Chromium-Salen Catalyzed Cross-Coupling of Phenols: Mechanism and Origin of the Selectivity

Controlled Aerobic Oxidative Dimerization of Hydroxystilbenoids by Chromium Catalysis

Aerobic oxidative dimerization of hydroxystilbenoids is described. A Cr-salen complex catalyzed the dimerization of hydroxystilbenoids in 1,1,1,3,3,3-hexafluoroisopropanol to form compounds comprising a natural product-like scaffold (quadrangularin) or its precursor depending on the aromatic substituents. The addition of a catalytic amount of scandium triflate [Sc(OTf)3] to the reaction system altered the reaction outcome to give a different natural product-like compound, a pallidol-type dimer.

Visible-light TiO2-catalyzed synthesis of dihydrobenzofurans by oxidative [3 + 2] annulation of phenols with alkenyl phenols

An oxidative strategy for the preparation of dihydrobenzofurans via heterogeneous photocatalysis is reported. This method leverages the surface interaction between the alkenyl phenol and the TiO2 solid surface, which enables direct activation by visible light without the need for pre-functionalization or surface modification. The resulting alkenyl phenoxyl radical is proposed to be selectively captured by a neutral phenol nucleophile, rendering β-5' coupling with excellent chemo- and regio-selectivity. The reaction proceeds under benign conditions, using an inexpensive, nontoxic, and recyclable photocatalyst under visible light irradiation with air as the terminal oxidant at room temperature.

Electrochemical Cross-Coupling between N-(4-Hydroxyphenyl)-sulfonamides and 2-Naphthols: Synthesis of 2,2'-Bis(arenol)s

We herein present an electrochemical method for the dehydrogenative cross-coupling of N-(4-hydroxyphenyl)-sulfonamides and 2-naphthols. This transformation provides a direct and scalable approach to a wide range of C1-symmetric 2,2'-bis(arenol)s with moderate to high yields under mild conditions. Preliminary attempts with the asymmetric variant of this reaction were also performed with ≤55% ee for the synthesis of 2,2'-bis(arenol)s. Control experiments were conducted to propose a plausible mechanism for the reaction.

Csp2-H functionalization of phenols: an effective access route to valuable materials via Csp2-C bond formation

In the vast majority of top-selling pharmaceutical and industrial products, phenolic structural motifs are highly prevalent. Non-functionalized simple phenols serve as building blocks in the synthesis of value-added chemicals. It is worth mentioning that lignin, being the largest renewable biomass source of aromatic building blocks in nature, mainly consists of phenolic units, which enable the production of structurally diverse phenols. Given their remarkable applicability in the chemical value chain, many efforts have been devoted to increasing the molecular complexity of the phenolic scaffold. Among the key techniques, direct functionalization of Csp2-H is a powerful tool, enabling the construction of new Csp2-C bonds in an economical and atomic manner. Herein we present and summarize the large plethora of direct Csp2-H functionalization methods that enables scaffold diversification of simple, unprotected phenols, leading to the formation of new Csp2-C bonds. In this review article, we intend to summarize the contributions that appeared in the literature mainly in the last 5 years dealing with the functionalization of unprotected phenols, both catalytic and non-catalytic. Our goal is to highlight the key findings and the ongoing challenges in the stimulating and growing research dedicated to the development of new protocols for the valorization of phenols.

Synthesis of Phenol-Pyridinium Salts Enabled by Tandem Electron Donor-Acceptor Complexation and Iridium Photocatalysis

Herein, we describe a dual photocatalytic system to synthesize phenol-pyridinium salts using visible light. Utilizing both electron donor-acceptor (EDA) complex and iridium(III) photocatalytic cycles, the C-N cross-coupling of unprotected phenols and pyridines proceeds in the presence of oxygen to furnish pyridinium salts. Photocatalytic generation of phenoxyl radical cations also enabled a nucleophilic aromatic substitution (SNAr) of a fluorophenol with an electron-poor pyridine. Spectroscopic experiments were conducted to probe the mechanism and reaction selectivity. The unique reactivity of these phenol-pyridinium salts were displayed in several derivatization reactions, providing rapid access to a diverse chemical space.

