Coinage metal aluminyl complexes: probing regiochemistry and mechanism in the insertion and reduction of carbon dioxide.

The synthesis of coinage metal aluminyl complexes, featuring M-Al covalent bonds, is reported via a salt metathesis approach employing an anionic Al(i) ('aluminyl') nucleophile and group 11 electrophiles. This approach allows access to both bimetallic (1 : 1) systems of the type ( t Bu3P)MAl(NON) (M = Cu, Ag, Au; NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) and a 2 : 1 di(aluminyl)cuprate system, K[Cu{Al(NON)}2]. The bimetallic complexes readily insert heteroallenes (CO2, carbodiimides) into the unsupported M-Al bonds to give systems containing a M(CE2)Al bridging unit (E = O, NR), with the µ-κ1(C):κ2(E,E') mode of heteroallene binding being demonstrated crystallographically for carbodiimide insertion in the cases of all three metals, Cu, Ag and Au. The regiochemistry of these processes, leading to the formation of M-C bonds, is rationalized computationally, and is consistent with addition of CO2 across the M-Al covalent bond with the group 11 metal acting as the nucleophilic partner and Al as the electrophile. While the products of carbodiimide insertion are stable to further reaction, their CO2 analogues have the potential to react further, depending on the identity of the group 11 metal. ( t Bu3P)Au(CO)2Al(NON) is inert to further reaction, but its silver counterpart reacts slowly with CO2 to give the corresponding carbonate complex (and CO), and the copper system proceeds rapidly to the carbonate even at low temperatures. Experimental and quantum chemical investigations of the mechanism of the CO2 to CO/carbonate transformation are consistent with rate-determining extrusion of CO from the initially-formed M(CO)2Al fragment to give a bimetallic oxide that rapidly assimilates a second molecule of CO2. The calculated energetic barriers for the most feasible CO extrusion step (ΔG ‡ = 26.6, 33.1, 44.5 kcal mol-1 for M = Cu, Ag and Au, respectively) are consistent not only with the observed experimental labilities of the respective M(CO)2Al motifs, but also with the opposing trends in M-C (increasing) and M-O bond strengths (decreasing) on transitioning from Cu to Au.

Computational Mechanistic Study of Brønsted Acid-Catalyzed Unsymmetrical 1,2,4,5-Tetrazines Synthesis.

Density functional theory (DFT) calculations were conducted to gain insight into the reaction mechanism of the Brønsted acid-catalyzed unsymmetrical 1,2,4,5-tetrazine synthesis. Various possible reaction pathways were considered, and the most favorable one can be characterized via sequential six steps, including addition of DCM to hydrazine 1 giving complex IM4, N-H bond activation in IM4 mediated by sulfur, AcOH-assisted substitution of 3 with sulfur-activated hydrazine 2, HNO2-assisted addition of nitrile to intermediate 8, cyclization, and intramolecular elimination leading to the final product 7. Among the six steps, sulfur activation of IM4 N-H bond is found to be the rate-determining step (RDS). The mechanism rationalizes the experimental observation that 2 equiv of sulfur leads to the best yield of product. Furthermore, we disclosed that the Brønsted acid additives (i.e., acetic acid and nitrous acid) served triple roles as catalyst, proton shuttle, and hydrogen bond donor and acceptor in the whole catalysis.

Mechanistic Study of the N-Quaternized Pyridoxal-Catalyzed Biomimetic Asymmetric Mannich Reaction: Insights into the Origins of Enantioselectivity and Diastereoselectivity.

Density functional theory calculations have been performed to gain insights into the catalytic mechanism of the N-quaternized pyridoxal (i.e., 1a)-mediated biomimetic asymmetric Mannich reaction of tert-butyl glycinate 3 with N-diphenylphosphinyl imine 2a to give the diamino acid ester 4a in high yield with excellent enantiomeric and diastereomeric selectivity (Science 2018, 360, 1438). The study reveals that the whole catalysis can be characterized via three stages: (i) the catalyst 1a reacts with the tert-butyl glycinate 3 to generate the active carbanion complex IM3. (ii) IM3 then reacts with the N-diphenylphosphinyl imine 2a giving the imine intermediate IM8. (iii) IM8 undergoes hydrolysis to give the final product anti-4a and regenerate the catalyst 1a for the next catalytic cycle. Each stage is kinetically and thermodynamically feasible for experimental realization. The hydrolysis step in the stage III is predicted to be the rate-determining step during the whole catalytic cycle. Furthermore, the origins of the enantioselectivity and diastereoselectivity for the target reaction, as well as the deactivation of the catalyst 1b, are also discussed.

