Chiral bisphosphine Ph-BPE ligand: a rising star in asymmetric synthesis.

Chiral 1,2-bis(2,5-diphenylphospholano)ethane (Ph-BPE) is a class of optimal organic bisphosphine ligands with C2-symmetry. Ph-BPE with its excellent catalytic performance in asymmetric synthesis has attracted much attention of chemists with increasing popularity and is growing into one of the most commonly used organophosphorus ligands, especially in asymmetric catalysis. Over two hundred examples have been reported since 2012. This review presents how Ph-BPE is utilized in asymmetric synthesis and how powerful it is as a chiral ligand or even a catalyst in a wide range of reactions including applications in the total synthesis of bioactive molecules.

Nonplanar Aromaticity of Dinuclear Rare-Earth Metallacycles.

While the concept of metalla-aromaticity has well been extended to transition organometallic compounds in diverse geometries, aromatic rare-earth organometallic complexes are rare due to the special (n - 1)d0 configuration and high-lying (n - 1)d orbitals of rare-earth centers. In particular, nonplanar cases of rare-earth complexes have not been reported so far. Here, we disclose the nonplanar aromaticity of dinuclear scandium and samarium metallacycles characterized by various aromaticity indices (nucleus-independent chemical shift, isochemical shielding surface, anisotropy of induced current density, and isomerization stabilization energy). Bonding analyses (Kohn-Sham molecular orbital, adaptive natural density partitioning, multicenter bond indices, and principal interacting orbital) reveal that three delocalized π orbitals, predominantly contributed by the 2-butene tetraanion ligand, result in the formation of six-electron conjugated systems. Guided by these findings, we predicted that the lutetium and gadolinium analogues of dinuclear rare-earth metallacycles should be aromatic, which have been verified by the successful synthesis of real molecules. This work extends the concept of nonplanar aromaticity to the field of rare-earth metallacycles and illuminates the path for designing and synthesizing various rare-earth metalla-aromatics.

Direct Synthesis of Phosphoryltriacetates from White Phosphorus via Visible Light Catalysis.

Organophosphorus compounds (OPCs) are widely used in many fields. However, traditional synthetic routes in the industry usually involve multistep and hazardous procedures. Therefore, it's of great significance to construct such compounds in an environmentally-friendly and facile way. Herein, a photoredox catalytic method has been developed to construct novel phosphoryltriacetates. Using fac-Ir(ppy)3 (ppy=2-phenylpyridine) as the photocatalyst and blue LEDs (456 nm) as the light source, white phosphorus can react with α-bromo esters smoothly to generate phosphoryltriacetates in moderate to good yields. This one-step approach features mild reaction conditions and simple operational process without chlorination.

Cascade C-H Activation and Two C-C Bond Forming in Reaction of Azalutetacyclopentadiene with 2-Methylbenzonitriles.

Azametallacyclopentadienes are an important class of metallacycles as the key intermediates in metal-promoted or catalyzed carbon-carbon coupling reaction of nitriles and alkynes. Rare-earth azametallacyclopentadienes have shown unique reactivity toward benzonitriles. The reaction chemistry of azalutetacyclopentadienes toward 2-methylbenzonitriles has been investigated in this work, showing different reactivity. Experimental and computational studies reveal that the reaction selectively initiates with the remote activation of the benzylic C-H bond by the Lu-N bond, followed by the intramolecular nucleophilic attack from the deprotonated benzylic carbon to form a C-C bond. Subsequently, the high ring strain promoted the generation of the uncoordinated carbanion dissociated from the lutetium center, which then undergoes intramolecular nucleophilic attack toward C≡N triple bond to give the final product containing fused 7-5-6-membered azalutetacycle. This work not only achieves highly selective three-step cascade transformation to form a unique class of rare-earth metallacycle, but also reveals a novel reaction pattern of unsaturated substrates with C-H bonds that can be activated.

Selective Cleavage of the Strong or Weak C-C Bonds in Biphenylene Enabled by Rare-Earth Metals.

