The mechanism of the N–N cross-coupling of nitroarene and aniline catalyzed by 1,2,2,3,4,4-hexamethylphosphetane oxide (1PO) as well as the prediction of a better catalyst was theoretically investigated using DFT and DLPNO-CCSD­(T) calculations. An active species 1P is generated through deoxygenation of 1PO by diphenylsilane. Then, 1P extracts one oxygen atom from nitroarene to produce nitrosoarene. In this deoxygenation step, the [3 + 1] cheletropic addition is a rate-determining step with the ΔG 0≠ and ΔG 0 values of 28.8 and −7.3 kcal/mol, respectively. Next, nitrosoarene exclusively undergoes a dehydrative condensation reaction with aniline to form an azo-cation intermediate, which is the origin of the high selectivity in this cross-coupling reaction. In this step, 2,4,6-trimethylbenzoic acid plays an essential role to significantly reduce the ΔG 0≠ value from 41.1 to 14.8 kcal/mol. Subsequently, 1P reacts with the azo cation to form a stable hydrazinylphosphonium species through the nucleophilic attack of the phosphorus atom to the cationic nitrogen atom. The phosphonium center preferably accepts a hydroxyl group from water to ensure the formation of hydrazine in the subsequent step. In the [3 + 1] cheletropic addition step, the highest occupied molecular orbital (HOMO) of 1P plays an important role. The small-ring scaffold of 1P raises the HOMO energy compared to acyclic phosphorus compounds to achieve high activity of 1P. Substitution of a dimethylamino group for the methyl group in 1P was theoretically predicted to improve the activity by further increasing the HOMO energy.

Mechanistic study on the 1,3,2-diazaphospholene (1)-catalyzed reduction reaction of allyl 2-phenylacrylate 4 with HBpin or ammonia borane (AB) was systematically performed by the density functional theory (DFT) method. When HBpin is employed as the reductant, the reductive Ireland–Claisen (IC) rearrangement reaction occurs. First, the active species P-hydrido-1,3,2-diazaphospholene 3 is generated through the metathesis reaction of 1 with HBpin. Next, the terminal CC double bond of 4 is inserted into the P–H bond of 3 to produce 6a through the 1,2-addition (Markovnikov) step, which is followed by the pinB–H bond activation to afford key boron enolate 8. Then, 8 undergoes the [3,3] rearrangement that is followed by the alcoholysis reaction with methanol leading to the final product γ,δ-unsaturated carboxylic acid. The [3,3] rearrangement step is the rate-determining step with the Gibbs energy barrier (ΔG ≠) and Gibbs reaction energy (ΔG) of 23.9 and −27.5 kcal/mol, respectively. When AB is employed as the reductant, the transfer hydrogenation reaction occurs through two comparable pathways, 1,2- and 1,4-transfer hydrogenation pathways. The former pathway directly leads to the hydrogenation product with the ΔG ≠ and ΔG values of 22.4 and −27.7 kcal/mol, respectively. The latter pathway produces an enolate intermediate (rate-determining step, ΔG ≠/ΔG = 24.1/–0.3 kcal/mol) first, which then prefers to undergo the enol–keto tautomerism instead of the [3,3] rearrangement to afford the hydrogenation product. Obviously, the generation of the boron enolate plays a crucial role in the reductive IC rearrangement reaction because it prevents the enol–keto tautomerism.

Perfluorochemicals (PFCs), especially perfluorooctanoic acid (PFOA), have contaminated the ground and surface waters throughout the world. Efficient removal of PFCs from contaminated waters has been a major challenge. This study developed a novel UV-based reaction system to achieve fast PFOA adsorption and decomposition without addition of sacrificial chemicals by using synthetic photocatalyst sphalerite (ZnS-[N]) with sufficient surface amination and defects. The obtained ZnS-[N] has the capability of both reduction and oxidation due to the suitable band gap and photo-generated hole-trapping properties created by surface defects. The cooperated organic amine functional groups on the surface of ZnS-[N] play a crucial role in the selective adsorption of PFOA, which guarantee the efficient destruction of PFOA subsequently, and 1 μg L–1 PFOA could be degraded to <70 ng L–1 after 3 h in the presence of 0.75 g L–1 ZnS-[N] under 500 W UV irradiation. In this process, the photogenerated electrons (reduction) and holes (oxidation) on the ZnS-[N] surface work in a synergistic manner to achieve complete defluorination of PFOA. This study not only provides promising green technology for PFC-pollution remediation but also highlights the significance of developing a target system capable of both reduction and oxidation for PFC degradation.

Sn-beta zeolite is a promising catalyst for the direct synthesis of lactide from lactic acid; here, we clarify the relationship between its local structure and activity through the interplay of experiments and DFT calculations. The open sites (HO-Sn-(OSi) 3 with adjacent Si−OH) play a key role in the high activity of Sn-beta zeolite, where the reaction rate significantly decreases from 0.2258 mmol min −1 g cat −1 to 0.0724 mmol min −1 g cat −1 if the open sites were masked with Na + . Density functional theory (DFT) calculations show that the decrease in reaction rate comes from the weaker interaction between substrate and the open Sn site by Na + exchange, which leads to the significant increase in the Gibbs energy barrier from 34.0 kcal/mol to 57.2 kcal/mol (rate-determining step). Other activities of heteroatom M-beta zeolites (where M = Ti, Zr, Hf) were also investigated, where the open sites were found to be crucial for the high activity in the direct synthesis of lactide as well. The information provided by this work is valuable for precise design of efficient catalysts for the one-step synthesis of lactide.

No abstract