Manganese-Catalyzed Regioselective C-H Lactonization and Hydroxylation of Fatty Acids with H2O2

Catalyst-Controlled Chemoselective γ-C(sp3)-H Lactonization of Carboxylic Acid: Methyl versus Methylene

Despite recent advances in ligand-enabled C(sp3)-H functionalization of native substrates, controlling chemoselectivity in the presence of methyl and methylene C(sp3)-H bonds remains a significant challenge. Herein, we report the first example of the Pd(II)-catalyzed chemoselective lactonization of γ-methyl and methylene C(sp3)-H bonds of carboxylic acids. Exclusive chemoselectivity of methyl or methylene γ-lactonization was achieved by using two different classes of Quinoline-Pyridone ligands. The bidentate ligand coordinating with Pd(II) via five-membered chelation favors γ-methyl C-H lactonization, whereas the ligand forming six-membered chelation affords γ-methylene C-H lactonization exclusively. Taking into account our previous findings, we show that the impact of ligand bite angle on chemoselectivity is different for five-membered and six-membered cyclopalladation processes. This method provides simple and versatile access to γ-lactones, including spiro- and fused ring systems. Deuterium incorporation experiments suggest that this observed chemoselectivity arises from both the C-H activation and C-O bond forming steps.

Versatile Copper-Catalyzed γ-C(sp3)-H Lactonization of Aliphatic Acids

Site-selective C(sp3)-H oxidation is of great importance in organic synthesis and drug discovery. γ-C(sp3)-H lactonization of free carboxylic acids provides the most straightforward means to prepare biologically important lactone scaffolds from abundant and inexpensive carboxylic acids; however, a versatile catalyst for this transformation with a broad substrate scope remains elusive. Herein, we report a simple yet broadly applicable and scalable γ-lactonization reaction of free aliphatic acids enabled by a copper catalyst in combination with inexpensive Selectfluor as the oxidant. This lactonization reaction exhibits compatibility with tertiary, benzylic, allylic, methylene, and primary γ-C-H bonds, affording access to a wide range of structurally diverse lactones such as spiro, fused, and bridged lactones. Notably, exclusive γ-methylene C-H lactonization of cycloalkane carboxylic acids and cycloalkane acetic acids was observed, giving either fused or bridged γ-lactones that are difficult to access by other methods. δ-C-H lactonization was only favored in the presence of tertiary δ-C-H bonds. The synthetic utility of this methodology was demonstrated by the late-stage functionalization of amino acids, drug molecules, and natural products, as well as a two-step total synthesis of (iso)mintlactones (the shortest synthesis reported to date).

Electrochemical Lactonization Enabled by Unusual Shono-Type Oxidation from Functionalized Benzoic Acids

A novel method for electrochemical lactonization via C(sp3)-H functionalization was developed. This metal- and oxidant-free strategy enabled the efficient synthesis of various lactones. Gram-scale reaction and derivatization of the lactone product demonstrated the synthetic utility of this methodology. Mechanistic studies using control experiments and CV curves elucidated the proposed intramolecular HAT and the oxidative cyclization pathway. An unusual Shono-type oxidation was realized through this electrochemical approach, proceeding without a traditional nucleophilic addition process.

Bioinspired Non-Heme Mn Catalysts for Regio- and Stereoselective Oxyfunctionalizations with H2 O2

In recent years, metalloenzymes-mediated highly selective oxidations of organic substrates under mild conditions have been inspiration for developing synthetic bioinspired catalyst systems, capable of conducting such processes in the laboratory (and, in the future, in industry), relying on easy-to-handle and environmentally benign oxidants such as H2 O2 . To date, non-heme manganese complexes with chiral bis-amino-bis-pyridylmethyl and structurally related ligands are considered as possessing the highest synthetic potential, having demonstrated the ability to mediate a variety of chemo- and stereoselective oxidative transformations, such as epoxidations, C(sp3 )-H hydroxylations and ketonizations, oxidative desymmetrizations, kinetic resolutions, etc. Furthermore, in the past few years non-heme Mn based catalysts have become the major platform for studies focused on getting insight into the molecular mechanisms of oxidant activation and (stereo)selective oxygen transfer, testing non-traditional hydroperoxide oxidants, engineering catalytic sites with enzyme-like substrate recognition-based selectivity, exploration of catalytic regioselectivity trends in the oxidation of biologically active substrates of natural origin. This contribution summarizes the progress in manganese catalyzed C-H oxygenative transformations of organic substrates, achieved essentially in the past 5 years (late 2018-2023).

Highly Enantioselective Catalytic Lactonization at Nonactivated Primary and Secondary γ-C-H Bonds

Chiral oxygenated aliphatic moieties are recurrent in biological and pharmaceutically relevant molecules and constitute one of the most versatile types of functionalities for further elaboration. Herein we report a protocol for straightforward and general access to chiral γ-lactones via enantioselective oxidation of strong nonactivated primary and secondary C(sp3)-H bonds in readily available carboxylic acids. The key enabling aspect is the use of robust sterically encumbered manganese catalysts that provide outstanding enantioselectivities (up to >99.9%) and yields (up to 96%) employing hydrogen peroxide as the oxidant. The resulting γ-lactones are of immediate interest for the preparation of inter alia natural products and recyclable polymeric materials.

Hydrogen Bonding-Assisted and Nonheme Manganese-Catalyzed Remote Hydroxylation of C-H Bonds in Nitrogen-Containing Molecules

The development of catalytic systems capable of oxygenating unactivated C-H bonds with excellent site-selectivity and functional group tolerance under mild conditions remains a challenge. Inspired by the secondary coordination sphere (SCS) hydrogen bonding in metallooxygenases, reported herein is an SCS solvent hydrogen bonding strategy that employs 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as a strong hydrogen bond donor solvent to enable remote C-H hydroxylation in the presence of basic aza-heteroaromatic rings with a low loading of a readily available and inexpensive manganese complex as a catalyst and hydrogen peroxide as a terminal oxidant. We demonstrate that this strategy represents a promising compliment to the current state-of-the-art protection approaches that rely on precomplexation with strong Lewis and/or Brønsted acids. Mechanistic studies with experimental and theoretical approaches reveal the existence of a strong hydrogen bonding between the nitrogen-containing substrate and HFIP, which prevents the catalyst deactivation by nitrogen binding and deactivates the basic nitrogen atom toward oxygen atom transfer and the α-C-H bonds adjacent to the nitrogen center toward H-atom abstraction. Moreover, the hydrogen bonding exerted by HFIP has also been demonstrated not only to facilitate the O-O bond heterolytic cleavage of a putative Mn^III-OOH precursor to generate Mn^V(O)(OC(O)CH₂Br) as an active oxidant but also to affect the stability and the activity of Mn^V(O)(OC(O)CH₂Br).