The α-effect and competing mechanisms: the gas-phase reactions of microsolvated anions with methyl formate

Benchmark ab initio characterization of the complex potential energy surfaces of the HOO- + CH3Y [Y = F, Cl, Br, I] reactions

The α-effect is a well-known phenomenon in organic chemistry, and is related to the enhanced reactivity of nucleophiles involving one or more lone-pair electrons adjacent to the nucleophilic center. The gas-phase bimolecular nucleophilic substitution (SN2) reactions of α-nucleophile HOO- with methyl halides have been thoroughly investigated experimentally and theoretically; however, these investigations have mainly focused on identifying and characterizing the α-effect of HOO-. Here, we perform the first comprehensive high-level ab initio mapping for the HOO- + CH3Y [Y = F, Cl, Br and I] reactions utilizing the modern explicitly-correlated CCSD(T)-F12b method with the aug-cc-pVnZ [n = 2-4] basis sets. The present ab initio characterization considers five distinct product channels of SN2: (CH3OOH + Y-), proton abstraction (CH2Y- + H2O2), peroxide ion substitution (CH3OO- + HY), SN2-induced elimination (CH2O + HY + HO-) and SN2-induced rearrangement (CH2(OH)O- + HY). Moreover, besides the traditional back-side attack Walden inversion, the pathways of front-side attack, double inversion and halogen-bond complex formation have also been explored for SN2. With regard to the Walden inversion of HOO- + CH3Cl, the previously unaddressed discrepancies concerning the geometry of the corresponding transition state are clarified. For the HOO- + CH3F reaction, the recently identified SN2-induced elimination is found to be more exothermic than the SN2 channel, submerged by ∼36 kcal mol-1. The accuracy of our high-level ab initio calculations performed in the present study is validated by the fact that our new benchmark 0 K reaction enthalpies show excellent agreement with the experimental data in nearly all cases.

Solvent-induced dual nucleophiles and the α-effect in the SN2 versus E2 competition

We have quantum chemically investigated how microsolvation affects the various E2 and SN2 pathways, their mutual competition, and the α-effect of the model reaction system HOO-(H2O)n + CH3CH2Cl, at the CCSD(T) level. Interestingly, we identify the dual nature of the α-nucleophile HOO- which, upon solvation, is in equilibrium with HO-. This solvent-induced dual appearance gives rise to a rich network of competing reaction channels. Among both nucleophiles, SN2 is always favored over E2, and this preference increases upon increasing microsolvation. Furthermore, we found a pronounced α-effect, not only for SN2 substitution but also for E2 elimination, i.e., HOO- is more reactive than HO- in both cases. Our activation strain and quantitative molecular orbital analyses reveal the physical mechanisms behind the various computed trends. In particular, we demonstrate that two recently proposed criteria, required for solvent-free nucleophiles to display the α-effect, must also be satisfied by microsolvated HOO-(H2O)n nucleophiles.

Shapeshifting Nucleophile Singly Hydrated Hydroperoxide Anion Leads to the Occurrence of the Thermodynamically Unfavored SN2 Product

Single water molecules alone may introduce unusual features into the kinetics and dynamics of chemical reactions. The singly hydrated hydroperoxide anion, HOO-(H2O), was found to be a shapeshifting nucleophile, which can be transformed to HO- solvated by hydrogen peroxide HO-(HOOH). Herein, we performed direct dynamics simulations of its reaction with methyl iodide to investigate the effect of individual water molecules. In addition to the normal SN2 product CH3OOH, the thermodynamically unfavored proton transfer-induced HO--SN2 path (produces CH3OH) was also observed, contributing ∼4%. The simulated branching ratio of the HO--SN2 path exceeded the statistical estimation (0.6%) based on the free energy barrier difference. The occurrence of the HO--SN2 path was attributed to the shallow entrance channel well before a submerged saddle point, thus providing a region for extensive proton exchange and ultimately leading to the formation of CH3OH. In comparison, changing the leaving group from Cl to I increased the overall reaction rate as well as the proportion of the HO--SN2 path because the CH3I system has a smaller internal barrier. This work elucidates the importance of the dynamic effect introduced by a single solvent molecule to alter the product channel and kinetics of typical ion-molecule SN2 reactions.

