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

Abstract

The fragmentation of gaseous ion/neutral complexes [R +⋯C 6H 5CH 2CH 2CH 2C 6H 4–R′] with (i) R = R′ = C 4H 9, (ii) R = C 4H 9 and R′ = CH 3 and (iii) R = C 6H 11 and R′ = H has been studied by CI(CH 4)-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-C 4H 9-C 6H 4)CH 2CH 2CH 2(C 6H 4-4- tert-C 4H 9) + 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-C 4H 9 groups and requires the presence of the meta-( tert-C 4H 9)-substituted diphenylpropane as the neutral constituent of the eventually fragmenting I/N complex. As a consequence, it appears that the reactive complex [C 4H 9 +⋯C 6H 5CH 2CH 2CH 2(C 6H 4-3- tert-C 4H 9)] is formed prior to the generation of the expected “ para-isomer”, [C 4H 9 +⋯C 6H 5CH 2CH 2CH 2(C 6H 4-4- tert-C 4H 9)]. Isobutyl analogues, such as [(4- iso-C 4H 9-C 6H 4)CH 2CH 2CH 2(C 6H 4-4- iso-C 4H 9) + H] +, do not show evidence for the intermediacy of “isomerized” I/N complexes containing a tert-C 4H 9 + ion. The fragmentation of ion–neutral complexes containing C 6H 11 + ions, formed from the [M+H] + ions of (4-cyclohexyl)- and of 4-(1-methylcyclopentyl)-substituted 1,3-diphenylpropane, indicate that the C 6H 11 + ions only partially retain their structural identity: while the secondary isomer, (CH 2) 5 > CH +, predominantly transfers a proton in competition to hydride abstraction, indicating its stronger Bronsted acidity, the tertiary isomer, (CH 2) 4 > C +CH 3, 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.