Selective cleavage of C-C bonds within arene rings is of great interest but remains elusive, especially for the molecules possessing the active and inert C-C bonds. Here, we report that the active and inert C-C bonds of biphenylene could be controllably cleaved by the reaction of biphenylene, potassium graphite, and rare-earth complexes with different metal centers. For scandium, the bond activation occurs at the Caryl-Caryl single bond, yielding 9-scandafluorene. For Lu, the reaction goes through ring contraction of the aromatic ring in biphenylene to provide benzopentalene dianionic lutetium. The origin of the selectivity and the reaction mechanism were illustrated by the isolation of intermediates and DFT calculations.

Progress of Azametallacyclopentadienes in the New Century.

As the key intermediates in metal-promoted/catalyzed C-C bond coupling reactions of nitriles and alkynes, azametallacyclopentadienes, M(N=CR1 -CR2 =CR3 ), are an important class of azametallacycles. Although the first authentic azametallacyclopentadienes were documented in 1986, their chemistry towards solid-state structures, intrinsic reactivity, and synthetic application was rarely investigated for a long time. At the beginning of this century, seminal works about the applications of azametallacyclopentadienes in the synthesis of heterocycles, including multi-substituted pyridines, isoquinolines, furans, and pyrroles were reported. Subsequently, a series of new complexes with this motif, namely the Group 4, aluminum, actinide, and rare-earth azametallacyclopentadienes were isolated and structurally characterized. Among them, the rare-earth azametallacyclopentadiene expresses high reactivity towards unsaturated molecules, such as nitriles, isocyanides, and Mo(CO)6 to provide novel fused metallacycles. In this Concept, we reviewed the advances in the preparation, reactivity, and synthetic application of azametallacyclopentadienes in the past twenty years.

Photochemical Benzylation of White Phosphorus.

Organophosphorus compounds (OPCs) have wide application in organic synthesis, material sciences, and drug discovery. Generally, the vast majority of phosphorus atoms in OPCs are derived from white phosphorus (P4). However, the large-scale preparation of OPCs mainly proceeds through the multistep and environmentally toxic chlorine route from P4. Herein, we report the direct benzylation of P4 promoted by visible light. The cheap and readily available benzyl bromide was used as a benzylation reagent, and tetrabenzylphosphonium bromide was directly synthesized from P4. In addition, the metallaphotoredox catalysis strategy was applied to functionalize P4 for the first time, which significantly improved the application range of the substituted benzyl bromide.

Insight into the Ligand-to-Ligand Charge-Transfer Process in Rare-Earth-Metal Diradical Complexes.

While a ligand-to-ligand charge-transfer (LLCT) process is an important way to understand the interactions between metal-bridged radicals for late-transition-metal complexes, there is little clear and evident observation of the LLCT process for rare-earth-metal complexes. In this work, rare-earth-metal diradical complexes supported by diazabutadiene (DAD) ligands [(DAD)2RE(BH4)] [RE = Yb (1), Sm (2)] were synthesized and studied. The coordination geometries of 1 and 2 are different due to the different ionic radii. Reduction of 1 or 2 generated monoradical complexes, with one of their DAD radical anions being reduced. In all of the complexes, Sm and Yb remain at the 3+ valence state. In their UV-vis spectra, the LLCT transition of 1 could be clearly observed, but complex 2 did not show the same transition. These results could be related to the geometric structures of the complexes as well as exchange coupling between diradicals, thus clearly expanding the model for late-transition-metal-bridged diradicals to rare-earth systems experimentally.

Overview of 1,5-Selective Click Reaction of Azides with Alkynes or Their Synthetic Equivalents.

