O-Benzoylhydroxylamines as Alkyl Nitrene Precursors: Synthesis of Saturated N-Heterocycles from Primary Amines

Beyond Tertiary Amines: Introducing Secondary Amines by Palladium/Norbornene-Catalyzed Ortho Amination

Since the discovery of the palladium/norbornene (Pd/NBE)-catalyzed ortho amination in 2013, escaping the limitation of only yielding tertiary anilines has been a long-standing challenge. Here, we describe that, by carefully choosing the phosphine ligand and NBE mediator, the installation of a N-mono-alkylamino group becomes feasible. The reaction tolerates a wide range of aryl iodide substrates and various N-mono-tertiary alkylamine-derived electrophiles. Both ipso alkenylation and alkynylation can be realized. The synthetic utility of this method is exemplified by the formation of primary amino group via selective deprotection and streamlined access to N-heterocycles. Preliminary success of installing a bulky N-secondary alkylamino group and a mechanistic understanding of the decomposition pathways of mono N-alkylamine electrophiles have been obtained.

Examining the effects of additives and precursors on the reactivity of rhodium alkyl nitrenes generated from substituted hydroxylamines

In this study, the reactivity of the alkyl nitrenes, generated from the substituted hydroxylamine precursors, was determined using the same rhodium catalyst. The results revealed that in competitive C-H insertion experiments, the regioselectivity between benzylic and tertiary C-H bonds could be modulated by adding Brønsted acids or changing the substituents on oxygen. This study enhances our understanding of the metallonitrene structures and provides valuable insights for further development of selective N-heterocycle syntheses.

Chemical insights into the synthetic chemistry of five-membered saturated heterocycles-a transition metal-catalyzed approach

Drug design and delivery is primarily based on the hunt for new potent drug candidates and novel synthetic techniques. Recently, saturated heterocycles have gained enormous attention in medicinal chemistry as evidenced by the medicinal drugs listed in the FDA Orange Book. Therefore, the demand for novel saturated heterocyclic syntheses has increased tremendously. Transition metal (TM)-catalyzed reactions have remained the prime priority in heterocyclic syntheses for the last three decades. Nowadays, TM catalysis is well adorned by combining it with other techniques such as bio- and/or enzyme-catalyzed reactions, organocatalysis, or using two different metals in a single catalysis. This review highlights the recent developments of the transition metal-catalyzed synthesis of five-membered saturated heterocycles.

Photoinduced Decarbonylative Rearrangement of Diazabicyclo[2.2.2]Octenones: A Photochemical Approach of Diazabicyclo[4.1.0]heptene Skeleton from Masked o-Benzoquinone

We report a photoinduced decarbonylative rearrangement of diazabicyclo[2.2.2]octenone in the facile synthesis of a functionalized diazabicyclo[4.1.0]heptene skeleton, a unique derivative of the hydropyridazine type structure which could be found in a variety of biologically active natural products. The scope of functional group compatibility in the photoreaction was examined by taking advantage of the easy access of the heterobicyclo[2.2.2] structure from the Diels-Alder reaction of masked o-benzoquinones. 4-Phenyl-1,2,4-triazoline-3,5-dione served as the dienophile which provided the adjacent N-N unit in hexahydropyridazine-type products of subsequent photorearrangement.

Theoretical Exploration of Energy Transfer and Single Electron Transfer Mechanisms to Understand the Generation of Triplet Nitrene and the C(sp3)-H Amidation with Photocatalysts

Mechanistic explorations and kinetic evaluations were performed based on electronic structure calculations at the CASPT2//CASSCF level of theory, the Fermi's golden rule combined with the Dexter model, and the Marcus theory to unveil the key factors regulating the processes of photocatalytic C(sp³)-H amidation starting from the newly emerged nitrene precursor of hydroxamates. The highly reactive nitrene was found to be generated efficiently via a triplet-triplet energy transfer process and to be benefited from the advantages of hydroxamates with long-range charge-transfer (CT) excitation from the N-centered lone pair to the 3,5-bis(trifluoromethyl)benzoyl group. The properties of the metal-to-ligand charge-transfer (MLCT) state of photocatalysts, the functionalization of chemical moieties for substrates involved in the charge-transfer (CT) excitation, such as the electron-withdrawing trifluoromethyl group, and the energetic levels of singlet and triplet reaction pathways may regulate the reaction yield of C(sp³)-H amidation. Kinetic evaluations show that the triplet-triplet energy transfer is the main driving force of the reaction rather than the single electron transfer process. The effects of electronic coupling, molecular rigidity, and excitation energies on the energy transfer efficiency were further discussed. Finally, we investigated the inverted behavior of single-electron transfer, which is correlated unfavorably to the catalytic efficiency and amidation reaction. All theoretical explorations allow us to better understand the generation of nitrene with visible-light photocatalysts, to expand highly efficient substrate sources, and to broaden our scope of available photosensitizers for various cross-coupling reactions and the construction of N-heterocycles.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Photoredox/Iron Dual-Catalyzed Insertion of Acyl Nitrenes into C-H Bonds

In this work, the use of N-acyloxybenzamides as efficient acyl nitrene precursors under photoredox/iron dual catalysis is reported. The resulting acyl nitrenes could be captured by various types of C-H bonds and S- or P-containing molecules. Mechanism investigations suggested that the formation of the acyl nitrene from the N-acyloxybenzamide occurs by a photoredox process, and it is believed that in this redox process oxidative N-H bond cleavage of the N-acyloxybenzamide occurs prior to reductive N-O bond cleavage of the N-acyloxybenzamide.

