Superelectrophilic Activation of 4-Heterocyclohexanones. Implications for Polymer Synthesis. A Theoretical Study

Abstract

The stability and the reactivity of mono- and diprotonated 4-heterocyclohexanones as well as cyclohexanone in triflic acid have been studied at the PBE0/aug-cc-pvtz//PBE0/6-31+G** level of theory. In all cases the first protonation is an exergonic process occurring at a carbonyl oxygen except for 4-piperidone where a nitrogen atom is protonated fist. Second protonation is only slightly endergonic for all studied molecules except for cyclohexanone where the second protonation is very unfavorable thermodynamically. According to calculations, diprotonated 4-heterocyclohexanones are much more active in the reactions of triflic acid mediated polyalkoxyalkylation with aromatic hydrocarbons compared to monoprotonated ones. The increase of the reactivity of diprotonated 4-heterocyclohexanones is due to inductive effect rather than through space electrostatic influence as follows from the electronic structure analysis of dications. Moreover, the second protonation reduces the possibility of an aldol condensation side reaction, reducing the enol electrophilicity rendering heterocyclohexanones as promising monomers for superacid mediated polyhydroxyalkylation.

Cyclization Reactions of 1‐Amino‐5‐trifluoromethyl‐5‐thienyl‐1‐azapenta‐1,4‐dien‐3‐ones under Superelectrophilic Conditions: Synthesis of Novel Benzothiophenols, Cyclopentenols and Dihydrodiazepinols

“…Trifluoromethyl ketones are known to enhance polymerization reactions under superelectrophilic conditions. [10] For the three types of transformations presented above we suggest the following reasonable mechanisms.…”

Acid‐Promoted Chemoselective Introduction of Amide Functionality onto Aromatic Compounds Mediated by an Isocyanate Cation Generated from Carbamate

“…Intrinsic reaction coordinate (IRC) computations of the transition structures verified the reactants, intermediates, and products on the potential energy surface (PES). Bulk solvation effects (self‐consistent reaction field, SCRF) were simulated by using the CPCM method in TfOH as a solvent (eps=77.4,25 rsolv=2.5985274,25 density=1.696,26 epsinf=1.882384 (the value for acetic acid was used)). Single‐point energies were calculated by using CPCM‐M06/6‐311++G(d,p) on the basis of the optimized structures.…”

Superacid‐Catalyzed Reactions

“…The protonated intermediate 64 is highly unstable when the ring has an electronwithdrawing substituent. 248 Furthermore, dicationic intermediates 67, 68, and 69 generated in FSO 3 H-SbF 5 -SO 2 ClF solution have been directly observed by low-temperature NMR spectroscopy at À80 C. 247,248,253 Recent computational studies have shown 255 that the first protonation of 4-heterocyclohexanones at the carbonyl oxygen in triflic acid is exergonic (4-piperidone is an exception protonated at the N atom). Aminoacetals proved to be more reactive in triflic acid reacting with benzene at room temperature to give diphenyl-substituted products of alkylation 246 [Eq.…”

“…THF has also been homopolymerized [936][937][938] and copolymerized with ethylene oxide and propylene oxide in the presence of Nafion-H. [939][940][941] Triflic acid has become a widely applied catalyst in various polymerization processes, and a few selected characteristic examples are discussed here. It has been concluded in recent studies (PBE0/aug-cc-PVTZ//PBE0/6-31þG* level of theory) 255,952 that C,O and O,O diprotonated species derived from molecules with electron-withdrawing or moderately electron-donating substituents (for example, hexafluoroacetone) do not participate in polycondensation. [942][943][944][945] A polyaldolic condensation of acetone can be induced by triflic acid to give a solid resin with a polyenic structure resembling poly(methylacetylene) having some functional groups 946 (289).…”