When Does an Intermediate Become a Transition State? Degenerate Isomerization Without Competing Racemization During Solvolysis of (S)-1-(3-Nitrophenyl)ethyl Tosylate

摘要

(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 halftimes for the solvolysis reaction was used to calculate a rate constant krac ≈ 4 × 10^-6 s^-1. This is 80 fold smaller than kiso = 3.2 × 10^-4 s^-1 for isomerization that exchanges oxygen-16 and oxygen-18 of >3-NO2C6H4¹³CH(Me)OS(¹⁸O)2Tos during solvolysis and 10 fold smaller than the minimum value of krac = 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.