Direct Hydrodecarboxylation of Aliphatic Carboxylic Acids: Metal- and Light-Free

Collective Total Synthesis of Four Ganoderma Meroterpenoids Based on an Intramolecular Aldol Strategy

The total synthesis of four Ganoderma meroterpenoids (lingzhiol, sinensilactam A, lingzhilactone B, and applanatumol I) has been accomplished from a known olefinic lactone in much shorter steps (4-8 steps) and markedly improved overall yields (15-27%) compared to previous syntheses. The key steps are highly regio- and diastereoselective intramolecular aldol reactions to prepare bicyclic lactone intermediates and a decarboxylative radical cyclization to install the unique tetracyclic ring system of lingzhiol.

Heterocoupling Two Similar Benzyl Radicals by Dual Photoredox/Cobalt Catalysis

Transition-metal-regulated radical cross coupling enables the selective bonding of two distinct transient radicals, whereas the catalytic method for sorting two almost identical transient radicals, especially similar benzyl radicals, is still rare. Herein, we show that leveraging dual photoredox/cobalt catalysis can selectively couple two similar benzyl radicals. Using easily accessible methylarenes and phenylacetates (benzyl N-hydroxyphthalimide (NHPI) esters) as benzyl radical sources, a range of unsymmetrical 1,2-diarylethane classes via the 1°-1°, 1°-2°, 1°-3°, 2°-2°, 2°-3° and 3°-3° couplings were obtained with broad functional group tolerance. Besides the photochemical continuous flow synthesis, the one-pot procedure that directly uses phenylacetic acids and NHPI as the starting materials to avoid the pre-preparation of benzyl NHPI esters for the gram-scale synthesis is also feasible and affords good yields, showcasing the synthetic utility of our protocol.

Light-Mediated Direct Decarboxylative Giese Aroylations without a Photocatalyst

Previous light-mediated approaches to the direct decarboxylative Giese aroylation reaction have mainly relied on the use of a photocatalyst and a reductive quenching pathway. By exploiting a mechanistically distinct oxidative protocol, we have successfully developed a photocatalyst-free, light-mediated direct Giese aroylation methodology.

Direct C-H amidation of 1,3-azoles: light-mediated, photosensitiser-free vs. thermal

We have developed one thermal and one light-mediated method for direct Minisci-type C-H amidation of 1,3-azoles, which are applicable to thiazoles, benzothiazoles, benzimidazoles, and for the first time, imidazoles. The new visible light-mediated approach can be rendered photosensitiser/photocatalyst-free and likely proceeds via an electron donor-acceptor (EDA) complex, the first direct Minisci-type amidation to do so.

Redox-Neutral Multicatalytic Cerium Photoredox-Enabled Cleavage of O-H Bearing Substrates

The need for synthetic methodologies capable of rapidly altering molecular structure are in high demand. Most existing methods to modify scaffolds rely on net exothermicity to drive the desired transformation. We sought to develop a general strategy for the cleavage of C-C bonds β to hydroxyl groups independent of inherent substrate strain. To this end we have applied a multicatalytic cerium photoredox-based system capable of activating O-H bonds in lactols to deliver formate esters. The same system is also capable of effecting hydrodecarboxylation and hydrodecarbonylation reactions. Initial mechanistic probes demonstrate atomic chlorine (Cl⋅) is generated under the reaction conditions, but substrate activation through cerium-alkoxides or -carboxylates cannot be ruled out.

Asymmetric synthesis of the fully functionalized six-membered A-ring of siphonol A

A synthetic study toward the construction of the fully functionalized six-membered A-ring of siphonol A is described. The salient features include the introduction of a six-membered ring system through a HWE reaction, the construction of a stereocenter at C5 via a hetero-Diels-Alder reaction, and the installation of the fully functionalized six-membered A-ring of siphonol A through photolytic decarboxylation.

Direct decarboxylative Giese amidations: photocatalytic vs. metal- and light-free

A direct intermolecular decarboxylative Giese amidation reaction from bench stable, non-toxic and environmentally benign oxamic acids has been developed, which allows for easy access to 1,4-difunctionalised compounds which are not otherwise readily accessible. Crucially, a more general acceptor substrate scope is now possible, which renders the Giese amidation applicable to more complex substrates such as natural products and chiral building blocks. Two different photocatalytic methods (one via oxidative and the other via reductive quenching cycles) and one metal- and light-free method were developed and the flexibility provided by different conditions proved to be crucial for enabling a more general substrate scope.

