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

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 2 H and 13 C. 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 CH 3 . and C 2 H 4 . 1,3‐Indandione and 3‐methylenephthalide ions with identical alkylidene moieties (i.e. 1 +. ⇌3 +. and 2 +. ⇌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 +. ⇌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 +. ⇌2 +. of the diketone ions but rather from the n ‐propylidenephthalide ions 4 +. . This follows from CID‐MIKE spectrometry of the [M CH 3 ] + ions of 1–4 and two reference C 10 H 7 O 2 + ( m/z 159) ions of authentic structures (protonated 2‐methylene‐1,3‐indandione and protonated 1,4‐naphthoquinone). The characteristic CID fragmentation of the C 10 H 7 O 2 + ions is rationalized. Finally, the multistep isomerization of ionized 1,3‐indandiones apparently also extends to higher homologues [e.g. 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’.