Diversification of Amidyl Radical Intermediates Derived from C-H Aminopyridylation

The N-activating substituents typically encountered in C-H amination chemistry are challenging to remove and have limited scope for synthetic elaboration. Here, we demonstrate that N-benzylaminopyridinium species provide a platform for synthetic elaboration via reductive N-N bond activation to unveil electrophilic N-centered radicals. These reactive intermediates can be trapped either via anti-Markovnikov olefin carboamination to provide access to tetrahydroisoquinolines or via aza-Rubottom chemistry with silyl enol ethers to provide α-amino ketones.

Imbuing an Old Heterocycle with the Power of Modern Catalysis: An Isoxazolidin-5-one Story

Isoxazolidin-5-ones have been regarded as β-amino acid surrogates owing to their labile N-O bond. While many efforts have been devoted to the catalytic enantioselective synthesis of the core of this heterocycle, its further transformation has been less explored, especially in the context of catalysis. This review summarizes the author's research on the development of catalytic reactions using isoxazolidin-5-ones as substrates. Asymmetric catalysis has proven effective for C-C bond formation at the carbonyl α-carbon. Catalytic asymmetric allylation and direct Mannich-type reactions have been developed. Further, the resulting products have been readily converted into the corresponding quaternary β^2,2-amino acids. Moreover, isoxazolidin-5-ones have been identified as alkyl nitrene precursors in the presence of a suitable metal catalyst. The generated metallonitrene undergoes either the electrophilic amination of the aromatic ring or aliphatic C-H insertion, affording a series of cyclic β-amino acids. A remarkable difference in chemoselectivity between rhodium and copper alkyl nitrenes has also been demonstrated, highlighting the unique nature of the underexplored reactive intermediates. The various linear and cyclic β-amino acids obtained through the study are likely to find great utility in a broad range of chemical sciences.

Ligand-Enabled, Copper-Catalyzed Electrophilic Amination for the Asymmetric Synthesis of β-Amino Acids

Catalytic asymmetric nitrene transfer has emerged as a reliable method for the synthesis of nitrogen-containing chiral compounds. Herein, we report the copper-catalyzed intramolecular asymmetric electrophilic amination of aromatic rings. The reactive intermediate is a copper-alkyl nitrene generated from isoxazolidin-5-ones. Copper catalysis promotes three classes of asymmetric transformations, namely, asymmetric desymmetrization, parallel kinetic resolution, and kinetic resolution, expanding the repertoire of alkyl nitrene transfer and providing various cyclic and linear β-amino acids in their enantioenriched forms.

Asymmetric Synthesis of Enantiopure Pyrrolidines by C(sp3 )-H Amination of Hydrocarbons

The asymmetric synthesis of enantiopure pyrrolidines is reported via a streamlined strategy relying on two sequential C-H functionalizations of simple hydrocarbons. The first step is a regio- and stereoselective catalytic nitrene C-H insertion. Then, a subsequent diastereoselective cyclization involving a 1,5-hydrogen atom transfer (HAT) from a N-centered radical leads to the formation of pyrrolidines that can then be converted to their free NH-derivatives.

Generation and application of Cu-bound alkyl nitrenes for the catalyst-controlled synthesis of cyclic β-amino acids

The advent of saturated N-heterocycles as valuable building blocks in medicinal chemistry has led to the development of new methods to construct such nitrogen-containing cyclic frameworks. Despite the apparent strategic clarity, intramolecular C–H aminations with metallonitrenes have only sporadically been explored in this direction because of the intractability of the requisite alkyl nitrenes. Here, we report copper-catalysed intramolecular amination using an alkyl nitrene generated from substituted isoxazolidin-5-ones upon N–O bond cleavage. The copper catalysis exclusively aminates aromatic C(sp²)–H bonds among other potentially reactive groups, offering a solution to the chemoselectivity problem that has been troublesome with rhodium catalysis. A combined experimental and computational study suggested that the active species in the current cyclic β-amino acid synthesis is a dicopper alkyl nitrene, which follows a cyclisation pathway distinct from the analogous alkyl metallonitrene.

Copper-catalysed conditions have been developed for the chemoselective synthesis of cyclic β-amino acids.

Direct intramolecular C(sp3)–H bond sulfonamidation to synthesize benzosultam derivatives under metal-free conditions

An efficient intramolecular C(sp3)–H bond amination under metal-free conditions has been developed to provide a straightforward method for the synthesis of 5-membered cyclic sulfonamides (benzosultams).