Comprehensive mechanistic study of ion pair S_N2 reactions of lithium isocyanate and methyl halides

Abstract

ABSTRACT:The anionic S N 2 reactions NCO Ϫ ϩ CH 3 X and ion pair S N 2 reactions LiNCO ϩ CH 3 X (X ϭ F, Cl, Br, and I) at saturated carbon with inversion and retention mechanisms were investigated at the level of MP2/6-311ϩG(d,p). There are two possible reaction pathways in the anionic S N 2 reactions, but eight in the ion pair S N 2 reactions. Calculated results suggest that the previously reported T-shaped isomer of lithium isocyanate does not exist. All the retention pathways are not favorable based on the analysis of transition structures. Two possible competitive reaction pathways proceed via two six-member ring inversion transition structures. It is found that there are two steps in the most favorable pathway, in which less stable lithium cyanate should be formed through the isomerization of lithium isocyanate and nucleophilic site (N) subsequently attacks methyl halides from the backside. The thermodynamically and kinetically favorable methyl isocyanate is predicted as major product both in the gas phase anionic and the ion pair S N 2 reactions. In addition, good correlations between the overall barriers relative to separated reactants, ⌬H ovr , with geometrical looseness parameter %L and the heterolytic cleavage energies of the COX and LiON (or LiOO) bonds are observed for the anionic and ion pair S N 2 reactions. The trend of variation of the overall barriers predicts the leaving ability of X increase in the order: F Ͻ Cl Ͻ Br Ͻ I. The polarized continuum model (PCM) has been used to evaluate the solvent effects on the two inversion pathways with six-member transition structures for the reactions of LiNCO ϩ CH 3 X. The calculations in solution indicate that solvent effects will retard the rate of reactions and the predicted product, methyl isocyanate, is same as the one in the gas phase.

Novel ionic liquids and plastic crystals utilizing the cyanate anion

“…In fact, this potential reactivity is one aspect of these new anions that has prompted our interest in the development and study of cyanatebased ionic liquids. For example, the lithium and sodium OCN salts can be reacted with alkyl halides to form alkyl-NCO compounds, 37 while polymer-bound alkyl halides such as chloromethylated polystyrene can be converted to polymeric isocyanate crosslinkers by reaction with potassium cyanate. 38 There are also examples of the cyanate functional group in organic synthesis, such as in the phenol-catalysed synthesis of urea derivatives, obtained through addition of methyl aniline to aryl cyanates, 39 or as an intermediate in the synthesis of pyranose frameworks, where an allyl cyanate undergoes a concerted [3,3] sigmatropic rearrangement to an allyl isocyanate, which can then be transformed into allylureas or anallylacetamides.…”

Theoretical Study on the Identity Ion Pair SN2 Reactions of LiX with CH3SX (X = Cl, Br, and I): Structure, Mechanism, and Potential Energy Surface

“…In the retention pathway, the coordination of the lithium cation is on the same side of sulfur to both entering and leaving halide ions (see Figure , 2 ‘), which is similar to the geometries of retention TSs in the ion pair S N 2(C) reactions 16a. There are more elongations of S−X bond distances (0.337−0.363 Å) and remarkable decreases of X−S−X angles (54.2−76.6°) in the retention LiX/CH 3 SX TSs ( 2 ‘ a − c ) relative to the inversion TSs ( 2a − c ), respectively.…”

“…The inversion TSs with inclusion Li cation show smaller deformation from the stable intermediate found in the anionic substitution reactions at sulfur . The bridging actions of Li cation only cause two halogen anions to bend toward it with a decrease of the X−S−X angle by about 35°, which is much smaller than that in the inversion TSs LiX/CH 3 X in the ion pair S N 2(C),16a where there is a remarkable deformation from the linear TS geometry [X···CH 3 ···X] - ⧧ found in the anionic S N 2(C) reactions and the Li cation causes a large decrease of the X−C−X angle by about 90°. These may be the main reasons the central barrier heights in the inversion pathway, Δ H ⧧ cent (inv), for the ion pair S N 2(S) are much lower than those in the ion pair S N 2(C) reactions (see Table ).…”

“…They also extended the work to the higher alkyls and discussed some steric effects for the ion pair S N 2(C)reactions 15b. More recently, Ren et al reported a series of theoretical studies on the ion pair S N 2(C) and S N 2(N) and addressed the influence of Li + on the geometries and relative energies of the stationary points on the PESs.…”

Nucleophilic Aliphatic Substitution

“…Structure (69) The S N 2 reactions of a free cyanate ion or a lithium isocyanate ion pair with methyl halides have been investigated at the MP2/6-311+G(d,p) level of theory. 107 Although the ion-pair reactions are slower than the free-ion reactions, the calculated H = decreases in the order MeF > MeCl > MeBr > MeI as expected for both the free-ion and ion-pair reactions. The calculations also show that both the free-ion and ion-pair reactions occur with inversion, rather than with retention of configuration, via the transition states (70) and (71), respectively.…”