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Cobalt-bis(dinitrogen) complexes [LCo(N2)2]− (3,4) with simple and commercially available bidentate phosphine ligands (L: Cy2PCH2CH2PCy2) were synthesized and structurally characterized. Further N2...

A series of structurally well-defined iron–dinitrogen complexes (LFe–N2) bearing cyclopentadienyl-phosphine ligands (L) were synthesized and analyzed by single-crystal X-ray structural analysis, IR...

Rare-earth metal boroxides are not only rare and of great synthetic challenge but also important theoretically due to their unrevealed bonding characteristics. In this paper, we report the synthesi...

By applying the potassium salts of cyclopentadienyl-phosphine ligands LK to CoCl

Synthesis and structural characterization of inverse-sandwich complexes, whose planar conjugated ligand is a cyclobutadiene, have attracted much attention for their unique chemical bonding modes an...

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, Mossbauer 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.

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.

Direct conversion of dinitrogen (N2) into organic compounds, not through ammonia (NH3), is of great significance both fundamentally and practically. Here we report a highly efficient scandium-media...

A series of trinuclear and dinuclear Cr(I)–N2 complexes bearing cyclopentadienyl-phosphine ligands were synthesized and characterized. Further reduction of the Cr(I)–N2 complexes generated anionic ...

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-Mobius 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.

As aromaticity is one of the most fundamental concepts in chemistry, the construction of aromatic systems has long been an important subject. Herein, we report the synthesis and characterization of a tris-spiroaromatic complex, hexalithio spiro vanadacycle 2. The delocalization of the four electrons within the two V 3d orbitals and the π* orbitals of the three biphenyl ligands leads to a 40π Craig-Möbius aromatic system with three metalla-aromatic rings, as revealed by both experimental measurements and theoretical analyses. For comparison, if Cr is used instead of V, a similar Craig-Möbius aromatic system can not be generated. In this case, pentalithio spiro chromacycle 3 is obtained, and the Cr center uses its two 3d orbitals to form two independent metalla-aromatic rings. This work presents a type of aromatic systems that will contribute to both aromaticity theory and organometallic chemistry., Spiroaromatic compounds are advantageous platforms for designing expanded aromatic systems. Herein, the authors present a tris‐spiro metalla‐aromatic Vanadium compound which forms a 40π Craig‐Möbius aromatic system.

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.

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.

A series of butadienyl organomagnesium complexes, including 1-Li-4-(MgR)-butadienes, 1,4-bis(MgR)-butadienes, and 1-Mg-4-(MgR)-butadiene were synthesized starting with 1,4-dilithio butadienes via the transmetalation reaction between alkenyllithium bond and alkenylmagnesium bond. Single-crystal X-ray structural analysis of these butadienyl organomagnesiates revealed unique bonding modes. Study on their structural transformation demonstrated that 1-Li-4-(MgR)-butadiene is the key intermediate in the reaction process leading to the formation of 1,4-bis(MgR)-butadiene and 1-Mg-4-(MgR)-butadiene.

This perspective includes the synthesis and future challenges of aromatic dianion metalloles and their analogues., Metalloaromatic species are unique and important both experimentally and theoretically. Significant progress has been made during the past few decades. New aromatic systems have challenged and extended the concept of aromaticity remarkably. In this perspective, recent results on the study of the dianion aromatic metalloles and their corresponding analogues are reviewed. These include the dilithio group 14 metalloles, group 13 metalloles and transition metal metalloles. X-ray crystallography has made a key contribution to the understanding of the structures. Various theoretical tools, such as NICS and AdNDP, make it possible to measure the aromaticity beyond Hückel’s rule. The dianion butadiene skeletons play a key role in these metalloles and can be regarded as non-innocent ligands, which accept the electrons from the metal center and thus form the aromatic rings. By simply changing the central metals to different metals, the metallole analogues such as dicupra[10]annulenes and spiroaromatic palladoles can also be generated, which opens a door to synthesize other metalla-macrocyclic aromatics. Key challenges and envisioned opportunities for the future, such as applying these dianion metalloles as novel ligands of transition metals and generating new types of organometallic aromatic system, are also discussed.

