Solvolysis of cyclopentyl p-bromobenzenesulfonate in aqueous hexafluoroisopropyl alcohol. Deuterium rate effects, stereochemistry of substitution and elimination, and reaction mechanism

Seib, R. C.; Shiner, V. J.; Sendijarević, Vahid; Humski, K.

doi:10.1021/ja00494a019

Cited by 14 publications

(4 citation statements)

References 2 publications

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“…Shiner argued that since the tight ion pair arising from the protonation predominantly collapses to 15 instead of reacting with solvent, then the S N 1 reaction of 15 must involve a rate-limiting step that is later than formation of the tight ion pair, and so at the formation of a solvent-separated ion pair. This idea influenced the interpretation of a substantial history of observations . The flaw in this argument, as suggested by the present work, is the assumption that there is only one tight ion pair operative in the system.…”

Section: Conclusion and Perspectivementioning

confidence: 78%

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Solvation Dynamics and the Nature of Reaction Barriers and Ion-Pair Intermediates in Carbocation Reactions

Roytman

Singleton

2020

J. Am. Chem. Soc.

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Additions of acids to 1,3-dienes are conventionally understood as involving discrete intermediates that undergo an ordinary competition between subsequent pathways to form the observed products. The combined experimental, computational, and dynamic trajectory study here suggests that this view is incorrect, and that solvation dynamics plays a critical role in the mechanism. While implicit solvent models were inadequate, QM/QM′ trajectories in explicit solvent provide an accurate prediction of the experimental selectivity in the addition of HCl to 1,3-pentadiene. Trajectories initiated from a protonation saddle point on the potential of mean force surface are predominantly unproductive due to a gating effect of solvation that allows diene protonation only when the incipient ion pair is neither too solvent-stabilized nor too little. Protonation then leads to relatively unsolvated ion pairs, and a majority of these collapse rapidly to the 1,2-product, without barrier and without achieving equilibrium solvation as intermediates. The remainder decay slowly, at a rate consistent with equilibrium solvation as true intermediates, affording a mixture of addition products. Overall, an accurate description of the nature and pathway selectivity of the ion pair intermediates in carbocation reactions must allow for species lacking equilibrium solvation. Potential reinterpretations of a series of historically notable observations in carbocation reactions are discussed.

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Section: Conclusion and Perspectivementioning

confidence: 78%

“…We would suggest that a nonequilibrium solvated ion pair is initially formed from protonation of propene by 14 , and that this ion pair rapidly collapses to 15 . This then undercuts the original support for the importance of solvent-separated ion pairs in the solvolysis of 15 and other secondary systems …”

Section: Conclusion and Perspectivementioning

confidence: 99%

Solvation Dynamics and the Nature of Reaction Barriers and Ion-Pair Intermediates in Carbocation Reactions

Roytman

Singleton

2020

J. Am. Chem. Soc.

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“…59 Thus, as their ability to ionize substrates increases, their corresponding ability to dissociate ion pairs decreases. The properties of the pure solvents give rise to an interesting observation; in pure TFE, the rate of alcoholysis of 42 is 10 times greater than that in HFIP, 60 and more than 1000 times greater than PFTB! This would suggest that the RDS of the triflate solvolysis has changedinstead of ionization, the RDS is now nucleophile trapping.…”

Section: ■ Common Ion Effectsmentioning

confidence: 99%

Search for a Symmetrical C–F–C Fluoronium Ion in Solution: Kinetic Isotope Effects, Synthetic Labeling, and Computational, Solvent, and Rate Studies

et al. 2015

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Recently, we reported evidence for the generation of a symmetrical fluoronium ion (a [C-F-C](+) interaction) in solution from a cage-like precursor, relying heavily on a single isotopic-labeling experiment. Paraphrasing the axiom that a strong claim must be met by as much evidence as possible, we seek to expand upon our initial findings with comprehensive labeling studies, rate measurements, kinetic isotope effect (KIE) experiments, synthetic studies, and computations. We also chronicle the development of the system, our thought process, and how it evolved from a tantalizing indication of fluoronium ion assistance in a dibromination reaction to the final, optimized system. Our experiments show secondary KIE experiments that are fully consistent with a transition state involving fluorine participation; this is also confirmed by a significant remote isotope effect. Paired with DFT calculations, the KIE experiments are indicative of the trapping of a symmetrical intermediate. Additionally, starting with an epimeric in-triflate precursor that hydrolyzes through a putative frontside SNi mechanism involving fluorine participation, KIE studies indicate that an identical intermediate is trapped (the fluoronium ion). Studies also show that the rate-determining step of the fluoronium forming SN1 reaction can be changed on the basis of solvent and additives. We also report the synthesis of a nonfluorinated control substrate to measure a relative anchimeric role of the fluorine atom in hydrolysis versus μ-hydrido bridging. After extensive testing, we can make the remarkable conclusion that our system reacts solely through a "tunable" SN1 mechanism involving a fluoronium ion intermediate. Alternative scenarios, such as SN2 reactivity, do not occur even under forced conditions where they should be highly favored.

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“…NMR (C6D6) 1.87 (dd, J = 1.9, 0.7 Hz, Me5C5), 1.25 (d, J = 9.5 Hz, PMe3), 0.80, C6HU), -18.67 (d, J = 36.7 Hz, Ir-H); ,3C NMR (C6D6) 92.36 (d, J = 3.4 Hz, C5(CH3)5), 19.69 (d, J = 35.7 Hz, P(CH3)3), 10.75 (s, C5(CH3), 44.58 (d, J = 4 Hz, C6Hn (ß-C)), 43.96 (d, J = 2 Hz, C6H" 03-C)), 32.92 (s, C6H" ( -C)), 32.85 (s, C6H" ( -C)), 28.33 (s, C6Hn (5-C)), 3.27 (d, / = 7.1 Hz, C6H" ( -C)); FDMS,m¡e 488,486. The intermediate formed on irradiation of 5, presumably (MeC5)(Me3P)Ir, also reacts with neopentane. Irradiation of 5 in neopentane solvent gives, after 5.3 h of irradiation time (80% NMR yield after 83% conversion), a new complex once again seen (9) (a) Geoffrey, G. L.; Wrighton, M. S. "Organometallic Photochemistry"; Academic Press: New York, 1979. (b) Pierantozzi, R.; Geoffrey, G. L. Inorg.…”

Section: Scheme Imentioning

confidence: 99%

Carbon-hydrogen activation in completely saturated hydrocarbons: direct observation of M + R-H .fwdarw. M(R)(H)

Janowicz¹,

Bergman²

1982

J. Am. Chem. Soc.

620

305

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Solvolysis of cyclopentyl p-bromobenzenesulfonate in aqueous hexafluoroisopropyl alcohol. Deuterium rate effects, stereochemistry of substitution and elimination, and reaction mechanism

Cited by 14 publications

References 2 publications

Solvation Dynamics and the Nature of Reaction Barriers and Ion-Pair Intermediates in Carbocation Reactions

Solvation Dynamics and the Nature of Reaction Barriers and Ion-Pair Intermediates in Carbocation Reactions

Search for a Symmetrical C–F–C Fluoronium Ion in Solution: Kinetic Isotope Effects, Synthetic Labeling, and Computational, Solvent, and Rate Studies

Carbon-hydrogen activation in completely saturated hydrocarbons: direct observation of M + R-H .fwdarw. M(R)(H)

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