Fast interannular proton transfer in gaseous protonated α,ω-diphenylalkanes: stereocontrol by the cyclohexane-1,4-diyl unit

Abstract

The fast ring-to-ring proton transfer in protonated 1,4-bis(ω-phenylalkyl)cyclohexanes [(5a)+ H]⁺, [(5b+ H)]⁺, and [(6)+ H]⁺ has been studied by chemical ionization mass spectrometry. The cyclohexane-1,4-diyl unit exerts a distinct stereocontrol upon the rate of proton exchange. Complete proton equilibration is found for protonated cis-dibenzylcyclohexane [(5a)+ H]⁺, whereas in the trans isomer [(5b)+ H]⁺ the interannular proton transfer is strongly decelerated. The trans-bis(β- phenylethyl) homologue [(6)+ H]⁺ is found to represent an intermediate case. By comparison with simpler protonated diphenylalkanes the rate of the interannular proton transfer for metastable ions [(5b)+ H]⁺ and [(6)+ H]⁺ is estimated to lie within the limits 5 × 10⁵ and 10⁶s^–1.