Noriko Tsuchida

DFT Study of Internal Alkyne-to-Disubstituted Vinylidene Isomerization in [CpRu(PhC≡CAr)(dppe)]+

Internal alkyne-to-vinylidene isomerization in the Ru complexes ([CpRu(η(2)-PhC≡CC(6)H(4)R-p)(dppe)](+) (Cp = η(5)-C(5)H(5); dppe = Ph(2)PCH(2)CH(2)PPh(2); R = OMe, Cl, CO(2)Et)) has been investigated using a combination of quantum mechanics and molecular mechanics methods (QM/MM), such as ONIOM(B3PW91:UFF), and density functional theory (DFT) calculations. Three kinds of model systems (I-III), each having a different QM region for the ONIOM method, revealed that considering both the quantum effect of the substituent of the aryl group in the η(2)-alkyne ligand and that of the phenyl groups in the dppe ligand is essential for a correct understanding of this reaction. Several plausible mechanisms have been analyzed by using DFT calculations with the B3PW91 functional. It was found that the isomerization of three complexes (R = OMe, CO(2)Et, and Cl) proceeds via a direct 1,2-shift in all cases. The most favorable process in energy was path 3, which involves the orientation change of the alkyne ligand in the transition state. The activation energies were calculated to be 13.7, 15.0, and 16.4 kcal/mol, respectively, for the three complexes. Donor-acceptor analysis demonstrated that the aryl 1,2-shift is a nucleophilic reaction. Furthermore, our calculation results indicated that an electron-donating substituent on the aryl group stabilizes the positive charge on the accepting carbon rather than that on the migrating aryl group itself at the transition state. Therefore, unlike the general nucleophilic reaction, the less-electron-donating aryl group has an advantage in the migration.

A computational study of the role of hydrogen bonds in SN1 and E1 reactions

The reaction between tertiary butyl chloride and water clusters was examined by applying density functional theory calculations. The carbonium ion t‐Bu+ that is normally sandwiched between the water clusters was found to be absent, such that a CO covalent bond was formed in the intermediate (Int1) after heterolysis. An (H2O)4 cluster is able to bridge the front and rear of the central carbon and promotes heterolysis. A correlation between bond interchanges at the central carbon and proton relays is presented. Stereochemical scrambling in the solvolysis products is discussed in terms of this correlation. In addition, an E1 pathway for the elimination product, iso‐butene, is found from Int1.

A computational study of interactions between acetic acid and water molecules

Density functional theory calculations were performed for the title reactions to elucidate the difference between the strong cyclic hydrogen bond of (MeCOOH)2 and the electrolytic dissociation, MeCOOH⇌MeCOO− + H+, as a weak acid. The association of water clusters with acetic acid dimers strengthens the cyclic hydrogen bond. A nucleophilic attack of the carboxylic carbon by a water cluster leads to a first zwitterionic intermediate, MeCOO− + H3O+ + (HO)3CMe. The intermediate is unstable and is isomerized to a neutral interacting system, MeCOOH…(HO)3CMe + H2O. The ethanetriol, (HO)3CMe is transformed to an acetic acid monomer. The monomer may be dissociated to give a second zwitterionic intermediate with reasonable proton‐relay patterns and energy changes. In proton relay reaction channels, H in MeCOOH is not an acidic proton but is always a hydroxy proton.

Active Role of Hydrogen Bonds in Rupe and Meyer-Schuster Rearrangements.

Rupe and Meyer-Schuster rearrangements for the R2C(OH) [Formula: see text] C⋮C [Formula: see text] H + H3O(+) and (H2O)9 model (R = methyl and phenyl groups) have been investigated by the use of density functional theory calculations. In the substrate R2C(OH) [Formula: see text] C⋮CH catalyzed by H3O(+)(H2O), three reaction channels, the two rearrangements and SN (nucleophilic substitution), were predicted by the frontier molecular orbital theory. The SN (the OH-group exchange) path was found to have a large activation energy. For 2-methylbut-3-yn-2-ol (R = Me), the Rupe rearrangement has been found to be much more favorable than the Meyer-Schuster rearrangement. For 1,1-diphenylprop-2-yn-1-ol (R = Ph), the occurrence of Meyer-Schuster rearrangement is very likely with the small activation energy. Both rearrangements do not involve the carbonium ion intermediates. However, the calculated geometries of the first transition state are carbonium-ion-like. Dehydration and hydration may occur via the intermolecular proton relay along the hydrogen-bond chains. Minimal models were proposed to represent reaction mechanisms of both rearrangements.

