Chromium Aminocarbene Research Articles

Modular Synthesis of Fischer Biscarbene Complexes of Chromium

Aromatic and (hetero)aromatic chromium aminocarbenes were lithiated on the (hetero)aromatic ring and transmetalated with ZnBr2. The subsequent Pd(PPh3)4- or PdCl2(XantPhos)-catalyzed Negishi reaction with dibromoarenes or chromium alkoxy-/aminocarbenes bearing bromine atoms yielded bisaminocarbene or mixed bisalkoxyaminocarbene complexes of chromium. This methodology allows easy access to the biscarbene complexes containing poly(hetero)aromatic bridges, such as biphenylene, bithienylene, tetrathienylene, 1,1′-ferrocenylene, and others. In total, 18 new biscarbene complexes, including two containing both chromium and tungsten, were synthesized in this fashion. The electrochemical behavior of the biscarbenes obtained exhibits electronic communication between the metals.

N-Heterocycles from chromium aminocarbenes.

The initial [2 + 2]-cycloadduct between a chromium aminocarbene and a tethered alkene undergoes a β-hydrogen elimination very efficiently when triphenylphosphine is added as a ligand. The reaction gives cyclic enamines or homoenamines depending on the substitution on the alkene.

Structural flexibility of 2-hetaryl chromium aminocarbene complexes: Experimental and theoretical evidence

Chromium aminocarbene complexes [(CO)5CrC(N(CH3)2)R], RN-methyl-2-pyrrolyl, 2-thienyl and 2-furyl were prepared and structurally characterized. X-ray study indicated two stable rotamers in case of compounds containing 2-thienyl and 2-furyl substituents. DFT calculations well interpreted structural parameters of all studied complexes. Calculations of potential energy surfaces (PES) for neutral species confirmed existence of two energy minima on the potential energy surfaces. Calculated rotation barriers increase in the series 2-furyl, 2-thienyl and N-methyl-2-pyrrolyl, respectively. The PES scans for compounds in one-electron reduced states essentially reveal opposite behavior to the one observed for neutral complexes. Connection is drawn between the observed structural flexibility and electrochemical properties of the compounds.

Theoretical Predictions of Redox Potentials of Fischer-Type Chromium Aminocarbene Complexes

Redox potentials of series of chromium aminocarbene complexes with general formulas [(CO)5Cr═C(R)N(CH3)2] and [(CO)4Cr═C(R)N(CH2CHCH2)2] were calculated using DFT methods for both metal-localized oxidation and ligand-localized reduction processes. The electrostatic contribution of solvation was approximated by the polarizable continuum model (PCM); specific interactions of the complexes with counterions of supporting electrolyte were considered by explicitly including these ions in the model. The theoretical redox potentials were correlated with experimental values, and the qualities of the results of the approaches used were compared. It was shown that both sets of calculated redox potentials reproduce the experimental data well. The mean average error of the calculated redox potentials was 0.088 V with the counterions and 0.111 V without the counterions. The best results were obtained for oxidation processes, where the mean average error decreased from 0.110 to 0.059 V due to the inclusion of the counterions.

ChemInform Abstract: N‐Heteropolycyclic Compounds from the Formal Intramolecular [4 + 1]‐Cycloaddition of Chromium Aminocarbenes.

Chromium aminocarbenes tethered to dienes of all three electronic natures undergo an efficient intramolecular (4 + 1)-cycloaddition to give N-heteropolycyclic compounds. Ligands on chromium had a profound effect on the course of the reaction.

N-Heteropolycyclic Compounds from the Formal Intramolecular (4 + 1)-Cycloaddition of Chromium Aminocarbenes

Chromium aminocarbenes tethered to dienes of all three electronic natures undergo an efficient intramolecular (4 + 1)-cycloaddition to give N-heteropolycyclic compounds. Ligands on chromium had a profound effect on the course of the reaction.

Synthesis, characterization and electrochemical investigation of hetaryl chromium(0) aminocarbene complexes

Fischer carbene complexes are useful precursors in organic synthesis and promising catalysts. For both these purposes, a fine “tuning” of their thermodynamic properties and reactivity is of high interest. Therefore a detailed insight into the structure–reactivity correlation is necessary for aimed design and synthesis of compounds with required specific properties.In this contribution a new series of aminocarbene complexes of chromium(0) was synthesized, where the originally used phenyl substituent in the compound I pentacarbonyl[(N,N-dimethylamino)(phenyl)carbene]chromium(0), was replaced by various five-membered heteroaromatic rings: furan (II), thiophene (III) and N-Me-pyrrol (IV), attached either by their position 2 or 3 (Fig. 1). The compounds were characterized by 1H and 13C NMR and elemental analysis. In the second part of this work an electrochemical investigation in acetonitrile followed in order to understand more deeply the oxidation and reduction process and to analyze the relative contribution of inductive and mesomeric effects of the heterocycles to the reduction of the chromium aminocarbenes. Their electrochemical behavior was compared with that obtained for the phenyl derivative I, the interpretation was correlated with quantum chemical calculations and the differences were discussed.

ChemInform Abstract: Chromium Aminocarbene Complexes in Organic Synthesis

Abstract The synthesis, the physical and spectroscopic properties, and the thermal and photochemical reactions of chromium aminocarbene complexes are reviewed, with an emphasis on the authors' research in the area.

Ferrocene Derivatives in Voltammetric Investigations of Intramolecular Electronic Interactions

Many organic and organometallic molecules (including polynuclear complexes, supramolecular systems as well as molecules of biological significance) posses two or more redox centers. The localization of the redox centers, the extent of their intramolecular interactions and the distribution of electron density along the molecule, which is further influenced by substitution, are decisive for the chemical properties and potential utilization of such molecules. In this contribution we report about how the suitable electrochemical properties (reversibility and stability) of ferrocene backbone as a sensitive redox probe or an electrochemical marker were utilized for a) investigations of mutual electronic interactions of various molecular parts of hybrid phosphine ligands, their complexes and P-chelated chromium(0) aminocarbenes; b) testing of alkyne bridges as building blocks for connecting redox centers; c) experimental studies concerning electron density distribution along the purine heterocycle. Electrochemistry in this field represent a very useful approach as a unique source of experimental data.

Study of the Origin of the Hindered Rotation of an Aryl Ring in Chromium Aminocarbene Complexes Bearing an Aromatic Ring Attached to the Carbene Carbon Atom

The substituted chromium aminocarbene complexes cis-tetracarbonyl[(η2-N-allyl-N-allylamino)(4-X-phenyl)carbene]chromium(0) (X = MeO, Me, H, Cl, CO2Me, CF3) were prepared by the reaction of the corresponding N,N-diallylamides with Na2Cr(CO)5 and Me3SiCl. Using dynamic NMR, the rotational barrier of the aryl ring in these complexes was established. The obtained values are higher for the complexes bearing electron acceptors compared to those bearing donor substituents and correspond with the Hammett equation. This indicates that the restricted rotation of aryl ring in the chromium aminocarbene complexes is a balance between steric and conjugation effects; while the former increases the barrier, the latter decreases it by stabilizing the transition state of the rotation. This finding is compatible with the fact that in the related cis-tetracarbonyl[(N-alkyl-η2-N-allylamino)(phenyl)carbene]chromium(0) complexes (alkyl = Me, Et, i-Pr) the rotational barrier grows steadily with the growing steric demand of the s...

Fischer chromium carbene complexes as nucleophiles in palladium-catalyzed allylic substitution reactions

The Pd-catalyzed reaction of carbanions derived from chromium aminocarbene complexes with allylic acetates and carbonates smoothly affords the corresponding allyl-substituted aminocarbenes. On the contrary, the same reaction of the anion derived from pentacarbonyl[(methoxy)methylcarbene]chromium(0) with cinnamyl acetate affords 5-methoxy-1-phenylhexa-1,5-diene, the product of 1-methoxyethen-1-yl group transfer.

Insertion of alkynes into diterpenoid chromium aminocarbenes: synthesis of ring-C aromatic steroidal analogues

The insertion of alkyl- or aryl-substituted alkynes into chromium aminocarbenes derived from podocarpic acid gives good to excellent yields of cyclopentaannulated products. The presence of a heteroatom bonded directly to the alkyne lowers the yield of the indanones. Although no steroidal derivatives could be isolated from the use of acetylene or its synthons, the electron-deficient alkyne ethyl 4,4-dimethylpentyn-2-oate gave a good yield, as did ethynylferrocene. Novel diterpenoid ferrocenyl quinones were synthesised by reacting diterpenoid chromium alkoxycarbenes with ethynylferrocene.

Insertion reactions of alkenes with diterpenoid chromium aminocarbenes

Insertion reactions of electron-deficient alkenes with chromium aminocarbenes derived from podocarpic acid generally give aryl ketone products derived from ring opening of an aminocyclopropane and subsequent enamine hydrolysis, the exception being alkenyl sulphones which give products derived from insertion of the carbene into the β-CH bond of the alkene. Increasing steric hindrance due to the substituents on the aminocarbene nitrogen appears to result in higher yields of the insertion products. However, other factors such as stabilisation of the intermediate tetracarbonylaminocarbene may explain why morpholinocarbenes give superior yields of the insertion products. Propenoic acid, propenal or nitropropene give a 13-formyl-substituted diterpenoid. Electron-rich alkenes do not undergo insertion reactions with these aminocarbenes at 110°C.

Synthesis of chromium aminocarbene complexes of diterpenoids

Chromium aminocarbene complexes of podocarpane diterpenoids have been synthesised in good to excellent yields, either by aminolysis of the corresponding alkoxycarbene, or by treatment of the morpholino amide with disodium pentacarbonylchromium and subsequent elimination of silyloxide.

A matrix isolation and flash photolysis study of the cyclization reactions of chromium amino carbenes

The photochemistry of chromium amino carbene complexes have been studied using IR and UV–vis spectroscopy in matrices at 12 K and using UV–vis spectroscopy following laser flash photolysis in solution at room temperature (r.t.). The complexes studied can be divided into two classes. The Class 1 complexes are (CO) 5 Cr[C(NMe 2 )Me], (CO) 5 Cr[C(NBz 2 )Me], (CO) 5 Cr[C(NMe 2 )Ph] which are known to have a low or no photo-reactivity with imines to form β-lactams. Class 2 complexes are (CO) 5 Cr[C(NMe 2 )H], (CO) 5 Cr[C(NBz 2 )H], (CO) 5 Cr[C(NH 2 )Me] and these complexes are known to undergo efficient photochemical reactions with imines to form β-lactams. The proposed active intermediate in the reaction to form β-lactams is a metal ketene complex. For both classes of complexes no ketene complex was observed upon irradiation in either the cryogenic matrix or the solution studies. The only photochemical process which was observed in all experiments was CO loss and the major product for all complexes was assigned to be cis -(CO) 4 Cr[(N(R 2 )R′]. This species behaved differently for the two classes of complexes. In the matrix isolation experiments on Class 2 complexes this species reacted with N 2 , ethene or CO present in the matrix cage, whereas for Class 1 complexes it did not. In the flash photolysis studies on Class 1 complexes the rate of reaction of this transient species with CO showed only a moderate dependence on the nature of the solvent. The rate of reaction for the same process measured for the transient species formed from Class 2 complexes showed a significantly greater dependence on the solvent. We propose that for Class 1 complexes the vacant site formed by photo-dissociation of a cis -CO ligand is blocked, whereas for the Class 2 complexes it is not. Therefore, in the latter case the site is open to reaction with active species in the cryogenic matrices and the solvent acts as a ‘token’ ligand in the solution studies. The difference in the reactivity of cis -(CO) 4 Cr[C(NR 2 )R′] for the two classes of complex may aid the explanation of the differences in their synthetic photochemistry.

Asymmetric Synthesis of Activated α-Amino Esters via Chromium Carbene Complex Photochemistry

Optically active chromium aminocarbene complexes were α-alkylated and then photolyzed in the presence of 2,4,5-trichlorophenol to produce a number of optically active activated α-amino esters. These underwent coupling with amino acid esters or a soluble PEG-supported dipeptide to introduce the chromium−carbene-complex-derived amino acid residue into the peptide.

Use of Particle Beam Liquid Chromatography/Mass Spectrometry for the Characterization of Chromium Aminocarbene Complexes

Chromium carbenes are used in organic synthesis as reagents to obtain molecules of biological interest. In this work a particle beam (PB) coupled liquid chromatography/mass spectrometry (LC/MS) system was used for the characterization of a mixture of chromium aminocarbene complexes. Three of them are new compounds. The separation of the examined complexes was performed using normal-phase chromatography. The acquisition of electron ionization, positive-ion and negative-ion chemical ionization mass spectra of the eluates was accomplished. The capability of PB LC/MS technique in the on-line characterization of organometallic compounds is highlighted. In addition, the fragmentation patterns of all the compounds considered are discussed. © 1997 John Wiley & Sons, Ltd.

Use of Particle Beam Liquid Chromatography/Mass Spectrometry for the Characterization of Chromium Aminocarbene Complexes

Use of Particle Beam Liquid Chromatography/Mass Spectrometry for the Characterization of Chromium Aminocarbene Complexes

ChemInform Abstract: Reactions of Complex Ligands. Part 70. An Intramolecular Alkyne Insertion/Carbonylation/Cyclization Sequence of Chromium Aminocarbene Complexes: A Novel Access to Indole and Indenoindole Skeletons.

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Incorporation of Chromium Aminocarbene Complex-Derived Amino Acids into Soluble Poly(ethylene glycol) (PEG)-Supported Peptides

Incorporation of Chromium Aminocarbene Complex-Derived Amino Acids into Soluble Poly(ethylene glycol) (PEG)-Supported Peptides