Significant Enhancement in the Efficiency and Selectivity of Iron‐Catalyzed Oxidative Cross‐Coupling of Phenols by Fluoroalcohols

Significant enhancement of both the rate and the chemoselectivity of iron‐catalyzed oxidative coupling of phenols can be achieved in fluorinated solvents, such as 1,1,1,3,3,3‐hexafluoropropan‐2‐ol (HFIP), 2,2,2‐trifluoroethanol (TFE), and 1‐phenyl‐2,2,2‐trifluoroethanol. The generality of this effect was examined for the cross‐coupling of phenols with arenes and polycyclic aromatic hydrocarbons (PAHs) and of phenol with β‐dicarbonyl compounds. The new conditions were utilized in the synthesis of 2′′′‐dehydroxycalodenin B in only four synthetic steps.     

Metal‐ and Reagent‐Free Highly Selective Anodic Cross‐Coupling Reaction of Phenols

The direct oxidative cross‐coupling of phenols is a very challenging transformation, as homo‐coupling is usually strongly preferred. Electrochemical methods circumvent the use of oxidizing reagents or metal catalysts and are therefore highly attractive. Employing electrolytes with a high capacity for hydrogen bonding, such as methanol with formic acid or 1,1,1,3,3,3‐hexafluoro‐2‐propanol, a direct electrolysis in an undivided cell provides mixed 2,2′‐biphenols with high selectivity. This mild method tolerates a variety of moieties, for example, tert‐butyl groups, which are not compatible with other strong electrophilic media but vital for later catalytic applications of the formed products.     

Transition‐Metal‐Free Oxidative Cross‐Coupling of Tetraarylborates to Biaryls Using Organic Oxidants

Readily prepared tetraarylborates undergo selective (cross)‐coupling through oxidation with Bobbitt's salt to give symmetric and unsymmetric biaryls. The organic oxoammonium salt can be used either as a stoichiometric oxidant or as a catalyst in combination with in situ generated NO₂ and molecular oxygen as the terminal oxidant. For selected cases, oxidative coupling is also possible with NO₂/O₂ without any additional nitroxide‐based cocatalyst. Transition‐metal‐free catalytic oxidative ligand cross‐coupling of tetraarylborates is unprecedented and the introduced method provides access to various biaryl and heterobiaryl systems.     

Pioneering Metal‐Free Oxidative Coupling Strategy of Aromatic Compounds Using Hypervalent Iodine Reagents

We started our hypervalent iodine research about 30 years ago in the mid‐1980s. We soon successfully developed the single‐electron‐transfer oxidation ability of a hypervalent iodine reagent, specifically, phenyliodine(III) bis(trifluoroacetate) (PIFA), toward aromatic rings of phenyl ethers for forming aromatic cation radicals. This was one of the exciting and unexpected events in our research studies so far, and the discovery was reported in 1991. It also led to the next challenge, developing the metal‐free oxidative couplings for C–H functionalizations and direct couplings between the C–H bonds of valuable aromatic compounds in organic synthesis. In order to realize the effective oxidative coupling, pioneering new aromatic ring activations was essential and several useful methodologies have been found for oxidizable arenes. The achievements regarding this objective obtained in our continuous research are herein summarized with classification of the aromatic ring activation strategies.     

Transition‐Metal‐Free BF3‐Mediated Oxidative and Non‐Oxidative Cross‐Coupling of Pyridines

We report a BF₃‐mediated direct alkynylation of pyridines at C(2) by using a variety of alkynyllithium reagents (oxidative cross‐coupling). Moreover, we have developed a novel transition‐metal‐free cross‐coupling method between alkylmagnesium reagents and 4‐substituted pyridines, such as isonicotinonitrile and 4‐chloropyridine, by employing BF₃?OEt₂ as a promoter. The combination of these methods enabled us to efficiently prepare a range of di‐, tri‐, and tetrasubstituted pyridines.     

Intramolecular Metal‐Free Oxidative Aryl–Aryl Coupling: An Unusual Hypervalent‐Iodine‐Mediated Rearrangement of 2‐Substituted N‐Phenylbenzamides

Hypervalent‐iodine‐mediated oxidative coupling of the two aryl groups in either 2‐acylamino‐N‐phenyl‐benzamides or 2‐hydroxy‐N‐phenylbenzamides, with concomitant insertion of the ortho‐substituted N or O atom into the tether, has been described for the first time. This unusual metal‐free rearrangement reaction involves an oxidative C(sp²)? C(sp²) aryl–aryl bond formation, cleavage of a C(sp²)? C(O) bond, and a lactamization/lactonization. Furthermore, unsymmetrical diaryl compounds can be easily obtained by removing the tether within the cyclized product.     