Theoretical study of the role of solvent H2O in neopentyl and pinacol rearrangements

The neopentyl and the pinacol rearrangements as examples of Wagner–Meerwein rearrangements were investigated by the use of DFT calculations. As the first reaction, a model of neopentyl chloride (1b) and (H2O)12 was employed. In the reaction, the patterns of CCl scission, methyl migration, and COH formation were analyzed. The calculations have shown that the 2‐methyl‐2‐butanol (6) is formed in two steps with the transient intermediate, neopentyl alcohol (3). The first step is the nucleophilic substitution reaction and is the rate‐determining one. The second step is the dual migration of methyl and OH2 groups. The primary and tertiary carbocations were calculated to be absent in the neopentyl rearrangement starting from the hydrolysis. As the second reaction, the pinacol rearrangement of two substrates 2,3‐dimethyl‐2,3‐butanediol (7) and 2,3‐diphenyl‐2,3‐butanediol (12) was investigated. Acidic aqueous solvent was modeled by H3O+ and 12H2O. The reaction paths were promoted by a hydrogen‐bond circuit of H3O+(H2O)2 and were determined as completely concerted processes. Protonated species and carbocations as intermediates also do not intervene during the pinacol rearrangement. Active functions of proton relays along the hydrogen bonds in the two rearrangements were demonstrated.

A ruthenium tellurocarbonyl (CTe) complex with a cyclopentadienyl ligand: systematic studies of a series of chalcogenocarbonyl complexes [CpRuCl(CE)(H2IMes)] (E = O, S, Se, Te)

The first tellurocarbonyl complex with a half-sandwich structure [CpRuCl(CTe)(H2IMes)] was synthesized by a ligand substitution reaction. The practically complete series of the CpCE complexes [CpRuCl(CE)(H2IMes)] (E = O, S, Se, Te) were systematically explored. The tellurium atom in the CTe complex could be smoothly replaced with lighter chalcogen atoms.

Revisiting Hydrogen [1,5] Shifts in Cyclopentadiene and Cycloheptatriene as Bimolecular Reactions.

Hydrogen [1,5] shifts are pericyclic reactions and take place typically in 1,3-pentadiene. However, because of structure restriction, the symmetry-allowed thermal reactions of 1,3-cyclopentadiene (CPD) and 1,3,5-cycloheptatriene (CHT) suffer large energy barriers, ΔU(‡) = +26.9 kcal/mol and ΔU(‡) = +37.5 kcal/mol by density-functional theory (B3LYP/6-31G*) calculations, respectively. This theoretical study has shown that exo [4+2] and [6+4] cycloadduct dimers involve novel hydrogen-shift channels. After hydrogen migration, one-center adducts are obtained, which undergo Cope rearrangements leading to the second one-center adducts. From the intermediates, reverse routes lead to CPD and CHT with [1,5] migrated hydrogens. A correlation between cycloadditions and [1,5] and [3,3] sigmatropic rearrangements in pericyclic reactions is also discussed.

Correlation between the rate order and the number of molecules in the reaction of trimethyl phosphite with water in acetonitrile solvent.

Density functional theory calculations of the title reaction, P(OCH₃)₃ + (H₂O)(n) in CH₃CN, were conducted, where n is the number of water molecules. Two routes, the routes suggested by (A) Aksnes and (B) Arbuzov, were traced with various n values. Both routes consist of two transition states (TSs) and one intermediate. Route B was found to be more likely than route A. In the former, the activation free energy (ΔG(‡)) of n = 3 is slightly smaller than that of n = 2. The n = 3 TS geometry is composed of a nucleophile H₂O, a proton donor H₂O, and an auxiliary one. Indeed, the geometry appears to be plausible for ready proton relays along hydrogen bonds, but it is inconsistent with the observed third-order rate constant. Catalytic water molecules were added to the n = 2 and 3 bond-interchange circuits. Then route B with n = 2 + 2 was found to be best. By n = 2 + 10 and n = 3 + 12 models, the n = 2 based route B was confirmed to be likely.

R/X exchange reactions in cis-[M(R)2{P(X)(NMeCH2)2}2] (M = Pd, Pt), via a phosphenium intermediate

R/X exchange reactions in cis-[M(R)2{P(X)(NMeCH2)2}2] (M = Pd, Pt; R = aryl, alkyl; X = Cl, Br) were achieved for the first time to give cis-[M(X)2{P(R)(NMeCH2)2}2]. DFT calculations suggested that the exchange reaction proceeds via a phosphenium intermediate.