Untersuchung der nukleophilen Substitution von Porphyrinen mit lithiumorganischen Reagenzien: The first 3-D structure of a glycosyltransferase, which catalyse the formation of large cyclic amylose.

Abstract

The aim of this thesis was the investigation and development of new synthetic methods for functionalized asymmetric porphyrins with organolithium reagents. Specifically, the following results were obtained : First, an experimental verification of the reaction mechanism of the SNAr reaction of porphyrins with LiR was performed. For this the free bases of 5,15-diphenyl- and 5,15-dibutylporphyrin and the corresponding NiII-complexes were used as test cases. The intermediates in each step of the reaction with LiBu were systematically investigated using UV/vis spectroscopy, deuterium labeling, trapping of the intermediates with electrophiles, and variation of the pH of the reaction mixture. It was found, that the nucleophilic reaction of free bases and NiII-complexes proceeds through different intermediates which after the addition of LiBu showed similar UV/vis spectra typical for a phlorin but exhibited different chemical reactivity in the reaction with electrophiles. Here, only NiII-complexes reacted at the meso-position opposite to that of the LiR attack with alkyliodides. Furthermore, a direct oxidation (with DDQ under nonaqueous conditions) of the anionic intermediates of free bases allowed a dimerization giving directly meso-meso linked bisporphyrins. Upon lowering the pH-value of the reaction mixture, different UV/vis spectra were obtained for the intermediates resulting from free bases (phlorin) and NiII-complexes (porphodimethene). Additionally the investigation showed, that the intermediate resulting from NiII-complexes is stable under the reaction conditions in the presence of water and no protonation was observed. According to the results from the investigation of deuterium labeling experiments, the negative charge in the NiII-complexes is preferentially localized at the meso- position opposite to the LiR attack, whereas in the free base the negative charge should be more localized at the nitrogen atoms. A model using a conformational analysis of the intermediates was introduced to explain the different chemical reactivities of free bases and metal complexes. The regioselectivity for meso und betta positions is important for potential applications of the reaction. For a detailed study, 5,15-disubstituted and 5,10,15-trisubstited aryl- and alkylporphyrins were treated with phenyl- and alkyllithium reagents (alkyl- = Bu, Pri, Bus und But). Complete regioselectivity and excellent yields for the substitution of the meso positions was found with sterically undemanding organolithium reagents (LiBu, LiPh) whereas more byproducts were observed in the reactions with sterically hindered LiR (R = Pri, Bus und But) and the corresponding target compounds were obtained only in low yields. In addition, simple variation of the reaction conditions provided a route to directly meso-meso linked bisporphyrins in high yields. The influence of the porphyrin central ion on the reactivity of the intermediates was investigated by using LiI-, ZnII-, NiII- and PdII-complexes. Only the intermediates derived from NiII-complexes reacted with alkyl iodides indicating that the postulation that conformational effects govern the chemical reactivity of the intermediates was correct again. In the course of further studies on the reactivity various organolithium reagents with different reactivities such as alkyl-, alkenyl-, alkinyl-, phenyl-, benzyl- and heteroaryllithium were used for reaction with 5,15-diphenylporphyrin. Aryl- and n-alkyllithium reagents generally gave the best yields. On the basis of the results discussed above, a general synthetic method for functionalized A2B- and A2BC-porphyrins was developed. Especially noteworthy is the reaction of the monoanionic NiII-intermediates, prepared in situ by reaction with LiR, with alkyl iodides. This constitutes a one-flask synthesis that allows the regioselective introduction of two different substituents in the porphyrin system. Using the reaction discussed above, many novel porphyrins with functional groups like ?OH, -CHO, -CºCH, -COOH, -COOEt, -CN, -I and ?NH2 were prepared in good yields. In order to test the chemical reactivity of the functional groups introduced, the synthesized porphyrins were employed as reaction precursors for further connections with target molecules. All introduced functional groups showed a satisfactory chemical reactivity and can be used as reaction centers for the synthesis of complex tetrapyrrole systems with special chemical and physical properties. In addition, a new synthesis way leading to 5,5?,15,15?-tetrasubstituted functionalized asymmetric porphodimethenes was found. This reaction involved trapping of monoanionic NiII-intermediates with alkyl iodides with small substituents such as (ICH2)4CN, MeI, and NCCH2I. The structure and conformation of the novel porphodimethenes was investigated via NOE and 1H- 13C-NMR experiments. The present work has resulted in the development of several facile and efficient synthetic strategies for a regioselective introduction of almost any functional group into the porphyrin system allowing the synthesis of asymmetrically substituted porphyrins for further synthetic studies on tetrapyrroles.