Reductive Cross‐Coupling Reactions between Two Electrophiles

Reductive cross‐electrophile coupling reactions have recently been developed to a versatile and sustainable synthetic tool for selective C?C bond formation. The employment of cheap and abundant electrophiles avoids the pre‐formation and handling of organometallic reagents. In situ reductive coupling is effected in the presence of a transition‐metal catalyst (Ni, Co, Pd, Fe) and a suitable metallic reductant (Mn, Zn, Mg). This Concept article assesses the current state of the art and summarizes recent protocols with various combinations of alkyl, alkenyl, allyl, and aryl reagents and highlights key mechanistic studies.     

Source of Selectivity in Oxidative Cross‐Coupling of Aryls by Solvent Effect of 1,1,1,3,3,3‐Hexafluoropropan‐2‐ol

Solvents such as 1,1,1,3,3,3‐hexafluoroisopropanol (HFIP) with a high capacity for donating hydrogen bonds generate solvates that enter into selective cross‐coupling reactions of aryls upon oxidation. When electric current is employed for oxidation, reagent effects can be excluded and a decoupling of nucleophilicity from oxidation potential can be achieved. The addition of water or methanol to the electrolyte allows a shift of oxidation potentials in a specific range, creating suitable systems for selective anodic cross‐coupling reactions. The shift in the redox potentials depends on the substitution pattern of the substrate employed. The concept has been expanded from arene–phenol to phenol–phenol as well as phenol–aniline cross‐coupling. This driving force for selectivity in oxidative coupling might also explain previous findings using HFIP and hypervalent iodine reagents.     

Gold‐Catalyzed Homogeneous Oxidative Cross‐Coupling Reactions

Oxidizing gold? A gold(I)/gold(III) catalytic cycle is essential for the first oxidative cross‐coupling reaction in gold catalysis. By using Selectfluor for gold(I) oxidation, this chemistry reveals the synthetic potential of incorporating gold(I)/gold(III) catalytic cycles into contemporary gold chemistry and promises a new area of gold research by merging powerful gold catalysis and oxidative metal‐catalyzed cross‐coupling reactions.

     

Palladium‐Catalysed Direct Cross‐Coupling of Organolithium Reagents with Aryl and Vinyl Triflates

A palladium‐catalysed cross‐coupling of organolithium reagents with aryl and vinyl triflates is presented. The reaction proceeds at 50 or 70 °C with short reaction times, and the corresponding products are obtained with moderate to high yields, with a variety of alkyl and (hetero)aryl lithium reagents.     

tBuLi‐Mediated One‐Pot Direct Highly Selective Cross‐Coupling of Two Distinct Aryl Bromides

A Pd‐catalyzed direct cross‐coupling of two distinct aryl bromides mediated by tBuLi is described. The use of [Pd‐PEPPSI‐IPr] or [Pd‐PEPPSI‐IPent] as catalyst allows for the efficient one‐pot synthesis of unsymmetrical biaryls at room temperature. The key for this selective cross‐coupling is the use of an ortho‐substituted bromide that undergoes lithium–halogen exchange preferentially.     

Single‐Electron‐Transfer (SET)‐Induced Oxidative Biaryl Coupling by Polyalkoxybenzene‐Derived Diaryliodonium(III) Salts

Metal‐free oxidative C? C coupling by using polyalkoxybenzene‐derived diaryliodonium(III) salts as both the oxidant and aryl source has been developed. These salts can induce single‐electron‐transfer (SET) oxidation to yield electron‐rich arenes and subsequently transfer the polyalkoxyphenyl group into in situ generated aromatic radical cations to produce biaryl products. The reaction is promoted by a Lewis acid that activates the iodonium salts. It has been revealed that the reactivity of the salts under acidic conditions is quite different to their known behavior under basic conditions. The reactivity preference of a series of iodonium salts in the SET oxidation and their ligand transfer abilities have been systematically investigated and the results are summarized in this report.     

Secondary Alkyl Halides in Transition‐Metal‐Catalyzed Cross‐Coupling Reactions

Secondary, but second to none : The use of secondary alkyl halides in transition‐metal‐catalyzed cross‐coupling reactions (see scheme) has advanced significantly over the last five years. Selected examples of these transformations are examined, including mechanistic and stereochemical aspects.