Showing papers by "Asher Mandelbaum published in 1999"
TL;DR: The results show that the isobutane-CI protonation of the amino ethers and amino alcohols is a kinetically controlled process, occurring competitively at both basic sites of the molecules, despite the large difference between their proton affinities.
Abstract: The isobutane chemical ionization (CI) mass spectra of cis- and trans-1-butyl-3- and -4-dimethylaminocyclohexanols and of their methyl ethers exhibit abundant [MH - H(2)O](+) and [MH - MeOH](+) ions respectively. On the other hand, only the MH(+) ions of the cis-isomers exhibit significant [MH - H(2)O](+) and [MH - MeOH](+) ions under collision-induced dissociation (CID) conditions. The non-occurrence of water and methanol elimination in the CID spectra of the trans-isomers indicates retention of the external proton at the dimethylamino group in the MH(+) ions that survive after leaving the ion source and the first quadrupole of the triple-stage quadrupole ion separating system, and the trans-orientation of the two basic sites does not allow proton transfer from the dimethylamino group to the hydroxyl or methoxyl. Such transfer is allowed in the cis-amino alcohols and amino ethers via internal hydrogen-bonded (proton-bridged) structures, resulting in the elimination of water and methanol from the surviving MH(+) ions of these particular stereoisomers upon CID. The abundant [MH - ROH](+) ions in the isobutane-CI mass spectra of the trans-isomers indicates protonation at both basic sites, affording two isomeric MH(+) ions in each case, one protonated at the dimethylamino group and the other at the less basic oxygen function. These results show that the isobutane-CI protonation of the amino ethers and amino alcohols is a kinetically controlled process, occurring competitively at both basic sites of the molecules, despite the large difference between their proton affinities ( approximately 25 and approximately 35 kcal mol(-1); 1 kcal = 4.184 kJ). Copyright 1999 John Wiley & Sons, Ltd.
25 citations
TL;DR: In this article, it was shown that a variety of benzyl diethers undergo a unique rearrangement yielding relatively abundant m/z 181 C14H13+ ions, both in the ion source and under collision-induced dissociation (CID) conditions.
Abstract: Protonated molecules of a variety of benzyl diethers, produced by chemical ionization (CI), undergo a unique rearrangement yielding relatively abundant m/z 181 C14H13+ ions, both in the ion source and under collision-induced dissociation (CID) conditions. This highly general rearrangement involves an intramolecular C–C bond formation between the two benzyl groups, and the resulting C14H13+ ions have been shown by the analysis of their CID spectra to be an almost equimolar mixture of isomeric α-o-tolylbenzyl, α-p-tolylbenzyl and p-benzylbenzyl cation structures in all cases. This structural information suggests that this process may be viewed as gas-phase aromatic substitution of the non-charged benzyloxy group by the benzyl cation originating from the protonated ether function involving a series of π- (and/or ion–neutral) and σ-complexes. The extent of this fragmentation in alkane benzyl diethers PhCH2O(CH2)nOCH2Ph (n = 2–10,12) is strongly affected by the alkane chain length. Stereoisomeric benzyl diethers display an unusual steric effect: the trans-isomers give rise to more abundant C14H13+ ions than their cis-counterparts. The latter two effects are explained in terms of intramolecular hydrogen bonding between the two alkoxy groups. Bis(benzyloxy)benzenes and -naphthalenes exhibit very low abundance C14H13+ ions in contrast to the aliphatic analogues. This behavior is attributed to competing intramolecular benzylation involving the aromatic skeletons of these compounds.
16 citations
TL;DR: In this article, it was shown that the ion abundance ratio decreases with the bulkiness of the N-substituents, indicating lower rates of protonation at the amino group when the approach of the protonating reagent (C4H9 + ion in our measurements) to the nitrogen atom is increasingly hindered by the Nsubstitutes.
