Synthesis and Catalytic Activities of Porphyrin‐Based PCP Pincer Complexes

2,18‐Bis(diphenylphosphino)porphyrins undergo peripheral cyclometalation with group 10 transition‐metal salts to afford the corresponding porphyrin‐based PCP pincer complexes. The porphyrinic plane and the PCP‐pincer unit are apparently coplanar, with small strain. The catalytic activities of the porphyrin‐based pincer complexes at the periphery were investigated in the allylation of benzaldehyde with allylstannane and in the 1,4‐reduction of chalcone to discover the electronic interplay between the inner metal and the outer metal in catalysis.     

Multicomponent Synthesis of Unsymmetrical Unsaturated N‐Heterocyclic Carbene Precursors and Their Related Transition‐Metal Complexes

A low‐cost, modular, and easily scalable multicomponent procedure affording access in good yields and excellent selectivity (up to 93 %) to a wide range of (a)chiral unsymmetrical 1‐aryl‐3‐cycloalkyl‐imidazolium salts is disclosed. Electronic and steric properties of the corresponding unsymmetrical unsaturated N‐heterocyclic carbene (U₂‐NHC) ligands were evaluated and evidenced strong electron donor ability, high steric discrimination, and modular steric demand.     

[3,3]‐Sigmatropic Rearrangement/Allylboration/Cyclization Sequence: Enantioenriched Seven‐Membered‐Ring Carbamates and Ring Contraction to Pyrrolidines

The combination of in situ generated α‐isocyanato allylboronic esters and aldehydes afforded seven‐membered‐ring enecarbamates with high levels of diastereo‐ and enantiocontrol. They were easily converted into diversely substituted 1,3‐oxazepan‐2‐ones. An unprecedented rearrangement of 5‐acetoxy‐7‐aryl or styryl derivatives led to tetrasubstituted pyrrolidines. A computational study provides evidence on the feasibility of the proposed mechanism of this unusual ring contraction.     

Kinetic and Theoretical Studies on Ni0/N‐Heterocyclic Carbene‐Catalyzed Intramolecular Alkene Hydroacylation

A combined kinetic and theoretical study was conducted in order to clarify the details on the reaction mechanism for Ni⁰/It Bu‐catalyzed intramolecular alkene hydroacylation. The results confirm the hypothesis that this intramolecular hydroacylation proceeds through an oxanickelacycle key intermediate.     

Stable P‐Heterocyclic Carbenes: Scope and Limitations

The conjugate acids (PHCH⁺s) of P‐heterocyclic carbenes (PHCs) are prepared by formal [3+2] cycloaddition of a 1,3‐diphosphaallyl or 1,3‐phosphinophosphenium cation with various nitriles. The effect of the phosphorus substituent on the fate of the cyclization and on that of the counteranion and base in the subsequent deprotonation reaction are reported. Two PHCs that are indefinitely stable in the solid state are described. In solution, one of them, made from acetonitrile, undergoes a facile [3+2] cycloreversion, whereas the other, based on dimethyl cyanamide, is stable, presumably owing to its zwitterionic structure, which involves a tricoordinate pentavalent phosphorus atom. The reactivity of PHCs is strongly driven by the high electrophilicity of the phosphorus centers, as demonstrated by their reactivity with water and benzaldehyde. Although both PHCs reported in this paper are direct analogues of the least‐basic NHCs, their basicity is comparable to those of the more strongly basic NHCs (as determined by comparison of the carbonyl stretching frequencies of their corresponding cis‐[RhCl‐(CO)₂(L)] complexes).     

Promoting the Electrocatalytic Performance of Noble Metal Aerogels by Ligand‐Directed Modulation

Noble metal aerogels (NMAs) are an emerging class of porous materials. Embracing nano‐sized highly‐active noble metals and porous structures, they display unprecedented performance in diverse electrocatalytic processes. However, various impurities, particularly organic ligands, are often involved in the synthesis and remain in the corresponding products, hindering the investigation of the intrinsic electrocatalytic properties of NMAs. Here, starting from laser‐generated inorganic‐salt‐stabilized metal nanoparticles, various impurity‐free NMAs (Au, Pd, and Au‐Pd aerogels) were fabricated. In this light, we demonstrate not only the intrinsic electrocatalytic properties of NMAs, but also the prominent roles played by ligands in tuning electrocatalysis through modulating the electron density of catalysts. These findings may offer a new dimension to engineer and optimize the electrocatalytic performance for various NMAs and beyond.     

