- Mechanism of Propene and Water Elimination from the Oxonium Ion CH3CH=O+CH2CH2CH3
-
The site-selectivity in the hydrogen transfer step(s) which result in propene and water loss from metastable oxonium ions generated as CH3CH=O+CH2CH2CH3 have been investigated by deuterium-labelling experiments.Propene elimination proceeds predominantly by transfer of a hydrogen atom from the initial propyl substituent to oxygen.However, the site-selectivity for this process is inconsistent with β-hydrogen transfer involving a four-centre transition state.The preference for apparent α- or γ-hydrogen transfer is interpreted by a mechanism in which the initial propyl cation accessible by stretching the appropriate bond in CH3CH=O+CH2CH2CH3 isomerizes unidirectionally to an isopropyl cation, which then undergoes proton abstraction from either methyl group +CH2CH2CH3 CH3CH=O---+CH2CH2CH3 +CH(CH3)2> + CH3CH=CH2>>.This mechanism involving ion-neutral complexes can be elaborated to accommodate the minor contribution of expulsion of propene containing hydrogen atoms originally located on the two-carbon chain.Water elimination resembles propene loss insofar as there is a strong preference for selecting the hydrogen atoms from the α- and γ-positions of the initial propyl group.The bulk of water loss is explicable by an extension of the mechanism for propene loss, with the result that one hydrogen atom is eventually transferred to oxygen from each of the two methyl groups in the complex +CH(CH3)2>.This site-selectivity is strikingly different from that (almost random participation of the seven hydrogen atoms of the propyl substituent) encountered in the corresponding fragmentation of the lower homologue CH2=O+CH2CH2CH3.This contrast is explained in terms of the differences in the relative energetics and associated rates of the cation rearrangement and hydrogen transfer steps.
- Bowen, Richard D.,Suh, Dennis,Terlouw, Johan K.
-
p. 119 - 130
(2007/10/02)
-
- Unimolecular Reactions of Isolated Organic Ions: Reactions of the Immonium Ions CH2=N+(CH3)CH(CH3)2, CH2=N+(CH3)CH2CH2CH3 and CH2=N+(CH2CH2CH3)2
-
The reactions of metastable CH2=N+(CH3)C3H7 immonium ions have been investigated by means of 2H-labelling experiments and kinetic energy release measurements.Loss of C3H6, with specific β-H transfer, is the sole channel for dissociation of CH2=N+(CH3)CH(CH3)2.This process gives rise to a Gaussian metastable peak.The isomeric ion, CH2=N+(CH3)CH2CH2CH3, also expels C3H6; however, both α-H and γ-H as well as β-H transfer occurs in this case, and the reaction proceeds with an increased kinetic energy release.The role of ion-neutral complexes in C3H6 loss from CH2=N+(CH3)C3H7 ions is discussed.In addition, CH2=N+(CH3)CH2CH2CH3 eliminates C2H4.This fragmentation yields a broad dish-topped metastable peak, corresponding to a very large kinetic energy release (T1/2 ca. 73 kJ mol-1), and it involves specific and unidirectional γ-H transfer.A potential energy profile summarising the reactions of CH2=N+(CH3)CH2CH2CH3 and CH2=N+(CH3)CH(CH3)2 is constructed.The mechanisms by which immonium ions of this general class eliminate C3H6 and C2H4 have been further probed by studying the behaviour of the higher homologue, CH2=N+(CH2CH2CH3)2.The mechanistic conclusions derived from this work are found to be in excellent qualitative agreement with those of previous studies.
- Bowen, Richard D.,Colburn, Alex W.,Derrick, Peter J.
-
p. 2363 - 2372
(2007/10/02)
-
- Unimolecular Reactions of Isolated Organic Ions: the Chemistry of the Oxonium Ions CH3CH2CH2CH2(+)O=CH2 and CH3CH2CH2CH=O(+)CH3
-
The reactions of the metastable oxonium ions CH3CH2CH2CH2(+)O=CH2 and CH3CH2CH2CH=O(+)CH3 are reported and discussed.Both these isomers of C5H11O(+) expel predominantly CH2O (75 - 90percent of the metastable ion current), a moderate amount of C3H6 (5-15percent), a minor amount of CH3OH (2-8percent) and a very small proportion of H2O (0.5-3percent).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 confirms that the reactions of CH3CH2CH2CH2(+)O=CH2 and 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 CH3CH2CH2CH2(+)O=CH2 by a 1,5-H shift and to CH3CH2CH2CH=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 CH3CH2CH2CH2(+)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.
- Bowen, Richard D.,Derrick, Peter J.
-
p. 1197 - 1209
(2007/10/02)
-
- The Mechanism of Ethylene Elimination from the Oxonium Ions CH3CH2CH=O+CH2CH3 and (CH3)2C=O+CH2CH3
-
The reactions of the metastable oxonium ions CH3CH2CH=O+CH2CH3 and (CH3)2C=O+CH2CH3 are reported and discussed.Various mechanisms for ethylene elimination, which is the principal dissociation route for these ions, are considered.It is shown by means of 2H-labelling experiments and analysis of collision-induced dissociation spectra that routes involving ion-neutral complexes pre-empt 'conventional' mechanisms for these processes.In contrast, the behaviour of the lower homologues CH3CH2CH=OR+ and (CH3)2C=OR+ (R = H, CH3) is consistent with the operation of 'conventional' mechanisms for ethylene expulsion.This contrast is interpreted in energetic terms.The significance of these results for the chemistry of homologous and analogous 'onium' ions containing a Z+-R function (Z = O, S, NH, NCH3; R= CnH2n+1, n 2) is explained.
- Bowen, Richard D.,Derrick, Peter J.
-
p. 1033 - 1039
(2007/10/02)
-
- Alkene Loss from Metastable Methyleneimmonium Ions: Unusual Inverse Secondary Isotope Effect in Ion-Neutral Complex Intermediate Fragmentations
-
The mechanism of propene elimination from metastable methyleneimmonium ions is discussed.The first field-free region fragmentations of complete sets of isotopically labelled methyleneimmonium ions (H2C=N+R1R2 : R1 = R2 = n-C3H7; R1 = R2 = i-C3H7; R1 = n-C3H7; R2 = C2H5; R1 = n-C3H7; R2 = CH3; R1 = n-C3H7; R2 = H) were used to support the mechanism presented.The relative amounts of H/D transferred are quantitatively correlated to two distinct mathematical concepts which allow information to be deduced about influences on reaction pathways that cannot be measured directly.Propene loss from the ions examined proceeds via ion-neutral complex intermediates.For the di-n-propyl species rate-determining and H/D distribution-determining steps are clearly distinct.Whereas the former corresponds to a 1,2-hydride shift in a 1-propyl cation coordinated to an imine moiety, the latter is equivalent to a proton transfer to the imine occurring from the 2-propyl cation generated by the previous step.For the diisopropyl-substituted ions which directly form the 2-propyl cation-containing complex, the rate-determining hydride shift vanishes.The 2-propyl cation-containing complex can decompose directly or via an intermediate proton-bridged complex.Competition of these routes is not excluded by the experimental results.Assuming a 2:1:3 distribution, a preference for the α- and β-methylene of the initial n-propyl chain as the source of the hydrogen transferred is detected for n-propylimmonium ions containing a second alkyl chain R2.This preference shows a clear dependence on the steric influence of R2.During the transfer step isotopic substitution is found to affect the H/D distribution strongly.For the alternative route of McLafferty rearrangement leading to C2H4 loss, specific γ-H transfer is observed.
- Veith, Hans J.,Gross, Juergen H.
-
p. 1097 - 1108
(2007/10/02)
-