Theoretical studies of the formation of quaternary pyridinium mesylates

DFT studies of the formation of furanoid derivatives of ammonium chlorides

B3LYP/6-31+G** level computations were performed on the formation of ammonium salts during the reaction of (S)-1,4-anhydro-5-chloro-2,3,5-trideoxypentitol (1) (2S,5S)-2,5-anhydro-6-chloro-1,3,4,6-tetradeoxyhexitol (2) and methyl 5-chloro-2,3,5-trideoxy-β-D-pentofuranoside (3) with ammonia in order to describe the reaction pathway in detail. All the structures were fully optimized in the gas phase, in chloroform and water. In addition, the gas phase activation barrier heights were estimated at B3LYP/6-311++G**, MPWIK/6-31+G**, MPWIK/6-311++G** and MP2/6-311++G(2d,2p)//MPWIK/6-31+G** levels of theory. All the calculations in solvents were performed the using polarizable continuum model (PCM) and the B3LYP functional with the 6-31+G** basis set. A detailed description of all the stationary points is presented, and the conformational behavior of the five-membered ring is discussed in the gas phase and in the solvents. The conversion of the reactant complexes into ion pairs is accompanied by a strong energy decrease in the gas phase and in all the solvents. The overall process is strongly unfavorable in the gas phase, but takes place readily in high-polarity solvents.

The conformational behavior, geometry and energy parameters of Menshutkin-like reaction of O-isopropylidene-protected glycofuranoid mesylates in view of DFT calculations

The formation of pyridinium salts in the transformation of three O-isopropylidene-protected mesylates of furanoid sugar derivatives under pyridine action is considered at the B3LYP/6-31+G** computation level. All the structures were optimized in the gas phase, in chloroform and water. Activation barrier heights in the gas phase were also estimated at the B3LYP/6-311++G**, MPW1K/6-31+G** and MPW1K/6-311++G** levels. The conducted calculations, both in the gas phase (regardless of the computation level) and in solvents, revealed the barrier height increasing order as follows: 1>2>3 for the three reactions studied. The conformational behavior of the five-membered ring is discussed in the gas phase and in solvents. The fused dioxolane ring makes the furanoid ring less likely to undergo conformational changes. In the case of reaction 3, the furanoid ring shape does not change either in the gas phase or in solvents. All conformers are close to E0 or (0)E.

DFT studies of conversion of methyl chloride and three substituted chloromethyl tetrahydrofuran derivatives during reaction with trimethylamine

B3LYP/6-31+G** level computations were performed for the formation of four trimethylammonium salts in the reaction of methyl chloride (1a), (S)-1,4-andydro-5-chloro-2,3,5-trideoxypentitol (2a), (2S,5S)-2,5-andydro-6-chloro-1,3,4,6-tetradeoxyhexitol (3a) and methyl 5-chloro-2,3,5-trideoxy-β-D-pentofuranoside (4a) with trimethylamine. All the structures were fully optimized in the gas phase, in chloroform and water. In addition, B3LYP/6-311++G** and MPW1K/6-31+G** level calculations were carried out to estimate activation barrier heights in the gas phase. A detailed description of all stationary points is presented, and the conformational behavior of the THF ring is discussed. B3LYP and MPW1K activation barriers indicate the reaction between methyl chloride and trimethylamine to be the fastest, whereas reaction 4 is the slowest one, both in the gas phase and in solvents. THF ring conformation changes were observed for reactions 2 and 3 along the reaction pathway, whereas it was almost unchanged for reaction 4, in the gas phase. In the case of reactions 2 and 3, different shapes of the THF ring were found for the transition state geometry in the gas phase and in water. The ⁵E→E₄ and ³E→E₅ conformational changes were observed for reactions 2 and 3, respectively.

Graphical abstract

DFT studies of the conversion of four mesylate esters during reaction with ammonia

The energetics of the Menshutkin-like reaction between four mesylate derivatives and ammonia have been computed using B3LYP functional with the 6-31+G** basis set. Additionally, MPW1K/6-31+G** level calculations were carried out to estimate activation barrier heights in the gas phase. Solvent effect corrections were computed using PCM/B3LYP/6-31+G** level. The conversion of the reactant complexes into ion pairs is accompanied by a strong energy decrease in the gas phase and in all solvents. The ion pairs are stabilized with two strong hydrogen bonds in the gas phase. The bifurcation at C2 causes a significant activation barrier increase. Also, bifurcation at C5 leads to noticeable barrier height differentiation. Both B3LYP/6-31+G** and MPW1K/6-31+G** activation barriers suggest the reaction 2 (2a + NH3) to be the fastest in the gas phase. The reaction 4 is the slowest one in all environments.