Recent advances in oxidative phenol coupling for the total synthesis of natural products

Covering: 2008 to 2023This review will describe oxidative phenol coupling as applied in the total synthesis of natural products. This review covers catalytic and electrochemical methods with a brief comparison to stoichiometric and enzymatic systems assessing their practicality, atom economy, and other measures. Natural products forged by C-C and C-O oxidative phenol couplings as well as from alkenyl phenol couplings will be addressed. Additionally, exploration into catalytic oxidative coupling of phenols and other related species (carbazoles, indoles, aryl ethers, etc.) will be surveyed. Future directions of this particular area of research will also be assessed.

Modular Approach for the Synthesis and Bioactivity Profiling of 8,8'-Biflavones

In this work, we report the scalable and modular synthesis of a library of 55 monomeric and dimeric flavonoids including 14 8,8'-biflavones. The sterically demanding tetra-ortho-substituted axis of an acetophenone dimer key intermediate was constructed in a regioselective manner using Fe-mediated oxidative coupling. This step was systematically optimized and performed on up to multigram scale. The biological activities of this compound library were evaluated, including cytotoxicity against healthy and malignant human cell lines, antimicrobial activity against the apicomplexan parasite Toxoplasma gondii, and antioxidant capacity. A marked increase in activity for the 8,8'-dimeric structures compared to that of their monomeric counterparts was observed. Several biflavones were identified with high selectivity indices (low cytotoxicity and high antiprotozoal activity), showing that this class of natural products may serve as lead structures for further investigations.

A Fenton Approach to Aromatic Radical Cations and Diarylmethane Synthesis

Manipulating carbon-centered radicals to add to electron-deficient systems is a well-precedented process. By coupling the Fe(II)-mediated Fenton reaction with the Fe(III)-mediated single-electron oxidation of anisolic compounds, we demonstrate how electron-rich carbon-centered radicals can react with electron-rich arenes through a radical-polar cascade pathway. This bioinspired approach produces diarylmethane derivatives from simple unfunctionalized precursors.

A novel aminated lignin/geopolymer supported with Fe nanoparticles for removing Cr(VI) and naphthalene: Intermediates promoting the reduction of Cr(VI)

A novel, inexpensive and eco-friendly aminated lignin/geopolymer supported with Fe nanoparticles (Fe@N-L-GM) composite was successfully synthesized using kaolin and lignin as the major precursors. The prepared Fe@N-L-GM had larger specific surface area, rich oxygen-containing and nitrogen-containing functional groups, greater electron transfer ability and interconnective porous structure. The Fe@N-L-GM could be employed as the adsorbent of Cr(VI) and the activator of potassium peroxymonosulfate (PMS) for treatment of Cr(VI) and naphthalene (NAP) in wastewater. The adsorption and degradation results indicated that the maximum adsorption capacity of Cr(VI) could reach 65.83 mg g^-1, whereas the maximum NAP degradation efficiency could reach 97.81 %. The adsorbed Cr(VI) was mostly converted to the low toxic Cr(III) through the reduction of electron donors such as Fe(II), amino and hydroxyl groups. The quenching experiment results confirmed that ·OH might be the crucial ROSs in mediating NAP degradation. In the simultaneous removal experiment of Cr(VI) and NAP, the Cr(VI) removal rate was significantly improved in the presence of NAP, while phenol as the degradation intermediate of NAP might be the main substance for promoting the reduction of Cr(VI). This work provided the theoretical foundation and a new type of material for the simultaneous removal of heavy metal and persistent organic pollutants (POPs).

