Selective Formation and Cleavage of Carbon–Heteroatom Bonds using Visible-Light Photocatalysis

Abstract

Since the beginning of the 21st century, a renaissance of photochemistry has led to the development of new synthetic methods that use light as a sustainable reagent to achieve organic transformations. In particular, photocatalysis allows to harvest visible light and activate reagents and intermediates by means of catalytic amounts of a suitable chromophore. Due to the presence of heteroatoms in multiple natural products and active pharmaceutical ingredients, methodologies for the formation and cleavage of carbon-heteroatom bonds are highly valuable in synthetic organic chemistry. Cross-couplings are among the most powerful tools available to the synthetic organic chemist, enabling strategic bond constructions. Previously dominated by transition metal catalysis, crosscoupling reactions were revolutionized by photochemical approaches (Chapter 2). Photocatalysis allows to initiate radical couplings by selective reduction or oxidation of the coupling partners under mild reaction conditions. Alternatively, a photocatalyst (PC) can be used to regulate the reactivity of a transition metal catalyst. Photochemical strategies that do not require a PC rely on photoactive starting materials or formation of electron donor-acceptor complexes or catalytic intermediates that can be activated with light. Nickel is an appealing, sustainable alternative to palladium in transition metal-catalyzed crosscouplings, but the reaction of aryl halides and nucleophiles is hampered by the stability of nickel(II) intermediates. Visible light irradiation and a photocatalyst can enable nickel-catalyzed cross-coulings. While most dual nickel/photocatalytic methods rely on homogeneous iridium or ruthenium-polypyridyl complexes as photocatalysts, semiconductor materials emerged as sustainable, inexpensive alternatives. Carbon nitrides are a class of metal-free semiconductors that can be prepared from inexpensive, abundant commodity chemicals and absorb visible light. The carbon nitride CN-OA-m was combined with nickel catalysts to promote carbon–heteroatom cross-couplings. Aryl iodides and carboxylic acids are coupled using a combination of a nickel(II) salt, a bipyridyl ligand, CNOA-m and visible light irradiation (Chapter 3). The same approach was later extended to the coupling of aryl bromides with alcohols and aryl iodides with thiols (Chapter 4). The heterogeneous CN-OA-m is straightforward to separate from the reaction mixture and was reused several times without loss in reactivity. The ligand 5,5’-dicarbazolyl-2,2’-bipyridyl (czbpy) forms nickel complexes that absorb visible light up to 450 nm. A complex prepared in situ from NiCl2·glyme and czbpy can promote the coupling of aryl iodides with sodium sulfinates, carboxylic acids and sulfonamides under blue light irradiation, without an additional photocatalyst (Chapter 5). Polymerization of czbpy resulted in a polymeric ligand network that enables immobilization of nickel. The heterogeneous material catalyzes carbon–heteroatom cross-coupling under visible-light irradiation and was recycled multiple times. Benzyl ethers are convenient protective groups in carbohydrate chemistry due to their high stability. Harsh reaction conditions required for their cleavage by catalytic hydrogenation, Birch reduction or ozonolysis have poor functional-group compatibility and limit their use. Visible-light photocatalysis using iridium-polypyridyl complexes or organic dyes as PC are suitable for the cleavage of electron-rich para-methoxy benzyl ethers but do not affect benzylic ones. Upon visible light irradiation, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) forms a highly oxidizing triplet state that can cleave benzyl ethers. Combinations of DDQ and tert-butyl nitrite in presence of air catalyze the selective, oxidative cleavage of benzyl ethers in carbohydrate structures in presence of many other common functional groups (Chapter 6).