Secondary and tertiary 2-methylbutyl cations. 2. Addition of trifluoroacetic acid to methylbutene

Fǎrcaşiu, D.; Marino, Gaye; Hsu, Chang Samuel

doi:10.1021/jo00080a026

Cited by 23 publications

(28 citation statements)

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“…The results of the labeling study indicated that the ion X cannot have a symmetrical structure. The same conclusion was reached from a study of the addition of TFAH (neat, or containing potassium trifluoroacetate, or mixed with acetonitrile) and TFAD to alkene 6 and its isomers, 2-methyl-1-butene and 2-methyl-2-butene …”

Section: Introductionmentioning

confidence: 53%

Secondary and Tertiary 2-Methylbutyl Cations. 3. Theoretical Study of Structures and Interconversions of C₅H₁₁⁺ Ions in the Gas Phase¹^a

and

1997

J. Org. Chem.

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The structures, relative energies, and interconversion barriers and pathways for several isomeric C5H11 + carbocations were investigated by high level ab initio MO calculations with inclusion of electron correlation. The energy minima found were two conformations of the tertiary 2-methyl-2-butyl cation (2), bisected (2 b ) and asymmetric (2 as ), and three 1-protonated (corner-protonated) cyclopropanes: trans-2,3-dimethyl (trans-8), cis-2,3-dimethyl (cis-8), and 1,2-dimethyl (9).The bridged ions trans-8, cis-8, and 9 are higher in energy than the tertiary ion by 8.3, 9.2, and 10.6 kcal/mol, respectively, at the MP4SDTQ(FC)/6-31G**//MP2(FC)/6-31G** level, or 9.6, 10.6 and 12.4 kcal/mol after correcting for the zero-point energies (ZPE). Various conformations of the “open” 3-methyl-2-butyl cation correspond to transition states for the interconversion of the tertiary ion 2 as and the two 1-protonated-2,3-dimethylcyclopropanes. Hydrogen shift in 2 as can lead only to the cis-2,3-dimethyl isomer, cis-8; trans-8 is formed only from the cis isomer by ring opening and reclosure. The 1,3-protonated-1,2-dimethylcyclopropane (10) (edge-protonated) is the transition state for the interconversion of trans-8, and the third corner-protonated cyclopropane isomer (1-protonated-1,2-dimethylcyclopropane, 9). The latter can be regarded as the cyclic form of the 2-pentyl cation or of the 2-methyl-1-butyl cation. The calculations thus agree with the results of the experimental study on the cations generated in trifluoroacetic by predicting the existence of more than one “2-methylbutyl” cation, but disagree on another point, by predicting that the other cation besides 2 has a symmetrical structure, 8.

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Section: Introductionmentioning

confidence: 53%

Secondary and Tertiary 2-Methylbutyl Cations. 3. Theoretical Study of Structures and Interconversions of C₅H₁₁⁺ Ions in the Gas Phase¹^a

and

1997

J. Org. Chem.

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“…We found, however, that the results of high-level ab initio calculations on cations with an isopentane carbon skeleton and the results of studies conducted in our laboratory on these cations generated solvolytically in trifluoroacetic acid (TFA) disagreed in some key conclusions. An attempt at taking into account the solvent by introducing a dielectric medium in the calculations was able to correct only part of the discrepancy .…”

Section: Introductionmentioning

confidence: 78%

“…Therefore, a complex, rather than monatomic, anion had to be used in the calculation in order to ensure that only a part of the negative charge would directly face the cation. It was important, however, not to use a very stable anion when trying to model the type of interaction occurring in ionization in solvolytic media that provide good anion stabilization but have both low dielectric constants and some nucleophilicity (e.g., TFA). , At the same time, it was necessary that the anion be as small as possible to allow calculations at an adequate level of theory. Thus, our calculations were not meant to describe behavior in superacid solutions, although a comparison of the results with findings about carbocations in such media could indicate whether the cations are ion-paired there.…”

