Origin of regioselectivity in the reactions of nitronate and enolate ambident anions

Mechanism and Regioselectivity in Methylation of Nitronates [CH2NO2]−: Resonance vs Inductive Effects

Density functional theory calculations were performed to investigate the mechanism, regioselectivity, and the resonance and inductive effects in methylation of nitronate reactions in the gas-phase and in solutions (water, DMF,...

Glycine-derived nitronates bifurcate to O-methylation or denitrification in bacteria

Natural products with rare functional groups are likely to be constructed by unique biosynthetic enzymes. One such rare functional group is the O-methyl nitronate, which can undergo [3 + 2] cycloaddition reactions with olefins in mild conditions. O-methyl nitronates are found in some natural products; however, how such O-methyl nitronates are assembled biosynthetically is unknown. Here we show that the assembly of the O-methyl nitronate in the natural product enteromycin carboxamide occurs via activation of glycine on a peptidyl carrier protein, followed by reaction with a diiron oxygenase to give a nitronate intermediate and then with a methyltransferase to give an O-methyl nitronate. Guided by the discovery of this pathway, we then identify related cryptic biosynthetic gene cassettes in other bacteria and show that these alternative gene cassettes can, instead, facilitate oxidative denitrification of glycine-derived nitronates. Altogether, our work reveals bifurcating pathways from a central glycine-derived nitronate intermediate in bacteria.

The Reactivity of Ambident Nucleophiles: Marcus Theory or Hard and Soft Acids and Bases Principle?

The model reactions CH₃ X + (NH-CH=O)M ➔ CH₃ -NH-NH═O or NH═CH-O-CH₃  + MX (M = none, Li, Na, K, Ag, Cu; X = F, Cl, Br) are investigated to demonstrate the feasibility of Marcus theory and the hard and soft acids and bases (HSAB) principle in predicting the reactivity of ambident nucleophiles. The delocalization indices (DI) are defined in the framework of the quantum theory of atoms in molecules (QT-AIM), and are used as the scale of softness in the HSAB principle. To react with the ambident nucleophile NH═CH-O^- , the carbocation H₃ C⁺ from CH₃ X (F, Cl, Br) is actually a borderline acid according to the DI values of the forming C…N and C…O bonds in the transition states (between 0.25 and 0.49), while the counter ions are divided into three groups according to the DI values of weak interactions involving M (M…X, M…N, and M…O): group I (M = none, and Me₄ N) basically show zero DI values; group II species (M = Li, Na, and K) have noticeable DI values but the magnitudes are usually less than 0.15; and group III species (M = Ag and Cu(I)) have significant DI values (0.30-0.61). On a relative basis, H₃ C⁺ is a soft acid with respect to group I and group II counter ions, and a hard acid with respect to group III counter ions. Therefore, N-regioselectivity is found in the presence of group I and group II counter ions (M = Me₄ N, Li, Na, K), while O-regioselectivity is observed in the presence of the group III counter ions (M = Ag, and Cu(I)). The hardness of atoms, groups, and molecules is also calculated with new functions that depend on ionization potential (I) and electron affinity (A) and use the atomic charges obtained from localization indices (LI), so that the regioselectivity is explained by the atomic hardness of reactive nitrogen atoms in the transition states according to the maximum hardness principle (MHP). The exact Marcus equation is derived from the simple harmonic potential energy parabola, so that the concepts of activation free energy, intrinsic activation barrier, and reaction energy are completely connected. The required intrinsic activation barriers can be either estimated from ab initio calculations on reactant, transition state, and product of the model reactions, or calculated from identity reactions. The counter ions stabilize the reactant through bridging N- and O-site of reactant of identity reactions, so that the intrinsic barriers for the salts are higher than those for free ambident anions, which is explained by the increased reorganization parameter Δr. The proper application of Marcus theory should quantitatively consider all three terms of Marcus equation, and reliably represent the results with potential energy parabolas for reactants and all products. For the model reactions, both Marcus theory and HSAB principle/MHP principle predict the N-regioselectivity when M = none, Me₄ N, Li, Na, K, and the O-regioselectivity when M = Ag and Cu(I). © 2019 Wiley Periodicals, Inc.

