Add time:08/15/2019         Source:sciencedirect.com

The fragmentation of gaseous ion/neutral complexes [R+⋯C6H5CH2CH2CH2C6H4–R′] with (i) R = R′ = C4H9, (ii) R = C4H9 and R′ = CH3 and (iii) R = C6H11 and R′ = H has been studied by CI(CH4)-MIKE spectrometry of the corresponding alkyl-substituted 1,3-diphenylpropanes. Different from all other isomers containing two para-alkyl substituents, the [M+H]+ ion generated from the symmetrical ion [(4-tert-C4H9-C6H4)CH2CH2CH2(C6H4-4-tert-C4H9) + H]+ shows the characteristic fragmentation pattern of ion–neutral complexes containing a meta-alkyl-substituted 1,3-diphenylpropane. This indicates a proton-induced 1,2-shift of one or even both of the tert-C4H9 groups and requires the presence of the meta-(tert-C4H9)-substituted diphenylpropane as the neutral constituent of the eventually fragmenting I/N complex. As a consequence, it appears that the reactive complex [C4H9+⋯C6H5CH2CH2CH2(C6H4-3-tert-C4H9)] is formed prior to the generation of the expected “para-isomer”, [C4H9+⋯C6H5CH2CH2CH2(C6H4-4-tert-C4H9)]. Isobutyl analogues, such as [(4-iso-C4H9-C6H4)CH2CH2CH2(C6H4-4-iso-C4H9) + H]+, do not show evidence for the intermediacy of “isomerized” I/N complexes containing a tert-C4H9+ ion. The fragmentation of ion–neutral complexes containing C6H11+ ions, formed from the [M+H]+ ions of (4-cyclohexyl)- and of 4-(1-methylcyclopentyl)-substituted 1,3-diphenylpropane, indicate that the C6H11+ ions only partially retain their structural identity: while the secondary isomer, (CH2)5 > CH+, predominantly transfers a proton in competition to hydride abstraction, indicating its stronger Bronsted acidity, the tertiary isomer, (CH2)4 > C+CH3, mainly reacts by hydride abstraction. In spite of the partial isomerization, deuterium labelling experiments corroborate the usual regioselectivity of the hydride abstraction from the benzylic methylene groups in both cases.

▶ Isomerization of the neutral constituent of ion/neutral complexes ▶ Isomerization of the ionic constituent of ion/neutral complexes ▶ H+-induced 1,2-t-C4H9 shift affects the intra-complex reactivity of the neutral ▶ Cyclohexyl vs. methylcyclopentyl cations as constituents of I/N complexes ▶ Designed synthesis enables the generation of “tailored” I/N complexes

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