Competitive Formation of Helical Cycloocta- and Cyclododecapyrroles

Abstract

In connection with a study aimed at the evaluation of electronic effects in spiro-dicorrole (1a) and its binuclear Ni(II) complex (1b) we became interested in gem-dimethyl-substituted cyclotetrapyrrole (2a) and the corresponding Ni(II) complex (2b). Attempts to prepare 2a as the 12,13,16,17-tetraethyl-2,3,7, 8-tetramethyl derivative (5) by an acid-catalyzed (1 + 1) condensation of dimethyldipyrrylmethane 3 and diformylbipyrrole 4 resulted in the formation of the (2 + 2) and (3 + 3) condensation products, i.e., the cyclooctapyrrole 6 and the cyclododecapyrrole 7, respectively, rather than in that of the desired gem-dimethyl cyclotetrapyrrole. The cyclododecapyrrole 7, isolated as the major product, is among the largest cyclopolypyrroles known to date. These two new macrocycles have been structurally characterized by variable temperature 1D and 2D NMR experiments, as well as by single-crystal X-ray diffraction analysis. In solution both the cyclooctapyrrole 6 and cyclododecapyrrole 7 exhibit dynamic behavior. At 337 K 6 adopts a D(2)-symmetric conformation, whereas at 196 K two equivalent C(2) conformers that interconvert through the D(2)-symmetric intermediate are observed. The energy barrier for the interconversion process between these two degenerate conformers is found to be 10.6 kcal mol(-)(1). The solution dynamics of 7 could be described in an analogous manner, with the time-averaged conformation at 378 K displaying D(3)(h)() symmetry. X-ray analyses showed that for both macrocycles, 6 and 7, the solid state structures were nearly identical to the low-temperature solution conformers.

Spirodiporphyrins—As Binuclear Metal Complexes

“…It seems reasonable to assume that the skeletal rearrangement accompanied by extrusion of carbon dioxide is preceded by the establishment of the thermal conformational equilibrium 8 ⇄ 11 through which the remote carbonyl groups are brought into close proximity. Semiempirical PM3(tm) calculations carried out on the two dioxo conformers (with methyl instead of ethyl groups) lead to the conclusion that 8 and 11 are comparable in energy, while analogous examples19 suggest that only a low activation barrier separates 8 from 11 . Calculations show that the incorporation of Ni II into 11 , either to form 14 (arbitrarily assumed here) or to give a mononuclear nickel complex, brings the carbonyl groups even closer together and induces strain into the molecule.…”

Bimetallic Figure‐Eight Octaphyrins Split into Four‐Pyrrolic Macrocycles

“…In selected cases the restriction of the intramolecular mobility allowed the separation of two enantiomers 9. The second mechanism which seems to be operating for the figure‐eight molecules cyclooctapyrroles,11, 15, 22 41,43,45,47‐tetrathia‐[36]octaphyrin(1.1.1.1.1.1.1.1),23 and turcasarin21 involves a conveyor‐belt‐like movement of the whole ring, but excludes, however, racemization. Rearrangement of the whole figure‐eight ring of the molecule eventually takes place to reconstruct the geometry around “zigzag” spacers.…”

Die Spaltung zweikerniger Figure‐Eight‐Octaphyrine zu Tetrapyrrol‐Makrocyclen

“…Eine Trennung der beiden Enantiomere konnte in einigen Fällen durch Einschränkung der intramolekularen Beweglichkeit erreicht werden 9. Der zweite Mechanismus, der im Fall der Figure‐Eight‐Cyclooctapyrrole,11, 15, 22 41,43,45,47‐Tetrathia[36]octaphyrin(1.1.1.1.1.1.1.1)23 und Turcasarin21 vorzuliegen scheint, geht von einer fließbandartigen Bewegung des gesamten Rings aus, die jedoch keine Racemisierung ermöglicht. Die Wiederherstellung der Zickzack‐Anordnung der Stickstoffatome an den Kreuzungspunkten erfolgt dann schließlich durch eine konformative Umordnung des gesamten Moleküls.…”