Redox Divergent Synthesis of Fawcettimine‐Type Lycopodium Alkaloids

A new approach for synthesis of fawcettimine‐type Lycopodium alkaloids is described. A divergent strategy was achieved by applying stereoselective Diels–Alder reaction followed by redox‐controlled elaboration. Eventually, (?)‐8‐deoxyserratinine, (+)‐fawcettimine, (?)‐lycopoclavamine‐A, (?)‐serratine, (?)‐lycopoclavamine‐B and (?)‐serratanidine were successfully accessed.     

Coumaraz‐2‐on‐4‐ylidene: Ambiphilic N‐Heterocyclic Carbenes with a Tunable Electronic Structure

Herein, a coumaraz‐2‐on‐4‐ylidene ( 1 ) as a new example of an ambiphilic N‐heterocyclic carbene, having electronic properties that can be fine‐tuned, is reported. The N‐carbamic and aryl groups on the carbene carbon center provide exceptionally high electrophilicity and nucleophilicity simultaneously to the carbene center, as evidenced by the ⁷⁷Se NMR chemical shifts of their selenoketone derivatives and the CO stretching strengths of their rhodium carbonyl complexes. Since the precursors of 1 could be synthesized from various functionalized Schiff bases in a practical and scalable manner, the electronic properties of 1 can be fine‐tuned in a quantitative and predictable way by using the Hammett σ constant of the functional groups on aryl ring. The facile electronic tuning capability of 1 may be applicable to eliciting novel properties in main‐group and transition‐metal chemistry.     

Stereoselective [3+2] Carbocyclization of Indole‐Derived Imines and Electron‐Rich Alkenes: A Divergent Synthesis of Cyclopenta[b]indole or Tetrahydroquinoline Derivatives

An unprecedented stereoselective [3+2] carbocyclization reaction of indole‐2‐carboxaldehydes, anilines, and electron‐rich alkenes to obtain cyclopenta[b]indoles is disclosed. This pathway is different from the well‐established Povarov reaction: the formal [4+2] cycloaddition involving the same components, which affords tetrahydroquinolines. Moreover, by simply changing the Brønsted acid catalyst, this multicomponent coupling process could be divergently directed towards the conventional Povarov pathway to produce tetrahydroquinolines or to the new pathway (anti‐Povarov) to generate cyclopenta[b]indoles. Supported by computational studies, a stepwise Mannich/Friedel–Crafts cascade is proposed for the new anti‐Povarov reaction, whereas a concerted [4+2] cycloaddition mechanism is proposed for the Povarov reaction.     

α‐Cationic Phosphines: Synthesis and Applications

In coordination chemistry, typical ancillary ligands are anionic or neutral species. Cationic ones are exceptions and, when used, the positively charged groups are normally attached to the periphery and not close to the donating atom. However, this concept article highlights a series of recent experimental, as well as theoretical results, suggesting that the utility in catalysis of cationic phosphines with no spacer between the phosphorus atom and the positively charged group(s) has been largely overlooked. In fact, a growing number of studies indicate that, because of their specific architecture, these cationic ligands depict excellent π‐acceptor character that can exceed that of phosphites or polyfluorinated phosphines. This property has been used to increase the Lewis acidity of the metals they coordinate. Specifically, new extreme π‐acid catalysts, mainly based on Pt^II and Au^I, have been recently prepared and their superior performance demonstrated along several mechanistically distinct transformations. In this concept article the current state of the art is critically assessed and possible future directions of the topic discussed.     

Synthesis of 2H‐Azirines by Iridium‐Catalyzed Decarboxylative Ring Contraction of Isoxazol‐5(4H)‐ones

A phosphine‐free iridium‐catalyzed reaction of isoxazol‐5(4H)‐ones (isoxazolones) has been developed, and affords 2H‐azirines through decarboxylation and ring contraction. This method provides an efficient and environmentally benign protocol which could replace the conventional approaches used to synthesize 2H‐azirines.     

