When does an intermediate become a transition state? Degenerate isomerization without competing racemization during solvolysis of (S)-1-(3-nitrophenyl)ethyl tosylate

(S)-1-(3-Nitrophenyl)ethyl tosylate [(S)-2-OTs] was prepared in >99% enantiomeric excess and the change in the chiral purity of this compound was monitored during solvolysis in 50:50 trifluoroethanol/water. The barely detectable formation of 0.5% (R)-2-OTs after two half times for the solvolysis reaction was used to calculate a rate constant of k_rac ≈ 4 × 10^-6 s^-1. This is 80-fold smaller than k_iso = 3.2 × 10^-4 s^-1 for the isomerization that exchanges oxygen-16 and oxygen-18 of 3-NO₂C₆H₄ ¹³CH(Me)OS(¹⁸O)₂Tos during solvolysis and 10-fold smaller than the minimum value of k_rac = 4.6 × 10 ^-5 s^-1 predicted if isomerization and racemization products form by partitioning of a common ion-pair intermediate of a stepwise reaction. It is concluded that the isomerization reaction proceeds mainly by a pathway that avoids formation of this putative intermediate. It is suggested that the solvolysis reaction of 2-OTs may proceed by a stepwise preassociation mechanism where solvent "reorganization" precedes substrate ionization to form an ion-pair intermediate.