The SN2-SN1 spectrum. 3. Solvolyses of secondary and tertiary alkyl sulfonates in fluorinated alcohols. Further evidence for the SN2 (intermediate) mechanism

Abstract

Kinetic data are reported for solvolyses of secondary and tertiary alkyl tosylates in trifluoroethanol and hexa- fluoroisopropyl alcohol and also for 1-adamantylmethylcarbinyl (VI) and l-bicyclo(2.2.2)octy1 (VII) tosylates in a wide range of solvents. The relative solvolysis rates of 2-adamantyl (I), 1-adamantylmethylcarbinyl (VI), and l-bicyclo(2.2.2)octyl (VII) tosylates are essentially independent of solvent (in ethanol, methanol, water, trifluoroethanol, hexafluoroisopropyl alcohol, acetic acid, formic acid, and trifluoroacetic acid). Thus these three substrates are good models for sN1 (k, or limiting) mechanistic behavior; they respond almost identically to changes in solvent ionizing power and are insensitive to changes in solvent nucleophilicity. In contrast relative solvolysis rates of 2-adamantyl and 2-propyl tosylates vary with solvent 105-fold from 134 in hexafluoroisopropyl alcohol to 0.001 1 in ethanol. For straight-chain secondary alkyl tosylates, logarithms of solvolysis rate constants in hexafluoroisopropyl alcohol correlate with u* and give a large negative p* value (-9.1). Other solvents give less negative p* values (e.g., CF3C02H, p* = -7.3; CF3CH20H, p* = -5.2; H20, p* = -4.3) and smaller 2-adamantyl/2-propyl (2-AdOTs/Z-PrOTs) rate ratios. Increasing amounts of nucleophilic solvent assistance in the more nucleophilic solvents leads to decreased electron demand by the cationic center (i.e., less negative p*), and solvolyses of 2-propyl become more rapid than 2-adamantyl. Solvent effects on the relative reactivity of secondary alkyl tosylates (ROTS) are correlated accurately by using the linear free energy relationship: log (k/kO)ROTs = Q'log (k/k&.~d~~~ (k/ko)~.~d~~~ + (1 - e') log (k/k0)2.~~~ where k refers to any solvent, ko refers to 80% ethanol/water (v/v), and Q'is an adjustable blending parameter. The high precision of correlations using this equation for 2-butyl, 2-pentyl, 3-pentyl, 4-heptyl, cyclopentyl, cyclohexyl, and cycloheptyl tosylates provides evidence for a gradual change of mechanism from SN2 (one-stage) through sN2 (intermediate) to SN1 mechanisms. Solvolyses of 3-methyl-2-butyl tosylate show significant sensitivity to nucleophilic solvent assistance (Q' = 0.42). Solvolyses of pinacolyl (11), 2-exo-norbornyl (111), 2-endo-norbornyl (IV), menthyl (V), and cyclooctyl tosylates are either k, or kA (not k, as proposed by others), since they respond less to changes in solvent ionizing power than the k, mechanistic models (I, VI, VII).