Asymmetric Synthesis of α-Chloroamides via Photoenzymatic Hydroalkylation of Olefins.

Photoenzymatic intermolecular hydroalkylations of olefins are highly enantioselective for chiral centers formed during radical termination but poorly selective for centers set in the C-C bond-forming event. Here, we report the evolution of a flavin-dependent "ene"-reductase to catalyze the coupling of α,α-dichloroamides with alkenes to afford α-chloroamides in good yield with excellent chemo- and stereoselectivity. These products can serve as linchpins in the synthesis of pharmaceutically valuable motifs. Mechanistic studies indicate that radical formation occurs by exciting a charge-transfer complex templated by the protein. Precise control over the orientation of molecules within the charge-transfer complex potentially accounts for the observed stereoselectivity. The work expands the types of motifs that can be prepared using photoenzymatic catalysis.

Graphitic Carbon Nitride as a Photocatalyst for Decarboxylative C(sp2)-C(sp3) Couplings via Nickel Catalysis.

The development of robust and reliable methods for the construction of C(sp2)-C(sp3) bonds is vital for accessing an increased array of structurally diverse scaffolds in drug discovery and development campaigns. While significant advances towards this goal have been achieved using metallaphotoredox chemistry, many of these methods utilise photocatalysts based on precious-metals due to their efficient redox processes and tuneable properties. However, due to the cost, scarcity, and toxicity of these metals, the search for suitable replacements should be a priority. Here, we show the use of commercially available heterogeneous semiconductor graphitic carbon nitride (gCN) as a photocatalyst, combined with nickel catalysis, for the cross-coupling between aryl halide and carboxylic acid coupling partners. gCN has been shown to engage in single-electron-transfer (SET) and energy-transfer (EnT) processes for the formation of C-X bonds, and in this manuscript we overcome previous limitations to furnish C-C over C-O bonds using carboxylic acids. A broad scope of both aryl halides and carboxylic acids is presented, and recycling of the photocatalyst demonstrated. The mechanism of the reaction is also investigated.

Metallaphotoredox-catalysed sp(3)-sp(3) cross-coupling of carboxylic acids with alkyl halides.

In the past 50 years, cross-coupling reactions mediated by transition metals have changed the way in which complex organic molecules are synthesized. The predictable and chemoselective nature of these transformations has led to their widespread adoption across many areas of chemical research. However, the construction of a bond between two sp(3)-hybridized carbon atoms, a fundamental unit of organic chemistry, remains an important yet elusive objective for engineering cross-coupling reactions. In comparison to related procedures with sp(2)-hybridized species, the development of methods for sp(3)-sp(3) bond formation via transition metal catalysis has been hampered historically by deleterious side-reactions, such as ß-hydride elimination with palladium catalysis or the reluctance of alkyl halides to undergo oxidative addition. To address this issue, nickel-catalysed cross-coupling processes can be used to form sp(3)-sp(3) bonds that utilize organometallic nucleophiles and alkyl electrophiles. In particular, the coupling of alkyl halides with pre-generated organozinc, Grignard and organoborane species has been used to furnish diverse molecular structures. However, the manipulations required to produce these activated structures is inefficient, leading to poor step- and atom-economies. Moreover, the operational difficulties associated with making and using these reactive coupling partners, and preserving them through a synthetic sequence, has hindered their widespread adoption. A generically useful sp(3)-sp(3) coupling technology that uses bench-stable, native organic functional groups, without the need for pre-functionalization or substrate derivatization, would therefore be valuable. Here we demonstrate that the synergistic merger of photoredox and nickel catalysis enables the direct formation of sp(3)-sp(3) bonds using only simple carboxylic acids and alkyl halides as the nucleophilic and electrophilic coupling partners, respectively. This metallaphotoredox protocol is suitable for many primary and secondary carboxylic acids. The merit of this coupling strategy is illustrated by the synthesis of the pharmaceutical tirofiban in four steps from commercially available starting materials.

Towards Uniform Iodine Catalysis: Intramolecular C-H Amination of Arenes under Visible Light.

A photochemical catalytic amination of arenes is presented. The reaction proceeds under benign iodine catalysis in the presence of visible light as the initiator and provides access to a range of differently substituted arylamines. A total of 29 examples demonstrate the broad applicability of this mild oxidation method. The scope of the reaction could further be expanded to silyl-tethered derivatives, which undergo intramolecular amination upon formation of seven-membered heterocycles. Cleavage of the silicon tether provides access to the corresponding 3-substituted anilines.