Reductive elimination of Cu(III) intermediates is often proposed as a key step in many copper-catalyzed or -mediated formation of C-C or C-heteroatom bonds. However, there still lacks concrete evidence on this key step, mainly because Cu(III) complexes are usually too unstable to be isolated and structurally characterized. In this work, novel organocopper(III) spiro complexes and their analogous organocopper(I) spiro complexes were synthesized and structurally characterized. Single-crystal X-ray structural analysis revealed that the spiro Cu(III) atom adopted a distorted square-planar geometry while its corresponding spiro Cu(I) atom was tetrahedrally coordinated. A redox transformation between these spiro Cu(I) and Cu(III) complexes was experimentally observed by reacting with reductants or oxidants, respectively. As concrete evidence, the organocopper(III) spiro compounds were found to form C-C bonds intramolecularly via reductive elimination.

Since the concept of aromaticity represents one of the most fundamental principles in chemistry, the search for unprecedented and exciting aromatic systems, therefore, continues to drive research in this area. Herein we report the synthesis and characterization of spiro metalla-aromatics, in which the transition metal (Pd, Pt, or Rh) is the spiro atom, that cross-conjugates two aromatic five-membered metallacycles. These spiro metalla-aromatics tend to take square planar geometries, with the dihedral angle being influenced by the steric repulsion between the α-positioned substituents. Rationalized and classified via both experimental measurements (X-ray structural analysis, NMR spectroscopy, XPS, etc.) and theoretical analysis (DFT calculation, ISE, AICD, NICS, and CMOs), all these fundamental observations extend the concept of aromaticity and organometallic chemistry.

Metallacyclopentadienes, which possess two M–C(sp2) bonds and feature the structure of M(CR1CR2–CR3CR4), are an important class of five-membered metallacycles. They are considered as both reactive intermediates in the stoichiometric and catalytic transformations of organic molecules and useful precursors to main group element compounds, and have received considerable attention in organometallic chemistry, coordination chemistry and synthetic organic chemistry over the past six decades because of their unique metallacyclic structure. This review comprehensively presents the synthesis, structure and reactivity of the s-, p-, d- and f-block metallacyclopentadienes distributed in the whole periodic table. In addition, their application in synthetic organic chemistry and polymer chemistry is summarized. This review aims to be beneficial for the design and synthesis of novel metallacyclopentadienes, and for promoting the rapid development of metallacyclic chemistry.

Although transition metallacyclopropenes have been extensively explored for more than 40 years, their analogues of rare-earth metals have remained elusive. Herein, we report the synthesis of three isolable scandacyclopropenes, thus representing the first well-defined rare-earth metallacyclopropenes. Structural characterization and DFT calculations revealed a delocalized three-center two-electron (3c-2e) aromatic system. When scandacyclopropenes were treated with phenylacetylene or TMSN3, the scandium complex of bis-phenylacetylide or bis-azide was obtained, respectively. The reaction of scandacyclopropene with phenazine could provide the binuclear ring-opening scandium complex via 1,4-insertion of phenazine into one Sc-C bond and subsequent metathesis reaction. However, insertion of TMSNCO or N2O into one Sc-C bond of scandacyclopropenes gave a five- or six-membered scandacycle. In addition, scandacyclopropenes can serve as a two-electron reductive agent for PhSSPh and PhNNPh. These results show that scandacyclopropenes exhibit diversified and unique reactivity toward small molecules because of the strongly nucleophilic alkenediyl dianion and highly strained three-membered ring.

Herein, we present the realization of a class of unprecedented aromatic structures 2: metalla-aromatics with two independent and perpendicular aromatic rings spiro-fused by a transition-metal spiro atom, of which their corresponding organic analogues are impossible. Tetralithio spiro manganacycles 2 are readily synthesized from 1,4-dilithio-1,3-butadienes 1 and MnCl2 in the presence of lithium. The aromaticity of 2 is supported by experimental measurements (X-ray structural analysis, NMR) and theoretical analyses (NICS, ACID, MOs). The spiro atom Mn in 2 uses its 3dxz and 3dxy orbitals to form the two perpendicular manganacycles, which are two independent 6π aromatic systems. Theoretical analyses reveal that the Li cations play an indispensable role in governing their geometric and electronic structures and hence their aromaticity. Therefore, this work contributes not only to enrich the concept of aromaticity, but also to deepen the understanding of the fundamental chemical bonding.

Selective reduction of 1,5-diazacyclooctatetraenes with potassium or a Co(I) complex was achieved to provide the first structurally well-defined aromatic 1,5-diazacyclooctatetraenyl dianion or a novel dinuclear Co(II) complex bridged by a bipyrrolinyl dianion, respectively.

