO₂ with epoxide into cyclic carbonate over ZnCo-F/KI.

The Influence of Weakly Coordinating Cations on the O-H⋅⋅⋅O- Hydrogen Bond of Silanol-Silanolate Anions

The reaction of a saline phosphazenium hydroxide hydrate with siloxanes led to a novel kind of silanol-silanolate anions. The weakly coordinating behavior of the cation renders the formation of silanol-silanolate hydrogen bonds possible, which otherwise suffer from detrimental silanolate-oxygen cation interactions. We investigated the influence of various weakly coordinating cations on silanol-silanolate motifs, particularly with regard to different cation sizes. While large cations favor the formation of intramolecular hydrogen bonds resulting in cyclic structures, the less bulky tetramethyl ammonium cation encourages the formation of polyanionic silanol-silanolate chains in the solid state.

Catalytic, Kinetic, and Mechanistic Insights into the Fixation of CO2 with Epoxides Catalyzed by Phenol-Functionalized Phosphonium Salts

A series of hydroxy-functionalized phosphonium salts were studied as bifunctional catalysts for the conversion of CO2 with epoxides under mild and solvent-free conditions. The reaction in the presence of a phenol-based phosphonium iodide proceeded via a first order rection kinetic with respect to the substrate. Notably, in contrast to the aliphatic analogue, the phenol-based catalyst showed no product inhibition. The temperature dependence of the reaction rate was investigated, and the activation energy for the model reaction was determined from an Arrhenius-plot (Ea =39.6 kJ mol-1 ). The substrate scope was also evaluated. Under the optimized reaction conditions, 20 terminal epoxides were converted at room temperature to the corresponding cyclic carbonates, which were isolated in yields up to 99 %. The reaction is easily scalable and was performed on a scale up to 50 g substrate. Moreover, this method was applied in the synthesis of the antitussive agent dropropizine starting from epichlorohydrin and phenylpiperazine. Furthermore, DFT calculations were performed to rationalize the mechanism and the high efficiency of the phenol-based phosphonium iodide catalyst. The calculation confirmed the activation of the epoxide via hydrogen bonding for the iodide salt, which facilitates the ring-opening step. Notably, the effective Gibbs energy barrier regarding this step is 97 kJ mol-1 for the bromide and 72 kJ mol-1 for the iodide salt, which explains the difference in activity.

Molecular design and experimental study on synergistic catalysts for the synthesis of cyclocarbonate from styrene oxide and CO2

Taking the reaction between styrene oxide and CO₂ to yield cyclocarbonate as the target, the activities of synergistic catalysts, which are composed of Br⁻ and alcohol compounds serving as hydrogen bond donors (HBDs), were predicted by DFT calculations and confirmed by subsequent experiments.