Nickel-Catalyzed Site-Selective Intermolecular C(sp3 )-H Amidation

Fe-Catalyzed α-C(sp3)-H Amination of N-Heterocycles

Nitrogen-heterocycles are privileged structures in both marketed drugs and natural products. On the other hand, C-H amination reactions furnish unconventional and straightforward approaches for the construction of C-N bonds. Yet, most of the known methods rely on precious metal catalysts. Herein we report a site-selective intermolecular C(sp3)-H amination of N-heterocycles, catalyzed by inexpensive FeCl2, which allows the functionalization of a wide range of pharmaceutically relevant cyclic amines. The C-H amination occurs site-selectively in α-position to the nitrogen atom, even when weaker C-H bonds are present, and furnishes Troc-protected aminals or amidines. The method employs the N-heterocycle as limiting reagent and is applicable to the late-stage functionalization of complex molecules. Its synthetic potential was further illustrated through the derivatization of α-aminated products and the application to a concise total synthesis of the reported structure for senobtusin. Mechanistic studies allowed to propose a plausible reaction mechanism involving a turnover-limiting Fe-nitrene generation followed by fast H atom transfer and radical rebound.

Ni-Catalyzed Enantioselective Reductive Cyclization/Amidation and Amination of 1,6-Enynes and 1,7-Enynes

Transition-metal-catalyzed hydroamination of unsaturated hydrocarbons is an appealing synthetic tool for the construction of high value-added chiral amines. Despite significant progress in the asymmetric hydroamination of alkenes, allenes, and 1,3-dienes, asymmetric hydroamination of 1,6-enynes or 1,7-enynes remains rather limited due to the enormous challenges in controlling the chemoselectivity and stereoselectivity of the reaction. Herein, we report a Ni-catalyzed chemo- and enantioselective reductive cyclization/amidation and amination of 1,6-enynes and 1,7-enynes using dioxazolones or anthranils as nitrene-transfer reagents. This mild, modular, and practical protocol provides rapid access to a variety of enantioenriched 2-pyrrolidone and 2-piperidone derivatives bearing an aminomethylene group at the 4-position in good yields (up to 83 %) with excellent enantioselectivities (46 examples, up to 99 % ee). Mechanistic experiments and density functional theory calculations indicate that the reaction is initiated by hydronickelation of alkynes followed by migratory insertion into alkenes, rather than by a [2+2+1] oxidative addition process of nickel to alkenes and alkynes.

Acridine/Lewis Acid Complexes as Powerful Photocatalysts: A Combined Experimental and Mechanistic Study

A class of in-situ generated Lewis acid (LA) activated acridine complexes is reported, which act as potent photochemical catalysts for the oxidation of a variety of protected secondary amines. Acridine/LA complexes exhibit tunable excited state reduction potentials ranging from +2.07 to 2.38 V vs. SCE. The ytterbium triflate complex of 3,6-di-t-butyl-9-mesitylacridine catalyzes a photochemical Giese-type reaction of Boc-protected secondary amines with challenging conjugate acceptors such as acrylates, that are inaccessible to the analogous acridinium (t-Bu-Mes-Acr⊕) catalyzed reaction. The mechanism of this reaction was investigated using a suite of physical organic probes including intramolecular 13C kinetic isotope effects (KIEs), variable time normalization analysis (VTNA) kinetics, determination of redox potentials, and computational studies. In the reaction catalyzed by t-Bu-Mes-Acr⊕, mechanistic studies are consistent with single-electron transfer (SET) from the ground-state reduced t-Bu-Mes-Acr• to the α-keto radical intermediate as the first irreversible step in the catalytic cycle. Intriguingly, we find that the reduced acridine/LA complexes are better ground state reductants (-0.72 to -0.74 V vs SCE) relative to t-Bu-Mes-Acr• (-0.59 V vs SCE) and predict that the increased substrate reactivity stems from a lower energy barrier for this key SET event.

Heterogeneous Photocatalytic Strategies for C(sp3 )-H Activation

Activation of ubiquitous C(sp3 )-H bonds is extremely attractive but remains a great challenge. Heterogeneous photocatalysis offers a promising and sustainable approach for C(sp3 )-H activation and has been fast developing in the past decade. This Minireview focuses on mechanism and strategies for heterogeneous photocatalytic C(sp3 )-H activation. After introducing mechanistic insights, heterogeneous photocatalytic strategies for C(sp3 )-H activation including precise design of active sites, regulation of reactive radical species, improving charge separation and reactor innovations are discussed. In addition, recent advances in C(sp3 )-H activation of hydrocarbons, alcohols, ethers, amines and amides by heterogeneous photocatalysis are summarized. Lastly, challenges and opportunities are outlined to encourage more efforts for the development of this exciting and promising field.

