Dennis Suh

ClH⋯O and O⋯H⋯O bonded intermediates in the dissociation of low energy methyl glycolate radical cations

Abstract Low energy methyl glycolate radical cations HOCH2C(O)OCH3+, 1, abundantly lose HCO, yielding protonated methyl formate HC(OH)OCH3+. Tandem mass spectrometry based experiments on 2H, 13C and 18O labelled isotopologues show that this loss is largely (about 75%) atom specific. Analysis of the atom connectivity in the product ions indicates that the reaction proceeds analogously to the loss of HCO and CH3CO from ionized acetol HOCH2C(O)CH3+ and acetoin HOCH(CH3)C(O)CH3+, respectively. The mechanism, it is proposed, involves isomerization of 1 to the key intermediate CH2O⋯ HC(O)OCH3+, an H-bridged ion-dipole complex of neutral formaldehyde and ionized methyl formate. Next, charge transfer takes place to produce CH3OC(H)O⋯HC(H)O+, an H-bridged ion-dipole complex of ionized formaldehyde and neutral methyl formate, followed by proton transfer to generate the products. Preliminary ab initio calculations executed at the UMP3/6-31G∗//6-31G∗+ZPVE level of theory are presented in support of this proposal. The non-specific loss of HCO from 1 (about 25%) is rationalized to occur via the same mechanism, but after communication with isomeric dimethyl carbonate ions CH3OC(O)OCH3+, 2, via the O⋯H⋯O bonded intermediate [CH2O⋯H⋯OCOCH3]+. The latter pathway is even more important in the formation of CH2OH+ ions from 1 which, it is shown, is not a simple bond cleavage reaction, but may involve consecutive or concerted losses of CH3 and CO2 from the above O⋯H⋯O bonded species. Ionized methyl lactate HOCH(CH3)C(O)OCH3+, the higher homologue of 1, shows a unimolecular chemistry which is akin to that of 1.

Low energy acetoin ions CH3C(O)C(H)(OH)CH3.+ decompose to CH3CO. and CH3CHOH+ via a remarkable “hidden rearrangement”

Abstract Tandem mass spectrometry based experiments on D and 18 O labelled acetoin isotopomers show that the title reaction is not just a simple bond cleavage, but rather involves rearrangement via ion—dipole complexes; the intermediacy of a charge-transfer complex is proposed to account for the observed product ratios.

The chemistry of ionized ethyl glycolate, HOCH 2 CO 2 C 2 H 5 •+ , and its enol isomer, HOCH=C(OH)OC 2 H 5 •+ . Formation of ionized and neutral trihydroxyethylene

The unimolecular reactions of ionized ethyl glycolate, HOCH2CO2C2H5•+, and its enol isomer, HOCH=C(OH)OC2H5•+, have been studied by a variety of techniques including 2H-, 13C- and 18O-labelling experiments, kinetic energy release information, analysis of collision-induced dissociation and neutralization–reionization mass spectra and thermochemical measurements. The metastable ionized enol eliminates C2H4 essentially exclusively by β-hydrogen transfer to give (HO)2C=CHOH•+, which itself expels H2O with very high selectivity (∼99%). The metastable ionized keto isomer also eliminates C2H4, but minor amounts (∼5%) of competing fragmentations resulting in expulsion of H2O, HOCO• and HCO• are also observed. Moreover, in this case, loss of C2H4 no longer involves specific β-hydrogen transfer. This behavior is interpreted in terms of rearrangement of the ionized keto form via a 1,5-H shift to the distonic ion HOCH2C+(OH)OCH2CH2•, which undergoes a further 1,5-hydrogen shift to form HOCH=C(OH)OC2H5•+, from which C2H4 is lost to give (HO)2C=CHOH•+. The chemistry of these C4H8O3•+ species, in which unidirectional tautomerism of HOCH2CO2C2H5•+ to HOCH=C(OH)OC2H5•+ via two 1,5-H shifts is important, contrasts sharply with the behavior of the lower homologues, HOCH2CO2CH3•+ and HOCH=C(OH)OCH3•+, for which the analogous tautomerism via 1,4-H shifts does not occur. The mechanism for loss of C2H4 from HOCH=C(OH)OC2H5•+ is essentially the same as that observed for ionized phenyl ethyl ether in that the reaction proceeds via an exothermic proton transfer in an ethyl cation–radical complex. Neutralization–reionization experiments show that neutral trihydroxyethylene in the gas phase is a remarkably stable species which does not tautomerize to glycolic acid, HO–CH2–COOH. Using G2(MP2) theory, the enthalpy of formation, ΔHf298, of the neutral molecule (most stable conformer) was found to be −449.4 kJ mol−1 while that of the corresponding ion was calculated to be 299.6 kJ mol−1.

Mechanism of propene and water elimination from the oxonium ion CH3CHO+CH2CH2CH3

The site-selectivity in the hydrogen transfer step(s) which result in propene and water loss from metastable oxonium ions generated as CH3CHO+CH2CH2CH3 have been investigated by deuteriumlabelling 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 CH3CHO+CH2CH2CH3 isomerizes unidirectionally to an isopropyl cation, which then undergoes proton abstraction from either methyl group {CH3CHO+CH2CH2CH3→ CH3CHO+CH2CH2CH3→[CH3CHO +CH(CH3)2]→[CH3CHOH+ CH3CHCH2]}. 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 [CH3CHO +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 CH2O+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.

The mechanism of alkene elimination from the oxonium ions (CH 3 CH 2 ) 2 C=OH + , CH 3 CH 2 CH 2 (CH 3 )C=OH + and (CH 3 CH 2 CH 2 ) 2 C=OH +

The reactions of the metastable oxonium ions (CH3CH2)2C=OH+,CH3CH2CH2(CH3)C=OH+ and (CH3CH2CH2)2C=OH+ have been studied by 13C-labelling experiments. The mechanism of alkene elimination from these oxonium ions is discussed in the light of earlier studies on the behaviour of their lower homologues, (CH3)2C=OH+ and CH3CH2CH=OH+, which eliminate ethylene. Propene loss from (CH3CH2)2C=OH+ must entail skeletal rearrangement leading to CH3CH2CH2(CH3)C=OH+ or related structures. These isomerisation sequences may be formulated in three plausible ways. The first two possibilities involve 1,2-H shifts in conjunction with ring-closures to either protonated oxiranes or oxetanes, followed by ring-opening in the opposite sense, thus breaking the original C–O bond and allowing the hydroxy function to migrate along the carbon chain. Alternatively, a combination of 1,2-H and 1,2-alkyl shifts permits the carbon skeleton to be isomerised via the isomeric oxonium ion, CH3CH2(CH3)CHCH=OH+, without disrupting the C–O bond. Both (CH3CH2)213C=OH+ and CH3CH2CH2(CH3)13C=OH+ lose C3H6 with closely similar high selectivities (88 and 89%, respectively). This observation shows that the first two routes compete very poorly with the third pathway in which rearrangement of the carbon skeleton occurs without migration of the oxygen function. Extension of the mechanistic investigation to include propene and butene expulsion from metastable (CH3CH2CH2)2C=OH+ shows that this preference for retaining the initial C–O connection is general: (CH3CH2CH2)213C=OH+ eliminates C3H6 and C4H8 with extremely high selectivities (∼99%).