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) approximately equal to 4 x 10-6 s-1. This is 80-fold smaller than kiso = 3.2 x 10-4 s-1 for the 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 k(rac) = 4.6 x 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.
Tsuji
Yutaka
Y
Department of Biochemistry and Applied Chemistry, Kurume National College of Technology, Komorinomachi, Kurume 830-8555, Japan.
Richard
John P
JP
D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009578:Nitrobenzenes / Q000138:chemical synthesis / Q000737:chemistry*; D012680:Sensitivity and Specificity; D012995:Solubility; D013237:Stereoisomerism; D014270:Trifluoroethanol / Q000737:chemistry; D014867:Water / Q000737:chemistry
C504943:1-(3-nitrophenyl)ethyl tosylate; D009578:Nitrobenzenes; D014867:Water; D014270:Trifluoroethanol