Mass spectrometric fragmentation of isomeric 2-alkyl-substituted 1,3-indandiones and 3-alkylidenephthalides: a seven-step consecutive isomerization of regular and distonic molecular radical cations

Kuck D (1994)
Organic Mass Spectrometry 29(3): 113-125.

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Abstract
The electron impact-induced fragmentation of 2,2-dimethyl- and 2-ethyl-1,3-indandione, 1 and 2, and their isomers, 3-isopropylidene- and 3-propylidenephthalide, 3 and 4, respectively, was studied in detail by mass-analysed ion kinetic energy (MIKE) and collision-induced dissociation (CID-MIKE) spectrometry, including H-2 and C-13-labelled analogues of 1 and 2. In all regimes of internal energy, the molecular ions 1+. - 4+. interconvert by up to seven consecutive, reversible isomerization steps prior to the main fragmentation processes, viz. loss of CH3. and C2H4 . 1,3-Indandione and 3-methylenephthalide ions with identical alkylidene moieties (i.e. 1+. half arrow right over half arrow left 3+. and 2+. half arrow right over half arrow left 4+.) equilibrate rapidly and completely prior to fragmentation, whereas these pairs of isomers interconvert only slowly via a five-step rearrangement of the indandione ions 1+. half arrow right half arrow left 2+.. Distinct from the behaviour of simpler ionized carbonyl species, a 1,2-C shift of a (formally) neutral carbonyl group is found to occur along with that of a protonated one. Also distinct from simpler cases, methyl loss does not take place from the ionized enol intermediates formed within the interconversion 1+. half arrow right over half arrow left 2+. of the diketone ions but rather from the n-propylidenephthalide ions 4+.. This follows from CID-MIKE spectrometry of the [M - CH3]+ ions of 1-4 and two reference C10H7O2+ (m/z 159) ions of authentic structures (protonated 2-methylene-1,3-indandione and protonated 1,4-naphthoquinone). The characteristic CID fragmentation of the C10H7O2+ ions is rationalized. Finally, the multistep isomerization of ionized 1,3-indandiones apparently also extends to higher homologues [eg. 5+. from 2-ethyl-2-methyl-1,3-indandione (5) and 6+. from 2,2-diethyl-1,3-indandione (6)]: the ionized phthaloyl group of 1,3-indandione radical cations 1+., 2+., 5+. and 6+., originally attached with its two acyl functionalities to the same carbon of the aliphatic chain, performs, in fact, a 'multi-step migration'.
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Kuck D. Mass spectrometric fragmentation of isomeric 2-alkyl-substituted 1,3-indandiones and 3-alkylidenephthalides: a seven-step consecutive isomerization of regular and distonic molecular radical cations. Organic Mass Spectrometry. 1994;29(3):113-125.
Kuck, D. (1994). Mass spectrometric fragmentation of isomeric 2-alkyl-substituted 1,3-indandiones and 3-alkylidenephthalides: a seven-step consecutive isomerization of regular and distonic molecular radical cations. Organic Mass Spectrometry, 29(3), 113-125.
Kuck, D. (1994). Mass spectrometric fragmentation of isomeric 2-alkyl-substituted 1,3-indandiones and 3-alkylidenephthalides: a seven-step consecutive isomerization of regular and distonic molecular radical cations. Organic Mass Spectrometry 29, 113-125.
Kuck, D., 1994. Mass spectrometric fragmentation of isomeric 2-alkyl-substituted 1,3-indandiones and 3-alkylidenephthalides: a seven-step consecutive isomerization of regular and distonic molecular radical cations. Organic Mass Spectrometry, 29(3), p 113-125.
D. Kuck, “Mass spectrometric fragmentation of isomeric 2-alkyl-substituted 1,3-indandiones and 3-alkylidenephthalides: a seven-step consecutive isomerization of regular and distonic molecular radical cations”, Organic Mass Spectrometry, vol. 29, 1994, pp. 113-125.
Kuck, D.: Mass spectrometric fragmentation of isomeric 2-alkyl-substituted 1,3-indandiones and 3-alkylidenephthalides: a seven-step consecutive isomerization of regular and distonic molecular radical cations. Organic Mass Spectrometry. 29, 113-125 (1994).
Kuck, Dietmar. “Mass spectrometric fragmentation of isomeric 2-alkyl-substituted 1,3-indandiones and 3-alkylidenephthalides: a seven-step consecutive isomerization of regular and distonic molecular radical cations”. Organic Mass Spectrometry 29.3 (1994): 113-125.
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