Build and release of molecular strain as a tool in organic synthesis

Abstract

Over the course of this work, the build and release of molecular strain was used as a tool in organic synthesis. In the first research project, a proof-of-concept for the synthesis of O-heterocy-cles from photochemically generated strained intermediates was provided. α-Aminoacetophenones were identified as a versatile platform for the systematic investigation of the light-driven Norrish-Yang cyclisation. The influence of pro-tecting groups on the heteroatom and the influence of sterically demanding sub-stituents on the success of the cyclisation were investigated. Consecutively, the synthesis of several 3-azetidinols and a 3-oxetanol was demonstrated. Use of an electron-deficient trifluoroacetophenone enabled the formation of stable hemi ketals from the synthesised heterocycles. The increased acidity of the formed hemi ketals enabled the ring-opening reaction of 3-azetidinols towards diox-olanes without the need for external activation. Transformation of 3-oxetanols into dioxolanes was efficiently realised in a cobalt catalysed dynamic kinetic res-olution, providing high degrees of diastereo- and enantioselectivity. During the second research project, the build and release of molecular strain was used not for the generation of molecular complexity, but for the storage of light energy in chemical bonds. The previous investigation of the Norrish-Yang cyclisa-tion inspired the use of structurally rigid systems, which resulted in the develop-ment of an isomer pair ortho-methylacetophenone ⇄ benzocyclobutenol. A unique beneficial effect of a trifluoromethyl group on the photocyclisation was asserted and the mechanism of the transformation was elucidated. The overall robustness of the reaction was confirmed, merely a detrimental influence of oxy-gen onto the reaction was found. All formed side and degradation products were unambiguously identified. Comparison of several previously unknown benzocy-clobutenols resulted in the emergence of an isomer-pair with several benefits compared to literature known reference systems for the chemical storage of light energy. By addition of catalytic amounts of organic base, the stored energy was found to be releasable under ambient conditions and an immobilised base was utilised to showcase the cyclability of the system. Finally, the successful charging of the system with solar light was demonstrated, underlining its applicability.