Open chain oligopyrroles are regarded as a unique scaffold for helices because of their intense absorptions in the visible region and redox activity due to their π-conjugated system. They can provide hydrogen bonding network and strong metal coordinating site that limit conformational flexibility leading to a unique stereochemical structure. A number of linear tetrapyrroles including bile pigments have been known to generate a helicate that was originally coined for helical metal complexes of oligopyridines.¹⁾ Proceeding to study on long chain oligopyrroles with more than five pyrrolic units is a logical extension aimed at developing new functional materials, but such compounds are difficult to prepare and their handling under ambient conditions is frequently problematic.²⁾ Since only a few tetrapyrrolic helicates have so far been shown to induce helical sense bias,³⁾ we are focusing on the generation of one-handed helicates of long chain oligopyrroles without using optical resolution. We have developed an efficient synthetic method for stable hexapyrroles by reacting bis(azafulvene) with two parts of gem-dimethyldipyrrylmethane-1-carboxaldehyde. These hexapyrrole-α,ω-dicarboxaldehydes are readily metallated with divalent metal ions to give dinuclear single helicates of two turns and a pair of stereogenic centers are easily introduced by converting terminal aldehyde groups into the imine groups with a variety of optically active amines.⁴⁾ We have demonstrated that very high helical sense bias can be induced in this way. Here, we describe stereochemical feature and chiroptical properties of dinickel helicates of benzenehexapyrrole as longer homologues of hexapyrrole.

BisNi complexes of benzenehexapyrrole-α,ω-dicarboximines 1b, 1c, and 1d were prepared in good yields by reacting the corresponding α,ω-dicarboxaldehyde 1a with benzylamine, (S)-(–)-1-phenylethylamine, and (R)-(–)-1-cyclohexylethylamine, respectively. In these complexes, a pair of 4N metal coordination planes are helically distorted and the π-conjugated plane in the central part is twisted. These three components of local helicity designated as P or M specify their stereochemical feature. It was found that fast rotation around the C(pyrrole)-C(benzene) bond and slow flipping of the terminal formimidoylpyrrole are taking place to cause some conformational isomers in equilibrium at ambient temperature. For example, a single diastereomer of C₂-symmetry was observed in the ¹H NMR spectrum of 1d at 20 ˚C in CDCl₃ but three diastereomers of 1c in a ratio of 85(C₂) : 13(C₁) : 2(C₂) were discerned. VT ¹H NMR and CD study indicated that 1d undergoes rapid interchange between a major (P, P, P)-closed C₂ form and a minor (P, M, P)-open C₂ form. Two pyrroles in the 1,4-dipyrrylbenzene unit take a syn conformation in a closed form and anti in an open form. Either of both forms was exclusively taken in the case of 3d or 2d having 2,3-dimethoxy or 2,5-dimethoxy substituents, respectively, at the 1,4-phenylene spacer. The positive or negative CD first Cotton effect observed for 1d (3d) or 2d, respectively, was indicative of the handedness of the twisted π-conjugated central unit. In any case, it is noteworthy that the (R)-(–)-1-cyclohexylethylimine group exclusively induces the P-helical sense of the Ni coordination sites. If a pair of Ni coordination sites take opposite helical sense, it is asymmetric in the presence of stereogenic centers at the termini of the oligopyrrole chain and favors an open form. Such a (P, –, M)-open C₁ form appeared as a substantial contribution (>98%) of 2c and as a minor component accompanying a major (M, M, M)-closed C₂ form of 1c. These results indicate that combination of the aromatic spacer of the central part and the N-substituent of the terminal imine part is crucial in obtaining one-handed helicates of the oligopyrroles.

References

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