Drawing inspiration from photosynthesis, scientists challenged the use of visible light for the activation of organic molecules and the discovery unknown solar-driven transformations. Plethora of organic compounds have been investigated in terms of their photochemical activity. However, the fact that diazo compounds possess their intrinsic chromophore (-C=N+=N-) puts them in a unique position among many reagents that can absorb visible light and hence are suitable for solar-driven transformations. To date, they are extensively studied in metal catalyzed reactions and proved to undergo a range of unusual transformations, furnishing even complex molecules with impressive stereoselectivities. As traces of toxic metals are unacceptable by pharmaceutical industry, greener methodologies are in high demand.
The main objective of this proposal is to unveil general reactivity modes of diazo compounds under visible light-irradiation. We will develop their solar-driven reactions and determine reactive intermediates (singlet or triplet carbenes, radicals, others) generated during these processes. The use of photocatalysts for solar-driven chemistry of diazo compounds will enable generation of reactive intermediates even from otherwise unreactive substrates.
Processes induced by solar energy fall into two categories: direct photolysis and photocatalysis. However, application of these approaches in diazo chemistry is rare. It was only in 2018 when Jurberg and Davies discovered that decomposition of aryl diazoacetates to free carbenes under blue light irradiation is feasible as these donor-acceptor diazo compounds absorb light in the wavelength region of 400-500 nm. As a consequence, their reactivity under blue light irradiation has been extensively studied. Indeed, visible light-induced cyclopropanation, cyclopropenation, benzannulation, cross-coupling, as well as C-H, O-H, N-H insertion reactions have been recently reported. However, all these studies are limited to a sole group of aryl diazoacetates which decompose to carbenes and as such undergo various transformations
Therefore, we will study reactivity of diazo compounds under visible light-irradiation and develop their solar-driven reactions. We will elaborate direct photolysis of diazo compounds absorbing in the blue region in C-H insertions. For compounds unreactive under visible light-irradiation, photocatalyzed transformations represent an alternative.
In the first step, direct photolysis of diazo compounds other than aryl diazoacetates, which we have already proven, theoretically and experimentally, absorb in the blue region, will be studied mainly in C-H insertions. Thus, if few examples support our hypothesis that other diazo compounds can also generate carbenes providing they absorb visible light, we will focus on synthetically useful transformations, including: photolysis of 2-(N-PG-amino)-2-diazo-acetamides, [2,3]-sigmatropic rearrangement, generation of cyclopropenes.
In subsequent stages, photocatalyzed transformations will be evaluated preceded by establishing redox properties of diazo compounds (by cyclic voltammetry), based on which photoredox catalysts will be chosen. We will test our hypothesis in model reactions, in: alkylation of electron rich double bonds, cyclizations, cyclopropenes as radical species acceptors.
Summing up, the use of diazo compounds as a source of reactive intermediates under visible light irradiation will be fully defined. Once established, we will elaborate on synthetic transformations utilizing developed methodologies based on direct photolysis and photocatalysis. This type of universal procedures are well sought by any synthetic chemist as they will allow not only to establish desired photochemical methods but also will give an in-depth look into chemistry of diazo compounds, a concern that is rarely seen. When combined, their easy synthesis and high reactivity make them reagents of choice for the solar-driven synthesis of complex molecules.
Enormous potential of diazo photoreactivity and our experience in this field as well as in photochemistry will assure realization of this project in a timely manner.