Proton-induced reactions of [2.2]-paracyclophane in the gas phase. A study by FT-ICR-spectrometry and DFT calculation

Abstract

The reactions of the [2-2]-paracyclophane 1 and the [2-2]-metaparacyclopane 2 in the gas phase after protonation by CI(CH₄) or CI(I-C₄H₁₀) were studied by FT-ICR mass spectrometry. The ions C₁₆H₁₇⁺ produced in the external ion source of the FT-ICR instrument were transferred into the ICR cell containing the neutral reactant, and the reactions were analyzed measuring the efficiency of the transfer of a proton to a series of bases with known proton affinity and gas phase basicity as well as the efficiency of the ion-molecule reaction with ethyl vinyl ether. Both reaction types show that the ions C₁₆H₁₇⁺ produced by chemical ionization (CI) consist of two sets of isomeric ions A and B which exhibit distinctly different behavior on deprotonation and of the reaction with ethyl methyl ether. Isomer(s) A (about 65% of the ion population) react efficiently with this vinyl ether by an addition/elimination process typical of primary and secondary benzylic carbenium ions, while isomer B (about 35% of the ion population) undergoes only an ineffective deprotonation by the vinyl ether. By bracketing deprotonation, it is shown that A is actually composed of two isomers A₁ and A₂ with slightly different proton affinity and gas phase basicity. These two ions have been identified using CA-mass spectrometry as protonated 3-phenethylstyrene (A₁) and protonated 4-phenethylstyrene (A₂). The CA-mass spectrum of the isomer B indicated that these ions C₁₆H₁₇⁺ correspond to protonated 1-(ethyl phenyl)-1-phenyl-ethene. This agrees with the rather strong basicity of the conjugated base of ions B, which results in a slow deprotonation. A protonated 1-(ethyl phenyl)-1-phenyl-ethene can arise from a protonated 2-phenethylstyrene by H- and subsequent phenyl shifts, but requires the preceding rearrangement of the protonated [2.2]-paracyclophane into the protonated isomer “[2.2]-orthocyclophane” – the 1,5-dibenzocyclooctadiene. The possibility of such a deep-sited rearrangement was studied by the computation of the relevant reaction routes applying DFT-methods at the level B3LYP/6-311+g(3d,2p)//B3LYP/3-21g) to analyze the reaction mechanisms.

Keywords

Cyclophanes chemical ionization mass spectrometry FT-ICR fragmentation DFT computations

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The basis set B3LYP/3-21g, which has been used for the optimization of the ion structures is quite small. However, we are only interested in the energy of isomeric ions of a similar structure. We have tested tne computation using the commonly used basis set B3LY/6-31+g with some ions, and the difference of the values of the relative of )H0 were found below ±5 kJ/mol.

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We calculated also a dissociation route of 5iH+, where the initially arising primary carbenium ion stabilizes by a H-abstraction from the phenethyl group in the orthoposition. This would generate the protonated stilbene 7 directly, but the activation enthalpy of 91 kJ/mol of this process is markedly higher than the dissociation to 6c.