Halogen-atom-transfer (XAT) processes have revolutionized the use of ubiquitous halide reagents in organic chemistry. This mini-review focuses on recent C−C bond forming reactions that have exploited α-aminoalkyl radicals as metal-free XAT procedures.

Abstract

The merging of photocatalysis with halogen-atom transfer (XAT) processes has proven to be a versatile tool for the generation of carbon-centered radicals in organic synthesis. XAT processes are unique in that they generate radicals without requiring the use of strong reductants necessary for the traditional single electron transfer (SET) activation of halides. Pathways to achieve XAT in synthetic applications can be categorized into three major sections: i) heteroatom-based activators, ii) metal-based activators, and iii) carbon-based activators among which α-aminoalkyl radicals have taken the center stage. Access to these α-aminoalkyl radicals as XAT reagents has gained significant attention in the past few years due to the robustness of the reactions, the simplicity of the reagents required, and the broadness of their applications. Generation of these α-aminoalkyl radicals is simply achieved through the single electron oxidation of tertiary amines, which after deprotonation at the α-position generates the α-aminoalkyl radicals. Due to the wide scope of tertiary amines available and the tunable nucleophilicity of α-aminoalkyl radical formed, this strategy has become an attractive alternative to heteroatom/metal-based radicals for XAT. In this minireview, we focus our attention on recent (2020–2023) developments and uses of this robust technology to mediate XAT processes.

C−H annulation with alkynes has been demonstrated to be one powerful strategy for the expedient construction of ring systems, while the use of unsymmetrical alkynes often led to elusive regioselectivity. Herein, we summarized recent exploration of this field, including the development of directing groups, catalytic systems, and versatile alkynes, which has led to prosperous achievements toward the application in drugs, natural products, and materials.

Abstract

Selective and concise construction of ring systems that are ubiquitous skeletons across chemistry, drugs and materials, is indispensable for human life. Of note, directed C−H annulation with alkynes for the expedient delivery of ring systems holds great importance, featuring step- and atom-economy, mild conditions, and broad substrate scope. However, regioselectivity issues remained when using unsymmetrical alkynes for the directed C−H annulation. Herein, we summarized recent achievements towards solving this problem by developing directing groups, metal catalysts, and alkynes with versatile and traceless functionality that ensure the overall regioselectivity, enantioselectivity, efficiency, and synthetic application. We hope this concept will promote the further development of the precise construction of functional molecules using C−H annulation with unsymmetrical alkynes.

This review provides the overview of the fundamental concepts on ketone properties, and summarizes the recent advances of visible light-induced asymmetric reactions of carbonyl compounds for synthesizing chiral alcohols, which are introduced by the type of catalytic system, including single catalyst and synergetic catalysts. It is hopeful to provide guidance and assistance for the development of this field in future.

Abstract

Visible light-induced photocatalysis has been widely investigated, which offers exciting opportunities to build new catalytic platforms that are unattainable under ground state conditions. Asymmetric photocatalysis has been a longstanding challenge due to the high reactivity of photogenerated intermediates leading to strong background reaction. Carbonyl group is an important fundamental scaffold in organic synthesis. The photocatalytic asymmetric transformations of carbonyl compounds for synthesizing enantioenriched secondary and tertiary alcohols are of significant value but remain problematic. Even so, a series of intriguing works concerning this topic have been reported in recent year. This review summarizes the advances in this area, mainly dividing into single and synergetic catalyst systems, and the mechanism of each reaction is discussed.

Disclosed, herein, a flexible multidentate ligand (L) for Co/Mn-based supramolecular materials, serving as a reusable catalyst for C-/N-alkylation of alcohols with higher reaction efficiency and selectivity compared to individual components via its flexible binding sites, diverse hydrogen bond donor-acceptor fragments, rigidity, variable coordination mode, and cooperativity.

Abstract

Despite the progress on cobalt and manganese catalyzed C−C and C−N bond-forming methodologies, the associated catalyst reusability remains with some unresolved issue, which needs to be addressed. Disclosed herein, a flexible multidentate proton-responsive ligand (L) bearing 2,6-bis(1H-benzo[d]imidazol-2-yl)pyridine (BBP), 6-(1H-benzo[d]imidazol-2-yl)picolinic acid (BPA), and benzene-1,2-diamine (BDA) for Co/Mn-based mono- and bi-metallic supramolecular materials for C-/N-alkylation of alcohols. The flexible binding sites and different hydrogen bond donor-acceptor fragments of L brings the rigidity and self-assembling to ordered crystalline supramolecular materials, which prevented the coordinatively saturated active sites and thus providing much higher reaction efficiency and selectivity, which is highly unlikely in the case of comparable individual components. The easy synthesis, efficient reactivity and selectivity through cooperativity, broad substrate scope, and efficient recycling via recharging of metals make the catalyst and the protocol economical and sustainable. Importantly, the design strategy based on metallo-organic hydrogen bonded coordination assembly has the potential to contribute to the development of supramolecular materials for various advanced catalytic applications.

Plasmonic photocatalysis: A boosting hot-electron-hole separation driven by interfacial contact potential enables high photocatalytic activity on Au_core-Ag_shell bimetallic nanoparticles towards the four-electron reduction of 4-NTP to 4,4′-DMAB.

Abstract

Plasmonic photocatalysis under visible-light irradiation has long been regarded as a very promising strategy for inducing chemical transformations. However, the efficient utilization of these hot electrons on monometallic nanoparticles to induce chemical reaction remains a challenging subject. Here, we study plasmonic hot electron activity of Au_core-Ag_shell bimetallic nanoparticles towards the four-electron reduction of 4-nitrothiophenol to 4,4′-dimercaptoazobenzene. Our results show that Au_core-Ag_shell nanoparticles possess a higher catalytic activity than pure Au and Ag nanoparticles and the photocatalytic transformation is strongly dependent on the thickness of Ag shell. The plasmonic catalytic activity could be explained by a boosting hot-electron-hole separation driven by the contact potential at the bimetallic interface. This work provides new opportunities to enhance the efficient utilization of hot electron for plasmonic photocatalysis reaction.

Catalytic Hydrodeoxygenation of Guaiacol: By constructing a first-principles microkinetic model from over 300 DFT models of the intermediates, we report that guaiacol HDO exhibits highly desirable deoxygenation and hydrogenation kinetics over Ni(111) at industrial temperatures.

Abstract

The mechanism behind the hydrodeoxygenation (HDO) of guaiacol on Co(0001), Ni(111), Cu(111), Pd(111), and Pt(111) was investigated by constructing a first-principles microkinetic model from density functional theory (DFT) models for 68 possible intermediates over each surface. We report that the most energetically favorable pathway for this process is the demethylation of guaiacol to catechol over Ni(111), which exhibits highly desirable deoxygenation and hydrogenation kinetics at industrial temperatures. Guaiacol readily undergoes hydrogenation over Pt(111) and Pd(111), but the products exhibit slow desorption from the surfaces at standard operation temperatures. Furthermore, the deoxygenation pathway is hindered by the high energy barrier associated with the scission of the C_alkyl−O bond.

Au(III) π-allyl complexes have recently been shown to be readily accessible and stable, as well as highly reactive towards nucleophiles, including in catalytic transformations. They display structural features and reactivity profiles that differ noticeably from the related Pd(II) π-allyl complexes, making them complementary and attractive for synthesis.

Abstract

π-Allyl complexes of transition metals are key species in organometallic chemistry and homogeneous catalysis. Palladium(II) π-allyl complexes in particular, have gained a lot of attention, but their isoelectronic gold(III) counterparts long remained elusive. However, this situation changed during the last few years. This concept article describes the preparative routes, characterization, structure and reactivity of such species, together with their catalytic applications. The influence of the ancillary ligand at gold, either (P,C)/(N,C)-cyclometalated or (P,N)-hemilabile, is analysed in detail. The Au(III) and Pd(II) π-allyl complexes are also compared to highlight the similarities and differences.

The balance of activity and selectivity in liquid alkane oxidation is challenging to design supported Fe catalysts. When Fe species leach to the solvent, uncontrolled free radical chain reactions happen. Herein, we have constructed isolated Fe(3+) species by forming a strong Fe-O-Si bond for the selective oxidation of cyclohexane to cyclohexanone by H2O2. Compared to the supported FeOx clusters, the strong Fe-O-Si bond between isolated Fe(3+) species and SiO2 prevents the leaching of Fe in strong oxidation (H2O2) reaction conditions and dominates the non-free radical mechanism. The turnover frequency over the 10FeOx/SBA-15 reached 15.2 h-1, higher than the reported Fe-based catalysts. The high selectivity of cyclohexanone is maintained at different conversions. Moreover, the (SiO)xFe3+(OH)3-x-OOH active species were detected by Raman and FTIR and are generated from the oxidation of isolated Fe species. The strong Fe-O-Si bond and non-free radical mechanism by (SiO)xFe3+(OH)3-x-OOH active species induce high activity and selectivity for the oxidation of many other alkanes.

Here we report that two very important catalytic transformations i. e., N-alkylation of amines with alcohols and β-alkylation of secondary alcohols with primary alcohols that is generally carried out with transition metal-based catalysts can be performed with a catalytic amount of base under air in a closed vessel without using transition metals or any other additives generating only water as byproduct.

Abstract

Borrowing hydrogen (BH) reactions are very useful for the sustainable synthesis of C−C and C−N bonds. They generally operate with transition metal-based catalysts along with stoichiometric/catalytic amounts of added base. Here we report that two catalytic transformations, generally carried out with the BH methodology, i. e. N-alkylation of amines with alcohols and β-alkylation of secondary alcohols with primary alcohols, can be performed very effectively with just catalytic amounts of base under air without using any transition metal-based catalyst. The mechanism is proposed to be based on air oxidation of the alcohol to aldehyde followed by condensation to an unsaturated intermediate which undergoes transfer hydrogenation with alcohol to the product.

The catalytic C−H borylation of (hetero)arenes involving borenium species has witnessed fast development in the past decade. This review aims at summarize recent progress made in this field, with an emphasis on synthetic application and reaction mechanisms.