Browsing by Author "Jogun, Kurt H."

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Open Access
Structure and reactivity of aromatic σ-complexes (cyclohexadienylium ions) : a correlated experimental and theoretical study
(1987) Effenberger, Franz; Reisinger, Friedrich; Schönwälder, Karl-Heinz; Bäuerle, Peter; Stezowski, John J.; Jogun, Kurt H.; Schöllkopf, Klaus; Stohrer, Wolf-Dieter
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.
Open Access
Synthesis, structure, and spectral behavior of donor acceptor substituted biphenyls
(1983) Effenberger, Franz; Agster, Wolfgang; Fischer, Peter; Jogun, Kurt H.; Stezowski, John J.; Daltrozzo, Ewald; Kollmannsberger- von Nell, Georg
Reaction of the activated halonitrobenzenes 2a-f with the bis- and tris(dialkylamino)benzenes 1a-e affords, via a direct nucleophilic substitution, the highly substituted biphenyls 3a-o; the lesser substituted biphenyls 5 and 6 were prepared by an Ullmann reaction. All these biphenyls are deeply colored; the dark red color can be assigned to an intramolecular charge transfer. A crystal structure determination was carried out for 2,4,6-tripyrrolidino-2',4',6'-trinitrobiphenyl (3a): space group C2/c, a = 16.071 (2) A, b = 14.545 (1) A, c = 20.177 (2) A, {3 = 91.361 (9)°, Z = 8 (temperature ≈ 120 K). The dihedral angle between the two arene rings was found to be only 52.5°, despite the four bulky substituents in the o,o'-positions. With this far-from-orthogonal torsional angle about the biphenyl linkage, the strong intramolecular charge transfer from the π system of the donor into the π system of the acceptor arene becomes easily understandable. A PPP calculation with the torsional angles taken from the X-ray structure analysis satisfactorily reproduces the experimental absorption spectrum of 3a. The shift of the long-wavelength absorption between the individual biphenyls 3a-o, 5, and 6 likewise is accounted for satisfactorily by the calculation; it depends primarily on the intrinsic donor strength of the different NR2 moieties (pyrrolidino > dimethylamino > piperidino > morpholino). The IH NMR spectra (in dilute solution) likewise mirror this gradation in Nr2 donor capacity; they also show that steric hindrance at the biphenyl linkage is mainly the result of interaction between the o-dialkylamino groups and the C6 skeleton of the acceptor arene. This is borne out by the crystal structure analysis.