P. Ulrich Biedermann

Conformational Space and Dynamic Stereochemistry of Overcrowded Homomerous Bistricyclic Aromatic Enes − A Theoretical Study

The conformational spaces and dynamic stereochemistry of representative overcrowded homomerous bistricyclic aromatic enes (1, X = Y) are investigated, applying the semiempirical PM3 method. The experimental energy barriers for E,Z isomerizations, enantiomerizations, and conformational inversions of 1 and related compounds, derived from DNMR and other kinetic studies, are reviewed. This study focuses on the analysis of the minima, transition states, and dynamic mechanisms of the conformational isomerizations of bifluorenylidene (2), dixanthylene (3), dithioxanthylene (9), and bi-5H-dibenzo[a,d]cyclohepten-5-ylidene (11). The four representative bistricyclic enes differ in the sizes of their central rings and in their bridging groups. The mechanisms of the interconversions of the twisted, anti-folded, and syn-folded conformations and of thermal E,Z isomerizations (topomerizations), enantiomerizations, and conformational inversions (including combinations) are elucidated. The calculated energy barriers for E,Z topomerizations of 2, 3, 9, and 11 are 25.3, 16.4, 24.3, and 39.3 kcal/mol, respectively. The corresponding barriers for enantiomerizations or conformational inversions are 4.9, 15.9, 24.3, and 37.6 kcal/mol, respectively. In most cases, the agreement with experimentally determined values is within 1−3 kcal/mol. New mechanisms are proposed for the E,Z isomerizations and conformational inversions of anti-folded 3, 9, and 11, involving low-symmetry folded/twisted transition states and the respective syn-folded intermediates.

Cyclic Conjugation Effects in Cyclacenes

Abstract Cyclic conjugation is studied in Huckel and Mobius cyclacenes as well as in a few other related open-chain and cyclic belt-shaped hydrocarbons (e.g., zig-zag cyclarenes). the main conclusions are that (a) the energy effect of the conjugation along the perimeter of cyclacenes is relatively large, (b) conjugation along the 6 -, 10 - and 14-membered cycles has a much greater effect, in spite of the fact that these cycles embrace bonds that formally are not involved in cyclic conjugation. Mobius cyclacenes are non-alternant hydrocarbons and their odd-cycles have a significant contribution to conjugation. the contribution of conjugation in individual cycles to the total resonance energy is briefly discussed.

ENANTIOMERIZATION OF ENVIRONMENTALLY SIGNIFICANT OVERCROWDED POLYCHLORINATED BIPHENYLS (PCBS)

The rotational barriers of overcrowded PCBs are predicted by ab initio methods including electron correlation, thus settling the controversy between theory and experiment. For 2,2′,3,3′,4,6′-hexachlorobiphenyl (PCB 132), an enantiomerization barrier of 185 kJ/mol is calculated by B3LYP/6-31G*, in excellent agreement with the experimental data. Chirality 9:350–353, 1997.

Electrostatic chiral distinction: Tetrahedral model dimers

Although chiral distinction plays a pervasive role in chemistry, a complete understanding of how this takes place is still lacking. In this work, we expand the earlier described minimal requirement of so called four-point interactions (vide infra). We focus on chiral point charge model systems as a means to aid in the dissection of the underlying, operative principles. We also construct models with defined symmetry characteristics. By considering extensive constellations of diastereomeric complexes, we are able to identify emerging principles for chiral distinction. As previously postulated, all the diastereomeric complexes, regardless of their nominal contact-points, possess a chiral distinction energy. In the comparison of complexes, we find that, contrary to chemical intuition, the magnitude of chiral distinction does not correlate with the stability of the complexes, i.e., consideration of low energy complexes may not be an effective way to evaluate chiral distinction. Similarly, we do not find a correlation between the number of contact-points and chiral distinction. Moreover, favorable interactions and facile chiral distinction appear to be unrelated. We also see some tendency for greater chiral distinction in less symmetric systems, although this may not be general. These studies can now form the basis to fold in higher levels of complexity into the models so as to gain further insights into the nature of chiral distinction.

Thermochromism of overcrowded bistricyclic aromatic enes (BAEs). A theoretical study

A criterion for thermochromism has been derived from ab initio DFT B3LYP/6-31G* calculations of the anti-folded, syn-folded and twisted conformations of thermochromic and non-thermochromic overcrowded bistricyclic aromatic enes (BAEs) 1–8: the thermochromic species is the twisted conformation, a necessary condition being a small energy difference, <30 kJ mol−1, between the global minimum anti-folded and the thermochromic conformations.

Selenium and Tellurium Tricyclics. Conformational Effects on 77Se and 125Te NMR Spectra

A series of selenium and tellurium tricyclics (1,2) have been synthesized and their 77Se and 125Te NMR chemical shifts determined. Semiempirical MNDO-PM3 calculations on representative 1 and 2 have been used to characterize their folded and planar conformations. The results indicate a pronounced effect of the conformations on these chemical shifts.

Two-point contact chiral distinction—a theoretical appraisal

Ab initio calculations reveal chiral distinction in two-point contact CHFCIBr dimers, with chiral distinction energy of 1.5 kJ mol-1 between the SR and SS dimers fully optimized at the MP2/6-311++G** level.

