2-Benzylindane radical cations in the gas phase (Part II): Substituent effects on regio- and stereoselective progressive hydrogen scrambling occurring prior to unimolecular fragmentation

Abstract

In the present and in the preceding article, the unimolecular fragmentation of the radical cations of 2-benzylindane and eleven derivatives bearing meta- or para-substituents at the benzylic moiety have been studied with respect to the hydrogen exchange that precedes their fragmentation by the McLafferty reaction. Extensive regio- and stereospecific deuterium labelling confirmed that the hydrogen exchange involves the two cis-oriented H atoms at the benzylic C-1 and C-3 positions of the indanyl moiety and the two ortho-H atoms of the benzyl group. This 4-H exchange is perfectly stereoselective for the most electron-rich congeners [X=OCH3,N(CH3)2 and 3′,5′-(OCH3)2] but less stereoselective for the other analogs. Energy- and lifetime-dependent measurements with the deuterium-labelled isotopologs revealed that the extent of H/D exchange progresses with the ions’ lifetime much faster in the meta- than in the para-isomers. With exception of the meta-dimethylamino derivative, equilibration (complete scrambling) of the four H atoms is reached in the long-lived, metastable ions, whereas the para-dimethylamino isomer does not undergo any exchange at all. The relative rate of the H/D exchange is rationalized on the basis of the energy profiles calculated for the reversible γ-H transfer that generates the corresponding distonic arenium-ion intermediates and for the overall fragmentation by the McLafferty reaction. The ipso-protonolysis of the benzyl residue giving rise to the loss of the corresponding arenes specifically from some of the para-isomers was also found to reflect the regio- and stereoselective 4-H exchange.