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ABSTRACT: Two Prins reactions were investigated by the use of DFT calculations. A model composed of R-CH=CH2 + H3O(+)(H2O)13 + (H2C=O)2, R = Me and Ph, was adopted to trace reaction paths. For both alkenes, the concerted path forming 1,3-diols was obtained as the rate determining step (TS1). TS stands for a transition state. From the 1,3-diol, a bimolecular elimination (TS2) leads to the allylic alcohol as the first channel. In the second channel, the 1,3-diol was converted via TS3 into an unprecedented hemiacetal intermediate, HO-CH2-O-CH(R)-CH2-CH2-OH. This intermediate undergoes ring closure (TS4), affording the 1,3-dioxane product. The intermediate is of almost the same stability as the product, and two species were suggested to be in a state of equilibrium. While the geometry of TS1 appears to be forwarded to that of a carbocation intermediate, the cation disappeared through the enlargement of the water cluster. Dynamical calculations of a classical trajectory using the atom-centered density matrix propagation molecular dynamics model on the four TSs were carried out, and results of IRC calculations were confirmed by them.
Beilstein Journal of Organic Chemistry 01/2013; 9:476-85. · 2.52 Impact Factor
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ABSTRACT: A representative azulene formation from an active troponoid precursor (2-methoxytropone) and an active methylene compound (malononitrile) has been analyzed both experimentally and theoretically. (2)H-Tracer experiments using 2-methoxy[3,5,7-(2)H(3)]tropone (2-d(3)) and malononitrile anion give 2-amino-1,3-dicyano[4,6,8-(2)H(3)]azulene (1-d(3)) in quantitative yield. New and stable (2)H-incorporated reaction intermediates have been isolated, and main intermediates have been detected by careful low-temperature NMR measurements. The detection has been guided by mechanistic considerations and B3LYP/6-31(+)G(d) calculations. The facile and quantitative one-pot formation of azulene 1 has been found to consist of a number of consecutive elementary processes: (a) The troponoid substrate, 2-methoxytropone (2), is subject to a nucleophilic substitution by the attack of malononitrile anion (HC(CN)(2)(-)) to form a Meisenheimer-type complex 3, which is rapidly converted to 2-troponylmalononitrile anion (5). (b) The anion 5 is converted to an isolable intermediate, 2-imino-2H-cyclohepta[b]furan-3-carbonitrile (6), by the first ring closure in the reaction. (c) A nucleophilic addition of the second HC(CN)(2)(-) toward the imine 6 at the C-8a position produces the second Meisenheimer-type adduct 7. (d) The second ring closure leads to 1-carbamoyl-1,3-dicyano-2-imino-2,3-dihydroazulene (11). A base attacks the imine 11, which results in generation of a conjugate base 12 of the final product, azulene 1.
The Journal of Organic Chemistry 05/2012; 77(12):5318-30. · 4.45 Impact Factor
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ABSTRACT: Tautomerization of purine in the water cluster was investigated by the use of DFT calculations. The correlation between the reaction paths and the number of water molecules (n) was examined. For n = 3 and n = 4, concerted reaction paths were obtained. However, for n = 5, a stepwise path including an ion pair intermediate was found with small activation energies. The n = 4 + 3 and n = 4 + 3 + 9 models were calculated to give further small activation energies, where n = 4 constitutes the reaction center and +3 and +3 + 9 denote the number of catalytic water molecules. The combination of the in-plane deprotonation at the N9 site and the out-of-plane protonation at the N7 site makes the n = 4 model probable. Three protonated n = 4 + 3 + 9 routes, a, b, and c, composed of purineH(+)(H(2)O)(4+3+9) were investigated. The n = 4 + 3 moiety is also included in the three routes, and the route c (with the N1 protonation) was found to be most favorable. The purine tautomerization was found to involve the Zundel cation in the ion pair intermediate.
The Journal of Physical Chemistry A 12/2011; 116(4):1289-97. · 2.95 Impact Factor
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ABSTRACT: Through variable-temperature solution-state NMR and molten- and solid-state CP/MAS (13)C NMR spectra, thiotropolone is found to exist as two rapidly equilibrated tautomeric structures, thione and enethiol, even in the solid state far below the melting point. The crystal structure shows an almost perpendicular packing, suggesting that the intramolecular hydrogen bond is dominant.
The Journal of Organic Chemistry 07/2011; 76(13):5457-60. · 4.45 Impact Factor
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ABSTRACT: The Reimer-Tiemann (R-T) reaction was investigated by DFT calculations. A model composed of CHCl(3), PhO(-)(Na(+))H(2)O and [NaOH(H(2)O)(2)](2) was employed for geometry optimizations. A K(+)-containing model was also investigated. The dichlorocarbene reagent, which has been thought of for a long time, was found to intervene only transiently in the carbenoid form. In this form, the Na(+) (or K(+)) coordination to CCl(2) enhances its electrophilicity toward C(6)H(5)O(-). The counter ion also works to stabilize the precursor phenoxide ion and intermediates of the substituted phenoxides in the hexagonal pyramidal coordination. The Na(+)-containing reaction consists of seven elementary processes, (K(+), six ones) with extremely high exothermicity and spontaneity.
