The Role of Hydrogen Bonds in Baeyer−Villiger Reactions

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]

A Theoretical Study of Curing Reactions of Maleimide Resins through Michael Additions of Amines

A Theoretical Study of Curing Reactions of Maleimide Resins through Michael Additions of Amine

Hydrogen-Bond Networks for Hydrolyses of Anhydrides

Hydrogen-Bond Networks for Hydrolyses of Anhydride

A Three-Center Orbital Interaction in the Diels−Alder Reactions Catalyzed by Lewis Acids

Ab initio calculations were performed on title reactions between butadiene and acrolein with BCl3, AlCl3, GaCl3, InCl3, ZnCl2, SnCl2, and SnCl4. A dimethyl ether molecule is explicitly considered in various reaction systems to examine solvent effects. First, the reaction path of an AlCl3-promoting reaction was examined thoroughly. This reaction has two channels. The first one involves a weak reactant-like complex (precursor) and a normal [4 + 2] addition. The second does three elementary processes, one-center addition, ring closing, and Claisen shift. The first channel is more favorable by 12.1 kcal/mol (B3LYP/6-311+G(2d,p) SCRF//B3LYP/6-31G* SCRF) than the second one. Then the first channels with other Lewis acids were traced with and without an ether molecule. The ether molecule has an appreciable effect not on geometries but on activation energies. BCl3 is desolvated and has extraordinarily strong catalytic ability. Even with the strongest catalyst, not a [2 + 4] but a normal [4 + 2] cycloaddition takes place. Except for BCl3, SnCl4 is the strongest Lewis acid with the ether molecule. The frontier orbital, LUMO, of acrolein is distorted in the course of the reaction so that the formation of two C−C covalent bonds is possible. The precursor formation and the one-center addition were discussed also by the frontier orbital theory

A Computational Study on Addition of Grignard Reagents to Carbonyl Compounds

The mechanism of stereoselective addition of Grignard reagents to carbonyl compounds has been investigated using B3LYP density functional theory calculations. The study of the reaction of methylmagnesium chloride and formaldehyde in dimethyl ether revealed a new reaction path involving carbonyl compound coordination to magnesium atoms in a dimeric Grignard reagent. The structure of the transition state for the addition step shows that an interaction between a vicinal-magnesium bonding alkyl group and CO causes the C−C bond formation. The simplified mechanism shown by this model is in accord with the aggregation nature of Grignard reagents and their high reactivities toward carbonyl compounds. Concerted and four-centered formation of strong O−Mg and C−C bonds was suggested as a polar mechanism. When the alkyl group is bulky, C−C bond formation is blocked and the Mg−O bond formation takes precedence. A diradical is formed with the odd spins localized on the alkyl group and carbonyl moiety. Diradical formation and its recombination were suggested to be a single electron transfer (SET) process. The criteria for the concerted polar and stepwise SET processes were discussed in terms of precursor geometries and relative energies

A Three-Center Orbital Interaction in the Diels−Alder Reactions Catalyzed by Lewis Acids

Ab initio calculations were performed on title reactions between butadiene and acrolein with BCl3, AlCl3, GaCl3, InCl3, ZnCl2, SnCl2, and SnCl4. A dimethyl ether molecule is explicitly considered in various reaction systems to examine solvent effects. First, the reaction path of an AlCl3-promoting reaction was examined thoroughly. This reaction has two channels. The first one involves a weak reactant-like complex (precursor) and a normal [4 + 2] addition. The second does three elementary processes, one-center addition, ring closing, and Claisen shift. The first channel is more favorable by 12.1 kcal/mol (B3LYP/6-311+G(2d,p) SCRF//B3LYP/6-31G* SCRF) than the second one. Then the first channels with other Lewis acids were traced with and without an ether molecule. The ether molecule has an appreciable effect not on geometries but on activation energies. BCl3 is desolvated and has extraordinarily strong catalytic ability. Even with the strongest catalyst, not a [2 + 4] but a normal [4 + 2] cycloaddition takes place. Except for BCl3, SnCl4 is the strongest Lewis acid with the ether molecule. The frontier orbital, LUMO, of acrolein is distorted in the course of the reaction so that the formation of two C−C covalent bonds is possible. The precursor formation and the one-center addition were discussed also by the frontier orbital theory

Theoretical Study of Mutarotation of Glucose

Theoretical Study of Mutarotation of Glucos

Reaction Paths of Tautomerization between Hydroxypyridines and Pyridones

Tautomerization paths of 2(and 4)-hydroxypyridine (called here HP) to 2(and 4)-pyridone (called here PY) with water molecules were investigated by the use of density functional theory calculations. Potential energies were compared for a number of water molecules. The 2-HP molecule was found to be isomerized most readily and concertedly to the 2-PY one via proton relays with two water molecules. The reaction pattern is invariant even when outer water molecules are added. The 4-HP(H2O)n → 4-PY(H2O)n reaction model did not give small activation energies. However, a reaction of (4-HP)2(H2O)2 → (4-PY)2(H2O)2 was found to occur readily through a transient ion-pair intermediate. The conversion processes of (2-PY)2 to the tautomerization reacting system were discussed. The hydrogen-bond directionality regulates the tautomerization paths

Theoretical Study on the Mechanism of Formation of Cyanate Resins

Ab initio calculations of a formation reaction of a triazine ring were performed. From the model substrate, methyl cyanate, a concerted association path with C3h symmetry was first examined. In terms of energy changes, this path was found to be unlikely. Second, a stepwise path assisted by a water cluster was tested. But, this path was found to be of a relatively high amount of activation energy in the first additional step. Third, a zinc formate was used as a catalyst, and the reaction was computed to have a reasonable stepwise route for formation of the six-membered triazine ring. Fourth, the reaction promoted by a hydronium ion was shown to generate a ring-closure mechanism similar to that caused by the zinc catalyst. Thus, the crucial role of catalysts coordinated to the σ lone-pair orbital of the cyanate nitrogen atom was verified

Proton Transfers along Hydrogen Bonds in the Tautomerization of Purine

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 (<i>n</i>) was examined. For <i>n</i> = 3 and <i>n</i> = 4, concerted reaction paths were obtained. However, for <i>n</i> = 5, a stepwise path including an ion pair intermediate was found with small activation energies. The <i>n</i> = 4 + 3 and <i>n</i> = 4 + 3 + 9 models were calculated to give further small activation energies, where <i>n</i> = 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 <i>n</i> = 4 model probable. Three protonated <i>n</i> = 4 + 3 + 9 routes, a, b, and c, composed of purineH⁺(H₂O)₄₊₃₊₉ were investigated. The <i>n</i> = 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