Origin of the Microsolvation Effect on the Central Barriers of SN2 Reactions

We have quantitatively analyzed the microsolvation effect on the central barriers of microsolvated bimolecular nucleophilic substitution (S_N2) reactions by means of a two-step energy decomposition procedure. According to the first energy decompositions, an obvious increase in the central barrier for a microsolvated S_N2 reaction against its unsolvated counterpart can be mainly ascribed to the fact that the interaction between the solute and the conjunct solvent becomes less attractive from the reactant complex to the transition state. On the basis of the second energy decompositions with symmetry-adapted perturbation theory, this less attractive interaction in the transition state is primarily due to the interplay of the changes in the electrostatic, exchange, and induction components. However, the contribution of the change for the dispersion component is relatively small. A distinct linear correlation has also been observed between the changes of the total interaction energies and those of the corresponding electrostatic components for the microsolvated S_N2 reactions studied in this work. Moreover, the two-step energy decomposition procedure employed in this work is expected to be extensively applied to the gas phase reactions mediated by molecules or clusters.

Microsolvated Ion-Molecule SN2 Reactions with Dual Nucleophiles Induced by Solvent Molecules

Singly-hydrated HOO - anion was found to induce alternative nucleophile HO - via proton transfer from water molecule as react with CH 3 Cl recently. To investigate the generality of this effect, the competition between the solvent-induced HO - -S N 2 pathway and the normal HOO - -S N 2 pathway is studied for the microsolvated HOO - (H 2 O) n=1,2,3 + CH 3 X (X = F, Cl, Br, I) reaction by quantum chemistry calculation. Incremental hydration increases the barrier heights of both pathways and enlarges the barrier difference between them, which favors the HOO - -S N 2 pathway. Interestingly, the barrier difference is insensitive to the leaving group. Calculation shows the water induced HO - -S N 2 pathway is highly suppressed as the degree of hydration increases beyond two. The differential barrier under incremental hydration can be explained by solvent molecules stabilizing the HOMO level of HO - (HOOH)(H 2 O) n-1 nucleophile more than that of HOO - (H 2 O) n nucleophile. Comparison between these HO - -nucleophiles and HOO - -nucleophiles suggests that α-effect exists. Activation strain analysis attributes the barrier differences to the stronger distortion of the TS of HO - -S N 2 pathway than the counterparts of HOO - -S N 2 pathway. This work adds our understanding of the role of individual solvent molecules to induce new nucleophiles of the fundamental organic reaction.

Proton transfer-induced competing product channels of microsolvated Y-(H2O)n + CH3I (Y = F, Cl, Br, I) reactions

The potential energy profiles of three proton transfer-involved product channels for the reactions of Y^-(H₂O)_1,2 + CH₃I (Y = F, Cl, Br, I) were characterized using the B97-1/ECP/d method. These three channels include the (1) PT_CH3 product channel that transfers a proton from methyl to nucleophile, (2) HO^--induced nucleophilic substitution (HO^--S_N2) product channel, and (3) oxide ion substitution (OIS) product channel that gives CH₃O^- and HY products. The reaction enthalpies and barrier heights follow the order OIS > PT_CH3 > HO^--S_N2 > Y^--S_N2, and thus HO^--S_N2 can compete with the most favored Y^--S_N2 product channel under singly-/doubly-hydrated conditions, while the PT_CH3 channel only occurs under high collision energy and the OIS channel is the least probable. All product channels share the same pre-reaction complex, Y^-(H₂O)_n-CH₃I, in the entrance of the potential energy profile, signifying the importance of the pre-reaction complex. For HO^-/Y^--S_N2 channels, we considered front-side attack, back-side attack, and halogen-bonded complex mechanisms. Incremental hydration increases the barriers of both HO^-/Y^--S_N2 channels as well as their barrier difference, implying that the HO^--S_N2 channel becomes less important when further hydrated. Varying the nucleophile Y^- from F^- to I^- also increases the barrier heights and barrier difference, which correlates with the proton affinity of the nucleophiles. Energy decomposition analyses show that both the orbital interaction energy and structural deformation energy of the transition states determine the S_N2 barrier change trend with incremental hydration and varying Y^-. In brief, this work computes the comprehensive potential energy surfaces of the HO^--S_N2 and PT_CH3 channels and shows how proton transfer affects the microsolvated Y^-(H₂O)_1,2 + CH₃I reaction by competing with the traditional Y^--S_N2 channel.