Organic & Biomolecular Chemistry 05/2011; 9(14):5109-14. · 3.70 Impact Factor
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ABSTRACT: 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.
The Journal of Physical Chemistry A 10/2010; 114(43):11699-707. · 2.95 Impact Factor
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12/2009; , ISBN: 9780470682531
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ABSTRACT: By the use of DFT calculations, the title rearrangement, Ph-NH-NH-Ph () --> H(2)N-C(6)H(4)-C(6)H(4)-NH(2) (2), was studied for the first time. Although it is a classical reaction (found in 1862), its mechanism is almost entirely unknown. There are three complexities associated with this mechanism. The first is the various rate orders for substituted hydrazobenzenes. The second is the product distribution. The third is the result of the kinetic isotope effect which is difficult to interpret. A reaction model, , (H(3)O(+))(2) and (H(2)O)(10) was used to trace the reaction path. Two hydronium ions were included, because there are two nitrogen atoms in . In the paths of the main reaction, (H(+))(2)-->H(+) + H(+), transient intermediates were found. Through their conversion, the second product, diphenyline (), was reached. For H(+), only the Claisen shift path was found, and the pi complex proposed by Dewar was not found. The absence is in accord with the kinetic result of Hammond and Shine. But the complex was revealed in the dimethoxyhydrazobenzene. Thus, while Dewar's pi complex was ruled out in 1950, it has been revived by the present calculations.
Organic & Biomolecular Chemistry 11/2009; 7(22):4631-40. · 3.70 Impact Factor
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Journal of Physical Organic Chemistry 06/2009; 22(11):1094 - 1103. · 1.96 Impact Factor
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ABSTRACT: The Cannizzaro reaction ("RX C") and the benzoin reaction ("RX B") were investigated by density functional theory calculations. Reaction models (benzaldehyde)2 + X(-) + (H2O)n [for X(-) = OH(-) (RX C) n = 8, and for X(-) = CN(-) (RX B) n = 8 or n = 14] were adopted. Three transition states (TSs) were obtained for RX C, and the rate-determining step was confirmed to be the hydride shift. The single electron transfer path was also obtained, which is supported by the C-O(-)CH[double bond, length as m-dash]O attraction. In RX B, seven TSs were obtained. The CC bond formation TS, TS4(B), was found to be the rate-determining step. However, the carbanion-formation TS (TS3(B)) and the CN(-) release TS (TS7(B)) are also of large activation free energies (DeltaG(double dagger)s). The result DeltaG(double dagger)(TS4(B)) > or = DeltaG(double dagger)(TS7(B)) approximately DeltaG(double dagger)(TS3(B)) was obtained with both n = 8 and n = 14 models. Proton relays along the linear hydrogen bonds are concerned with bond interchanges and promote well arranged and successive elementary processes in RXs C and B.
Organic & Biomolecular Chemistry 04/2009; 7(5):951-61. · 3.70 Impact Factor
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ABSTRACT: The title reaction was investigated by the use of ONIOM-RB3LYP calculations. A reaction system composed of alpha-chlorocyclohexanone, a methoxide ion and 8 MeOH solvent molecules was adopted. Two reaction channels, the semibenzilic acid mechanism (A) and cyclopropanone mechanism (B), were compared. B is found to be more favorable than A. The rate-determining step of B is the (MeOH)(3) addition transition state (TS3B) to the cyclopropanone intermediate. While TS3B involves a concerted function of MeO(-) addition and proton relays, it has a large activation energy. A new route was found, where the chloride ion evolved at the cyclopropane formation step (TS2B) works as a nucleophile to the cyclopropanone intermediate. Thus, a cyclopentane-carbonyl chloride intermediate is formed with a small activation energy. A new cyclopropanone mechanism is proposed.
Organic & Biomolecular Chemistry 10/2008; 6(17):3109-17. · 3.70 Impact Factor
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Annalen der Chemie und Pharmacie 11/2007; 2007(36):6070 - 6077. · 3.10 Impact Factor
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ABSTRACT: 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.
Journal of Computational Chemistry 08/2007; 28(9):1561-71. · 4.58 Impact Factor
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ABSTRACT: Density functional theory calculations were conducted on the title reactions with explicit inclusion of a variety of water molecules, H-CO-NMe2+MeOH+(H2O)n-->H-CO-OMe+HNMe2+(H2O)n. Geometries of transition states, reactant-like complexes and product-like ones were determined by the use of RB3LYP/6-31G(d) SCRF=dipole. Concerted paths were examined with n=0-3. Their Gibbs activation energies are larger than the experimental value. Stepwise paths were also investigated with n=2-4. The n=4 model has the energy close to the experimental value. However, when the catalytic water molecules were added to the n=4 one, the stepwise path was switched to the concerted one. A systematic comparison of the concerted path with n=2+1, 2+2, 2+3, 2+4, 2+5, 2+4+4, and 2+5+5 models was made, and the water-dimer based reaction path was found to be most favorable. The contrast between the concerted path of the amide solvolysis (and hydrolysis) and the stepwise one of the ester hydrolysis was discussed in terms of the frontier-orbital theory.