Abstract: tially protonated at one of the two sites. The latter ion abundance ratio decreases with the bulkiness of the N-substituents (by a factor of more than 10 between N,N-dimethyl- and N,N-diisopropyl-derivatives), indicating lower rates of protonation at the amino group when the approach of the protonating reagent (C4H9 + ion in our measurements) to the nitrogen atom is increasingly hindered by the N-substituents. Another effect of steric hindrance in the CI process involves enhanced formation of the molecular radical cations M + (presumably by a charge exchange mechanism), in competition with the usual protonation, in aminoethers with bulky N-substituents.
13 citations
TL;DR: In this article, the authors examined the mechanistic pathways of alcohol elimination in analogous stereoisomeric protonated mixed ethyl-methyl 3,6-diethers by an extensive deuterium labeling and collision-induced dissociation (CID) study.
Abstract: It has recently been shown that in the 3-methoxytricyclo[6.2.2.0 2,7 ]dodeca-9-ene system the 2,3-cis-isomer (endo-1) undergoes a unique multi-step methanol elimination under chemical ionization (CI) and collision-induced dissociation (CID) conditions, involving a 1,4-migration of a methoxy group from position 3 to 10, which is possible only in that particular stereoisomer. The epimeric 2,3-trans-stereoisomer (exo-1) and saturated analogues undergo elimination of MeOH by different pathways. In the present work the mechanistic pathways of alcohol elimination were examined in analogous stereoisomeric protonated mixed ethyl-methyl 3,6-diethers (2) by an extensive deuterium labeling and CID study. The cis-endo-isomer endo-2 with both endo-alkoxy groups undergoes methanol and ethanol elimination partly by the multi-step pathway, involving the 1,4-migration of an alkoxy group from position 3 or 6 to 10 or 9, respectively, under both CI and CID conditions. In the two trans-diethers, the elimination of alcohol involving the endo-alkoxyl occurs mainly via the multi-step pathway involving the 1,4-migration, whereas that involving the exo-alkoxy group takes place by different routes, which have been also investigated. The results of this work show the diversity and often complexity of mechanistic pathways of alcohol elimination from protonated cyclic secondary ethers, where the simple C-O bond cleavage leading to a secondary cation is a relatively high-energy process.
4 citations
TL;DR: In this article, a deuterium labeling study showed that the hydrogen atom involved in this elimination process originates at position 3 (formal 1,1-elimination) followed by a proton transfer from the methoxy group to the double bond, initiated by a 1,4methoxyl migration from C-3 to C-10.
Abstract: The eilmination of methanol from the MD + ion of 2,3-cis-3-methoxytricyclo[6220 2,7 ]dodeca-9-ene, endo-2, upon chemical ionization (CI) gives rise to both [MD - MeOD] + and [MD - MeOH] + ions Only the [MD - MeOH] + ion is formed under collision-induced dissociation (CID) conditions This is in contrast with the behavior of the 2,3-trans-stereoisomer exo-2 and with saturated analogs which undergo exclusive elimination of MeOD The unusual elimination of MeOH from endo-2 indicates transfer of the external deuteron in the MD + ion from the oxygen atom to the interior of the organic moiety and a back transfer of a hydrogen from the organic moiety to the oxygen atom prior to the C-O bond dissociation step A deuterium labeling study showed that the hydrogen atom involved in this elimination process originates at position 3 (formal 1,1-elimination) These results suggest a multi-step mechanism for this unique stereospecific methanol elimination, initiated by a proton transfer from the methoxy group to the double bond followed by a 1,4-methoxyl migration from C-3 to C-10 The proposed mechanism finds support in the Cl and CID study of deuterium-labeled analogs which have a methoxy group at position 10
3 citations
01 Jan 1999
TL;DR: The use of mass spectrometric techniques to distinguish and characterize isomeric substances is surveyed in this paper, where many examples of such techniques are provided. And the special case of recognition of enantiomers is also covered.
Abstract: The use of mass spectrometric techniques to distinguish and characterize isomeric substances is surveyed. Mass spectral differences between isomers can arise because of differences in ionization, ion kinetics, hydrogen transfer and other reactions, anchimeric assistance and stereo-electronic effects. The special case of recognition of enantiomers is also covered. Many examples are provided.