Nowadays, the click reaction of azides with alkynes has evolved rapidly and become one of the most efficient methods to synthesize 1,2,3-triazoles, which are an important class of N-containing heterocycles. While the 1,4-selective click reaction of azides with alkynes is well established to synthesize 1,4-substituted 1,2,3-triazoles, the corresponding 1,5-selective click reaction for the generation of 1,5-substituted-1,2,3-triazoles is much less explored, and there is no systematic review for the 1,5-selective click reaction. This timely review summarizes the discovery and development of 1,5-selective click reactions of azides with alkynes for the synthesis of 1,5-substituted 1,2,3-triazoles. The 1,5-selective click reactions will be divided into three types according to the critical reactive intermediates: metallacyclic intermediates, acetylide intermediate, and formal 1,5-selective azide-alkyne cycloaddition. The related mechanistic studies will also be involved in this review.

Metalla-aromatics: Planar, Nonplanar, and Spiro.

ConspectusThe concept of aromaticity is one of the most fundamental principles in chemistry. It is generally accepted that planarity is a prerequisite for aromaticity, and typically the more planar the geometry of an aromatic compound is, the stronger aromatic it is. However, it is not always the case, particularly when transition metals are involved in conjugation and electron delocalization of aromatic systems, i.e., metalla-aromatics. Because of the intrinsic nature of transition-metal orbitals, besides planar geometries, the most stable molecular structures of metalla-aromatic compounds could take nonplanar and even spiro geometries. In this Account, we outline several unprecedented types of metalla-aromatics developed recently in our research group.Around seven years ago, we found that 1,4-dilithio-1,3-butadienes, dilithio reagents with π-conjugation, could function as non-innocent ligands and react with low-valent transition-metal complexes, generating monocyclic metalla-aromatic compounds. Later on, by taking advantage of the unique behavior of dilithio reagents and the intrinsic nature of different transition metals, we have synthesized a series of metalla-aromatic compounds, of which four types are discussed here, and each of them represents the first of its kind. First, nearly planar aromatic dicupra[10]annulenes, a 10 π-electron aromatic system with two bridging Cu atoms participating in the orbital conjugation and electron delocalization, are synthesized by annulating two dilithio reagents with two Cu(I) complexes.Second, four kinds of spiro metalla-aromatics, featuring planar (with Pd, Pt, or Rh as the spiro atom) geometry with a whole 10π aromatic system, octahedral (tris-spiro metalla-aromatics with V as the spiro atom) geometry with an entire 40π Craig-Möbius aromatic system, tetrahedral (with Mn as the spiro atom) geometry having two independent and perpendicular 6π planar aromatic rings, and tetrahedral (with Mn as the spiro atom) geometry with one planar and one nonplanar 6π aromatic rings, respectively, are generated. In sharp contrast to spiroaromaticity with carbon acting as the spiro atom described in Organic Chemistry, the metal spiro atom herein takes part in orbital conjugation and electron delocalization.Third, nonplanar aromatic butadienyl diiron complexes are realized. Different from planar aromatic systems featuring delocalized π-type overlap, this nonplanar metalla-aromaticity is achieved by the novel σ-type overlap between the two Fe 3dxz orbitals and the butadienyl π orbital, forming a 6π aromatic system. Fourth, dinickelaferrocene, a ferrocene analogue with two aromatic nickeloles, is synthesized from our monocyclic aromatic dilithionickelole and FeBr2. The aromaticity of dinickelaferrocene and its nickelole ligands is realized by electron back-donation from the Fe 3d orbital to the π* orbital of nickeloles, which also deepens our understanding of the origin of aromaticity.The search for unprecedented and exciting aromatic systems, particularly with transition metals being involved, will continue to drive this intriguing research field forward. Given the synthetic strategies and various types of metalla-aromatics developed and described, diversified metalla-aromatics of interesting structures and reaction chemistry, novel chemical bonding modes, and useful functions can be expected.

Selective Coupling of Lanthanide Metallacyclopropenes and Nitriles via Azametallacyclopentadiene and η2-Pyrimidine Metallacycle.