Origin of the α-Effect in SN 2 Reactions

The α-effect is a term used to explain the dramatically enhanced reactivity of α-nucleophiles (R-Y-X:- ) compared to their parent normal nucleophile (R-X:- ) by deviating from the classical Brønsted-type reactivity-basicity relationship. The exact origin of this effect is, however, still heavily under debate. In this work, we have quantum chemically analyzed the α-effect of a set of anionic nucleophiles, including O-, N- and S-based normal and α-nucleophiles, participating in an SN 2 reaction with ethyl chloride using relativistic density functional theory at ZORA-OLYP/QZ4P. Our activation strain and Kohn-Sham molecular orbital analyses identified two criteria an α-nucleophile needs to fulfill in order to show α-effect: (i) a small HOMO lobe on the nucleophilic center, pointing towards the substrate, to reduce the repulsive occupied-occupied orbital overlap and hence (steric) Pauli repulsion with the substrate; and (ii) a sufficiently high energy HOMO to overcome the loss of favorable HOMO-LUMO orbital overlap with the substrate, as a consequence of the first criterion, by reducing the HOMO-LUMO orbital energy gap. If one of these two criteria is not fulfilled, one can expect no α-effect or inverse α-effect.

Single Solvent Molecules Induce Dual Nucleophiles in Gas-Phase Ion-Molecule Nucleophilic Substitution Reactions

Direct dynamics simulation of singly hydrated peroxide ion reacting with CH₃Cl reveals a new product channel that forms CH₃OH + Cl^- + HOOH, besides the traditional channel that forms CH₃OOH + Cl^- + H₂O. This finding shows that singly hydrated peroxide ion behaves as a dual nucleophile through proton transfer between HOO^-(H₂O) and HO^-(HOOH). Trajectory analysis attributes the occurrence of the thermodynamically and kinetically unfavored HO^--induced pathway to the entrance channel dynamics, where extensive proton transfer occurs within the deep well of the prereaction complex. This study represents the first example of a single solvent molecule altering the nucleophile in a gas-phase ion-molecule nucleophilic substitution reaction, in addition to reducing the reactivity and affecting the dynamics, signifying the importance of dynamical effects of solvent molecules.

Hydrazinolysis of aryl cinnamates and related esters: the α-effect arises from stabilization of five-membered cyclic transition state

A kinetic study is reported for nucleophilic substitution reactions of Y-substituted-phenyl cinnamates (1a–1h) with a series of primary amines including hydrazine in H2O containing 20 mol % DMSO at 25.0 °C. The Brønsted-type plot for the reaction of 2,4-dinitrophenyl cinnamate (1a) is linear with βnuc = 0.57 except hydrazine, which exhibits positive deviation from the linear correlation (i.e., the α-effect). The Brønsted-type plots for the reactions of 1a–1h with hydrazine and glycylglycine (glygly) are also linear with βlg = –0.71 and –0.87, respectively, when 1a is excluded from the linear correlation. Thus, the reactions have been concluded to proceed through a concerted mechanism on the basis of the linear Brønsted-type plots and magnitudes of the βnuc and βlg values. The α-effect shown by hydrazine is dependent on electronic nature of the substituent Y in the leaving group, e.g., it increases as the substituent Y becomes a weaker electron-withdrawing group (or as basicity of the leaving aryloxide increases), indicating that the α-effect is not due to destabilization of the ground state but mainly due to stabilization of the transition state. A five-membered cyclic TS structure, which could increase nucleofugality of the leaving aryloxide through H-bonding interaction, has been proposed to account for the leaving-group dependent α-effect found in this study. The theories suggested previously to rationalize the α-effect found for the related systems are also discussed.

