A comparison between the stereoselective thermal-induced and ionization-induced elimination of acetic acid from 2-butyl acetate

Green, Mark M.; McCluskey, Richard J.; Vogt, Julia E.

doi:10.1021/ja00372a026

Cited by 11 publications

(6 citation statements)

References 1 publication

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“…The elimination of acetic acid represents the predominant dissociation of the molecular ion, representing 75−80% of the fragment ion intensity in the metastable ion decompositions that we observe in the MIKE spectrum. This is homologous to the stereoselective elimination that Green and Vogt examined in detail for sec -butyl acetate, for which they propose a six-member cyclic transition state analogous to thermal syn-elimination from neutral acetates . In the 4-octyl system, however, the elimination is not regioselective.…”

Section: Resultsmentioning

confidence: 77%

Stereochemical Analysis of Deuterated Alkyl Chains by MS/MS

et al. 1999

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Vicinally deuterated sec-alkyl phenyl ethers, CH(3)(CH(2))(m)()CH(OPh)CHD(CH(2))(n)()CH(3), display significant differences in mass spectra between threo and erythro stereoisomers. MS/MS experiments, in which parent ions of a single mass are selected and their fragmentation patterns subsequently measured, show that alkene expulsion represents virtually the only decomposition pathway. Two types of MS/MS experiment are reported: mass-analyzed ion kinetic energy (MIKE) spectroscopy of metastable ions and collisionally activated decomposition (CAD) of stable ions. The expulsion of a deuterated alkene from a monodeuterated precursor yields ionized phenol, PhOH(*)()(+) (m/z 94). The expulsion of an undeuterated alkene yields PhOD(*)()(+) (m/z 95). Without exception, the ratios (PhOD(*)()(+)/PhOH(*)()(+)) from precursors in the threo series have values greater than their diastereomers in the erythro series. The ratio of ratios, r = PhOD(*)()(+)/PhOH(*)()(+) for the threo divided by PhOD(*)()(+)/PhOH(*)()(+) for the erythro, has a value of 1.2 for the 2-phenoxy-3-deuteriobutanes and larger values for all of the higher homologues up through the monodeuterated phenoxyoctanes (m + n = 4). The highest degree of stereoselectivity, r = 5.8, is measured for 3-phenoxy-4-deuteriohexane. Experiments with multiply deuterated analogues show that alkene elimination is highly regioselective, unlike the corresponding decompositions of ionized sec-alcohols or their acetates. The fact that a large fraction of ionized sec-alkyl phenyl ethers undergo stereospecific syn-elimination means that mass spectrometry has a useful capacity to distinguish one isotopically labeled diastereomer from another.

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Section: Resultsmentioning

confidence: 77%

Stereochemical Analysis of Deuterated Alkyl Chains by MS/MS

et al. 1999

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“…2 They also measured a comparable level of stereoselectivity in acetic acid elimination from ionized threo-and erythro-CH 3 CH(OCOCH 3 )CHDCH 3 (d 1 -sec-butyl acetate), 3 in which the stereocenters are adjacent, and subsequently examined the energetic dependence. 4 Ions e0.6 eV above threshold display stereoselectivity that decreases with energy, but in more highly excited ions the selectivity fluctuates (though the average does not vary from what is observed in the source mass spectra). In any event, other decompositions compete with the stereoselective pathways, which limits their utility for analytical purposes.…”

mentioning

confidence: 93%

“…In the fragmentation pattern of threo- and erythro-CH 3 CHClCH 2 CH 2 CHDCH 3 , for instance, they reported ion intensity ratios [M - DCl] •+ /[M - HCl] •+ of 0.36 for the former and 0.26 for the latter . They also measured a comparable level of stereoselectivity in acetic acid elimination from ionized threo- and erythro-CH 3 CH(OCOCH 3 )CHDCH 3 ( d 1 - sec -butyl acetate), in which the stereocenters are adjacent, and subsequently examined the energetic dependence . Ions ≤0.6 eV above threshold display stereoselectivity that decreases with energy, but in more highly excited ions the selectivity fluctuates (though the average does not vary from what is observed in the source mass spectra).…”

