Site-selective deuterium labeling of the tetrabutylammonium cation
摘要:
Four separate selectively deuterated samples of tetrabutylammonium iodide have been prepared in which each one of the four nonequivalent alkyl carbons is separately and fully deuterated. These samples were prepared for nuclear magnetic resonance (NMR) studies of the aggregation of ion pairs in low polarity solvents.
AbstractThe reactions of ionized di‐n‐butyl ether are reported and compared with those of ionized n‐butyl sec‐butyl and di‐sec‐butyl ether. The main fragmentation of metastable (CH3CH2CH2CH2)2O+. is C2H5⋅ loss (˜85%), but minor amounts (2–4%) of CH3⋅, C4H7⋅, C4H9⋅, C4H10 and C4H10O are also eliminated. In contrast, C2H5⋅ elimination is of much lower abundance (20 and 4%, respectively) from metastable CH3CH2CH2CH2OCH(CH3)CH2CH3+. and [CH3CH2(CH3)CH]2O+., which expel mainly C2H6 and CH3⋅ (35–55%). Studies on collisional activation spectra of the C6H13O+ oxonium ions reveal that C2H5⋅ loss from (CH3CH2CH2CH2)2O+. gives the same product, (CH3CH2CH2CH2+OCHCH3) as that formed by direct cleavage of CH3CH2CH2CH2OCH(CH3)CH2CH3+.. Elimination of C2H5⋅ from (CH3CH2CH2CH2)2O+. is interpreted by means of a mechanism in which a 1,4‐H shift to the oxygen atom initiates a unidirectional skeletal rearrangement to CH3CH2CH2CH2OCH(CH3)CH2CH3+., which then undergoes cleavage to CH3CH2CH2CH2+OCHCH3 and C2H5⋅. Further support for this mechanism is obtained from considering the collisional activation and neutralization‐reionization mass spectra of the (C4H9)2O+. species and the behaviour of labelled analogues of (CH3CH2CH2CH2)2O+.. The rate of ethyl radical loss is suppressed relative to those of alternative dissociations by deuteriation at the γ‐position of either or both butyl substituents. Moreover, C2H5⋅ loss via skeletal rearrangement and fragmentation of the unlabelled butyl group in CH3CH2CH2CH2OCH2CH2CD2CH3+. occurs approximately five times more rapidly than C2H4D⋅ expulsion via isomerization and fission of the labelled butyl substituent. These findings indicate that the initial 1,4‐hydrogen shift is influenced by a significant isotope effect, as would be expected if this step is rate limiting in ethyl radical loss.
Unimolecular reactions of isolated organic ions: The chemistry of the oxonium ions CH3CH2CH2CH2+O = CH2 and CH3CH2CH2CH = O+CH3
作者:Richard D. Bowen、Peter J. Derrick
DOI:10.1002/oms.1210281035
日期:1993.10
AbstractThe reactions of the metastable oxonium ions CH3CH2CH2CH2+O = CH2 and CH3CH2CH2 = O+ CH3 are reported and discussed. Both these isomers of C5H11O+ expel predominantly CH2O (75–90% of the metastable ion current), a moderate amount of C3H6 (5–15%), a minor amount of CH3OH (2–8%) and a very small proportion of H2O (0.5–3%). All these processes give rise to Gaussian metastable peaks. The kinetic energy releases associated with fragmentation of these oxonium ions are similar, but slightly larger for dissociation of CH3CH2CH2CH = O+CH3. The behaviour of labelled analogues confirm that the reactions of CH3CH2CH2CH = O+CH3 are closely related, but subtly different. Elimination of CH2O and C3H6 is intelligible by means of mechanisms involving CH3CH+CH2CH2OCH3. This open‐chain cation is accessible to CH3CH2CH2+O = CH2 by a 1,5‐H shift and to CH3CH2CH2‐CH = O+CH3 by two consecutive 1,2‐H shifts (or, possibly, a direct 1,3‐H shift). The rates of these 1,2‐, 1,3‐ and 1,5‐H shifts are compared with one another and also with the rates of CH2O and C3H6 loss from each of the two oxonium ions. The 1,5‐H shift that converts CH3CH+CH2CH2OCH3 formed from CH3CH2CH2CH = O+ CH3 into CH3CH2CH2+O = CH2 prior to CH2O elimination is essentially unidirectional. In contrast, the corresponding step converting C5H11O+ ions generated as CH3CH2CH2CH2+O = CH2 into CH3CH+ CH2CH2OCH3 competes effectively with expulsion of CH2O and C3H6. The implications of the latter finding for the degree of concert in the hydrogen transfer and carbon‐carbon bond fission steps in alkene losses from oxonium ions via routes that are formally isoelectronic with the retro ‘ene’ pericyclic process are emphasized.
Site-selective deuterium labeling of the tetrabutylammonium cation
Four separate selectively deuterated samples of tetrabutylammonium iodide have been prepared in which each one of the four nonequivalent alkyl carbons is separately and fully deuterated. These samples were prepared for nuclear magnetic resonance (NMR) studies of the aggregation of ion pairs in low polarity solvents.
Noncompeting metastable losses of methyl and ethylene from gaseous butanoic acid ions due to isomerization prior to methyl loss