Exploring new lanthanide metallacycles and finding their unique chemistry different from the analogues of transition metals are of great interest and importance. In this work, we reported the synthesis, characterization, and reactivity toward nitriles of two lanthanide metallacyclopropenes: lutetacyclopropene 2a and dysprosacyclopropene 2b. The selective coupling of 2a and three molecules of PhCN was found for the first time to provide the unexpected fused lutetacycle 3a with one 1,6-dihydropyrimidine ring. Mechanistic studies by DFT calculations reveal that the triple insertion of PhCN into 2a proceeds through four key steps: the insertion of the first PhCN into 2a giving azalutetacyclopentadiene IM1, the insertion of the second PhCN into the Lu-N bond of IM1, the intramolecular electrocyclization providing a highly strained η2-pyrimidine metallacycle, and the insertion of the third PhCN into the Lu-Csp3 bond. Isolation and characterization of two active intermediates, azalutetacyclopentadiene IM1 and η2-pyrimidine dysprosacycle, provide critical evidence for the formation of 3a. Furthermore, IM1 was also reported to react with TMSCN, isocyanides, or W(CO)6 to furnish the fused [4,5] lutetacycles. The chemistry of two lanthanide metallacyclopropenes with nitriles is significantly different from these metallacyclopropenes of scandium and other metals. Most notably, the azalutetacyclopentadienes, η2-pyrimidine complex, and other metallacycles all represent the first examples in rare-earth organometallic chemistry; the formation of these new lutetacycles provides concrete evidence for understanding the mechanism of transition metal promoted or catalyzed [2+2+2] cycloaddition between alkynes and nitriles.

Insertion Chemistry of Lutetacyclopropene toward Unsaturated C-O/C-N Bonds.

Although the reaction chemistry of transition metallacyclopropenes has been well-established in the last decades, the reactivity of rare-earth metallacyclopropenes remains elusive. Herein, we report the reaction of lutetacyclopropene 1 toward a series of unsaturated molecules. The reaction of 1 with one equiv. of PhCOMe, Ar1 CHO (Ar1 =2,6-Me2 C6 H3 ), W(CO)6 , and PhCH=NPh provided oxalutetacyclopentenes, metallacyclic lutetoxycarbene, and azalutetacyclopentene via 1,2-insertion of C=O, C≡O, or C=N bonds into Lu-Csp2 bond, respectively. However, the reaction between 1 and Ar2 N=C=NAr2 (Ar2 =4-MeC6 H4 ) gave an acyclic lutetium complex with a diamidinate ligand by the coupling of one molecule of 1 with two carbodiimides, irrespective of the amount of carbodiimide employed. More interestingly, when 1 was treated with two equiv. of Ar1 CHO, the reductive coupling of two C=O bonds was discovered to give a lutetium pinacolate complex along with the release of tolan. Remarkably, the reactivity of 1 is significantly different from that of scandacyclopropenes; these metallacycles derived from 1 all represent the first cases in rare-earth organometallic chemistry.

Synthesis, Structure, and Magnetic Properties of Rare-Earth Bis(diazabutadiene) Diradical Complexes.

New kinds of diradical rare-earth metal complexes supported by diazabutadiene (DAD) ligands, [(DAD)2LnN(TMS)2] (1; Ln = Dy, Lu; TMS = SiMe3), were synthesized and studied. They showed a new [radical-Ln-radical] alignment with distorted square-pyramidal geometry. Structural and density functional theory analysis illustrated the radical anionic nature of the ligands. Magnetic studies revealed antiferromagnetic coupling of the two radicals in 1-Lu. 1-Dy showed typical single-molecule-magnet (SMM) behavior with an effective energy barrier of 231 K, which is much higher than those of similar radical-containing SMMs. Magnetostructural analysis suggests that the anionic [N(TMS)2]- group plays a vital role in the SMM property. This study provides a new platform for further improving the performance of radical-Ln SMMs.

Carbodiimide-based synthesis of N-heterocycles: moving from two classical reactive sites to chemical bond breaking/forming reaction.