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confidence: 97%

Stereochemical Analysis of Monodeuterated Isomers By GC/MS/MS

et al. 1997

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GC/MS/MS techniques to quantitate mixtures of acyclic stereoisomers are explored for a set of secondary n-alkyl phenyl ethers vicinally substituted with deuterium. Examination of a set of test compounds using a commercial quadrupole ion trap instrument (GC/QIT) reveals large differences between threo and erythro diastereomers when trapped molecular ions expel neutral alkene as a result of collisionally activated dissociation (CAD). All of the monodeuterated compounds give PhOH•+ (m/z 94) and PhOD•+ (m/z 95) as the principal fragments under GC/QIT/CAD conditions. The threo isomers give consistently larger values of the ion intensity ratio r = m/z 95/m/z 94 than do the corresponding erythro, with a ratio of ratios r t /r e as high as 3. A nonlinear calibration curve shows that the threo decomposes to a lesser extent than the erythro in mixtures of the two stereoisomers; hence, the precision of quantitation is sensitive to experimental error. Sustained off-resonance irradiation experiments in an FT-ICR give virtually complete dissociation, which results in a linear calibration curve. Ion−molecule reactions of the fragment ions indicate that alkene expulsion does not take place via a six-member cyclic transition state, while the stereochemical selectivity appears to rule out the intermediacy of ion−neutral complexes. Threo- and erythro-2-phenoxy-3-deuterio-n-octane (and their mixtures) are proposed as test compounds for evaluating the capability of GC/MS/MS instrumentation to perform stereochemical analyses on subnanomole samples of isotopically labeled molecules.

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“…Gaseous radical cations show very similar selectivity. The isomer distribution inferred from the expulsion of neutral acetic acid from ionized acetate esters has been assessed by means of deuterium labeling, as depicted in eqs and , and gives comparable ratios, in the range 1.4−1.7:1 (depending on ion internal energies) when deuterium isotope effects are taken into consideration …”

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confidence: 99%

Stereochemical Preferences in 4-Center Syn-Eliminations from Gaseous Ions

et al. 2008

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Concerted unimolecular eliminations from ionized sec-alkyl aryl ethers (ROAr (+*)) display a preference for producing double bonds with trans geometry. This preference can be assessed quantitatively, provided that a regioselective variant can be found. Expulsion of neutral alkenes via syn-elimination to give ionized ArOH does not exhibit a pronounced preference with regard to the direction of elimination. By contrast, ionized 2-hexyl p-trifluoromethylphenyl ether eliminates neutral ArOH regioselectively, giving ionized 2-hexenes rather than ionized 1-hexene. Vicinally monodeuterated 2-hexyl and 3-hexyl ethers were prepared as pure diastereomers. Metastable ion decompositions of their gaseous radical cations are compared over two different time windows. The regioselectivity for the 2-hexyl ether allows the geometric preference for the double bonds to be estimated based on the difference between the erythro and threo monodeuterated diastereomers ( trans/ cis = 2.0 for producing ionized 2-hexene from parent ions with the lowest internal energies). The 3-hexyl ethers and ionized 2- and 3-phenoxyoctanes also undergo stereoselective elimination but give experimental values that reflect their lack of regioselectivity. Examination of erythro/ threo combinations shows that GC/MS/MS has the ability to quantitate stereochemistry in mixtures containing both positional and stereoisomers.

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A comparison between the stereoselective thermal-induced and ionization-induced elimination of acetic acid from 2-butyl acetate

Cited by 11 publications

References 1 publication

Stereochemical Analysis of Deuterated Alkyl Chains by MS/MS

Stereochemical Analysis of Deuterated Alkyl Chains by MS/MS

Stereochemical Analysis of Monodeuterated Isomers By GC/MS/MS

Stereochemical Preferences in 4-Center Syn-Eliminations from Gaseous Ions

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