Structure and reactivity of aromatic σ-complexes (cyclohexadienylium ions) : a correlated experimental and theoretical study

Steric and electronic properties for a series of σ-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-2,4,6-tripyrrolidinobenzene 5 are reported. With respect to the cyclohexadienylium ring, the H,H σ-complexes, 3, display a planar conformation whereas σ-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 σ-complexes 3,4, and 6. Calculations performed for other σ-complexes indicate that the ring conformation for σ-complexes with small substituents in the 2- and 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 at 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 σ-complexes can dissociate during rearomatization. Thus the stability and reactivity of σ-complexes are strongly dependent on their conformation. The large differences in pKa values of planar and nonplanar σ-complexes, as well as the unexpectedly high stability of phloroglucinophane σ-complexes, can be explained by the high inversion energy for the formation of the σ-complexes with the proton as the leaving group in the axial position