Competitive formation of helical cycloocta- and cyclododecapyrroles

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 inter ested in gem-dimethyl-substituted cyclotetrapyrrole (2a) and the correspond ing Ni(II) complex (2b). Attempts to prepare 2a as the 12,13,16,17-tetraeth yl-2,3,7,8-tetramethyl derivative (5) by an acid-catalyzed (1 + 1) condensa tion of dimethyldipyrryl-methane 3 and diformylbipyrrole 4 resulted in the formation of the (2 + 2) and (3 + 3) condensation products, i.e., the cyclo octapyrrole 6 and the cyclododecapyrrole 7, respectively, rather than in th at of the desired gem-dimethyl cyclotetrapyrrole. The cyclododecapyrrole 7, isolated as the major product, is among the largest cyclopolypyrroles know n to date. These two new macrocycles have been structurally characterized b y variable temperature 1D and 2D NMR experiments, as well as by single-crys tal X-ray diffraction analysis. In solution both the cyclooctapyrrole 6 and cyclododecapyrrole 7 exhibit dynamic behavior. At 337 K 6 adopts a D-2-sym metric conformation, whereas at 196 K two equivalent C-2 conformers that in terconvert through the D-2-symmetric intermediate are observed. The energy barrier for the interconversion process between these two degenerate confor mers 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 37 8 K displaying D-3h symmetry. X-ray analyses showed that for both macrocycl es, 6 and 7, the solid state structures were nearly identical to the low-te mperature solution conformers.