The selective C(sp3)−H bond functionalization is an ideal and atom-economical method in organic synthesis. In this work, 2-aminopyrimidines are generated from a Cu-catalyzed reaction between carbodiimides and diaryliodonium salts, by cleavage of four C(sp3)−H, one C−N, and one C=N bonds in the carbodiimides. It is the first triple C(sp3)−H bond functionalization neighboring a C=N bond. The selective synthesis of 2-aminopyrimidines is controlled by the amount of the diaryliodonium salts. The novel mechanism involving a C−N formation/1,5-H shift/1,7-H shift/6 π-electrocyclic ring-closing/aromatization is well elucidated by the detection of important intermediates and DFT calculations.

The selective construction of P-C bonds directly from P4 and nucleophiles is an ideal and step-economical approach to utilizing elemental P4 for the straightforward synthesis of organophosphorus compounds. In this work, a highly efficient one-pot reaction between P4 and 1,4-dilithio-1,3-butadienes was realized, which quantitatively affords phospholyl lithium derivatives. DFT calculations indicate that the mechanism is significantly different from that of the well-known stepwise cleavage of P-P bond in P4 activation. Instead, a cooperative nucleophilic attack of two Csp2 Li bonds on P4 , leading to simultaneous cleavage of two P-P bonds, is favorable. This mechanistic information offers a new view on the mechanism of P4 activation, as well as a reasonable explanation for the excellent yields and selectivity. This method could prove to be a useful route to P4 activation and the subsequent production of organophosphorus compounds.

A series of unprecedented metallacycles of indium, including indacyclopentadienes (2), spiro indacyclopentadienes (3), aromatic indacyclopentadienyl dianions with an In-Li bond (4), and aromatic indacyclopentadienyl dianions with an In-In bond (5), were synthesized and structurally characterized by single-crystal X-ray structural analysis for the first time. Both indacyclopentadienes 2 and 3 were synthesized by reaction of 1,4-dilithio-1,3-butadienes with InCl3 . Structural transformations between 2 and 3 were observed. Reduction of indacyclopentadienes 2 with an excess amount of lithium afforded aromatic indacyclopentadienyl dianions 4 with an In-Li bond or 5 with an In-In bond, respectively, depending on the substituents on the ring. The aromatic character of 4 and 5 was confirmed by both experimental measurements and theoretical analysis.

Alkenyl magnesium compounds have received much attention in synthetic organic chemistry because of their high reactivity. This review summarizes three types of alkenyl magnesium compounds which contain at least one [Mg-C=C] fragment, for example, (1) alkenyl Grignard reagents prepared by halogen-magnesium exchange reaction; (2) alkenyl magnesium carbenoids prepared by halogen-magnesium exchange reaction or sulfoxide-magnesium exchange reaction; (3) magnesacarbocycles containing at least one alkenyl magnesium bond prepared from Zr- or Ti-catalyzed cyclomagnesation or transmetalation, as well as their further reactions and applications.

Herein we report that 1,4-dilithio-1,3-butadienes, a type of 1,3-butadienyl dianion, can act as non-innocent ligands, taking electrons from low-valent transition metals. Dilithio reagents reacted with [{RhCl(cod)}2] to give dilithio rhodacycle 3 a. Single-crystal X-ray structural analysis revealed the structure of 3 a with averaged bond lengths. XPS data suggested that the oxidation state of Rh in 3 a was more likely to be Rh(3+). CDA/ECDA confirmed the electron-transfer process. (7)Li NMR spectra of 3 a and theoretical calculations revealed a considerable aromatic character. In this process, the dilithio compounds behaved as non-innocent ligands and formal oxidants. These results demonstrated that organolithium compounds with suitable π-conjugation could be used as electron acceptor.

