Mechanistic Investigation into the Cleavage of a Phosphomonoester Mediated by a Symmetrical Oxyimine‐Based Macrocyclic Zinc(II) Complex

Density functional calculations are utilized to explore the hydrolysis mechanisms of the phosphomonoester 4‐nitrophenyl phosphate catalyzed by a symmetrical zinc(II) complex. The formation process and properties of the active catalyst are verified. Eight plausible mechanisms are proposed and categorized into three groups. All of the proposed mechanisms, except for Mechanism 7 (see text), are S_N2‐type addition–substitution reaction pathways. Nucleophilic attack at the ortho position occurs in Mechanism 7 with a relatively high reaction barrier. Mechanisms 1 and 2 in the monocatalyst model, Mechanisms 5 to 7 in the sandwich‐dual‐catalyst model, as well as the nucleophilic addition–substitution step in Mechanism 8 are concerted reaction pathways, whereas the rest appear to occur in a stepwise manner. Meanwhile, the explicit solvent model is utilized to consider direct hydrogen bonds and solvation interactions and these results indicate that the added water molecule is involved in the hydrolysis process, but does not change the mechanisms significantly. Mechanism 8, with the lowest reaction barrier, is the most favored reaction pathway of the eight proposed mechanisms, although Mechanisms 1, 4, and 6 are in competition with Mechanism 8. In consideration of the zinc(II) complex concentration, Mechanism 1 is only the predominant reaction pathway at a low zinc(II) complex concentration; Mechanisms 4 and 6 tend to be more competitive with increasing concentration of the zinc(II) complexes, and Mechanism 8 is favored at high zinc(II) complex concentrations. Our calculated results are consistent with, and can be used to systematically interpret, experimental observations. More importantly, insightful suggestions are made regarding the catalyst design and selection of the reaction environment.     

Reaction mechanism of oxidative desulfurization of heterocyclic organic sulfides: a computational study

The potential energy surfaces of the oxidation of two model heterocyclic organic sulfides thiophene and benzothiophene were examined using H₂O₂ and HCO₃H as oxidants adopting CCSD(T), ωB97X-D, M06-2X and B3LYP at the 6-311+G (d,p) level of theory. Stationary points on the potential energy surfaces for the first and second oxidation reaction were fully optimized and characterized. The natural orbital population analysis was also performed to understand the charge distribution. The results suggest that the oxidation of benzothiophene is faster than that of thiophene using both oxidants, and HCO₃H is more efficient than H₂O₂ in oxidation of both sulfides, which are in accordance with the experimental observation. Such results may assist in understanding the reaction mechanism of the oxidative desulfurization of sulfides.     

Electrosynthesis of Chirality Conducting Poly[N‐(9‐fluorenylmethoxycarbonyl)‐L‐phenylalanine] with Good Blue Light‐Emitting Properties

Poly[N‐(9‐fluorenylmethoxycarbonyl)‐L‐phenylalanine] (PN9FPA) films with good fluorescence properties and chirality were prepared electrochemically by direct anodic oxidation of N‐(9‐fluorenylmethoxycarbonyl)‐L‐phenylalanine (N9FPA) in boron trifluoride diethyletherate (BFEE). Fourier transform infrared spectroscopy measurement showed that the polymerization of N9FPA occurred mainly at the C(2) and C(7) positions. The fluorescence spectra indicated that PN9FPA films were blue‐light emitters. In addition, the structures and properties of the monomer and the polymers were characterized and evaluated with CV, UV, TGA and SEM.     

Intermolecular Electronic Coupling of Organic Units for Efficient Persistent Room‐Temperature Phosphorescence

Although persistent room‐temperature phosphorescence (RTP) emission has been observed for a few pure crystalline organic molecules, there is no consistent mechanism and no universal design strategy for organic persistent RTP (pRTP) materials. A new mechanism for pRTP is presented, based on combining the advantages of different excited‐state configurations in coupled intermolecular units, which may be applicable to a wide range of organic molecules. By following this mechanism, we have developed a successful design strategy to obtain bright pRTP by utilizing a heavy halogen atom to further increase the intersystem crossing rate of the coupled units. RTP with a remarkably long lifetime of 0.28 s and a very high quantum efficiency of 5 % was thus obtained under ambient conditions. This strategy represents an important step in the understanding of organic pRTP emission.     

