Photocatalytic E → Z Isomerization of Alkenes

Abstract

The current renaissance of organic photochemistry and photocatalysis has emboldened non-specialists to explore this rich landscape in search of solutions to long-standing challenges in synthesis. Many iconic stoichiometric photochemical transformations that were often platforms for mechanistic explorations have found limited application in contemporary synthesis, likely due to a perceived difficulty in reaction execution and the dearth of scope. Fortunately, many of these valuable processes are now being translated into robust methods, often with the aid of catalysis. The selective E → Z photochemical isomerization of olefins exemplifies this growing trend. Whilst examples of photochemical isomerization reactions are ubiquitous in physical organic textbooks, their intricate mechanistic delineations often contrast starkly with highly restricted substrate scopes: In the case of carbogenic scaffolds, electron-rich styrenes and stilbenes continue to dominate despite their limited synthetic potential. In this Account, our recent development of a photocatalytic isomerization of activated olefins is discussed. Inspired by the antipodal Z → E process with retinal that constitutes the basis of the mammalian visual system, and a qualitative report describing the reverse E → Z directionality enabled by crystalline (–)-riboflavin localized in the eye, critical noncovalent interactions have been distilled allowing this latter biological process to be reengineered. The result is an expansive, operationally trivial isomerization of substrates containing the cinnamyl motif using an inexpensive, readily available photocatalyst. Reliant on a selective excitation mechanism, this process complements the pioneering work of Hammond, Arai, and others by expanding the scope of olefin isomerization. In the case of carboxylic acids, it was possible to generate the coumarin scaffold directly by exploiting the two discrete activation modes of riboflavin sequentially, namely energy transfer (E_T) followed by single electron transfer (SET). This first example of a “one catalyst, two activation modes” strategy constitutes a photocatalytic variant of covalent cascade catalysis and provides a new entry to an important class of pharmaceutical building blocks. Reaction development is placed in an historical context, and key findings from other laboratories that facilitated the study are highlighted.

1 Introduction

2 A Brief Introduction to Photochemistry

3 Photochemical Isomerization of Olefins

4 The Bioinspired Isomerization of Activated Olefins

5 Emulating Coumarin Biosynthesis with (–)-Riboflavin

6 Conclusions

Key words

alkene isomerization - photochemistry - physical organic - bioinspired - (–)-riboflavin

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