Photobiocatalysis

Biocatalysis for Practitioners

Wiley-VCH, Weinheim

Año: 2022; p. 319 - 359

The application of catalytic systems instead of stoichiometric ones supposed a change in paradigm in the way of thinking and planning chemical reactions. Catalysis enables the synthesis of useful compounds in a more economical, energy-saving fashion with the obvious advantage to be less detrimental to the environment. In this line, the so-called ?practical elegance? in the design of a synthetic route entails a logic elegance along with a practical applicability. Scientists have spent tremendous efforts in setting up novel catalytic methodologies to achieve valuable synthetic targets [1]. Photocatalysis, in particular, photoredox catalysis, has witnessed an impressive burst thanks to the recent developments in collecting energy from the visible region of the electromagnetic spectrum. Indeed, this is possible by using transition metal complexes ([Ru(bpy)3]2+, fac-Ir(ppy)3, etc.), organic dyes (rose Bengal [RB], methylene blue, eosin Y, rhodamine G, fluorescein, etc.), natural pigments (flavin, chlorophyll, etc.), and nanostructured semiconductors such as quantum dots (QDs), doped TiO2, perovskites, carbon nitrides, etc. (Scheme 12.1). With this power in hands, chemists have set up a myriad of synthetically relevant reaction systems, enabling reduction, oxidation, and formation of C?C and C-heteroatom, among others, powered by visible light. It must be emphasized that a great number of these reactions are difficult (or too sluggish) under dark conditions [2]. Meanwhile, biocatalysis has gained momentum owing to the outstanding selectivity and negligible price compared with other types of catalysis, and therefore, chemical industry already adopted such technology even for large-scale processes (see also Chapter 15). The reason behind this success may be the simple modification of the enzyme by mutagenesis or directed evolution (see also Chapter 2), thus improving acceptance and selectivity toward non-natural substrates and activity in nonconventional media (see also Chapter 9),such as organic solvents, ionic liquids, and deep eutectic solvents [3, 4]. There are several features that are exclusive to only one type of catalysis, for instance, the ?innate? or ?guided? reactivity achieved in metallocatalytic or photocatalytic processes, or the perfect enantioselectivity displayed by biocatalysis, or the robustness of and scalability of metaland organo-catalyzed reactions [5]. Considering this, one may envision an ideal process comprising the advantages of several (or at least more than one) sort of catalysis. This might come true by a chemically reasonable and practically feasible combination of catalysts, either sequentially or in cascade/domino processes (see Chapter 14) [6?8]. In the past few years, scientists have successfully merged metal- and photocatalysis [9?11], metal- and biocatalysis [12?15], organo- and biocatalysis [16], organo- and photocatalysis [17?19], and of course a combination of photo- and biocatalysis [20?22]. In 2018 and 2019, comprehensive literature surveys collecting proof of concepts and applications of photobiocatalysis have appeared, illustrating the diverse underlying mechanisms. In this chapter, we will focus on examples of photobiocatalytic systems that can be useful in synthetic chemistry emphasizing practical aspects to be considered for the laboratory practitioner. Along it, several fundamental terms coming from radical chemistry and photochemistry will often be used. As many readers may not be familiar with some of them, the most common ones can be found in Scheme 12.2.