Section: Resultsmentioning

confidence: 99%

Theoretical Studies of Carbocations in Ion Pairs. 1. The 2-Propyl Cation

Fǎrcaşiu

Hâncu

1997

J. Phys. Chem. A

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Ab initio calculations with large basis sets and electron correlation were applied to the study of geometry of the 2-propyl cation in ion pairs with trihydro fluoroborate (A) or dihydrolithiate (B) as anion. The goal was to model the ion pair formed by ionization in a solvent with good anion-stabilizing properties, but of low dielectric constant, like trifluoroacetic acid (TFA). The effect of the anion, seen already at long distances, was that the preferred cation conformation changed from C1,C3 staggered as in the isolated carbocation (chiral 2-propyl cation, 1) to C1,C3 eclipsed (C s symmetry, 2). The optimized cation geometry was essentially the same at the MP2(FC)/6-31G*, MP2(FC)/6-31++G*, and MP2(FU)/6-311G** levels, but the position of the anion above the cation was somewhat more sensitive to the basis set. The preferred anion position was in the plane bisecting the C1−C2−C3 angle of 2 and in the region “inside” that angle. This “inside” displacement became more pronounced as the interionic distance, d, was made shorter; at the same time, the anion moved slightly off the bisecting plane. Elimination within the ion pair to form propene occurred at d < 2.5 Å. When the anion was allowed to “fall” freely, the reaction pathway was determined by the initial position of the anion: elimination for a position “inside” the C1−C2−C3 angle and recombination to 2-fluoropropane (occurring at d = 1.5−1.7 Å) for a position “outside” the C1−C2−C3 angle. The equilibrium cation geometry did not change significantly in 2.B relative to 2.A, but the distortions toward elimination occurred at longer distances for the more basic anion B and elimination itself took place at d = 3.5 Å. The energy difference beween 1.A (optimized with the methyl groups held staggered) and 2.A at d = 3.4 Å was 2.39 and 3.25 kcal/mol at the MP2(FC)/6-31++G* and MP2(FU)/6-311G** levels, respectively. The equilibrium position of the anion paired with 1 was above the C1−C2 bond, close to the syn hydrogen at C1. Thus, methyl rotation along the lowest-energy pathway involves also a movement of the anion relative to the cation.

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Section: Resultsmentioning

confidence: 95%

“…Moreover, the experimental isotopic patterns were in agreement with the theoretical ones, indicating that only the linear oligomers were modified. In this case, reaction with TFA occured specifically on the end-groups, probably on the terminal double bonds according to Fărcaşiu et al [32] Although the TFA modification complicated the mass spectra, it allowed a clear separation of the isotopic patterns of the cyclic and linear oligomers in the mass spectrum without the need for chromatographic separation. Derivatization of PA with TFA is necessary to select independently linear oligomers for tandem mass spectrometry experiments with a low-resolution mass spectrometer.…”

Section: Modification Of Polymers By Tfamentioning

confidence: 95%

Solvent‐based and solvent‐free characterization of low solubility and low molecular weight polyamides by mass spectrometry: a complementary approach

Barrère

Hubert‐Roux

Lange

et al. 2012

Rapid Comm Mass Spectrometry

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Secondary and tertiary 2-methylbutyl cations. 2. Addition of trifluoroacetic acid to methylbutene

Cited by 23 publications

References 0 publications

Secondary and Tertiary 2-Methylbutyl Cations. 3. Theoretical Study of Structures and Interconversions of C₅H₁₁⁺ Ions in the Gas Phase¹^a

Secondary and Tertiary 2-Methylbutyl Cations. 3. Theoretical Study of Structures and Interconversions of C₅H₁₁⁺ Ions in the Gas Phase¹^a

Theoretical Studies of Carbocations in Ion Pairs. 1. The 2-Propyl Cation

Solvent‐based and solvent‐free characterization of low solubility and low molecular weight polyamides by mass spectrometry: a complementary approach

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Secondary and tertiary 2-methylbutyl cations. 2. Addition of trifluoroacetic acid to methylbutene

Cited by 23 publications

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Secondary and Tertiary 2-Methylbutyl Cations. 3. Theoretical Study of Structures and Interconversions of C5H11+ Ions in the Gas Phase1a

Secondary and Tertiary 2-Methylbutyl Cations. 3. Theoretical Study of Structures and Interconversions of C5H11+ Ions in the Gas Phase1a

Theoretical Studies of Carbocations in Ion Pairs. 1. The 2-Propyl Cation

Solvent‐based and solvent‐free characterization of low solubility and low molecular weight polyamides by mass spectrometry: a complementary approach

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Secondary and Tertiary 2-Methylbutyl Cations. 3. Theoretical Study of Structures and Interconversions of C₅H₁₁⁺ Ions in the Gas Phase¹^a

Secondary and Tertiary 2-Methylbutyl Cations. 3. Theoretical Study of Structures and Interconversions of C₅H₁₁⁺ Ions in the Gas Phase¹^a