Carbodiimides are a unique class of heterocumulene compounds that display distinctive chemical properties. The rich chemistry of carbodiimides has drawn increasing attention from chemists in recent years and has made them exceedingly useful compounds in modern organic chemistry, especially in the synthesis of N-heterocycles. This review has outlined the extensive application of carbodiimides in the synthesis of N-heterocycles from the 1980s to today. A wide range of reactions for the synthesis of various types of N-heterocyclic systems (three-, four-, five-, six-, seven-, larger-membered and fused heterocycles) have been developed on the basis of carbodiimides and their derivatives.

2-Butene Tetraanion Bridged Dinuclear Samarium(III) Complexes via Sm(II)-Mediated Reduction of Electron-Rich Olefins.

While reduction reactions are ubiquitous in chemistry, it is very challenging to further reduce electron-rich compounds, especially the anionic ones. In this work, the reduction of 1,3-butadienyl dianion, the anionic conjugated olefin, has been realized by divalent rare-earth metal compounds (SmI2), resulting in the formation of novel 2-butene tetraanion bridged disamarium(III) complexes. Density functional theory (DFT) analyses reveal two features: (i) the single electron transfer (SET) from 4f atomic orbitals (AOs) of each Sm center to the antibonding π*-orbitals of 1,3-butadienyl dianion is feasible and the new HOMO formed by the bonding interaction between Sm 5d orbitals (AOs) and the π*-orbitals of 1,3-butadienyl dianion can accept favorably 2e- from 4f AOs of Sm(II); (ii) the 2-butene tetraanionic ligand serves as a unique 10e- donating system, in which 4e- act as two σ-donation bonding interactions while the rest 6e- as three π-donation bonding interactions. The disamarium(III) complexes represent a unique class of the bridged bis-alkylidene rare-earth organometallic complexes. The ligand-based reductive reactivity of 2-butene tetraanion bridged disamarium(III) complexes demonstrates that 2-butene tetraanionic ligand serves as a 3e- reductant toward cyclooctatetraene (COT) to provide doubly COT-supported disamarabutadiene complexes. The reaction of the disamarium(III) complexes with Cp*Li produces the doubly Cp*-coordinated Sm(III) complexes via salt metathesis. In addition, the reaction with Mo(CO)6 affords the oxycyclopentadienyl dinuclear complex via CO insertion.

Cyclic Bis-alkylidene Complexes of Titanium and Zirconium: Synthesis, Characterization, and Reaction.

Transition-metal alkylidenes have exhibited wide applications in organometallic chemistry and synthetic organic chemistry, however, cyclic Schrock-carbene-like bis-alkylidenes of group 4 metals with a four-electron donor from an alkylidene have not been reported. Herein, the synthesis and characterization of five-membered cyclic bis-alkylidenes of titanium (4 a,b) and zirconium (5 a,b) are reported, as the first well-defined group 4 metallacyclopentatrienes, by two-electron reduction of their corresponding titana- and zirconacyclopentadienes. DFT analyses of 4 a show a four-electron donor (σ-donation and π-donation) from an alkylidene carbon to the metal center. The reaction of 4 a with N,N'-diisopropylcarbodiimide (DIC) leads to the [2+2]-cycloaddition product 6. Compound 4 a reacted with CO, affording the oxycyclopentadienyl titanium complex 7. These reactivities demonstrate the multiple metal-carbon bond character. The reactions of 4 a or 5 a with cyclooctatetraene (COT) or azobenzene afforded sandwich titanium complex 8 or diphenylhydrazine-coordinated zirconacyclopentadiene 9, respectively, which exhibit two-electron reductive ability.

Fragmentation Mechanism of White Phosphorus: A Theoretical Insight into Multiple Cleavage/Formation of P-P and P-C Bonds.