Visible-Light-Induced Oxidative Coupling of Phenols and Alkenylphenols with a Recyclable, Solid Photocatalyst

A photocatalytic method for phenol and alkenylphenol oxidative coupling is reported using an inexpensive heterogeneous titanium dioxide photocatalyst with air and visible light. During the coupling process, the Ti-substrate complex is activated under visible light through a ligand to metal charge transfer effect, and the diphenol adduct is proposed to form through a radical cation. The heterogeneous TiO2 catalyst remains stable throughout the reaction and can be easily removed and reused multiple times.

Edge-Decorated Polycyclic Aromatic Hydrocarbons by an Oxidative Coupling Approach

Oxidative phenol coupling reduces reliance on halo/metalated substrates used in conventional redox neutral couplings. A new strategy for constructing polycyclic aromatic hydrocarbons (PAHs) that incorporates oxidative phenol coupling is outlined in a three-stage approach: oxidative fragment coupling, linking of the two resultant units, and oxidative cyclization. The protocol allows rapid assembly of both planar and helical systems with a high degree of edge functionalization. The incorporation of 12 alkoxy groups on systems with 12 rings gave rise to lower optical gaps compared to systems with a lesser degree of edge functionalization.

Atmosphere- and Solvent-Controlled Coupling and Acetylation of Phenols Induced by Visible Light

A tunable coupling or acetylation of phenol derivatives with diacetyl was enabled through the switch of the atmosphere and solvent induced by visible light under metal-free conditions. Symmetric and asymmetric diphenols or binaphthols were obtained under oxygen in water or 1,1,1,3,3,3-hexafluoroisopropanol, whereas phenol acetates were formed under argon in the presence of diacetyl and acetic acid. The possibility to control the chemo- and regioselectivities enriches the synthetic versatility of photoreactions.

Atroposelective Nenitzescu Indole Synthesis

In the past decade, compounds bearing a stereogenic C-N axis have gained significant attention in fields ranging from ligand to drug design. Yet, the atroposelective synthesis of these molecules remains a considerable challenge. In contrast to recent methods using more advanced chiral catalysts, a very simply accessed Jacobsen-type chromium(III)-salen complex was used here as a chiral enantiopure Lewis acid catalyst for a highly atroposelective Nenitzescu indole synthesis. Mild reaction conditions afforded various 5-hydroxybenzo[g]indoles in up to 97 % yield. Moreover, through a simple work-up, very high enantiomeric excesses of up to 99 % could be obtained.

Mechanistic views and computational studies on transition-metal-catalyzed reductive coupling reactions

Transition-metal-catalyzed reductive coupling reactions have been considered as a powerful tool to convert two electrophiles into value-added products. Numerous related reports have shown the fascinating potential. Mechanistic studies, especially theoretical studies, can provide important implications for the design of novel reductive coupling reactions. In this review, we summarize the representative advancements in theoretical studies on transition-metal-catalyzed reductive coupling reactions and systematically elaborate the mechanisms for the key steps of reductive coupling reactions. The activation modes of electrophiles and the deep insights of selectivity generation are mechanistically discussed. In addition, the mechanism of the reduction of high-oxidation-state catalysts and further construction of new chemical bonds are also described in detail.

Pd-catalyzed Aerobic Cross-Dehydrogenative Coupling of Catechols with 2-Oxindoles and Benzofuranones: Reactivity Difference Between Monomer and Dimer

Persistent radicals, which are generated from 2-oxindole or benzofuranone dimers, are useful tools for designing the radical-based cross-coupling reaction to provide molecules containing a quaternary carbon. The persistent radical is accessible from both the dimer and monomer; however, the reactivity difference between these substrates for the oxidative cross-coupling reaction is not fully understood, most likely because of the mechanistic complexity. Here, we present details of an aerobic cross-dehydrogenative coupling (CDC) reaction using various monomers and catechols. UV-Vis analysis and mechanistic control experiments showed that the monomer is less reactive than the dimer under aerobic conditions. Our Pd(II)-BINAP-μ-hydroxo complex significantly improved the reactivity of the monomers for the aerobic CDC reaction with catechols, yielding results comparable to those of the corresponding dimer. The procedure, which enables the generation of the persistent radical in situ, is particularly useful when employing the monomer that is not readily converted to the corresponding dimer.

