Catalytic Strain Release of Photoproducts and its Application in Organic Chemistry

Abstract

Different photochemical processes such as isomerizations and cycloadditions have shown to be particularly useful in the construction of highly strained photoproducts. The effect of that ring strain in terms of chemical reactivity and ground state energy represents a big advantage in the field of organic chemistry since the relief of ring strain energy can serve as the driving force for subsequent reactions. The combination of the generation of a photogenerated strained adduct with a strain-releasing follow-up transformation eases the preparation of complex products under very mild and potentially eco-friendly reaction conditions. Therefore, in the present work sequences that combine literature-known photochemical isomerizations that lead to high-energy strained intermediates with a new subsequent catalytic downstream reaction were developed. In the first part, meta photocycloadducts were synthesized from alkyl benzyl allylamines under photoinduced reaction conditions and afterwards, the possibility of a vinylcyclopropane ring-opening was evaluated with different reaction partners under metal-base and visible light catalysis. Attempts for a formal [3+2] cycloaddition with different alkenes and alkynes led to a novel visible light-mediated reaction with Ir (III) complexes in combination with acetylenic sulfones. DFT-calculations and time-resolved optical spectroscopy studies substantiated a triplet energy transfer mechanism for this transformation. In the second part of this work, the ring-opening of a cyclopropanol photochemically synthesized from 2-propionylpyridine was investigated in terms of metal as well as photoredox catalyzed downstream reactions with different reaction partners. A literature-known Pd-catalyzed cross-coupling reaction with benzyl chlorides was identified as the most efficient reaction. The optimization of these reaction conditions for the use of the photogenerated cyclopropanol allowed the preparation of a new series of 2-pyridylketones.