When Does an Intermediate Become a Transition State? Degenerate Isomerization without Competing Racemization during Solvolysis of (S)-1-(3-Nitrophenyl)ethyl Tosylate
- 1 December 2006
- journal article
- research article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 128 (51) , 17139-17145
- https://doi.org/10.1021/ja066235d
Abstract
(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 krac ≈ 4 × 10-6 s-1. This is 80-fold smaller than kiso = 3.2 × 10-4 s-1 for the isomerization that exchanges oxygen-16 and oxygen-18 of 3-NO 2 C 6 H 4 13CH(Me)OS( 18O) 2 Tos 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.Keywords
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