An experimental and theoretical study of NSCl decomposition in the presence of trace amounts of water

An experimental and density functional theory study of the decomposition reaction of NSCl with water molecule(s) into NH 3 and SO 2 is described. Experiments involving the decomposition of NSCl in the presence of trace amounts of water showed that the formation of both HNSO and SO 2 is faster when the cell and the cell surface are fresh and more likely to be coated in adsorbed water. The density functional theory computational results suggest that the decomposition involves two main steps: reactions 1 NSCl + nH 2O --> HNSO + ( n - 1) H 2O + HCl and 2 HNSO + nH 2O --> NH 3 + SO 2 + ( n - 1)H 2O. The barrier for reaction 1 in which NSCl decomposes to form HNSO becomes substantially lower when the explicit hydrogen bonding of water molecules is considered. The water assisted decomposition reaction mechanism can help account for the experimentally observed conversion of NSCl to produce HNSO in the presence of water. The calculated barrier of reaction 2 (the decomposition of HNSO into NH 3 and SO 2) can also explain the experimental observation of SO 2 bands from the decomposition of NSCl in the presence of water.     

On the mechanism and stereochemistry of chiral lithium-carbenoid-promoted cyclopropanation reactions

An investigation into the mechanism and stereochemistry of chiral lithium-carbenoid-promoted cyclopropanation reactions by using density functional theory (DFT) methods is reported. Previous work suggested that this type of cyclopropanation reaction may proceed by competition between a methylene-transfer mechanism and a carbometalation mechanism. In this paper, it is demonstrated that the intramolecular cyclopropanation reactions promoted by chiral carbenoids 1 and 2 proceed by the methylene-transfer mechanism. The carbometalation mechanism was found to have a much higher reaction barrier and does not appear to compete with the methylene-transfer mechanism. The Lewis base group does not enhance the carbometalation pathway enough to compete with the methylene-transfer pathway. The present computational results are consistent with experimental observations for these cyclopropanation reactions. The factors governing the stereochemistry of the intramolecular cyclopropanation reaction by the methylene-transfer mechanism were examined to help elucidate the origin of the stereoselectivity observed in experiments. Both the directing group and the configuration at the C(1) centre were found to play a key role in the stereochemistry. Carbenoid 1 has a chiral C(1) centre of R configuration. The Lewis base group directs the cyclization of carbenoid 1 to form a single product. In contrast, the Lewis base group cannot direct the cyclization of carbenoid 2 to furnish a stereoselective product due to the S configuration of the chiral C(1) centre in carbenoid 2. This relationship of the stereochemistry to the chiral character of the carbenoid has implications for the design of new efficient carbenoid reagents for stereoselective cyclopropanation.     

Theoretical study of samarium (II) carbenoid (ISmCH2I) promoted cyclopropanation reactions with ethylene and the effect of THF solvent on the reaction pathways

A computational study of the cyclopropanation reactions of divalent samarium carbenoid ISmCH(2)I with ethylene is presented. The reaction proceeds through two competing pathways: methylene transfer and carbometalation. The ISmCH(2)I species was found to have a "samarium carbene complex" character with properties similar to previously investigated lithium carbenoids (LiCH(2)X where X = Cl, Br, I). The ISmCH(2)I carbenoid was found to be noticeably different in structure with more electrophilic character and higher chemical reactivity than the closely related classical Simmons-Smith (IZnCH(2)I) carbenoid. The effect of THF solvent was investigated by explicit coordination of the solvent THF molecules to the Sm (II) center in the carbenoid. The ISmCH(2)I/(THF)(n)() (where n = 0, 1, 2) carbenoid methylene transfer pathway barriers to reaction become systematically lower as more THF solvent is added (from 12.9 to 14.5 kcal/mol for no THF molecules to 8.8 to 10.7 kcal/mol for two THF molecules). In contrast, the reaction barriers for cyclopropanation via the carbometalation pathway remain high (>15 kcal/mol). The computational results are briefly compared to other carbenoid reactions and related species.     