Divergent Catalysis: Catalytic Asymmetric [4+2] Cycloaddition of Palladium Enolates

An asymmetric decarboxylative [4+2] cycloaddition from a catalytically generated chiral Pd enolate was developed, forging four contiguous stereocenters in a single transformation. This was achieved through a strategy termed divergent catalysis, wherein departure from a known catalytic cycle enables novel reactivity of a targeted intermediate prior to re-entry into the original cycle. Mechanistic studies including quantum mechanics calculations, Eyring analysis, and KIE studies offer insight into the reaction mechanism.

Visible Light Photoredox-Catalyzed Decarboxylative Alkylation of 3-Aryl-Oxetanes and Azetidines via Benzylic Tertiary Radicals and Implications of Benzylic Radical Stability

Four-membered heterocycles offer exciting potential as small polar motifs in medicinal chemistry but require further methods for incorporation. Photoredox catalysis is a powerful method for the mild generation of alkyl radicals for C-C bond formation. The effect of ring strain on radical reactivity is not well understood, with no studies that address this question systematically. Examples of reactions that involve benzylic radicals are rare, and their reactivity is challenging to harness. This work develops a radical functionalization of benzylic oxetanes and azetidines using visible light photoredox catalysis to prepare 3-aryl-3-alkyl substituted derivatives and assesses the influence of ring strain and heterosubstitution on the reactivity of small-ring radicals. 3-Aryl-3-carboxylic acid oxetanes and azetidines are suitable precursors to tertiary benzylic oxetane/azetidine radicals which undergo conjugate addition into activated alkenes. We compare the reactivity of oxetane radicals to other benzylic systems. Computational studies indicate that Giese additions of unstrained benzylic radicals into acrylates are reversible and result in low yields and radical dimerization. Benzylic radicals as part of a strained ring, however, are less stable and more π-delocalized, decreasing dimer and increasing Giese product formation. Oxetanes show high product yields due to ring strain and Bent's rule rendering the Giese addition irreversible.

Benzophenone as a cheap and effective photosensitizer for the photocatalytic synthesis of dimethyl cubane-1,4-dicarboxylate

The key intramolecular [2 + 2] photochemical cycloaddition step in the synthesis of dimethyl cubane-1,4-dicarboxylate is performed with substoichiometric amounts of the photosensitizer benzophenone. The reaction proceeds via a Dexter energy transfer process between the triplet excited state benzophenone and a well-known cubane precursor diene. The use of the cheap and widely available benzophenone as the photosensitizer enables lower energy light to be used than the traditional photochemical process.

Chemoselective Decarboxylative Protonation Enabled by Cooperative Earth-Abundant Element Catalysis

Decarboxylative protonation is a general deletion tactic to replace polar carboxylic acid groups with hydrogen or its isotope. Current methods rely on the pre-activation of acids, non-sustainable hydrogen sources, and/or expensive/highly oxidizing photocatalysts, presenting challenges to their wide adoption. Here we show that a cooperative iron/thiol catalyst system can readily achieve this transformation, hydrodecarboxylating a wide range of activated and unactivated carboxylic acids and overcoming scope limitations in previous direct methods. The reaction is readily scaled in batch configuration and can be directly performed in deuterated solvent to afford high yields of d-incorporated products with excellent isotope incorporation efficiency; characteristics not attainable in previous photocatalyzed approaches. Preliminary mechanistic studies indicate a radical mechanism and kinetic results of unactivated acids (KIE=1) are consistent with a light-limited reaction.

Direct Minisci-Type C–H Amidation of Purine Bases

A method for the C–H carboxyamidation of purines has been developed that is capable of directly installing primary, secondary, and tertiary amides. Previous Minisci-type investigations on purines were limited to alkylations and arylations. Herein, we present the first method for the direct C–H amidation of a wide range of purines: xanthine, guanine, and adenine structures, including guanosine- and adenosine-type nucleosides. The Minisci-type reaction is also metal-free, cheap, operationally simple, scalable, and applicable to late-stage functionalizations of biologically important molecules.

Photomediated Hydro- and Deuterodecarboxylation of Pharmaceutically Relevant and Natural Aliphatic Carboxylic Acids

Herein, we report a photomediated hydro- and deuterodecarboxylation of different primary, secondary, and tertiary carboxylic acids catalyzed by an organic pyrimidopteridine photoredox catalyst. The reaction was optimized by a statistical design of experiment (DoE). Under optimized reaction conditions, the conversion of commercially available nonsteroidal anti-inflammatory drugs (NSAIDs) in tablet form and on gram scale was realized. The scope of the application comprises primary, secondary, and tertiary aliphatic biologically active carboxylic acids. A deuterium incorporation of up to 95% by using D₂O as inexpensive deuterium source was achieved. A sensitivity assessment as well as experiments aiding the elucidation of the reaction mechanism are discussed.