Isomerization of the constituents of ion/neutral complexes during the fragmentation of protonated dialkyl-substituted 1,3-diphenylpropanes

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.

Figure optionsDownload high-quality image (123 K)Download as PowerPoint slideResearch highlights▶ 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