Regioselective Hydride Abstraction and Proton Transfer in Gaseous Ion/Molecule Complexes: Methyl Substituent Effects on the Fragmentation of Protonated 1-(4- tert -butylphenyl)-3-phenylpropanes

Abstract

Gas-phase protonolysis of 1-(4-tert-butylphenyl)-3-phenylpropanes bearing a methyl substituent at one of the arene rings gives rise to competing losses of isobutane and isobutene via intermediate, purely hydrocarbon, ion/molecule complexes [Me₃C⁺C₆H₅–CH₂CH₂CH₂–C₆H₄CH₃]. The hydride transfer within the complexes occurs preferentially from the CH₂ group of the methylated benzyl unit (in the order p-CH₃ > m-CH₃ > o-CH₃ ≈ H) and irrespective of the ring from which the tert-butyl group has been released originally. The reciprocal proton transfer gains importance with increasing proton affinity of the substituted benzene nucleus (p-CH₃ < o-CH₃ < m-CH₃), again independent of the original substitution pattern. Thus, the reactivity of the tert-butyl cation within the complex (as a Lewis and a Brønsted acid) is governed by the whole of the electrostatically bound 1,3-diarylpropane neutral.

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