Chromium Nitride Umpolung Tuned by the Locus of Oxidation

Oxidation of a series of Cr^V nitride salen complexes (Cr^VNSal^R) with different para-phenolate substituents (R = CF₃, tBu, NMe₂) was investigated to determine how the locus of oxidation (either metal or ligand) dictates reactivity at the nitride. Para-phenolate substituents were chosen to provide maximum variation in the electron-donating ability of the tetradentate ligand at a site remote from the metal coordination sphere. We show that one-electron oxidation affords Cr^VI nitrides ([Cr^VINSal^R]⁺; R = CF₃, tBu) and a localized Cr^V nitride phenoxyl radical for the more electron-donating NMe₂ substituent ([Cr^VNSal^NMe2]^•+). The facile nitride homocoupling observed for the Mn^VI analogues was significantly attenuated for the Cr^VI complexes due to a smaller increase in nitride character in the M≡N π* orbitals for Cr relative to Mn. Upon oxidation, both the calculated nitride natural population analysis (NPA) charge and energy of molecular orbitals associated with the {Cr≡N} unit change to a lesser extent for the Cr^V ligand radical derivative ([Cr^VNSal^NMe2]^•+) in comparison to the Cr^VI derivatives ([Cr^VINSal^R]⁺; R = CF₃, tBu). As a result, [Cr^VNSal^NMe2]^•+ reacts with B(C₆F₅)₃, thus exhibiting similar nucleophilic reactivity to the neutral Cr^V nitride derivatives. In contrast, the Cr^VI derivatives ([Cr^VINSal^R]⁺; R = CF₃, tBu) act as electrophiles, displaying facile reactivity with PPh₃ and no reaction with B(C₆F₅)₃. Thus, while oxidation to the ligand radical does not change the reactivity profile, metal-based oxidation to Cr^VI results in umpolung, a switch from nucleophilic to electrophilic reactivity at the terminal nitride.

Using Physical Organic Chemistry Knowledge to Predict Unusual Metabolites of Synthetic Phenolic Antioxidants by Cytochrome P450

Biotransformation, especially by human CYP450 enzymes, plays a crucial role in regulating the toxicity of organic compounds in organisms, but is poorly understood for most emerging pollutants, as their numerous "unusual" biotransformation reactions cannot retrieve examples from the textbooks. Therefore, in order to predict the unknown metabolites with altering toxicological profiles, there is a realistic need to develop efficient methods to reveal the "unusual" metabolic mechanism of emerging pollutants. Combining experimental work with computational predictions has been widely accepted as an effective approach in studying complex metabolic reactions; however, the full quantum chemical computations may not be easily accessible for most environmentalists. Alternatively, this work practiced using the concepts from physical organic chemistry for studying the interrelationships between structure and reactivity of organic molecules, to reveal the "unusual" metabolic mechanism of synthetic phenolic antioxidants catalyzed by CYP450, for which the simple pencil-and-paper and property-computation methods based on physical organic chemistry were performed. The phenol-coupling product of butylated hydroxyanisole (BHA) (based on spin aromatic delocalization) and ipso-addition quinol metabolite of butylated hydroxytoluene (BHT) (based on hyperconjugative effect) were predicted as two "unusual" metabolites, which were further confirmed by our in vitro analysis. We hope this easily handled approach will promote environmentalists to attach importance to physical organic chemistry, with an eye to being able to use the knowledge gained to efficiently predict the fates of substantial unknown synthesized organic compounds in the future.

