Palladium catalysis in the synthesis of covalently linked diporphyrins

The construction of two covalently linked diporphyrin series and a porphyrin-labeled nucleoside are described. The covalently linked systems comprise zinc/free-base and zinc/gold diporphyrins. Selected results from the photophysical studies on their properties regarding energy and electron transfer are reported, although the focus of this thesis is on synthesis and the development of synthetic methods. In order to obtain the diporphyrins in a precise state of metalation, the syntheses were based on a building block approach. The zinc/free-base series could be synthesized using known methods, whereas new methods had to be developed for the zinc/gold series. An efficient copper-free protocol for the palladium-catalyzed Sonogashira cross-coupling was needed for the assembly of the halide and alkyne substituted building blocks. Most studies devoted to the development of such cross-couplings have focused on ligands, whilst this work focuses on the choice of solvent and base. A model study was conducted to examine the dependence of the Heck alkynylation on the solvent and base. The model study included the screening of selected solvent-base combinations and kinetic experiments. The catalytic system throughout the investigation was formed from [Pd2(dba) 3&middotCHCl3]/AsPh3. The most successful modification proved to be the addition of methanol, as a cosolvent, in combination with an unhindered tertiary base. The success of the methanol additive is hypothesized to be caused by the presence of a rate determining deprotonation step, featuring a charge-separated transition state. Finally, the very high yielding and successful syntheses of the diporphyrin series and porphyrin-labeled nucleoside, using the newly developed conditions, are presented. For the first time, gold porphyrins substrates could efficiently be coupled in Heck alkynylations.