Imidozirconocene Complexes Research Articles

Evidence for the powerful catalytic ability of imidozirconocene complex from its epoxide ring cleavage reactions – A DFT mechanistic view#

Imidozirconocene complex is known for its bifunctional reactivity and catalytic ability and this complex mediates ring cleavage of epoxides. Cyclooctene oxide (1) Norbornene oxide (2) and 2,5-dimethyl cyclohexene oxide (3) undergo ring cleavage in the presence of imidozirconocene complex. Epoxide 1 has accessible β-hydrogens (type I) while epoxide 2 and 3 do not have them (type II). Normally type I epoxides undergo elimination while type II epoxides prefer insertion. All the insertion reactions lead to five-membered metallacycle formation and elimination results in thermodynamically stable allyl-alkoxy product. The insertion is a two-step process following either diradical or zwitterionic pathway, while elimination is a one-step concerted reaction. DFT (density functional theory) modelling of these reactions at B3LYP/LANL2DZ level show that epoxide 1 undergoes elimination in agreement with experiment. However, calculations indicate that epoxide (2) proceeds through diradical intermediate in contrast to experimental observations. Surprisingly, epoxide (3) that has both the β positions blocked by methyl groups undergoes elimination rather than insertion. AIM and EDA analyses offer further insights on the reaction mechanism and bifunctional reactivity of imidozirconozene complex. DFT calculations provide clear evidence for the bifunctional reactivity of imidozirconocene through bonding changes in the ring cleavage of epoxides Cyclooctene epoxide undergoes elimination and norbornene oxide follows insertion. Surprisingly, cyclohexene oxide which has both β positions blocked by methyl groups undergoes elimination instead of insertion due to carbocation stability.

Imidozirconocene‐Mediated Ring Cleavage of Epoxides – Evidence for Bifunctional Reactivity from DFT

AbstractThe ring cleavage of epoxides mediated by bis(cyclopentadienyl)(tert‐butylimido)zirconium (Cp2Zr=N–tBu)(THF) has been investigated using DFT calculations with a view to understand the mechanism of epoxide ring cleavage and the role of the imidozirconocene complex. Two types of epoxides have been chosen; some with and some without accessible β‐hydrogen atoms. Epoxides without accessible β‐hydrogen atoms undergo only insertion, whereas those with undergo both insertion and elimination. Although insertion is found to follow a zwitterionic mechanism, the elimination passes through a concerted intramolecular hydrogen transfer step. The role of the imidozirconocene complex has been elucidated by EDA and NBO analysis, which show that the vacant 1a1 molecular orbital of the Lewis acidic metal center of Cp2Zr=N–tBu fragment initiates the reaction as a Lewis acidic center and the Lewis basic imido nitrogen atom of the Cp2Zr=N–tBu fragment completes the reaction. The coordinate to covalent bond conversion and the involvement of the 1a1 and 2a1 molecular orbitals of the Cp2Zr=N–tBu fragment in the reaction have been examined. The formation of a five‐membered metallacycle in the insertion and the alkoxy allyl product in the elimination predicted by calculations concur with experimental observations. Topological analysis has been carried out to further elucidate the mechanism and to reveal the bifunctional reactivity of imidozirconocene in epoxide ring opening reactions.

Synthesis, Structural Characterization, and Quantitative Basicity Studies of Lithium Zirconimidate Complexes

Synthesis, Structural Characterization, and Quantitative Basicity Studies of Lithium Zirconimidate Complexes

Carboamination: Additions of Imine C=N Bonds Across Alkynes Catalyzed by Imidozirconium Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Zirconium‐Mediated Conversion of Amides to Nitriles: A Surprising Additive Effect.

AbstractFor Abstract see ChemInform Abstract in Full Text.