Charged Behaviour from Neutral Ligands: Synthesis and Properties of N‐Heterocyclic Pseudo‐amides

Deprotonation of the 1‐isopropyl‐3‐(phenylamino)pyridin‐1‐ium iodide gives the corresponding neutral betaine, which is formalised as a pyridinium‐amido ligand when coordinated to a metal. Spectroscopic, structural and theoretical methods have been used to investigate the metal–ligand bonding, ligand dynamics and electron distribution. Collectively, the data show that the ligand can be characterised as a pseudo‐amide and is a strong donor akin to alkyl phosphines and N‐heterocyclic carbenes. Furthermore, rotation about both N substituent C? N bonds occurs, which is in contrast to the two alternative pyridinium positional isomers that exhibit neutral resonance structures. For comparison, compounds and complexes derived from norharman were prepared, which contain an additional C? C bond supporting conjugation and the accessibility of a neutral resonance structure. Notwithstanding the formal neutral structure, norharman‐derived ligands are comparably strong donors, and have the additional advantage of exhibiting stability to dioxygen and water.     

Visible‐Light‐Induced ortho‐Selective Migration on Pyridyl Ring: Trifluoromethylative Pyridylation of Unactivated Alkenes

The photocatalyzed ortho‐selective migration on a pyridyl ring has been achieved for the site‐selective trifluoromethylative pyridylation of unactivated alkenes. The overall process is initiated by the selective addition of a CF₃ radical to the alkene to provide a nucleophilic alkyl radical intermediate, which enables an intramolecular endo addition exclusively to the ortho‐position of the pyridinium salt. Both secondary and tertiary alkyl radicals are well‐suited for addition to the C2‐position of pyridinium salts to ultimately provide synthetically valuable C2‐fluoroalkyl functionalized pyridines. Moreover, the method was successfully applied to the reaction with P‐centered radicals. The utility of this transformation was further demonstrated by the late‐stage functionalization of complex bioactive molecules.     

A Convenient Synthesis of 1,2,4‐ and 1,3,4‐Azadiphospholes

A new synthetic route to functionalized neutral and anionic azadiphospholes from easily accessible starting materials is described. Equimolar reaction of Na(OCP) and N‐(2,6‐dimethylphenyl)pivalimidoyl chloride 2 a cleanly affords the imidoxy‐functionalized 1,2,4‐azadiphosphole 3 a . Using Na(OCP) and imidoyl chloride in a 2:1 ratio leads to an anionic four‐membered ring Na[ 4 a ], which has been structurally characterized. During 16 h at room temperature, Na[ 4 a ] rearranges to the anionic 1,3,4‐azadiphospholide Na[ 5 a ] with release of carbon monoxide. Applying the more sterically demanding N‐(2,6‐diisopropylphenyl)pivalimidoyl chloride allows isolation of the 1,3,4‐azadiphospholide Na[ 5 b ] in good yield (>70 %). Possible mechanisms leading to the new isomeric azadiphospholides have been investigated with the aid of high‐level composite calculations.     

Cyclic Amino(Ylide) Silylene: A Stable Heterocyclic Silylene with Strongly Electron‐Donating Character

An isolable heterocyclic silylene ( 4 ) with two different π‐donating substituents, namely a classical amino group and a more electropositive and stronger carbon‐based π‐donating phosphonium ylide, was synthesized and fully characterized. The combination of these two different π‐donating substituents confers high thermal stability and an unusual nucleophilic character on silylene 4 . Therefore, silylene 4 behaves as a strong donor ligand toward transition metals with a donating character comparable to N‐heterocyclic carbenes, in contrast to classical N‐heterocyclic silylenes, which generally present a weak donating character.     

Intramolecular Formal anti‐Carbopalladation/Heck Reaction: Facile Domino Access to Carbo‐ and Heterooligocyclic Dienes

An intramolecular domino process consisting of a formal anti‐carbopalladation followed by Heck reaction is realized. Complex oligo(hetero)cyclic scaffolds are efficiently obtained in one synthetic step from easily obtainable enyne precursors. In contrast to common syn‐carbopalladation reactions of alkyne units, the carbopalladation employed here is designed to afford an anti‐arrangement of the two new substituents across the emerging double bond. A prerequisite is that the residues next to the alkyne should lack any β‐hydrogen atoms. The method paves the way to tri‐ and tetrasubstituted double‐bond systems that have not been accessible by conventional Pd catalysis.     

Divergent Total Synthesis of Atisane‐Type Diterpenoids

Atisane‐type diterpenoids are the principal constituent of tetracyclic C₂₀‐diterpenoids, widely isolated from the plant kingdom with varying degrees of structural complexity and pharmacological activity. The tetracarbocyclic system with the unique bicyclo[2.2.2]octane skeleton of this natural product family has generated interest within the synthetic community. Divergent total synthesis is an effective tactic to synthesize several atisane‐type diterpenoids using structural interconversion from a common intermediate. This account summarizes the divergent total synthesis of atisane‐type diterpenoids.