Stereochemistry of trifluoroacetolysis of optically active 2-butyl tosylate. A test for hydrogen bridging

Allen, Annette D.; Ambidge, I. C.; Tidwell, T. T.

doi:10.1021/jo00172a017

Cited by 11 publications

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“…Schleyer and Bentley made the reasonable assumption that 2-PrOTs, and all other secondary substrates, react by the k c process in CF 3 COOH. Other researchers using other experimental procedures have verified this assertion (Paradisi and Bunnett 1981;Diaz et al 1968;Dannenberg et al 1981;Allen et al 1983). Schleyer and Bentley surmised from the k H /k D values in Table 1 that 2-PrOTs reacts by merged mechanisms in solvents other than trifluoroacetic acid.…”

Section: -Octyl Tosylatementioning

confidence: 58%

Do the solvolysis reactions of secondary substrates occur by the SN1 or SN2 mechanism: or something else?

Pagni

2011

Found Chem

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Primary and methyl aliphatic halides and tosylates undergo substitution reactions with nucleophiles in one step by the classic S N 2 mechanism, which is characterized by second-order kinetics and inversion of configuration at the reaction center. Tertiary aliphatic halides and tosylates undergo substitution reactions with nucleophiles in two (or more) steps by the classic S N 1 mechanism, which is characterized by first-order kinetics and incomplete inversion of configuration at the reaction center due to the presence of ion pairs. When the nucleophile is also the solvent, the substitution reaction is called a solvolysis, and both the S N 2 and S N 1 reactions now obey first-order kinetics. Schleyer and Bentley have provided solid, but not conclusive, evidence that secondary substrates undergo solvolysis by a merged mechanism, one that blends characteristics of both the S N 2 and S N 1 mechanisms. The following paper presents the history of their sustained pursuit of a merged mechanism and subsequent rebuttals to this claim. Several issues related to the philosophy and sociology of science are also discussed.

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Section: -Octyl Tosylatementioning

confidence: 58%

Do the solvolysis reactions of secondary substrates occur by the SN1 or SN2 mechanism: or something else?

Pagni

2011

Found Chem

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“…The reversible formation of a tight ion pair was shown best to explain predominant racemization in the product accompanied by some racemization of the reactant in the trifluoroacetolysis of 2-butyl tosylate (14, X = OTs). 48 Ionization with hydrogen participation to form a hydrogenbridged carbocation (15) proposed by other authors49 could not give a simple prediction of the results. 48 Our observation of extensive return from the [2«OTs~] tight ion pair further supports this conclusion.…”

Section: Discussionmentioning

confidence: 91%

Secondary and tertiary 2-methylbutyl cations. 1. Trifluoroacetolysis of 3-methyl-2-butyl tosylate

Fǎrcaşiu¹,

Marino²,

Harris³

et al. 1994

J. Org. Chem.

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Trifluoroacetolysis rates for 3-methyl-2-butyl tosylate (4) and kinetic isotope effects at C(l) (*h/*d = 1.083 per H atom), C(2) (fen/kn = 1.10), and C(3) (fen/kc = 1.82) were determined. The products are 2-methyl-2-butyl trifluoroacetate (5, 98.5%) and 3-methyl-2-butyl trifluoroacetate (6, 1.5%). GC-MS analysis of products from labeled tosylates 4-1-ds and 4-2-d showed that 42 % of the apparently unrearranged 6 had a methyl group shifted from the original C(3) to the original C(2), whereas 3.6% methyl shift occurred in 5. The results do not substantiate a ks-kA competition mechanism. Instead, two carbocations, the tertiary 2-methyl-2-butyl (1) and the nominally secondary 3-methyl-2-butyl (2) intervene. The intimate structure of 2 is not established, but a symmetrical, methyl-bridged ion (3) does not agree with the results. A high ß isotope effect does not require hydrogen assistance to ionization; ionization concerted with (assisted by) hydrogen migration is unimportant in formation of 1 (and 5) from 4. Instead, the reaction involves reversible formation of an intimate ion pair with subsequent rate-determining H shift (which for 2*OTs~is in competition with Me shift and ca. 25% elimination) followed by solvent capture. Methyl migration in 2 may occur in the solvent-separated ion pair; alternatively, methyl or hydrogen migration is conformationally determined. At least 9% of 1 is formed from 2 which has undergone methyl shift. Nucleophilic attack on 4 appears important only in strongly nucleophilic media like aqueous ethanol. The claim that nucleophilic solvent assistance is significant in solvolysis of other secondary alkyl substrates in TFA or 97% hexafluoro-2-propanol is evaluated. Such a conclusion cannot be accepted on the basis of rate correlations alone, (t.e without product studies to support it). The implications of our results for the trifluoroacetolysis of 2-butyl tosylate are briefly discussed.

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“…The products of the reaction are stable under the reaction conditions and neither revert to reactant nor change their stereochemistry once formed, unlike solvolysis in TFA. [3][4][5] Thus, these observations must be explained by the mechanistic pathway of the solvolysis reaction.…”

Section: Discussionmentioning

confidence: 99%

“…The ratio of the two transition states can be estimated to be about 4:1 by using cis and trans 2-butene as a model, [3] leading to a predicted rate constant of 1.8 × 10 À7 s À1 for the interconversion of the 2-butyl tosylate isomers through this pathway. This value is similar to the observed rate constant for the interconversion of the 2-butyl tosylate enantiomers (Fig.…”

Section: Discussionmentioning

confidence: 99%

“…No clear-cut conclusion has been reached despite a series of mechanistic studies with 2-propyl and 2-butyl substrates. [1][2][3][4][5] Tidwell, [3] Dannenberg [4] and Farcaşiu [5] studied the solvolysis of 2-butyl tosylate in trifluoroacetic acid (TFA). As a 1,2-hydride shift was observed during the reaction, these authors all agreed that a simple secondary substrate in TFA should react by a stepwise substitution mechanism with a true intermediate rather than through a concerted pathway.…”

Section: Introductionmentioning

confidence: 99%

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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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Stereochemistry of trifluoroacetolysis of optically active 2-butyl tosylate. A test for hydrogen bridging

Cited by 11 publications

References 0 publications

Do the solvolysis reactions of secondary substrates occur by the SN1 or SN2 mechanism: or something else?

Do the solvolysis reactions of secondary substrates occur by the SN1 or SN2 mechanism: or something else?

Secondary and tertiary 2-methylbutyl cations. 1. Trifluoroacetolysis of 3-methyl-2-butyl tosylate

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

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