C—H Bond Activation of Hydrocarbons by an Imidozirconocene Complex.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reactions of Imines with Azazirconacyclobutenes and Generation of Electron-Deficient Imidozirconocene Complexes

Electron-deficient imines are shown to insert into the zirconium-carbon bond of azazirconacyclobutenes to generate new six-membered ring metallacycles. On heating, these expanded zirconacycles undergo a retro-[4 + 2] cycloaddition to generate alpha,beta-unsaturated imines in excellent yields and novel, fully characterized electron-deficient imidozirconocene complexes.

C-H bond activation of hydrocarbons by an imidozirconocene complex.

Monomeric imidozirconocene complexes of the type Cp2(L)Zr=NCMe3 (Cp = cyclopentadienyl, L = Lewis base) have been shown to activate the carbon-hydrogen bonds of benzene, but not the C-H bonds of saturated hydrocarbons. To our knowledge, this singularly important class of C-H activation reactions has heretofore not been observed in imidometallocene systems. The M=NR bond formed on heating the racemic ethylenebis(tetrahydro)indenyl methyl tert-butyl amide complex, however, cleanly and quantitatively activates a wide range of n-alkane, alkene, and arene C-H bonds. Mechanistic experiments support the proposal of intramolecular elimination of methane followed by a concerted addition of the hydrocarbon C-H bond. Products formed by activation of sp2 C-H bonds are generally more thermodynamically stable than those formed by activation of sp3 C-H bonds, and those resulting from reaction at primary C-H bonds are preferred over secondary sp3 C-H activation products. There is also evidence that thermodynamic selectivity among C-H bonds is sterically rather than electronically controlled.

Selective Transformations of Organic Compounds by Imidozirconocene Complexes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Mechanisms of allene stereoinversion by imidozirconium complexes.

The zirconium-mediated stereoinversion of allenes has been investigated by studying the stereochemical behavior of metallacycles derived from [2 + 2] cycloaddition of enantioenriched allenes with chiral and achiral imidozirconocene complexes. Relative rates of metallacycle racemization were measured by circular dichroism, and intermediates in the selective stereoinversion of diphenylallene with a chiral imidozirconium complex were observed by NMR spectroscopy. Metallacycles derived from dialkylallenes are proposed to racemize via reversible beta-hydride elimination. Stereoinversion of diarylallene-derived metallacycles proceeds much more slowly and is thought to proceed through an eta4-azatrimethylenemethane transition state.

Selective transformations of organic compounds by imidozirconocene complexes.

Imidozirconocene complexes (Cp(2)Zr=NR(THF)) possess a rich reaction chemistry, in contrast to many imido complexes of later transition metals. The synthesis and structural characteristics of these compounds are briefly described, along with a qualitative molecular orbital description of the metallocene-imido interaction. Imidozirconocene complexes react at the Zr=N linkage with a variety of X-H bonds, and undergo overall [2 + 2] cycloaddition reactions with a wide range of unsaturated organic and organometallic compounds. Results of studies detailing the scope and mechanism of these reactions are presented herein, along with applications to catalytic hydroamination, imine metathesis, and asymmetric transformations.

Structural Factors that Influence the Course of Overall [2 + 2] Cycloaddition Reactions between Imidozirconocene Complexes and Heterocumulenes

A study has been carried out to examine the factors that influence the overall [2 + 2] cycloaddition reactions between imidozirconocene complexes Cp2(THF)ZrNR (R = 2,6-i-Pr2C6H3, 2,6-Me2C6H3, t-Bu) and small, unsaturated organic molecules. Steric factors in the cycloaddition reactions investigated were found to be significant in both the imido fragment and the heterocumulene reactant partner. When Cp2(THF)ZrNR (R = 2,6-Me2C6H3) was treated with symmetrical carbodiimides RNCNR (R = t-Bu, i-Pr, SiMe3, cyclohexyl, p-tolyl), the corresponding diazametallacycles formed in high yields. However, with the more sterically encumbered zirconocene imido complex Cp2(THF)ZrNR (R = 2,6-i-Pr2C6H3), diazametallacycle formation was observed only with the heterocumulenes 1,3-diisopropylcarbodiimide, 1,3-dicyclohexylcarbodiimide, and 1,3-di-p-tolylcarbodiimide. The 16-electron diazametallacyle Cp2Zr(N(2,6-i-Pr2C6H3)CN(Tol)N(Tol)) (9c) underwent rearrangement of the Zr−N bound moiety upon heating, forming a new metallacycle w...