Medium effect on the α-effect for nucleophilic substitution reactions of p-nitrophenyl acetate with benzohydroxamates and m-chlorophenoxide in DMSO–H2O mixtures as contrasts with MeCN–H2O mixtures: comparing two very different polar aprotic solvent components

A kinetic study is reported on nucleophilic substitution reactions of p-nitrophenyl acetate (1a) with three α-effect nucleophiles, benzohydroxamate (BHA–), p-methylbenzohydroxamate (MBHA–), and p-methyl-N-methylbenzohydroxamate (M2BHA–), and a reference nucleophile, m-chlorophenoxide (m-ClPhO–), in DMSO–H2O mixtures of varying compositions at 25.0 ± 0.1 °C. Second-order rate constants for the reactions with BHA– and MBHA– decrease upon addition of DMSO to the reaction medium up to 60 mol % DMSO and then increase thereafter only a little. In contrast, M2BHA– and m-ClPhO– become much more reactive as the DMSO content in the medium increases. Such contrasting medium effects on reactivity are consistent with the report that hydroxamic acids behave as OH acids in H2O but as NH acids in dipolar aprotic solvents (e.g., DMSO and MeCN). It has been concluded that BHA– and MBHA– form an equilibrium of a reactive form I with less reactive species II in DMSO–H2O mixtures and the position of the equilibrium is dependent on solvent compositions. BHA– and MBHA– exhibit the α-effect in H2O but not in in 90 mol % DMSO. In contrast, the α-effect yielded by M2BHA– increases steeply up to 70 mol % DMSO and then levels off thereafter.

Gas-phase alkyl and N-alkylamino cation affinities of anionic alpha-oxygen nucleophiles (H n XO-; X = N, P, As, O, S, Se, F, Cl, Br; n = 0-2): a theoretical G2(+)M study

In this work, we studied anionic alpha-oxygen nucleophiles in which there was an atom of with one or more unshared lone-pair electrons from groups 15-17 and periods 2-4 of the periodic table positioned adjacent to the negative attacking-oxygen (i.e., H _n XO^-; X = N, P, As, O, S, Se, F, Cl, Br; n = 0-2); these nucleophiles were termed "alpha-agents." Specifically, we investigated the gas-phase simple alkyl cation affinities (ACAs) and the simple N-alkylamino cation affinities (NAAMCAs) of these alpha-agents theoretically via the modified G2(+)_M method. Our calculations indicate that the O-C bond of CH₃OXH _n is somewhat similar to that of iPrOXH _n (n = 0, 1, 2; X = N, P, As, O, S, Se, F, Cl, Br). Also, steric repulsion is an important influence on these cation affinities. It appears that the kinetic gas-phase alpha effect in the S_N2 reactions of interest cannot be explained by the thermodynamic proton affinity or the affinity (SuA) of the anionic or neutral nucleophile (Nu) for the substrate cation, as the gas-phase ACA (NAAMCA) was found to be linearly correlated with the PA for the alpha-agents, similar to the previously reported linear correlation of the gas-phase ACA (NAAMCA) with the PA for normal nucleophiles.

Microsolvation effects on the reactivity of oxy-nucleophiles: the case of gas-phase SN2 reactions of YO-(CH3OH) n=1,2 towards CH3Cl

The modified G4(MP2) method was applied to explore microsolvation effects on the reactivity of four solvated normal oxy-nucleophiles YO^-(CH₃OH) _n=1,2 (Y = CH₃, C₂H₅, FC₂H₄, ClC₂H₄), and five α-oxy-nucleophiles YO^-(CH₃OH) _n=1,2 (Y = HO, CH₃O, F, Cl, Br), in gas-phase S_N2 reactions towards the substrate CH₃Cl. Based on a Brønsted-type plot, our calculations reveal that the overall activation barriers of five microsolvated α-oxy-nucleophiles are obviously smaller than the prediction from the correlation line constructed by four normal microsolvated ones to different degrees, and clearly demonstrate the existence of an α-effect in the presence of one or two methanol molecule(s). Moreover, it was found that the α-effect of the mono-methanol microsolvated α-nucleophile is stronger than that of the monohydrated α-nucleophile. However, the α-effect of YO^-(CH₃OH)₂ becomes weaker for Y = HO and CH₃O, whereas it becomes stronger for Y = F, Cl, Br than that of YO^-(H₂O)₂, which can be explained by analyses of the activation strain model in the two cases. It was also found that the rationale about the low ionization energy of α-nucleophile inducing the α-effect was not widely significant. Graphical abstract Variation of alpha-effect in the gas-phase S_N2 reaction with the microsolvation.