Reactivity of strained systems in alkyne cycloaddition reactions - synthesis of substituted cycloparaphenylenes

During the last decade, cycloparaphenylenes (CPPs) have attracted the attention of chemists. Indeed, this interesting class of molecules represents the shortest section of an armchair carbon nanotube, offering a potential template for the selective synthesis of carbon nanotubes. Cycloparaphenylenes are also interesting molecules themselves, with their distorted aromatic system and radially oriented p-orbitals providing particular optical and electronical properties which could find application in material sciences. The possibility to access substituted CPPs would open ways to new applications such as host-guest chemistry, chemosensors and nanoporous materials.
The [2+2+2] cycloaddition reaction is one of the most powerful methods to access substituted aromatic system in one step. The gain of aromaticity is one of the driving forces of the reaction. Therefore the aim of this thesis was to apply this reaction for the synthesis substituted CPPs, taking advantage of the gain of aromaticity to overcome the strain energy.
However, due to the complexity of such a system, this concept was first probed on a simpler model system. The principal requirement for the model was to present a strained aromatic system with a tethered diyne as precursor for the [2+2+2] cycloaddition reaction. Surprisingly, when this strained precursor was subjected to the Rh-catalyzed [2+2+2] cycloaddition reaction an unexpected change of reactivity was observed. Instead of a [2+2+2] cycloaddition a [(2+2)+2] cycloaddition took place leading to an ortho-terphenylene macrocyclic product. Therefore, the [2+2+2] cycloaddition reaction is not a suitable method to bring strain in a system such as originally devised for the synthesis of substituted CPPs. Nevertheless, this reaction can be used as a versatile method to introduce substituents to the system, allowing access to substituted CPPs by choosing substituents at a late stage of the synthesis.
As a consequence, we modified our initial strategy to incorporate another building block being able to accommodate the strain. Previous syntheses of CPPs relied on similar strategies, synthesizing a macrocyclic precursor which was aromatized in a last step. Combining such a building block with a diyne building block provided the precursor for the synthesis of substituted CPPs. Using the [2+2+2] cycloaddition reaction as key step allowed the synthesis of the smallest substituted CPPs in a highly modular way. The advantage of this strategy is that the substitution can be chosen at a late stage of the synthesis, providing a highly modular method to synthesize functionalized CPPs.