Zirconium-Mediated Metathesis of Imines: A Study of the Scope, Longevity, and Mechanism of a Complicated Catalytic System

By kinetically stabilizing imidozirconocene complexes through the use of a sterically demanding ligand, or by generating a more thermodynamically stable resting state with addition of diphenylacetylene, we have developed transition metal-catalyzed imine metathesis reactions that are mechanistically analogous to olefin metathesis reactions catalyzed by metal carbene complexes. When 5 mol % of Cp*Cp(THF)Zr=N(t)Bu is used as the catalyst precursor in the metathesis reaction between PhCH=NPh and p-TolCH=N-p-Tol, a 1:1:1:1 equilibrium mixture with the two mixed imines p-TolCH=NPh and PhCH=N-p-Tol is generated in C(6)D(6) at 105 degrees C. The catalyst was still active after 20 days with an estimated 847 turnovers (t(1/2) 170 m; TON = 1.77 h(-1)). When the azametallacyclobutene Cp(2)Zr(N(Tol)C(Ph)=C(Ph)) is used as the catalyst precursor under similar reaction conditions, a total of 410 turnovers are obtained after 4 days (t(1/2) 170 m; TON = 4.3 h(-1)). An extensive kinetic and equilibrium analysis of the metallacyclobutene-catalyzed metathesis of PhCH=N-p-Tol and p-F-C(6)H(4)CH=N-p-F-C(6)H(4) was carried out by monitoring the concentrations of imines and observable metal-containing intermediates over time. Numerical integration methods were used to fit these data to a detailed mechanism involving coordinatively unsaturated (16-electron) imido complexes as critical intermediates. Examination of the scope of reaction between different organic imines revealed characteristic selectivity that appears to be unique to the zirconium-mediated system. Several zirconocene complexes that could generate the catalytically active "CpCp'Zr=NAr" (Cp' = Cp or Cp*) species in situ were found to be effective agents in the metathetical exchange between different N-aryl imines. N-Alkyl aldimines were found to be completely unreactive toward metathesis with N-aryl aldimines, and metathesis reactions involving the two N-alkyl imines TolCH=NPr and PhCH=NMe gave slow or erratic results, depending on the catalyst used. Metathesis was observed between N-aryl ketimines and N-aryl aldimines, but for N-aryl ketimine substrates, the catalyst resting state consists of zirconocene enamido complexes, generated by the formal C-H activation of the alpha position of the ketimine substrates.

ChemInform Abstract: Generation, Alkyne Cycloaddition, Arene C‐H Activation, N‐H Activation, and Dative Ligand Trapping Reactions of the First Monomeric Imidozirconocene (Cp2Zr=NR) Complexes.

AbstractThe zirconocene methyl amide complexes (III) are prepared according to the scheme.

Generation, alkyne cycloaddition, arene carbon-hydrogen activation, nitrogen-hydrogen activation and dative ligand trapping reactions of the first monomeric imidozirconocene (Cp2Zr:NR) complexes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTGeneration, alkyne cycloaddition, arene carbon-hydrogen activation, nitrogen-hydrogen activation and dative ligand trapping reactions of the first monomeric imidozirconocene (Cp2Zr:NR) complexesPatrick J. Walsh, Frederick J. Hollander, and Robert G. BergmanCite this: J. Am. Chem. Soc. 1988, 110, 26, 8729–8731Publication Date (Print):December 1, 1988Publication History Published online1 May 2002Published inissue 1 December 1988https://doi.org/10.1021/ja00234a043RIGHTS & PERMISSIONSArticle Views1999Altmetric-Citations331LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (467 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts