Oxygen scrambling and stereochemistry during the trifluoroethanolysis of optically active 2-butyl 4-bromobenzenesulfonate

Dietze, Paul E.; Wojciechowski, Marek

doi:10.1021/ja00169a035

Cited by 25 publications

(29 citation statements)

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“…However, rotation of the cation, which would lead to racemization, does not occur". 13 The results of this earlier study are similar in some respects to data reported here. However, they do not exclude the possibility that 18 O-scrambling proceeds through an ion-pair reaction intermediate because of the following limitations of the earlier experiments, which are addressed in the present work.…”

Section: Discussionsupporting

confidence: 88%

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

Tsuji

Richard

2006

J. Am. Chem. Soc.

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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 halftimes for the solvolysis reaction was used to calculate a rate constant k rac ≈ 4 × 10 -6 s -1 . This is 80 fold smaller than k iso = 3.2 × 10 -4 s -1 for isomerization that exchanges oxygen-16 and oxygen-18 of 3-NO 2 C 6 H 4 13 CH(Me)OS( 18 O) 2 Tos 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.

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Section: Discussionsupporting

confidence: 88%

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

Tsuji

Richard

2006

J. Am. Chem. Soc.

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“…As the rate constants are so close to the vibrational limit for the formal existence of an intermediate, it is likely that the processes that govern oxygen exchange and hydride transfer will be subject to dynamical effects, as has been commented by Richard and Tsuji in the context of carbenium intermediates that are believed to have shorter lifetimes. [6,18,19] In this case, the lifetimes are shortened so much that the mechanistic description becomes one of competitive concerted processes with no common intermediate, but a series of closely related transition states on a shallow potential surface. This may be appropriate for the 2alkyl tosylates but does not seem to be enforced by the analysis presented here.…”

Section: Discussionmentioning

confidence: 99%

The solvolysis mechanism of simple secondary tosylates in 50% aqueous TFE

Williams

2016

J of Physical Organic Chem

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The solvolysis of simple secondary tosylates in 50% trifluoethanol has been investigated using stereochemical and isotopic labels. 2-butyl, 2-pentyl and 2-octyl tosylates all solvolyse at very similar rates (~1 × 10−5 s−1) at 30 °C. Slow racemisation of S-2-butyl tosylate (~4.6 × 10−7 s−1) was observed during solvolysis, but R-2-octyl tosylate did not show any significant racemisation. Competing rearrangement of 3-pentyl tosylate to 2-pentyl tosylate was observed during solvolysis and is attributed to 1,2-hydride transfer, which occurs at a rate sufficient to account for the difference in the rates of racemisation of 2-butyl and 2-octyl tosylates. The stereochemistry of the alcohol product was studied for the reaction of R-2-octyl tosylate by derivatising the corresponding alcohol to 4-nitrobenzoate and showed high but not complete stereoselectivity (92:8 inversion : retention of configuration). 18O isotope exchange at the leaving group tosylate showed that both 2-butyl (5.4 × 10−7 s−1) and 2-octyl (4.2 × 10−7 s−1) tosylates exchange at similar rates. Partitioning of a common carbenium ion intermediate accounts for all these data only if solvolysis proceeds through pre-organisation of the solvent with 1,2-hydride transfer and isotope exchange competing inefficiently with ion return and solvolysis. Alternatively, a series of parallel concerted reactions (solvolysis, 1,2-hydride transfer and isotope exchange) would also account for these data

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“…does 18 O scrambling of oxygen in 'unreacted' sulfonates indicate ion pair return on the main reaction pathway 20 or a side reaction via a concerted polar mechanism? 23,24 Indirect mechanistic probes lead to different interpretations or emphases, illustrated by the results in Table 5.…”

Section: Discussionmentioning

confidence: 99%

“…The evidence for a mechanistic change to rate-determining separation of contact ion pairs (i.e. return from contact ion pairs) 20 relies on ambiguous 18 O scrambling data 23,24 and on complex interpretations of secondary deuterium kinetic isotope effects, 20 in cases where an ion pair may not survive long enough to undergo one vibration. 21…”

Section: Discussionmentioning

confidence: 99%

Structural and solvent effects on rates of solvolysis of secondary alkyl substrates—an update

Bentley

Roberts

2004

J of Physical Organic Chem

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Rate constants for solvolyses of secondary alkyl tosylates in fluorinated media [including hexafluoropropan-2-ol (HFIP), hexafluoroacetone sesquihydrate, and trichloromethylbis(trifluoromethyl)carbinol] are reported. Rates of solvolysis of 2-adamantyl p-toluenesulfonate in 97% (w/w) HFIP-water at 25 C are neither retarded by the addition of NaOTs nor accelerated greatly by NaClO 4 . The -deuterium kinetic isotope effect for solvolyses of 1-(1-adamantyl)ethyl methanesulfonate in 20% acetone-water at 25 C is 1.14. Mechanistic implications of these results are discussed. RESULTSSalt effects on the solvolysis of 2-AdOTs in aqueous HFIP are given in Table 1. As neither 0.03 M LiClO 4 nor

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Oxygen scrambling and stereochemistry during the trifluoroethanolysis of optically active 2-butyl 4-bromobenzenesulfonate

Cited by 25 publications

References 0 publications

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

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

The solvolysis mechanism of simple secondary tosylates in 50% aqueous TFE

Structural and solvent effects on rates of solvolysis of secondary alkyl substrates—an update

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