Free energy profile and microkinetic modeling of base-catalyzed conjugate addition reaction of nitroalkanes to α,β-unsaturated ketones in polar and apolar solvents

Michael reactions involving nitroalkanes and enones are important carbon-carbon bond formation reactions. These reactions are base-catalyzed, and during the past 15 years, the asymmetric version using bifunctional amino-thiourea organocatalyst has been developed. In this work, the reaction of nitromethane and 4-phenyl-3-buten-2-one, catalyzed by the methoxide ion and piperidine as bases, was investigated by theoretical calculations. We obtained the theoretical free energy profile and did a microkinetic analysis of the catalytic cycle. The direct reaction of the CH₂NO₂^- ion and the enone is very favorable, with a free energy of activation of 21.1 kcal mol^-1 in methanol solvent. However, the generated MS2 product works like an inhibitor of the catalysis, and the effective barrier in the catalytic cycle becomes 25.5 kcal mol^-1, leading to slow kinetics at room temperature. In the case of the reaction in apolar solvent (toluene), we found a pathway involving isomerization from the CH₃NO₂ reactant to the CH₂NO₂H species, and the latter makes a nucleophilic attack on the enone. Piperidine works like a bifunctional catalyst. In this case, the barrier is very high (32.5 kcal mol^-1), indicating the importance of the polar environment to accelerate the reaction in the catalytic cycle. Graphical abstract Base-catalyzed conjugate addition reaction of nitroalkanes to α,β-unsaturated ketones.

Contrasting C- and O-Atom Reactivities of Neutral Ketone and Enolate Forms of 3-Sulfonyloxyimino-2-methyl-1-phenyl-1-butanones

The mechanisms of intramolecular cyclization of 3-sulfonyloxyimino-2-methyl-1-phenyl-1-butanones (1) under basic (DABCO and t-BuOK) and acidic (AcOH and TFA) conditions were investigated by means of experimental and computational methods. The ketone, enol, and enolate forms of 1 can afford different intramolecular cyclization products (2, 3, 4), depending on the conditions. The results of the reaction of 1 under basic conditions suggest intermediacy of neutral enol (DABCO) and anionic enolate (t-BuOK), while the results under acidic conditions (AcOH and TFA) indicate involvement of neutral ketones, which exhibit reactivities arising from both the oxygen lone-pair electrons (O atom reactivity) and carbon σ-electrons (C atom reactivity). The neutral enol in DABCO afforded 2H-azirine 4. On the other hand, the products (isoxazole 2 and oxazole 3) generated from the ketone form and from the enolate form are the same, but the reaction mechanisms are apparently different. The results demonstrate ambident-like reactivity of neutral ketone in the 3-sulfonyloxyimino-2-methyl-1-phenyl-1-butanone system.

Why Do Enolate Anions Favor O-Alkylation over C-Alkylation in the Gas Phase? The Roles of Resonance and Inductive Effects in the Gas-Phase SN2 Reaction between the Acetaldehyde Enolate Anion and Methyl Fluoride

Contributions by resonance and inductive effects toward the net activation barrier were determined computationally for the gas-phase SN2 reaction between the acetaldehyde enolate anion and methyl fluoride, for both O-methylation and C-methylation, in order to understand why this reaction favors O-methylation. With the use of the vinylogue extrapolation methodology, resonance effects were determined to contribute toward increasing the size of the barrier by about 9.5 kcal/mol for O-methylation and by about 21.2 kcal/mol for C-methylation. Inductive effects were determined to contribute toward increasing the size of the barrier by about 1.7 kcal/mol for O-methylation and 4.2 kcal/mol for C-methylation. Employing our block-localized wave function methodology, we determined the contributions by resonance to be 12.8 kcal/mol for O-methylation and 22.3 kcal/mol for C-methylation. Thus, whereas inductive effects have significant contributions, resonance is the dominant factor that leads to O-methylation being favored. More specifically, resonance serves to increase the size the barrier for C-methylation significantly more than it does for O-methylation.

Understanding the Reactivity and Regioselectivity of Methylation of Nitronates [R(1)R(2)CNO2](-) by CH3I in the Gas Phase

Methylation of [R(1)R(2)CNO2](-), where R(1) = R(2) = H (1), R(1) = CH3 and R(2) = H (2), R(1) = R(2) = CH3 (3), and R(1) + R(2) = c-(CH2)2 (4), by CH3I was studied by an ab initio MP2/CBS method, RRKM theory, and kinetic simulations. Contrary to a previous proposal for the reaction mechanism, C-methylation is the preferred pathway of thermodynamics and kinetics. This is corroborated by the agreement between the calculated and experimental reactivity trend 4 ≫ 3 > 2 > 1. The regioselectivity toward C-alkylation is explained by the much larger exothermicity of this reaction channel compared to that of O-alkylation. The increase in reactivity with an increase in the crowdedness of the central carbon atom is explained by differences in sp(3) character at this atom and the decrease in the vibrational frequency associated with pyramidalization around this carbon atom.