Negative activation energies for carbene additions to tetramethoxyethylene

Abstract

Activation energies were determined for the additions of three arylhalocarbenes to tetramethoxyethylene (TMeOE) and tetramethylethylene (TME) in pentane and decane. For addition of 3,5-dinitrophenylchlorocarbene to TMeOE in decane, E a =-10.4 ± 0.7 kcal/mol, the most negative activation energy yet encountered in a carbene-alkene addition reaction. Computational studies parallel and elucidate the experimental results.

Complexes and Negative Activation Energies in Arylhalocarbene/Alkene Additions: Activation Parameter Dependence on Alkane Solvent Chain Length

“…We have used this or comparable levels of theory successfully in previous computational studies of carbene− alkene additions with low activation barriers. 13,14,16,18 The computed activation parameters in the pentane model solvent for the nine carbene−alkene combinations under consideration here are presented in Table 5, along with corresponding experimental data obtained in pentane. 12 A representative TS involving F 5 -PhCCl adding to TME is illustrated in Figure 4b.…”

“…This supports our notion that longer alkane chain solvent assemblies stabilize all species, including π-complexes and TSs for addition, and lower activation enthalpies as well as entropies. 18 Moreover, the pentane to decane activation parameter changes for PhCCl and 3). These changes are nearly identical to the average activation parameter modifications found for the seven unimodal carbene/alkene Arrhenius correlations accompanying the change of reaction locus from pentane solution to the postulated decane solvent assemblies (ΔE a = −3.3 kcal/mol and ΔΔS ‡ = −11 eu, respectively; see above and Table 3).…”

“…41 We employed the MN12-SX functional 24 and 6-311+G(d) 25−29 basis sets in geometry optimizations of reactants, complexes, transition states, and products (MN12-SX/6-311+G(d)); this functional/basis set combination is economical to use and has performed well in prior analogous situations where computations of very low or even negative activation energies for cycloaddition TSs were at stake. 13,14,16,18 The MN12-SX exchangecorrelation functional includes screened Hartree−Fock exchange (X = 0.25) and improved treatments of medium-range and short-range electron correlation. 24,42 The stationary points located on the potential energy surfaces were characterized further by normal-mode analysis, and the (unscaled) vibrational frequencies formed the basis for the calculation of vibrational zero-point energy (ZPE) corrections.…”

“…16 Note that, by solvent "cages", 17 we imply enthalpy driven assemblies of (one or more) solvent molecules preferentially solvating a carbene, alkene, or carbene/alkene pair (as an encounter complex or transition state), with mutual cohesion provided by long-range electrostatic forces. 18,19 More recently, we examined carbene additions to tetramethoxyethylene, observing analogous significant decane solvent effects. 18 Presently, we have determined the activation parameters for the carbene/alkene additions of three reactive, electrophilic halocarbenes to three alkenes of varying reactivity, in both pentane and decane solvents.…”

“…18,19 More recently, we examined carbene additions to tetramethoxyethylene, observing analogous significant decane solvent effects. 18 Presently, we have determined the activation parameters for the carbene/alkene additions of three reactive, electrophilic halocarbenes to three alkenes of varying reactivity, in both pentane and decane solvents. The new experimental results, coupled with new extended computational studies and analysis, foster a more detailed mechanistic model for these iconic reactions.…”