Molecular-level understanding of metal-mediated white phosphorus (P4 ) activation is meaningful but challenging because of its direct relevance to the conversion of P4 into useful organophosphorus compounds as well as the complicated and unforeseeable cleavage process of P-P bonds. The related study, however, has still rarely been achieved to date. Here, a theoretical insight into the step-by-step process of three P-P bond cleavage/four P-C bond formation for [P3 +P1 ]-fragmentation of P4 mediated by lutetacyclopentadienes is reported. The unique charge-separated intermediate and the intermolecular cooperation between two lutetacyclopentadienes play a vital role in the subsequent P-P/P-C bond breaking/forming. It is found that, although the first P-C formation is involved in the assembly of the cyclo-P3 [R4 C4 P3 ]- unit, the construction of the aromatic five-membered P1 heterocycle [R4 C4 P]- is completed prior to the cyclo-P3 formation. The reaction mechanism has been carefully elucidated by analyses of the geometric structure, frontier molecular orbitals, bond index, and natural charge, which greatly broaden and enrich the general knowledge of the direct functionalization of P4 .

Trishomoaromatic (B3 N3 Ph6 ) Dianion: Characterization and Two-Electron Reduction.

Benzene, a common aromatic compound, can be converted into an unstable antiaromatic 8π-electron intermediate through two-electron reduction. However, as an isoelectronic equivalent of benzene, borazine (B3 N3 Ph6 ), having weak aromaticity, undergoes a totally different two-electron reduction to afford (B3 N3 R6 )2- homoaromatic compounds. Reported here is the synthesis of homoaromatic (B3 N3 Ph6 )2- by the reduction of B3 N3 Ph6 with either potassium or rubidium in the presence of 18-crown-6 ether. Theoretical investigations illustrate that two electrons delocalize over the three boron atoms in (B3 N3 Ph6 )2- , which is formed by the geometric and orbital reorganization and exhibits (π,σ)-mixed homoaromaticity. Moreover, (B3 N3 Ph6 )2- can act as a robust 2e reductant for unsaturated compounds, such as anthracene, chalcone, and tanshinones. This 2e reduction is of high efficiency and selectivity, proceeds under mild reaction conditions, and can regenerate neutral borazine.

Dinickelaferrocene: A Ferrocene Analogue with Two Aromatic Nickeloles Realized by Electron Back-Donation from Iron.

The first example of ferrocene analogues with two transition-metal metallole ligands of the general formula (η5 -C4 R4 M)2 Fe in a sandwich structure are reported. Specifically, dinickelaferrocene 2, a type of dimetallametallocene, is efficiently synthesized from the reaction of dilithionickelole 1 with FeBr2 or FeCl2 , presumably via a redox process, and is subjected to detailed experimental (single-crystal X-ray structural analysis, ICP-OES, magnetometry, 57 Fe Mössbauer, XPS) and theoretical (MOs, CDA, NICS, ICSS, and AICD) characterizations. Unlike ferrocene and its Cp ligands, the aromaticity of dinickelaferrocene and its nickelole ligands is accomplished by electron back-donation from the Fe 3d orbitals to the π* orbitals of nickelole. Taken together, this work describes a new class of metallaferrocene sandwich complexes and provides a novel approach to effect aromaticity that will contribute to further development of metallocene chemistry.

Butadienyl Diiron Complexes: Nonplanar Metalla-Aromatics Involving σ-Type Orbital Overlap.

A new class of nonplanar metalla-aromatics, diiron complexes bridged by a 1,3-butadienyl dianionic ligand, were synthesized in high yields from dilithio reagents and two equivalents of FeBr2 . The complexes consist of two antiferromagnetically coupled high-spin FeII centers, as revealed by magnetometry, Mössbauer spectroscopy, and DFT calculations. Furthermore, experimental (X-ray structural analysis) and theoretical analyses (NICS, ICSS, AICD, MOs) suggest that the complexes are aromatic. Remarkably, this nonplanar metalla-aromaticity is achieved by an uncommon σ-type overlap between the ligand p and metal d orbitals, in sharp contrast to the intensively studied planar aromatic systems featuring delocalized π-type bonding. Specifically, the σ-type interaction between the two Fe 3dxz orbitals and the butadienyl π orbital results in the formation of a six-electron conjugated system and hence enables the aromatic character.