Structure and Reactivity of Aromatic o-Complexes (Cyclohexadienylium Ions): A Correlated Experimental and Theoretical Study

Steric and electronic properties for a series of u-complexes have been examined by experimental and theoretical techniques. Crystal structures for three 2,4,6-tripyrrolidinocyclohexadienylium salts 3a, 3b, and 4a and for l-methyl-2,4,6tripyrrolidinobenzene 5 are reported. With respect to the cyclohexadienylium ring, the H,H u-complexes, 3, display a planar conformation whereas u-complexes 4 and 6 are bent. Steric interactions force the larger substituents of the tetrahedral carbon atom in 4 and 6 into the pseudoaxial position. The results of EH, MNDO, and 3-21G calculations are in agreement with the crystal structure determinations for 0-complexes 3,4, and 6. Calculations performed for other u-complexes indicate that the ring conformation for a-complexes with small substituents in the 2and 6-positions should be planar even in cases with different substituents on the tetrahedral carbon atom; with larger substituents, a bent conformation is favored with the larger substituent a t C1 axial. The activation energy for planarization of the bent structure is fairly high, and it is even higher for inversion. For stereoelectronic reasons, only the ligand in the axial position of bent u-complexes can dissociate during rearomatization. Thus the stability and reactivity of a-complexes are strongly dependent on their conformation. The large differences in pK, values of planar and nonplanar momplexes, as well as the unexpectedly high stability of phloroglucinophane a-complexes, can be explained by the high inversion energy for the formation of the u-complexes with the proton as the leaving group in the axial position. Electrophilic substitution (Scheme I) plays a n important role in the chemistry of aromatic compounds. With few exceptions,’ the intermediate u-complexes, 2, have not been isolated and characterized. Consequently, little is known about the structure and properties of this transient species. In addition to the observed rearomatization of the a-complexes to either educt l a and/or product lb, competing reactions a re possible and in a few cases are known.*-* The factors that determine the alternative reaction pathways a re not well understood. I t is, however, very probable that the conformations of 2 (Scheme 11) play an important role in determining their chemistry. For example, in planar conformation 2B, the C-R and C-R’ bonds a re conformationally equivalent, whereas in the nonplanar conformations 2A and 2C, the equatorial and axial bonds may be expected to show different reactivity. Drawing on the results of crystal structure determinations combined with theoretical calculations, we have examined the steric and electronic effects of substituents on the conformations accessible to u-complexes and the influence of conformation on their chemical properties. We report crystal structure determinations for 2,4,6-tripyrrolidinocyclohexadienylium perchlorate (3a), 2,4,6-tripyrrolidinocyclohexadienylium bromide (3b), l-methyl-2,4,6tripyrrolidinocyclohexadienylium perchlorate (4a), and 1methyl-2,4,6-tripyrrolidinobenzene (5 ) . The crystal structure of 5 was determined to obtain experimental data suitable for assessing the steric contributions to the conformations of o,o’-pyrrolidino substituted benzenes, the parent of the u-complexes 3, 4a, and 6. Structural data for 1 -bromo-2,4,6-tripyrrolidinocyclohexadienylium bromide (6) ,9 heptamethylcyclohexdienylium tetrachloroaluminate (7a),I0 and I-phenylhexamethylcyclohexadienylium tetrachloroaluminate (7b)” are also considered in the comparison of theoretical and experimental results. Electrophilic Aromatic Substitution. 3 1. For part 30 see: Effenberger, Studiengang Chemie der Universitat Bremen. F.; Schollkopf, K. Chem. Ber. 1985, 118, 4377-4384. ‘Institut fiir Organische Chemie der Universitat Stuttgart. Experimental Procedures and Theoretical Concepts The crystallographic data for the crystals of 3, 4a, and 5 are characterized in Table I. Diffraction intensities were measured with a Syntex PI autodiffractometer (Mo Ka radiation, X = 0.71069 A) operating in an w-scan mode (see Table I for further details). Three reference reflections, measured periodically, were used to monitor the stability of each crystal over the course of the experiment: where appropriate (Table I) corrections were made for variations. Data were corrected for Lorentz and polarization effects but not for absorption.