The Impact of Substituents on the Transition States of S_N2 and E2 Reactions in Aliphatic and Vinylic Systems: Remarkably Facile Vinylic Eliminations

Abstract

For a series of α and β substituted haloethanes and haloethenes, gas-phase experiments and computational modeling have been used to characterize their nucleophilic substitution and elimination reactions. Despite being less thermodynamically favorable, the vinylic eliminations have rate constants and computed barriers that are similar to those of analogous aliphatic eliminations. This is the result of the vinylic systems shifting to more E1(cb)-like transition states and exploiting the inherent greater acidity of vinylic hydrogens. In general, the α-substituents have a greater impact on the S(N)2 pathways and stabilize the transition states via field and polarizability effects. Substantial stabilization is also provided to the E2 transition states by the α-substituents, but they have surprisingly little impact on the geometries of the transition states of either pathway. The β-substituents generally lead to a strong bias toward elimination and greatly affect the synchronicity of the elimination (more E1(cb)-like) as well as its location on the reaction coordinate (early). The experimental and computational data are in good accord, and the full data set provides a comprehensive picture of substituent effects on solvent-free S(N)2 and E2 processes.

Intermolecular CH Bond Activation by a Cationic Iridium(III) Dichloride Phenanthroline Complex

“…[23] When astable signal is obtained, neutral reagents can be spiked into the helium through ac ustom gas-handlings ystem which has been previously described. [17] Kinetic measurements were completed by establishing an appropriate flow rate for the neutral reagent and varying the time between the isolation of the ion and the expulsion of all the ions to obtain amass spectrum. Te ndifferent time delays were used in each reaction.…”

“…[9,12,15,16] Here we report the first example of bimolecular C À Ha ctivation by ac ationic iridium(III) dichloride phenanthroline complex in the gas phase.This is a14-electron species with atetracoordinate iridium. Thework includes kinetic measurements and is supported by computational modeling using density functional theory.Using am odified ion-trap mass spectrometer, [17] iridium-(III) dichloride phenanthroline complex I was produced in the gas phase by electrospray ionization (ESI). While surveying its reactivity as ac atalyst for diazoester decomposition, we were surprised by the observation of arelatively rapid reaction with the cyclohexane that was used to dilute the diazo ester.W ith cyclohexane,…”

“…Using am odified ion-trap mass spectrometer, [17] iridium-(III) dichloride phenanthroline complex I was produced in the gas phase by electrospray ionization (ESI). While surveying its reactivity as ac atalyst for diazoester decomposition, we were surprised by the observation of arelatively rapid reaction with the cyclohexane that was used to dilute the diazo ester.W ith cyclohexane, I mainly gives addition with the loss of HCl, which we interpret as aC À Hactivation process followed by reductive elimination of HCl (Scheme 1, Figure 1).…”

Substitution reactions of gaseous ions in a three‐dimensional quadrupole ion trap

“…More recently, S N 2 reactions in the gas phase have also been investigated with quadrupole ion traps (also known as Paul traps) . These traps use a combination of direct‐ and alternating‐current electrical fields to confine the ions spatially.…”

“…10 More recently, S N 2 reactions in the gas phase have also been investigated with quadrupole ion traps (also known as Paul traps). [11][12][13][14] These traps use a combination of direct-and alternating-current electrical fields to confine the ions spatially. For minimizing the kinetic energy taken up by the ions and for limiting their trajectories to the center of the trap, modern quadrupole ion traps moreover apply collisional cooling by helium (p(He) ≈ 10 −3 mbar).…”