Modeling SN2 reactions in methanol solution by ab initio calculation of nucleophile solvent-substrate clusters

[Structure: see text]. Ab initio calculations were used to study the S(N)2 reactions of the CH3OCH2I molecule with a methoxide ion (CH3O-) and a methanol molecule by systematically building up the reaction system with explicit incorporation of the methanol solvent molecules. For the reaction of CH3OCH2I with a methoxide ion, the explicit incorporation of the methanol molecules to better solvate the methoxide ion led to an increase in the barrier to reaction. For the reaction of CH3OCH2I with a methanol molecule, the explicit incorporation of the methanol molecules led to a decrease in the barrier to reaction because of an inclination of this reaction to proceed with the nucleophilic displacements accompanied by proton transfer through the H-bonding chain. The H-bonding chain served as both acid and base catalysts for the displacement reaction. A ca. 10(15)-fold acceleration of the methanol tetramer incorporated S(N)2 reaction was predicted relative to the corresponding methanol monomer reaction. The properties of the reactions examined are discussed briefly.     

Density functional theory investigation of the remarkable reactivity of geminal dizinc carbenoids (RZn)(2)CHI (R = Et or I) as cyclopropanation reagents with olefins compared to mono zinc carbenoids RZnCHI(2), EtCHIZnR (R = Et or I)

Density functional theory calculations for the cyclopropanation reactions of several mono zinc carbenoids and their corresponding gem-dizinc carbenoids with ethylene are reported. The mono zinc carbenoids react with ethylene via an asynchronous attack on one CH2 group of ethylene with a relatively high barrier to reaction in the 20-25 kcal/mol range similar to other Simmons-Smith type carbenoids previously studied. In contrast, the gem-dizinc carbenoids react with ethylene via a synchronous attack on both CH2 groups of ethylene and substantially lower barriers to reaction (about 15 kcal/mol) compared to their corresponding mono zinc carbenoid. Both mono zinc and gem-dizinc carbenoid reactions can be accelerated by the addition of ZnI2 groups as a Lewis acid, and this lowers the barrier by another 1.0-5.1 kcal/mol and 0.0-5.5 kcal/mol, respectively, for addition of one ZnI2 group. Our results indicate that gem-dizinc carbenoids react with C=C bonds with significantly lower barriers to reaction and in a noticeably different manner than Simmons-Smith type mono zinc carbenoids. The three gem-dizinc carbenoids have a substantially larger positive charge distribution than those in the mono zinc carbenoids and, hence, a stronger electrophilic character for the gem-dizinc carbenoids.     

Water-catalyzed dehalogenation reactions of isobromoform and its reaction products

A combined experimental and theoretical study of the photochemistry of CHBr(3) in pure water and in acetonitrile/water mixed solvents is reported that elucidates the reactions and mechanisms responsible for the photochemical conversion of the halogen atoms in CHBr(3) into three bromide ions in water solution. Ultraviolet excitation at 240 nm of CHBr(3) (9 x 10(-)(5) M) in water resulted in almost complete conversion into 3HBr leaving groups and CO (major product) and HCOOH (minor product) molecules. Picosecond time-resolved resonance Raman (ps-TR(3)) experiments and ab initio calculations indicate that the water-catalyzed O-H insertion/HBr elimination reaction of isobromoform and subsequent reactions of its products are responsible for the production of the final products observed following ultraviolet excitation of CHBr(3) in water. These results have important implications for the phase-dependent behavior of polyhalomethane photochemistry and chemistry in water-solvated environments as compared to gas-phase reactions. The dissociation reaction of HBr into H(+) and Br(-) ions is the driving force for several O-H insertion and HBr elimination reactions and allows O-H and C-H bonds to be cleaved more easily than in the absence of water molecules. This water-catalysis by solvation of a leaving group and its dissociation into ions (e.g., H(+) and Br(-) in the examples investigated here) may occur for a wide range of chemical reactions taking place in water-solvated environments.