Stereochemistry of selenium- and tellurium-bridged heteromerous bistricyclic aromatic enes. The fluorenylidenechalcoxanthene series

The effects of selenium and tellurium bridges on the conformations and dynamic stereochemical behavior of heteromerous bistricyclic aromatic enes (1) were studied. 9-(9′H-Fluoren-9′-ylidene)-9H-selenoxanthene (9) and 9-(9′H-fluoren-9′-ylidene)-9H-telluroxanthene (10) were synthesized, applying Bartons two-fold extrusion diazo–thione coupling method, which is especially suited for heteromerous 1. The isopropyl derivatives 14, 15 and the benzannulated derivatives 16, 17, and 18 were prepared analogously. The structures of 9, 10, 14, 15, and 16–18 were established by 1H, 13C, 77Se, and 125Te NMR spectroscopy and in the cases of 9 and 10 also by X-ray analysis. The molecules of 9 and 10 adopted anti-folded and folded conformations with 56.3/62.0° and 10.2/8.0° (9) and 63.6 and 2.2° (10) folding dihedrals, higher than in 7 and 8. The degrees of pyramidalization of C9 and C9′ were 2.8/3.9° and 0.9/2.1° (9) and 8 and 15° (10). Considerable overcrowding was evident in the short Se10⋯C9 and Te10⋯C9 contact distances in 9 and 10. The crystal structures of 10 indicated relatively short intermolecular Te⋯Te distances (408 pm). The 13C NMR chemical shifts of 9, 10, 9-(9′H-fluoren-9′-ylidene)-9H-xanthene (12) and 9-(9′H-fluoren-9′-ylidene)-9H-thioxanthene (13) indicated a variation in C9 of the chalcoxanthenylidene moiety, ascribed to through space interactions of Se, Te and S with C9. The 77Se and 125Te NMR signals of 9–10 and 14–17 were shifted downfield relative to the homomerous 7 and 8. A DNMR study of 14 and 15 gave ΔGc‡ (conformational inversion) = 14.4 (14) and 19.4 kcal mol−1 (15) and ΔGc‡ (E,Z-topomerizations) > 21.6 kcal mol−1, indicating an increase of ΔGc‡ in the fluorenylidenechalcoxanthenes series (11): O < S < Se < Te. The fluorenylidene-derived 1 were found to show distinct behavior for conformational inversions and E,Z-isomerizations. Semiempirical PM3 calculations of 9 and 10 indicated that unevenly anti-folded conformations were most stable. Conformational inversions of 9 and 10 proceed via the twisted transition states corresponding to calculated barriers of 14.8 and 21.6 kcal mol−1 in excellent agreement with experiment. The E,Z-isomerizations proceed via orthogonally twisted biradical transition states with predicted barriers of 27.0 and 34.0 kcal mol−1 for 9 and 10, respectively.

From Overcrowded Polycyclic Aromatic Enes to Semifullerenes

Abstract Overcrowded polycyclic aromatic enes (1), e.g., bi-9H-fluoren-9-ylidene (2) and bi-4H-cyclopenta[def]-phenanthren-4-ylidene (3) are potential starting materials for the preparation of bowl-shaped fragments of fullerenes. Semiempirical MNDO-PM3 calculations of C26 Hn and C30 Hn (n = 12,14,16) species 2–14 are used to analyze energetic and steric effects on the dehydrocyclization and isomerization reactions of these molecules. the out-of-plane bending and pyramidalization in these species are ascribed to intramolecular overcrowding in the fjord and cove regions and to strain introduced by C5 rings in the PAH skeleton. Oxidative photocyclization reactions on Z-2,2′-bridged derivatives of 2 and 3 are briefly outlined.

Selenium- and tellurium-bridged overcrowded homomerous bistricyclic aromatic enes

The effects of selenium and tellurium bridges on the conformations of overcrowded homomerous bistricyclic aromatic enes were studied. The structures of the target molecules 9,9′-bi(9H-selenoxanthen-9-ylidene) (7) and 9,9′-bi(9H-telluroxanthen-9-ylidene) (8) were established by 1H, 13C, 77Se, 125Te NMR spectroscopy, and by X-ray analysis. The molecules adopted anti-folded conformations with 53.6 (7) and 53.1° (8) folding dihedrals between pairs of benzene rings of the tricyclic moieties, whereas the corresponding folding dihedral in 9-methylene-9H-selenoxanthene 20 was considerably lower, 32.4°. An X-ray analysis of 9,9′-bi(9H-selenoxanthene) (9) indicated an anti-folded conformation with a folding dihedral of 49.2° and short Se10⋯H9′ and Se10′⋯H9 distances. Compounds 7 and 8 exhibited low degrees of overcrowding in the fjord regions. Considerable overcrowding was evident in the short Se10⋯C9 and Te10⋯C9 contact distances in 7 and 8. The high shielding of the protons in the fjord regions of 7 and 8 revealed anti-folded conformations in solution. The 13C NMR chemical shifts of 7 and 8 were characterized by low-field absorptions of C9 and C9′. Semi-empirical PM3 calculations of the anti-folded, syn-folded, and twisted conformations indicated that anti-folded-7 and syn-folded-8 were the most stable conformations, respectively. The special stability of syn-folded-8 was attributed to the short intramolecular Te10⋯Te10′ distance (3.06 A). Compounds 7 and 8 were synthesized by reductive “dimerizations” of 9H-selenoxanthene-9-thione (13) and 9H-telluroxanthene-9-thione (17) with copper in boiling toluene. Compound 7 was also synthesized by diazo–thione coupling between 13 and 9-diazo-9H-selenoxanthene (14), followed by elimination of sulfur from the intermediate thiirane 15. 9,9′-Bi(9H-selenoxanthene) (9) and 9,9′-bi(9H-telluroxanthene) (10) were prepared by low valent titanium induced reductive “dimerizations” of 9H-selenoxanthen-9-one (11) and 9H-telluroxanthen-9-one (12), respectively, using TiCl4/Zn/pyridine–THF.