Copper-Catalyzed Selective Oxidative Cross-Coupling of Tryptophols and Tryptamines To Access Heterocyclic 3a,3a'-Bisindolines

The first example of cyclization cross-coupling of tryptophols and tryptamines has been realized by copper catalysis with air or oxone as the terminal oxidant, resulting in the direct construction of a new class of heterocyclic 3a,3a'-bisindolines in moderate to good yields with high chemoselectivities. A series of mechanistic control experiments were also conducted, indicating that the copper catalyst selectively coordinates with the nitrogen moiety of the tryptamine to initiate the oxidation, and a nucleophilic-alkylation process is proposed for the carbon-carbon bond-forming in the reaction. The novel synthetic strategies and molecular skeletons outlined in this work provide new ideas and concepts for the design of other useful reaction and potential drugs.

The Transformative Power of Biocatalysis in Convergent Synthesis

Achieving convergent synthetic strategies has long been a gold standard in constructing complex molecular skeletons, allowing for the rapid generation of complexity in comparatively streamlined synthetic routes. Traditionally, biocatalysis has not played a prominent role in convergent laboratory synthesis, with the application of biocatalysts in convergent strategies primarily limited to the synthesis of chiral fragments. Although the use of enzymes to enable convergent synthetic approaches is relatively new and emerging, combining the efficiency of convergent transformations with the selectivity achievable through biocatalysis creates new opportunities for efficient synthetic strategies. This Perspective provides an overview of recent developments in biocatalytic strategies for convergent transformations and offers insights into the advantages of these methods compared to their small molecule-based counterparts.

Biocatalytic oxidative cross-coupling reactions for biaryl bond formation

Biaryl compounds, with two connected aromatic rings, are found across medicine, materials science and asymmetric catalysis1,2. The necessity of joining arene building blocks to access these valuable compounds has inspired several approaches for biaryl bond formation and challenged chemists to develop increasingly concise and robust methods for this task3. Oxidative coupling of two C-H bonds offers an efficient strategy for the formation of a biaryl C-C bond; however, fundamental challenges remain in controlling the reactivity and selectivity for uniting a given pair of substrates4,5. Biocatalytic oxidative cross-coupling reactions have the potential to overcome limitations inherent to numerous small-molecule-mediated methods by providing a paradigm with catalyst-controlled selectivity6. Here we disclose a strategy for biocatalytic cross-coupling through oxidative C-C bond formation using cytochrome P450 enzymes. We demonstrate the ability to catalyse cross-coupling reactions on a panel of phenolic substrates using natural P450 catalysts. Moreover, we engineer a P450 to possess the desired reactivity, site selectivity and atroposelectivity by transforming a low-yielding, unselective reaction into a highly efficient and selective process. This streamlined method for constructing sterically hindered biaryl bonds provides a programmable platform for assembling molecules with catalyst-controlled reactivity and selectivity.

Cu-Catalyzed Synthesis of Benzoxazole with Phenol and Cyclic Oxime

A Cu-catalyzed straightforward synthesis of benzoxazoles from free phenols and cyclic oxime esters is reported. The mild reaction conditions tolerate various electron-withdrawing and electron-donating functional groups on both substrates, affording benzoxazoles in moderate to good yields. With this protocol, large-scale syntheses of Ezutromid and Flunoxaprofe in one or two steps are demonstrated. A catalytic mechanism, which includes Cu-catalyzed amination via inner-sphere electron transfer and consequent annulation, is proposed.

Cr-Catalyzed Direct ortho-Aminomethylation of Phenols

We developed a Cr-catalyzed strategy for the regioselective formation of C_sp²-C_sp³ bonds through the direct and efficient ortho-aminomethylation of N,N-dimethylanilines with phenols. The approach showed excellent site selectivity at the ortho-position of phenols and accommodated broad substrate scope and functional group compatibility for both N,N-dimethylanilines and phenols. Mechanistic studies revealed that the direct ortho-aminomethylation between N,N-dimethylanilines and phenols occurred via an ionic mechanism.

