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

Abstract

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.

Figure

Ammonium salt formation in a Menshutkin-like reaction between ammonia and (S)-1,4-andydro-2,3-dideoxy-5-O-mesylpentitol (2a)

Fig. 1

Structures of mesylate derivatives converted into ammonium salts

Scheme 1

Reactions of ammonium salt formation

Fig. 2

Rotamers exhibiting possible spatial arrangements of the OCH₃ group in relation to the heterocyclic oxygen atom. The preferred orientation is in the box

Fig. 3

Energy (E ₀) and pseudochemical potential (U ₀) profiles for reactions 1–4 calculated at B3LYP/6-31+G** level in the gas phase, chloroform and water

Fig. 4

Definition of the endocyclic torsion angles ϕ ₀–ϕ ₄

Fig. 5

Geometries of the stationary points and ΔE (kcal mol⁻¹) computed at the B3LYP/6-31+G** level for reactions 1 and 2 in the gas phase. Selected distances in Å, and valence angles in degrees

Fig. 6

Geometries of the stationary points and ΔE (kcal mol⁻¹) computed at the B3LYP/6-31+G** level for reactions 3 and 4 in the gas phase. Selected distances in Å, and valence angles in degrees

Fig. 7

Comparison of activation barriers for reactions 1 and 2 with different nucleophiles in the gas phase

Fig. 8

Geometry of the ion pair in reaction 1

Fig. 9

Geometries of the transition states optimized at the B3LYP/6-31+G** level in water. Selected distances in Å, and valence angles in degrees