Semibullvalene (SBV) and its aza analogue 2,6-diazasemibullvalene (NSBV) are theoretically interesting and experimentally challenging organic molecules because of four unique features: highly strained ring systems, intramolecular skeletal rearrangement, extremely rapid degenerate (aza-)Cope rearrangement, and the predicted existence of neutral homoaromatic delocalized structures. SBV has received much attention in the past 50 years. In contrast, after NSBV was predicted in 1971 and the first in situ synthesis was realized in 1982, no progress on NSBV chemistry was made until our results in 2012. We have been interested in the reaction chemistry of 1,4-dilithio-1,3-butadienes (dilithio reagents for short), especially for their applications in the synthesis of SBV and NSBV, because (i) the cyclodimerization of dilithio reagents could provide the potential eight-carbon skeleton of SBV from four-carbon butadiene units and (ii) the insertion reaction of dilithio reagents with C≡N bonds of two nitriles could provide a 6C + 2N skeleton that might be a good precursor for the synthesis of NSBV. Therefore, we initiated a journey into the synthesis and reaction chemistry of SBV and NSBV starting from dilithio reagents that has been ongoing since 2006. In this Account, we outline mainly our recent achievements in the synthesis, structural characterization, reaction chemistry, synthetic application, and theoretical/computational analysis of NSBV. Two efficient strategies for the synthesis of NSBV from dilithio reagents and nitriles via oxidant-induced C-N bond formation are described. Structural investigations of NSBV, including X-ray crystal structure analysis, determination of the activation barrier for the aza-Cope rearrangement, and theoretical analysis, show that the localized structure of NSBV is the predominant form and that the homoaromatic delocalized structure exists as a minor component in the equilibrium. We also discuss the reaction chemistry and synthetic applications of NSBV. Several novel reaction patterns have been explored, including thermolysis, C-N bond insertion, rearrangement-cycloaddition, oxidation, and nucleophilic ring-opening reactions. Diverse and interesting N-containing polycyclic skeletons can be constructed, such as nickelaazetidine, 1,5-diazatriquinacenes, and triazabrexadienes, which are not available by other means. Our results show that NSBV not only features a rapid aza-Cope rearrangement with a low activation barrier but also acts as unique synthetic reagent that is significantly different from aziridine. The strained rigid ring systems as a whole can be involved in the reactions. Our achievements highlight two significant advances: (i) the well-established efficient synthesis and isolation of NSBV has greatly accelerated the development of NSBV chemistry, and (ii) the previously unattainable molecules have become "normal" and routine starting materials for the synthesis of otherwise unavailable but interesting structures. We expect that our pursuits will inspire and help direct future chemical and physical research on NSBV.

The Zn(OTf)2 -catalyzed guanylation/amidation from readily available amino acid esters and carbodiimides is achieved to provide efficiently various cyclic oxoguanidines, including 2-amino-1H-imidazol-5(4H)-ones and 2-aminoquinazolin-4(3H)-ones in medium-to-high yields. It is the first time that an ammonium salt has been used in a guanylation reaction. The application of cyclic oxoguanidines to provide the conjugated heterocyclic compounds via oxidative C-N formation or aldol reaction is explored. The reaction mechanism is well elucidated by the isolation and characterization of three important intermediates.

Organolithium compounds can behave as reductants but never as oxidants in redox reactions. Reported herein is that 1,4-dilithio-1,3-butadienes reacted with [Ni(cod)2] (cod = 1,5-cyclooctadiene) to deliver dilithionickeloles. Single-crystal X-ray structural analysis revealed a coplanar structure of dilithionickeloles with an averaging of bond lengths. XPS data confirmed the oxidation state of Ni in dilithionickeloles was Ni(2+). (7)Li NMR spectra of dilithionickeloles and theoretical calculations revealed a considerable aromatic character. In this redox reaction, the dilithio dianionic compounds behaved as formal oxidants, thus oxidizing Ni(0) into Ni(2+). These results demonstrated that organolithium compounds with π-conjugation could be used as oxidants and could continue to accept extra electrons.

The phosphaguanylation reaction of phosphines with carbodiimides catalyzed by half-sandwich yttrium tris(trimethylsilylmethyl) ate complexes is achieved for the first time to efficiently prepare phosphaguanidines. The catalyst system of the yttrium ate complex displays better catalytic activity than those of neutral yttrium complexes.

β-Hydride abstraction is a well-accepted elementary step for catalytic cycles in organometallic chemistry. It is usually anticipated that alkylpalladium halides containing syn-β-hydrogen atoms will undergo β-hydride abstraction to afford the Heck-type products. However, this study discloses that the above general knowledge is only conditionally correct. Our experimental results demonstrate that the reductive elimination of alkylhalides from alkylpalladium halides containing syn-β-hydrogen atoms may surpass the β-hydride abstraction or even become exclusive in certain cases.

