Chromium Carbene Complexes Research Articles

Metal–carbene-guided twofold cross-coupling of ethers with chromium catalysis

Coupling by metal–carbene transfer enables the formation of several different bonds at the carbenoid site, enabling prochiral Csp3 centers that are fundamental three-dimensional substructures for medicines to be forged with increased efficiency. However, strategies using bulk chemicals are rare because of the challenge of breaking two unactivated geminal bonds. Herein, we report the reactivity of ethers to form metal–carbene intermediate by cleavage of α-Csp3–H/Csp3–O bonds, which achieve selective coupling with arylmagnesium bromides and chlorosilanes. These couplings are catalysed by cyclic (alkyl)(amino)carbene-chromium complex and enable the one-step formation of 1,n-arylsilyl alcohols and α-arylated silanes. Mechanistic studies indicate that the in-situ formed low-valent Cr might react with iodobenzene to form phenyl radical species, which abstracts the α-H atom of ether in giving α-oxy radical. The latter combines with Cr by breaking α-Csp3–O bond to afford metal–carbene intermediate, which couples with aryl Grignard and chlorosilane to form two σ-bonds.

Pd-catalyzed Oxidative Cross-coupling of Alkyl Chromium(0) Fischer Carbene Complexes with Organoboronic Acids.

Alkyl chromium(0) carbene complexes have been explored as the cross-coupling partners in the palladium-catalyzed reaction with aryl or alkenyl boronic acids. This coupling reaction displays the versatile reactivities of alkyl chromium(0) carbenes under palladium catalysis. Mechanistically, this transformation is proposed to involve deprotonation of the alkyl chromium carbene substrate to generate a vinyl chromium anion intermediate that undergoes transmetalation to organopalladium species and reductive elimination.

Electronic effect of ligands vs. reduction potentials of Fischer carbene complexes of chromium: a molecular electrostatic potential analysis

Molecular electrostatic potential at the chromium centre (VCr) emerges as a powerful predictor of reduction potential (E0).

3+3] Annulation of Vinyl Chromium(0) Carbene Complexes

3+3] Annulation of Vinyl Chromium(0) Carbene Complexes

Palladium-Catalyzed Reductive Cross-Coupling Reaction of Aryl Chromium(0) Fischer Carbene Complexes with Aryl Iodides

The first palladium-catalyzed reductive cross-couplings of aryl chromium(0) carbene complexes with aryl iodides have been realized. This coupling reaction shows excellent functional group tolerance and high efficiency. Mechanistically, aryl chromium(0) carbene complexes undergo transmetalation with arylpalladium species to generate palladium(II) carbene intermediates, which is followed by migratory insertion. The catalytic cycle is then completed by hydrogen transfer and reductive elimination. Consistent with the mechanistic hypothesis, density functional theory (DFT) calculations support the involvement of a palladium carbene intermediate, and carbene migratory insertion is a facile step with an energy barrier of 5.1 kcal/mol. The carbene transfer step and the hydrogen transfer step are confirmed as the rate-limiting steps in the catalytic cycle.

Palladium-Catalyzed [3+3] Annulation of Vinyl Chromium(0) Carbene Complexes through Carbene Migratory Insertion/Tsuji-Trost Reaction.

Vinyl chromium(0) Fischer carbene complexes were employed as the source of π-allylic palladium species for catalytic [3+3] annulation under palladium catalysis. Mechanistically, this transformation is proposed to involve carbene migratory insertion and intramolecular Tsuji-Trost reaction as the key steps. Substituted six-membered heterocyclic flavonones and quinolines are obtained, depending on the nucleophilic functional group on the coupling partners.

Palladium‐Catalyzed [3+3] Annulation of Vinyl Chromium(0) Carbene Complexes through Carbene Migratory Insertion/Tsuji–Trost Reaction

AbstractVinyl chromium(0) Fischer carbene complexes were employed as the source of π‐allylic palladium species for catalytic [3+3] annulation under palladium catalysis. Mechanistically, this transformation is proposed to involve carbene migratory insertion and intramolecular Tsuji–Trost reaction as the key steps. Substituted six‐membered heterocyclic flavonones and quinolines are obtained, depending on the nucleophilic functional group on the coupling partners.

Pd-Catalyzed Cross-Coupling of Terminal Alkynes with Chromium(0) Fischer Carbene Complexes.

Alkyl chromium(0) carbene complexes typically undergo cycloadditions with alkynes to afford carbo- or heterocyclic compounds. In the presence of Pd catalyst, it is demonstrated that a traditional cycloaddition pathway is completely altered: instead of cycloaddition, oxidative cross-coupling reaction of alkyl chromium(0) carbene complexes with terminal alkynes occurs. The coupling reaction exhibits excellent functional group tolerance and good efficiency.

A four-step route to synthetic equivalents of ortho-xylylenes: Dötz benzannulation, desilylation, bromo-dehydroxylation, and sultine formation. A concise approach to oxygenated linearly fused polycyclic aromatics

A new route has been reported for the synthesis of densely oxygenated polycyclic aromatic compounds via cycloaddition approach. This strategy involves the Dötz benzannulation and Diels-Alder reaction as key steps. Naphthalene synthons required here were generated by Dötz benzannulation between aryl chromium carbene complexes and symmetrical internal alkyne.

C–H Bond Functionalization of Benzoxazoles with Chromium(0) Fischer Carbene Complexes

An efficient C–H bond functionalization of benzoxazoles with chromium(0) Fischer carbene complexes under catalyst-free conditions has been developed. A series of benzoxazoles and chromium(0) carbene complexes are compatible with the reaction. This transformation provides an alternative way for C–H bond alkylation of benzoxazoles. In the reaction mechanism the elimination of the Cr(CO)5 fragment seems more favored than the elimination of an alkoxy group, which is in sharp contrast to the previous reports on the reaction of organolithium reagents with chromium(0) Fischer carbene complexes.

Understanding the Mechanism of the Divergent Reactivity of Non-Heteroatom-Stabilized Chromium Carbene Complexes with Furfural Imines: Formation of Benzofurans and Azetines.

The mechanisms of the reaction between non-heteroatom-stabilized alkynyl chromium carbene complexes prepared in situ and furfural imines to yield benzofurans and/or azetines have been explored by means of density functional theory method calculations. The reaction proceeds through a complex cascade of steps triggered by a nucleophilic addition of the imine nitrogen atom. The formation of two benzofuran regioisomers has been explained in terms of competitive nucleophilic attacks to different positions of the carbene complex. Each of these regioisomers can be obtained as the major product depending on the starting materials. The overall sequence could be controlled to yield benzofurans or azetines by adjusting the substituents present in the initial carbene complex. This mechanistic information allowed for the preparation of new benzofurans and azetinylcarbenes in good yields.

N-Phosphanyl- and N,N′-Diphosphanyl-Substituted N-Heterocyclic Carbene Chromium Complexes: Synthesis, Structures, and Catalytic Ethylene Oligomerization

The chromium(II) complexes [CrCl2(t-BuNHC,P-κC)2] (1), [CrCl2(MesNHC,P-κC)2] (2), [CrCl2(DippNHC,P-κC)2] (3), and [CrCl2(P,NHC,P-κC)2] (4) containing the N-phosphanyl- or N,N′-diphosphanyl-substituted N-heterocyclic carbene (NHC) hybrid ligands t-BuNHC,P (1-(di-tert-butylphosphino)-3-tert-butylimidazol-2-ylidene), MesNHC,P (1-(di-tert-butylphosphino)-3-mesitylimidazol-2-ylidene), DippNHC,P (1-(di-tert-butylphosphino)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene), and P,NHC,P (1,3-bis(di-tert-butylphosphino)imidazol-2-ylidene), respectively, were prepared from CrII ([CrCl2(thf)2]) or CrIII ([CrCl3(thf)3] or [Cr(Me)Cl2(thf)3]) precursors. The solid-state structures of these four complexes show square-planar CrII centers, with two trans chloride and two monodentate CNHC donors. Alkylation of 3 and 4 with [Mg(benzyl)2(thf)2] led to the formation of the σ complexes [Cr(benzyl)3(DippNHC,P-κC,κP)] (5) and [Cr(benzyl)3(P,NHC,P-κC,κP)] (6), respectively, with five-coordinate distorted-square-pyramidal CrIII coordin...

Confocal and fluorescence lifetime imaging sheds light on the fate of a pyrene-tagged carbon monoxide-releasing Fischer carbene chromium complex.

The synthesis of a new pyrene-containing Fischer carbene complex is described. The complex has a broad absorbance spectrum between 300 and 400 nm and, on excitation at 345 nm in CH2Cl2 solution, emission is observed at 395 and 415 nm. Emission is also observed in PBS buffer, but in this case the resulting spectra are much broader. Confocal and fluorescence lifetime imaging indicate that emission occurs on treating HeLa cells with the complex and co-localisation studies demonstrate that this is from the mitochondria and lipid-rich regions of the cell.

Water freezing as a regiocontrol element in the multicomponent assembly of cyclic enones.

Regioselective synthesis of dialkoxy 2-cyclopentenones and 2-cyclohexenones with novel substitution patterns has been accomplished by the one-pot combination of three simple starting materials (chromium carbene complex, Weinreb acetamide lithium enolate and 1-alkoxyallenyllithium) under either anhydrous conditions or water-promoted solidification of the reaction mixture. These results revealed an unprecedented water-freezing effect that plays a key role to completely reverse the regioselectivity of the intramolecular carbometalation of an allene moiety.

Negishi Cross-Coupling Reaction as a Simple and Efficient Route to Functionalized Amino and Alkoxy Carbene Complexes of Chromium, Molybdenum, and Tungsten

Synthesis of functionalized phenyl, 2-thienyl, and N-methyl-pyrrol-2-yl Fischer carbene complexes of chromium and tungsten was achieved via a Negishi cross-coupling reaction of metalated aminocarbenes in the presence of a palladium catalyst. The reverse approach, i.e., coupling bromocarbenes with organozinc reagents, is also feasible and is particularly useful with alkoxycarbene complexes, metalation of which is not possible. Both these procedures are also suitable for the cross-coupling of molybdenum amino- and alkoxycarbene complexes. The presented methodology enables an access to the new amino and alkoxy carbene complexes bearing aldehyde, ketone, nitrile, and ester functionality.

Studies towards the synthesis of medermycin via Dötz benzannulation.

The C-arylglycosides are available in enantiomerically pure form via the Dötz benzannulation reaction between Fischer alkenyl chromium carbene complexes and alkynes; it also could be converted to a precursor of medermycin by O-carbamate directed ipso bromination and nitrile substitution in good overall yields.

Synthesis and biological evaluation of kendomycin and its analogues.

Ansa compounds are gifts from microbes with intriguing molecular structures and highly potent bioactivities. One of the ansa compounds, kendomycin, has an oxa-metacyclophane skeleton with a quinone methide core and a fully substituted tetrahydropyran ring. Beyond a common synthetic strategy for construction of the ansa skeleton (i.e., elongation of an alkyl chain from an aromatic core followed by macrocyclization), we challenged a new method for construction of the ansa skeleton via simultaneous macrocyclization and benzannulation (using an intramolecular Dötz benzannulation). Understanding the reactivity of various Fischer-type ω-alkynyloxy chromium carbene complexes with kendomycin analogue syntheses led to achievement of the total synthesis of kendomycin. Investigations of structure-activity relationships revealed the need for an ansa skeleton for antimicrobial activity. Therefore, we envisage that this intramolecular Dötz benzannulation will enable divergent syntheses of ansa compounds which have important bioactive potential.

分子内デッツ反応を用いたアンサ骨格の構築：ケンドマイシンの全合成

Ansa-compounds have intriguing molecular structures and highly potent bioactivities. One of the ansa-compounds, kendomycin, has an oxa-metacyclophane skeleton with quinone methide core and a fully substituted tetrahydropyran ring. In contrast to a common synthetic strategy for construction of the ansa-skeleton via elongation of an alkyl chain from an aromatic core followed by macrocyclization, we challenged a new method for construction of the ansa-skeleton via simultaneous macrocyclization and benzannulation (using intramolecular Dötz benzannulation). Understanding the reactivity of various Fischer-type ω-alkynyloxy chromium carbene complexes with kendomycin analogs syntheses led to achievement of the total synthesis of kendomycin. Investigations of structure-activity relationships revealed the need for an ansa-skeleton for antimicrobial activity.

Fac and mer dppe-substituted Fischer carbene complexes of chromium: X-ray, DFT and electrochemical study

X-ray crystallography data, DFT calculations and cyclic voltammograms in synergy shed light on the mer ↔ fac isomerization process of Cr carbenes of the type [(CO)3(dppe)CrC(X)R] with R = 2-thienyl or 2-furyl and X = OEt or NHCy, both in the neutral and oxidized states. Cyclohexylamino-containing complexes are easier oxidized and reduced than ethoxy-containing complexes. Dppe-containing complexes are considerably easier oxidized and reduced than non-phosphine containing Cr–Fischer carbene complexes of the type [(CO)5CrC(X)R] with X = OEt, NHBu, NMe2 or N(CH2Ph)2 and R = phenyl, thienyl, furyl, NMe-pyrrole, H or Me. The oxidation and reduction potential of the [(CO)3(dppe)CrC(X)R] complexes are not very sensitive to the influence of the heteroarene rings 2-thienyl or 2-furyl.

Total synthesis of (+)-antofine and (-)-cryptopleurine.

The tylophorine alkaloid anticancer compounds antofine and cryptopleurine have been synthesized in optically active form. Both syntheses employ optically pure α-amino acids as the starting materials, require only seven steps from known 2-ethynylpyrrolidine or 2-ethynylpiperidine derivatives, and are free of protecting groups. Key steps include an alkyne hydration and a chromium carbene complex based net [5+5]-cycloaddition step. Alkyne hydration was accompanied by racemization of the resulting β-aminoketone under most of the conditions examined, and successful minimization of this side reaction was achieved through careful pH control and choice of metal additive. Final ring closure involves a Bischler-Napieralski reaction using a carbamate (antofine) or urea (cryptopleurine) precursor.