Isomeric Alkyl Cation /Arene Complexes in the Gas Phase Antonello Filippi,* [a] Graziella Roselli, [b] Gabriele Renzi, [b] FeliceGrandinetti, [c] and Maurizio Speranza [a] Abstract: The kinetics and the stereo- chemistry of the protonation-induced unimolecular isomerization of (S)-()- 1-D 1 -3-(p-tolyl)butane have been inves- tigated in the gas phase in the 100± 160 8C range. The process leads to the almost exclusive formation of the rele- vant meta isomer with complete race- mization and partial 1,2-H shift in the migrating sec-butyl group. These results, together with the relevant activation parameters, point to the occurrence of low-energy, tightly bound isomeric sec- butyl cation/toluene complexes of de- fined structure and stability along the isomerization coordinate. The existence and the h 1 -type structure of these low- energy intermediate species are con- firmed by ab initio calculations on closely related systems at the MP2(full)/6-311G**//HF/6-31  G** level of theory. Their role in the relevant energy surface clearly emerges from the comparison of the present results with those concerning sec-butylation of tol- uene carried out under comparable ex- perimental conditions. Keywords: arenium ion isomeriza- tion ¥ chirality ¥ electrophilic substitution ¥ gas-phase reactions ¥ kinetics Introduction Noncovalent ion-neutral complexes (INC), first postulated in the study of bimolecular reactions, are widely recognized as intermediates in unimolecular isomerization and fragmenta- tion of excited ions. [1±3] Their occurrence in electrophilic aromatic substitutions is nowadays supported by a variety of refined theoretical, spectroscopic, and mass spectrometric techniques, which provided precious information on their lifetime. [4±7] However, other INC key features, such as the mutual orientation of their components and the nature of the forces holding them together, usually escape precise deter- mination because of intrinsic limitations of the available experimental methodologies. [8] This lack of information is particularly unsatisfactory since, in principle, the nature and the dynamics of the INC components may control their evolution to products and, thus, determine the selectivity of the process. This paper is aimed at providing a novel interpretation of the factors governing the positional selec- tivity in a representative gas-phase electrophilic aromatic substitution, that is the sec-butylation of toluene, based on a careful investigation of the nature and the dynamics of the INC involved. Some insights into the nature and the relative stability of the noncovalent INC structures involved in these reactions were given by Audier and co-workers who calculated the potential energy profiles (PES) of several arene alkylations in the gas phase at the HF/6-31G**//HF/3-21G level of theory. [6] According to these calculations, the INC involved in these reactions correspond to energy minima with a loose p- structure, where the alkyl cation and the aromatic ring lie in quasi-parallel planes and the formally charged carbon is centered above the ring centroid (the ™a-complex∫ in Fig- ure 1). The stabilization energies of these a-complexes relative to the separated components never exceed 13± 15 kcalmol 1 . The relative rotation of the two components of the complex is partially hindered by activation barriers from about 3 (sec-alkyl cation flipping over the arene ring) to about 10 kcalmol 1 (the arene ring flipping over the sec-alkyl cation). Instead, the INC involved in the fragmentation of the s-bonded alkylarenium intermediates (e.g. I) to the relevant arenium ion/alkene pair corresponds to an energy minimum with an even looser p-structure held together by a C  -H ¥¥¥ p interaction between the arenium ion and the double bond of the alkene moiety (the ™b-complex∫ in Figure 1). No precise information about the relative stability of the a- and b- complexes conceivably involved in the I fragmentation was provided. Nevertheless, following Audier×s indications, [6] the two complexes can be considered as almost equally stable. [a] Dr. A. Filippi, Prof. M. Speranza Facolta ¡ di Farmacia, Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive (No. 64) Universita ¡ degli Studi di Roma ™La Sapienza∫ P.le A. Moro 5, 00185 Roma (Italy) Fax: ()0649913602 E-mail: antonello.filippi@uniroma1.it [b] Dr. G. Roselli, Prof. G. Renzi Dipartimento di Scienze Chimiche Universita ¡ degli Studi di Camerino V. S. Agostino 1, 62032 Camerino (Mc) (Italy) [c] Prof. F. Grandinetti Dipartimento di Scienze Ambientali and Instituto Nazionale di Fisica della Materia (INFM) Universita ¡ della Tuscia Largo dell'Universita ¡, 01100 Viterbo (Italy) FULL PAPER ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/chem.200204281 Chem. Eur. J. 2003, 9, 2072±2078 2072