Prediction of the kinetic stability of N‐alkyl‐X‐pyridinium ions in dichloromethane

Abstract

Geometries of 20 N‐[4,4′‐bis (dimethylamino)benzhydryl]‐X‐pyridinium ions (DMABh‐X‐pyridinium ions) and of corresponding heterolytic transition structures have been optimized at the M06‐2X/6‐311+G(2d,p) level of theory in the presence of the IEFPCM model with dichloromethane as a solvent. Using the optimized geometries, free energies of activation (∆G‡model) for the model heterolysis of the DMABh‐X‐pyridinium ions in dichloromethane at 20°C have been obtained. A very good correlation between ∆G‡model values for the model heterolysis of five reference DMABh‐X‐pyridinium ions and literature ∆G‡ values for the heterolysis of the same ions in dichloromethane (r = 0.992) validates the procedure for predicting experimental reactivities of the ions (∆G‡calc) in dichloromethane. Accordingly, the correlation has been employed to obtain predicted free energies of activation (∆G‡calc) for heterolyses of other 15 DMABh‐X‐pyridinium ions in dichloromethane from corresponding ∆G‡model values. The high quality of the whole set of the predicted reactivities is confirmed by a very good correlation plot of log kcalc (obtained from ∆G‡calc) versus σ (r = 0.995). Using the previously established correlation equation log k = sf (Ef + Nf), predicted nucleofugality parameters (Nfcalc), which quantitatively describe leaving group abilities, have been determined for various X‐pyridines in dichloromethane, and now can be used not only for comparing leaving group abilities of X‐pyridines with those of other leaving groups on the existing nucleofugality (Nf) scale but also for estimating the reactivity, that is, the kinetic stability, of diverse N‐alkyl‐X‐pyridinium ions in dichloromethane at 20°C.

Nucleofugality of diphenylphosphinate and kinetic stabilities of secondary and tertiary benzylic diphenylphosphinates in aqueous solvents

“…25 The existing nucleofugality (N f ) scale includes numerous leaving groups of different functionalities and presently covers a reactivity range of 15 units, that is, 15 orders of magnitude. [25][26][27][28][29][30][31][32][33][34][35] Thus, the scale can be used for comparing leaving group abilities in the wide range of reactivity. Furthermore, Equation (1) can also be employed for predicting the kinetic stability of substrates in various solvents at 25 • C if reactivity parameters (namely, E f and N f ) of two constituent parts (i.e., of an electrofuge and a nucleofuge) of substrates are known.…”

“…Although diverse leaving groups have been included in the nucleofugality scale, [25][26][27][28][29][30][31][32][33][34][35] phosphorus leaving groups have remained somehow beyond the scope of previous studies. Therefore, we set out to investigate solvolytic behavior of benzhydryl diphenylphosphinates and to determine N f values of the DPP leaving group in several hydroxylic solvents.…”

“…As mentioned in Section 1, Equation ( 1) and existing reactivity parameters (N f , s f , and E f ) can also be used for predicting solvolytic reactivities of substrates in various solvents at the semiquantitative level. 25,[33][34][35] Accordingly, combining the nucleofuge-specific parameters of DPP leaving group (Table 2) with previously determined electrofugalities (E f ) of different electrofuges, 25,47,48 the kinetic stability of corresponding alkyl diphenylphosphinates toward heterolysis in some solvent at 25 • C can be estimated in terms of both log k values and reaction half-lives (t 1/2 ). For illustration, the poor 9-fluorenyl electrofuge (E f = −9.92) 47 can be combined with the DPP nucleofuge to afford 9-fluorenyl diphenylphosphinate (5-DPP).…”

“…The existing nucleofugality ( N f ) scale includes numerous leaving groups of different functionalities and presently covers a reactivity range of 15 units, that is, 15 orders of magnitude 25 – 35 . Thus, the scale can be used for comparing leaving group abilities in the wide range of reactivity.…”

“…Thus, the scale can be used for comparing leaving group abilities in the wide range of reactivity. Furthermore, Equation (1) can also be employed for predicting the kinetic stability of substrates in various solvents at 25°C if reactivity parameters (namely, E f and N f ) of two constituent parts (i.e., of an electrofuge and a nucleofuge) of substrates are known 25, 33 – 35 . The knowledge of kinetic stabilities of substrates in various solvents can be very useful in handling substrates during synthesis, purification and storage.…”