Visible-Light-Induced Regio- and Stereoselective C(sp2)-H Trifluoroethylation of Enamides with 2,2,2-Trifluoroethyl Iodide.

A photoredox-catalyzed regio- and stereoselective trifluoroethylation reaction of enamides using commercially available 2,2,2-trifluoroethyl iodide as trifluoroethylating agents has been developed, furnishing geometrically defined and synthetically and physiochemically pivotal ß-trifluoroethylated enamides bearing a diverse range of functional groups.

The Effect of Aging Time on the Properties of Mg-Al-CO3 Layered Double Hydroxides and Its Application as a Catalyst Support for TiO2.

By using chloride salts as the raw materials, Mg-Al-CO3 layered double hydroxides (Mg-Al-CO3 LDHs) with Mg/Al molar ratio 2:1 were prepared via a coprecipitation method. The effect of the aging time on the crystallinity, particle size, chemical composition, morphology, specific surface area, and pore size distribution of Mg-Al-CO3 LDHs were studied in details. The effect of aging time during LDHs preparation process on adsorption capacity of LDHs and their calcined products (mixed metal oxide, MMO) were investigated by removing the pollutant acid red 1 (AR1) from aqueous solution. The results showed that LDHs prepared at 90 degrees C for 4 h (LDH-4) had a higher specific area of 103.7 m2/g and LDHs prepared at 90 degrees C for 8 h (LDH-8) exhibited well-formed hexagonal crystals with a relative smooth surface. The LDH-4 and its calcined product are more effective in removing AR1 than LDH-8, which is related to the differences in their specific surface area. Meanwhile, anatase TiO2-coated MMO with MMO/TiO2 mass ratios at 1:1 and.2:1 were prepared via a chemical precipitation route in the presence of LDH, the composites showed an efficiently photocatalytic activity in the removal of AR1.

[Correlation of the inner diameter parameters of the spermatic vein in different positions and states of the varicocele patient with the results of seminal examination].

OBJECTIVE: To study the correlation of the inner diameter parameters of the spermatic vein in different positions and states of the varicocele (VC) patient with the results of seminal examination. METHODS: A total of 149 VC patients underwent ultrasonography, routine semen examination, and sperm morphological analysis. The parameters obtained from ultrasonography included the bilateral testis volume in a supine position, the largest spermatic vein diameter in a supine position at rest (DSR), the largest spermatic vein diameter in a supine position following Valsalva manoeuvre (DSV), the largest spermatic vein diameter in an upright position at rest (DUR), and the largest spermatic vein diameter in an upright position following Valsalva manoeuvre (DUV). Then we calculated the parameters △DS=DSV－DSR, △DU=DUV－DUR, △DR=DUR－DSR, and △DV=DUV－DSV and analyzed the correlation of the above parameters with the results of semen examination using the ROC curve. RESULTS: Based on the results of semen examination, 119 (79.87%) of the patients were allocated to the abnormal group and the other 30 (20.13%) to the normal group. Statistically significant differences were observed between the two groups in △DU (P=0.007), △DR (P=0.0001), and △DV (P=0.04), but not in DSR (P=0.35), DSV (P=0.34), DUR (P=0.06), DUV (P=0.12), and △DS (P=0.64), nor in the volume of the testis affected (P=0.323). The area under the ROC curve was 0.55 for DSR, 0.57 for DSV, 0.64 for DUR, 0.62 for DUV, 0.49 for △DS, 0.28 for △DU, 0.86 for △DR, and 0.69 for △DV. The corresponding cutoff values were 2.25, 2.51, 2.48, 2.63, 0.30, 0.23, 0.25, and 0.20, the corresponding sensitivities of semen detection were 50.42%, 65.55%, 60.50%, 60.50%, 49.90%, 29.41%, 79.83%, and 65.55%, and the corresponding specificities were 56.67%, 63.33%, 63.33%, 63.33%, 56.67%, 33.33%, 80%, and 63.33%, respectively. CONCLUSIONS: The difference between the largest spermatic vein diameters in supine and upright positions at rest provides a high diagnostic accuracy for semen detection and helps to predict abnormality in seminal examination for VC patients.