Chromium-Salen Complex/Nitroxyl Radical Cooperative Catalysis: A Combination for Aerobic Intramolecular Dearomative Coupling of Phenols

We describe an aerobic intramolecular dearomative coupling reaction of tethered phenols using a catalytic system consisting of a chromium-salen (Cr-salen) complex combined with a nitroxyl radical. This novel catalytic system enables formation of various spirocyclic dienone products including those unable to be accessed by previously reported methods efficiently under mild reaction conditions.

Bioinspired Selective Synthesis of Heterodimer 8-5' or 8-O-4' Neolignan Analogs

The bioinspired synthesis of heterodimer neolignan analogs is reported by single-electron oxidation of both alkenyl phenols and phenols individually, followed by a combination of the resultant radicals. This oxidative radical cross-coupling strategy can afford heterodimer 8-5' or 8-O-4' neolignan analogs selectively with the use of air as the terminal oxidant and copper acetate as the catalyst at room temperature.

Enantioselective Synthesis of 3,3'-Disubstituted 2-Amino-2'-hydroxy-1,1'-binaphthyls by Copper-Catalyzed Aerobic Oxidative Cross-Coupling

A challenging direct asymmetric catalytic aerobic oxidative cross-coupling of 2-naphthylamine and 2-naphthol, using a novel Cu^I /SPDO system, has been successfully developed for the first time. Enantioenriched 3,3'-disubstituted NOBINs were achieved and could be readily derived to divergent chiral ligands and catalysts. This reaction features high enantioselectivities (up to 96 % ee) and good yields (up to 80 %). The DFT calculations suggest that the F-H interactions between CF₃ of L17 and H-1,8 of 2-naphthol, and the π-π stacking between the two coupling partners could play vital roles in the enantiocontrol of this cross-coupling reaction.

Copper Catalyzed Oxidative Arylation of Tertiary Carbon Centers

We describe herein a Cu(OTf)₂ catalyzed oxidative arylation of a tertiary carbon-containing substrate including aryl malononitriles, 3-aryl benzofuran-2-ones, and 3-aryl oxindoles. In some cases, the nitrile groups of the aryl malononitriles undergo further reactions leading to lactones or imines. These reaction conditions are applicable for a range of arenes, including phenols, anilines, anisoles, and heteroarenes. Mechanistic studies support the formation of a cationic intermediate via a two-electron oxidation.

Oxidative Photocatalytic Homo- and Cross-Coupling of Phenols: Nonenzymatic, Catalytic Method for Coupling Tyrosine

The oxidative photocatalytic method for phenol-phenol homo-coupling and cross-coupling is described and isolated yields of 16-97% are obtained. Measured oxidation potentials and computed nucleophilicity parameters support a mechanism of nucleophilic attack of one partner onto the oxidized neutral radical form of the other partner. Understanding of this model permitted development of cross-coupling reactions between nucleophilic phenols/arenes and easily oxidized phenols with high selectivity and efficiency. A highlight of this method is that one equivalent of each coupling partner is utilized. Building on these findings, a non-enzymatic, catalytic method for coupling tyrosine was also developed.

Biomimetic Total Synthesis of (±)-Carbocyclinone-534 Reveals Its Biosynthetic Pathway

Carbocyclinone-534 is a new antibiotic produced after the metabolism of tapinarof. We identify a biomimetic total synthesis of carbocyclinone-534 in eight steps by taking advantage of an intermolecular Diels-Alder homodimerization/dehydrogenation/intramolecular Diels-Alder cycloaddition cascade. This synthetic sequence provides direct experimental evidence for revealing the biosynthetic pathway of carbocyclinone-534.

The Diaryliodonium(III) Salts Reaction With Free-Radicals Enables One-Pot Double Arylation of Naphthols

The chemoselective reaction of the C- followed by the O-centered naphthyl radicals with the more electron-deficient hypervalent bond of the diaryliodonium(III) salts is described. This discovered reactivity constitutes a new activation mode of the diaryliodonium(III) salts which enabled a one-pot doubly arylation of naphthols through the sequentialC s p 2 -C s p 2 /O-C s p 2 bond formation. The naphthyl radicals were generated in the reaction by the tetramethylpiperidinyl radical (TMP·) which resulted from the homolytic fragmentation of the precursor TMP2O. Experimental and DFT calculations provided a complete panorama of the reaction mechanism.

Salen-Based Conjugated Microporous Polymers for Efficient Oxygen Evolution Reaction

Exploring high-performance electrocatalysts, especially non-noble metal electrocatalysts, for the oxygen evolution reaction (OER) is critical to energy storage and conversion. Herein, we report for the first time that conjugated microporous polymers (CMPs) incorporating salen can be used as OER electrocatalysts with outstanding performances. The best OER electrocatalyst (salen-CMP-Fe-3) exhibits a low Tafel slope of 63 mV dec^-1 and an overpotential of 238 mV at 10 mA cm^-2 . DFT and Grand Canonical Monte Carlo calculations confirmed that the significantly improved electrocatalytic properties can be attributed to the intrinsic catalytic activity of the salen moiety and the enrichment effect of the pore structures. This work demonstrates that salen-based conjugated polymers are a type of metal-coordinated porous polymer that show excellent catalyst performance.

Total Synthesis of Pyrolaside B: Phenol Trimerization through Sequenced Oxidative C-C and C-O Coupling

A facile method to oxidatively trimerize phenols using a catalytic aerobic copper system is described. The mechanism of this transformation was probed, yielding insight that enabled cross-coupling trimerizations. With this method, the natural product pyrolaside B was synthesized for the first time. The key strategy used for this novel synthesis is the facile one-step construction of a spiroketal trimer intermediate, which can be selectively reduced to give the natural product framework without recourse to stepwise Ullmann- and Suzuki-type couplings. As a result, pyrolaside B can be obtained expeditiously in five steps and 16 % overall yield. Three other analogues were synthesized, thus highlighting the utility of the method, which provides new accessibility to this area of chemical space. A novel xanthene was also synthesized through controlled Lewis acid promoted rearrangement of a spiroketal trimer.

Vanadium-Catalyzed Oxidative Intramolecular Coupling of Tethered Phenols: Formation of Phenol-Dienone Products

A mild and efficient method for the vanadium-catalyzed intramolecular coupling of tethered free phenols is described. The corresponding phenol-dienone products are prepared directly in good yields with low catalyst loadings. Electronically diverse tethered phenol precursors are well tolerated, and the catalytic method was effectively applied as the key step in syntheses of three natural products and a synthetically useful morphinan alkaloid precursor.

Bulky 2,6-disubstituted aryl siloxanes and a disilanamine

The crystal structures of 5-bromo-1,3-di-tert-butyl-2-[(tri-methyl-sil-yl)-oxy]benzene, C17H29BrOSi, (I), 1,3-di-tert-butyl-2-[(tri-methyl-sil-yl)-oxy]benzene, C17H30OSi, (II), and N-(2,6-diiso-propyl-phen-yl)-1,1,1-trimethyl-N-(tri-methyl-sil-yl)silanamine, C18H35NSi2, (III), are reported. Compound (I) crystallizes in space group P21/c with Z' = 1, (II) in Pnma with Z' = 0.5 and (III) in Cmcm with Z' = 0.25. Consequently, the mol-ecules of (II) are constrained by m and those of (III) by m2m site symmetries. Despite this, both (I) and (II) are distorted towards mild boat conformations, as is typical of 2,6-di-tert-butyl-substituted phenyl compounds, reflecting the high local steric pressure of the flanking alkyl groups. Compound (III) by contrast is planar and symmetric, and this lack of distortion is compatible with the lower steric pressure of the flanking 2,6-diisopropyl substituents.

Correlation between the C-C Cross-Coupling Activity and C-to-Ni Charge Transfer Transition of High-Valent Ni Complexes

High-valent Ni complexes have proven to be good platforms for diverse cross-coupling reactions that are otherwise difficult to be achieved with conventional low-valent catalysts. However, their reductive elimination (RE) activities are still significantly variable by up to 5 orders of magnitude, depending on the supporting ligand and oxidation state of the Ni center. To elucidate frontier molecular orbitals (FMOs) that determine the RE activity of the Ni center, the electronic structures of cycloneophyl (CH₂C(CH₃)₂-o-C₆H₄) Ni^III and Ni^IV complexes have been characterized by utilizing various transition metal-based spectroscopic techniques such as electronic absorption, magnetic circular dichroism, electron paramagnetic resonance, resonance Raman, and X-ray absorption spectroscopies. In combination with density functional theory computations, the spectroscopic analyses have shown that the energies of the C-to-Ni charge-transfer (CT) electronic transitions are strongly correlated to the rates of C-C bond-forming RE reaction. This correlation suggests that the kinetic barrier of the RE reaction is determined by energy cost for internal CT (ICT) from the coordinated carbon moiety to the Ni center, and that FMOs involved in the RE reaction and the C-to-Ni CT electronic transitions are essentially identical. This FMO determination has led us to discover that photoexcitation to the C-to-Ni CT excited states accelerates the C-C cross-coupling reaction by up to 10⁵ times, as the CT electronic transition can substitute for the rate-determining ICT step of the RE reaction at the ground electronic state.

Cobalt-Catalyzed C-H Acetoxylation of Phenols with Removable Monodentate Directing Groups: Access to Pyrocatechol Derivatives

An efficient cobalt-catalyzed C-H acetoxylation of phenols has been developed by using PIDA (phenyliodine diacetate) as a sole acetoxy source to synthesize pyrocatechol derivatives for the first time. The key feature of this method is the use of earth-abundant metal cobalt as the green and inexpensive catalyst for the acetoxylation of C(sp²)-H bonds under neutral reaction conditions. Furthermore, the gram-scale reaction and late-stage functionalization demonstrated the usefulness of this method.

Synthetic Applications of Oxidative Aromatic Coupling-From Biphenols to Nanographenes

Oxidative aromatic coupling occupies a fundamental place in the modern chemistry of aromatic compounds. It is a method of choice for the assembly of large and bewildering architectures. Considerable effort was also devoted to applications of the Scholl reaction for the synthesis of chiral biphenols and natural products. The ability to form biaryl linkages without any prefunctionalization provides an efficient pathway to many complex structures. Although the chemistry of this process is only now becoming fully understood, this reaction continues to both fascinate and challenge researchers. This is especially true for heterocoupling, that is, oxidative aromatic coupling with the chemoselective formation of a C-C bond between two different arenes. Analysis of the progress achieved in this field since 2013 reveals that many groups have contributed by pushing the boundary of structural possibilities, expanding into surface-assisted (cyclo)dehydrogenation, and developing new reagents.

Aerobic Catalyzed Oxidative Cross-Coupling of N,N-Disubstituted Anilines and Aminonaphthalenes with Phenols and Naphthols

The cross-coupling of N,N-dialkyl aniline and aminonaphthalenes with phenols and naphthols using a Cr-salen catalyst under aerobic conditions was developed. Notably, air serves as an effective oxidant affording products in high selectivity. Initial mechanistic studies suggest an outer-sphere oxidation of the aniline/aminonaphthalene partner, followed by nucleophilic attack of the phenol/naphthol. Single products were observed in most cases, whereas mixtures of C-C and C-O coupled products arose from reactions involving aminonapthalene and sterically unencumbered phenols.