Sulfur-Directed α-C(sp3)-H Amidation of Pyrrolidines with Dioxazolones under Rhodium Catalysis

Site-selective functionalization of saturated N-heterocycles such as pyrrolidines is a central topic in organic synthesis and drug discovery. We herein report the sulfur-assisted rhodium(III)-catalyzed sp3 C-H amidation of pyrrolidines with dioxazolones as amidating agents. The amenability of the thioamide directing group is elucidated by a series of control experiments.

Amidative β-Scission of Alcohols Enabled by Dual Catalysis of Photoredox Proton-Coupled Electron Transfer and Inner-Sphere Ni-Nitrenoid Transfer

The photoredox/Ni dual catalysis is an appealing strategy to enable unconventional C-heteroatom bond formation. While significant advances have been achieved using this system, intermolecular C(sp3)-N bond formation has been relatively underdeveloped due to the difficulty in C(sp3)-N reductive elimination. Herein, we present a new mechanistic approach that utilizes dioxazolones as the Ni(II)-nitrenoid precursor to capture carbon-centered radicals by merging proton-coupled electron transfer (PCET) with nickel catalysis, thus forming synthetically versatile N-alkyl amides using alcohols. Based on mechanistic investigations, the involvement of (κ2-N,O)Ni(II)-nitrenoid species was proposed to capture photoredox PCET-induced alkyl radicals, thereby playing a pivotal role to enable the C(sp3)-N bond formation.

Alicyclic-Amine-Derived Imine-BF3 Complexes: Easy-to-Make Building Blocks for the Synthesis of Valuable α-Functionalized Azacycles

A new strategy to access α-functionalized alicyclic amines via their corresponding imine-BF3 complexes is reported. Isolable imine-BF3 complexes, readily prepared via dehydrohalogenation of N-bromoamines in a base-promoted/18-crown-6 catalyzed process followed by addition of boron trifluoride etherate, undergo reactions with a wide range of organometallic nucleophiles to afford α-functionalized azacycles. Organozinc and organomagnesium nucleophiles add at ambient temperatures, obviating the need for cryogenic conditions. In situ preparation of imine-BF3 complexes provides access to α-functionalized morpholines and piperazines directly from their parent amines in a single operation. α-Functionalized morpholines can be elaborated further, for instance by installing a second substituent in the α'-position.

A Formal γ-C-H Functionalization of Carboxylic Acids Guided by Metal-Nitrenoids as an Unprecedented Mechanistic Motif

Harnessing the key intermediates in metal-catalyzed reactions is one of the most essential strategies in the development of selective organic transformations. The nitrogen group transfer reactivity of metal-nitrenoids to ubiquitous C-H bonds allows for diverse C-N bond formation to furnish synthetically valuable aminated products. In this study, we present an unprecedented reactivity of iridium and ruthenium nitrenoids to generate remote carbocation intermediates, which subsequently undergo nucleophile incorporation, thus developing a formal γ-C-H functionalization of carboxylic acids. Mechanistic investigations elucidated a unique singlet metal-nitrenoid reactivity to initiate an abstraction of γ-hydride to form the carbocation intermediate that eventually reacts with a broad range of carbon, nitrogen, and oxygen nucleophiles, as well as biorelevant molecules. Alternatively, the same intermediate can lead to deprotonation to afford β,γ-unsaturated amides in a less nucleophilic solvent.

Comproportionation and disproportionation in nickel and copper complexes

Disproportionation and comproportionation reactions have become increasingly important electron transfer events in organometallic chemistry and catalysis. The renewed interest in these reactions is in part attributed to the improved understanding of first-row metals and their ability to occupy odd and even oxidation states. Disproportionation and comproportionation reactions enable metal complexes to shuttle between various oxidation states, a matter of utmost relevance for controlling the speciation and catalytic turnover. In addition, these reactions have a direct impact in the thermodynamic and kinetic stability of the corresponding metal complexes. This review covers the relevance and impact of these processes in electron transfer reactions and provides valuable information about their non-negligible influence in Ni- and Cu-catalysed transformations.

Regio- and Enantioselective Catalytic δ-C-H Amidation of Dioxazolones Enabled by Open-Shell Copper-Nitrenoid Transfer

Controlling regio- and enantioselectivity in C-H functionalization reactions is of paramount importance due to their versatile synthetic utilities. Herein, we describe a new approach for the asymmetric δ-C(sp³)-H amidation catalysis of dioxazolones using a Cu(I) precursor with a chiral bisoxazoline ligand to access six-membered lactams with high to excellent regio- and enantioselectivity (up to >19:1 rr and >99:1 er). Combined experimental and computational mechanistic studies unveiled that the open-shell character of the postulated Cu-nitrenoids enables the regioselective hydrogen atom abstraction and subsequent enantio-determining radical rebound of the resulting carbon radical intermediates. The synthetic utility of this asymmetric cyclization was demonstrated in the diastereoselective introduction of additional functional groups into the chiral δ-lactam skeleton as well as in the rapid access to biorelevant azacyclic compounds.