Four types of alkenyl organocopper(I) aggregates linked by 1,3-butadienyl and/or 1,3,5,7-octatetraenyl moieties were selectively realized in good isolated yields. All these organocopper(I) aggregates were structurally characterized by single-crystal X-ray structural analysis. These unprecedented aggregates, stabilized by multiple Cu-Cu interactions and the conjugated 1,3-butadienyl or 1,3,5,7-octatetraenyl bridges, could undergo controlled structural transformations. The 1,4-dicopper 1,3-butadienyl aggregate 3 could be efficiently transformed to aggregate 2, while LiI could disaggregate the 1,3-butadienyl-1,3,5,7-octatetraenyl aggregate 4 to 1,3,5,7-octatetraenyl aggregate 5 and 1,3-butadienyl aggregate 2. Preliminary reaction chemistry and synthetic applications of these organocopper(I) aggregates were also investigated.

A variety of ester-substituted cyclopentadiene derivatives have been synthesized by one-pot reactions of 1,4-dilithio-1,3-butadienes, CO, and acid chlorides. Direct deprotonation of the ester-substituted cyclopentadienes with Ae[N(SiMe3 )2 ]2 (Ae=Ca, Sr, Ba) efficiently generated members of a new class of heavier alkaline earth (Ca, Sr, Ba) metallocenes in good to excellent yields. Single-crystal X-ray structural analysis demonstrated that these heavier alkaline earth metallocenes incorporated two intramolecularly coordinated ester pendants and multiply-substituted cyclopentadienyl ligands. The corresponding transition metal metallocenes, such as ferrocene derivatives and half-sandwich cyclopentadienyl tricarbonylrhenium complexes, could be generated highly efficiently by metathesis reactions. The multiply-substituted cyclopentadiene ligands bearing an ester pendant, and the corresponding heavier alkaline earth and transition-metal metallocenes, may have further applications in coordination chemistry, organometallic chemistry, and organic synthesis.

This study sheds light on the cleavage and reorganization of C(sp(3))-H and C=N bonds of carbodiimides in a three-component reaction of terminal alkynes, sulfur, and carbodiimides by a combination of methods including 1) isolation and X-ray analysis of six-membered-ring lithium species 2-S, 2) trapping of the oxygen-analogues (B-O and D-O) of both four-membered-ring intermediate B-S and ring-opening intermediate D-S, 3) deuterium labeling studies, and 4) theoretical studies. These results show that 1) the reaction rate-determining step is [2+2] cycloaddition, 2) the C=N bond cleavage takes place before C(sp(3))-H bond cleavage, 3) the hydrogen attached to C6 in 2-S originates from the carbodiimide, and 4) three types of new aza-heterocycles, such as 1,2-dihydrothiopyrimidines, N-acyl 2,3-dihydropyrimidinthiones, and 1,2-dihydropyrimidinamino acids are constructed efficiently based on 2-S. All results strongly support the idea that the reaction proceeds through [2+2] cycloaddition/4π electrocyclic ring-opening/1,5-H shift/6π electrocyclic ring-closing as key steps. The research strategy on the synthesis, isolation, and reactivity investigation of important intermediates in metal-mediated reactions not only helps achieve an in-depth understanding of reaction mechanisms but also leads to the discovery of new synthetically useful reactions based on the important intermediates.

The chemistry of magnesium organocuprates, including their synthesis, structures, and reactions, remains underexplored. In this work, by taking advantage of the high reactivity and ready availability of magnesiacyclopentadienes, a series of magnesiacyclopentadiene-based organocuprates were synthesized and structurally characterized. A variety of CuX salts (X=Cl, Br, I, or alkynyl) were successfully applied to react with magnesiacyclopentadienes. Besides CuX salts, AgX salts (X=Cl, alkynyl) also undergwent the above reaction to afford the corresponding magnesium organoargentates. Single-crystal X-ray structural analysis and DFT calculations of these butadienyl magnesium organocuprates revealed unique structural characteristics and bonding modes. These results are also very useful to understand the transmetalation process, since the product can be viewed as the resting-state intermediate of a transmetalation reaction between organomagnesium compounds and coinage-metal salts. Preliminary information on the reaction chemistry of these magnesium organocuprates is provided by their reactions with allyl bromide, benzoyl chloride, and CO2 .