The Journal of Physical Chemistry A 08/2007; 111(28):6296-303. · 2.95 Impact Factor
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ABSTRACT: A benzyne-tropothione reaction was studied experimentally and computationally. Three isomeric products were detected by a careful experiment using two benzyne sources. The three equimolar products were identified. The expected symmetry-allowed [4+2] or [8+2] cycloadduct was not detected. In order to explain the unexpected products, density functional calculations and complete active space self-consistent field (CASSCF) calculations were carried out. The benzyne is, first, added to the tropothione via one-center C-S bond formation. Then a singlet biradical intermediate is formed. In the biradical, an alpha hydrogen atom of the tropothione moiety is moved to the benzyne moiety. A closed-shell intermediate is generated. This allene-type intermediate is isomerized to the second intermediate. The intramolecular proton shift in the latter leads to the three products. The biradical character of the benzyne has a key role in the present reaction and was discussed in reference to other benzyne reactions.
The Journal of Organic Chemistry 05/2007; 72(8):2832-41. · 4.45 Impact Factor
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ABSTRACT: Various Baeyer-Villiger (B-V) oxidation reactions were examined by density functional theory calculations. Proton movements in transition states (TSs) of the two key steps, the nucleophilic addition of a peroxyacid molecule to a ketone (TS1) and the migration-cleavage of O-O (TS3), were discussed. A new TS of a hydrogen-bond rearrangement in the Criegee intermediate (TS2) was found. The hydrogen-bond directionality requires a trimer of the peroxyacid molecules at the nucleophilic addition of a peroxyacid molecule to a ketone TS (TS1). At the migration-cleavage of O-O TS (TS3), also three peroxyacid molecules are needed. Elementary processes of the B-V reaction were determined by the use of the (acetone and (H-CO-OOH)n, n=3) system. The geometries of the nucleophilic addition of a peroxyacid molecule to a ketone TS (TS1) and the migration-cleavage of O-O TS (TS3) in the trimer (n=3) participating are nearly insensitive to the substituent on the peroxyacid. The directionality is satisfied in those geometries. The migration-cleavage of O-O TS (TS3) was found to be rate-determining in reactions, [Me2C=O+(H-CO-OOH)3], [Me2C=O+(F3C-CO-OOH)3], and [Me2C=O+(MCPBA)3]. In contrast, the nucleophilic addition of a peroxyacid molecule to a ketone (TS1) is rate-determining in the reaction, [Ph(Me)C=O+(H-CO-OOH)3].
The Journal of Organic Chemistry 05/2007; 72(8):3031-41. · 4.45 Impact Factor
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Journal of Computational Chemistry. 01/2007; 28:1561-1571.
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ABSTRACT: Reaction paths for the title rearrangement along with its methyl analogue were investigated by density functional theory calculations. The reaction model is R-CO-CO-R + OH(-)(H2O)4 --> R2C(OH)-COO- + (H2O)4 (R = Me and Ph), where the water tetramer is employed both for solvation to OH- and for the proton relay along hydrogen bonds. The reaction is composed of OH- addition, C-C rotation, carbanion [1,2] migration, and proton relay toward the product anions. The rate-determining step was calculated to be the carbanion migration. Apparently, carbanion [1,2] migration is unlikely relative to the carbonium ion one. However, LUMOs of the 1,2-diketones have large and nodeless lobes at the reaction center, the C1-C2 bond. The specific LUMO character is reflected both in the [2+1]-like one-center nucleophilic addition and in the carbanion [1,2] shift. The proton relay involved in the isomerization from the oxo intermediate to the carboxylate was calculated to take place via the water tetramer.
The Journal of Organic Chemistry 04/2006; 71(5):1777-83. · 4.45 Impact Factor
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ABSTRACT: The first example of direct spectroscopic detection of transient species, 1,4-zwitterions, generated in a ketene-alkene reaction is reported. Also, a striking result of the intervention of an unprecedented "1,4-zwitterion neutral dimer" is presented in a new mechanistic pathway; the ketene-alkene reaction gives the product cyclobutanone from the initial cycloadduct alpha-methyleneoxetane.
Journal of the American Chemical Society 02/2006; 128(1):44-5. · 9.91 Impact Factor
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ABSTRACT: [reaction: see text] RB3LYP calculations were performed on the Beckman rearrangement by the use of three substrates, acetone oxime (1), acetophenone oxime (2), and cyclohexanone oxime (3). Acidic solvents were modeled by H+ (CH3COOH)3 and H3O+ (H2O)6, and reaction paths were determined precisely. For 1, a two-step process involving a sigma-type cationic complex was obtained. For 2, a three-step process with pi- and sigma-type complexes was found in H+ (CH3COOH)3 and a two-step process involving a sigma-type cationic complex was obtained in H3O+ (H2O)6. However, for 3, a concerted process without pi and sigma complexes was calculated, which leads to the product, epsilon-caprolactam. Three different mechanisms were explained in terms of FMO theory.
The Journal of Organic Chemistry 01/2006; 70(26):10638-44. · 4.45 Impact Factor
