Direct synthesis of (η-arene)-tricarbonylchromium complexes by arene displacement from tricarbonyl(η-1-methylpyrrole)chromium(0)

Arene displacement from the title complex occurs under unprecedented mild conditions in neutral media to afford (η⁶-arene)tricarbonylchromium complexes. The process is most effective with heteroaromatic compounds: the first synthesis of a tricarbonyl(η⁶-quinoline)chromium complex and the diastereoselective preparation of a (η⁶-indole) complex derived from L-tryptophan are reported as representative examples.     

Microwave-assisted synthesis of (η-arene)tricarbonylchromium complexes

We have undertaken a study of the microwave-assisted synthesis of (η⁶-arene)tricarbonylchromium complexes. Under microwave irradiation, the reactions of hexacarbonylchromium with arenes gave high yields of (η⁶-arene)chromium tricarbonyl complexes.     

Stereoselective synthesis of 3-amino-4-substituted-2-azetidinones via [2+2] cycloadditions of tricarbonyl(ηarene)chromium(0)complexed imines

The stereoselective [2+2] cycloaddition reaction between the chiral tricarbonyl(η⁶arene) chromium(0) complexed imines 1 and 6 and phthalimidoketene affords tricarbonyl (η⁶arene)chromium(0) complexed 3-phthalimido-2-azetidinones 3, 7 and 8, both in racemic and enantiopure form. Decomplexation and the cleavage of the phthalimido group give 3-amino-4-substituted-2-azetidinones 5 and 10. Some insights into the stereochemical outcome of the [2+2] cycloaddition process are discussed.     

Molecular polarizability of the organic substances and their complexes: LX. Molecular volume and spatial structure of the series of heterocycles, their structurally nonrigid derivatives, and chromium tricarbonyl π-complexes in solutions

We have analyzed the molar volumes and structures in solutions of a series of azines, azoles, conformationally nonrigid bis(2-substituted benzimidazol-1-yl)methanes, and chromium tricarbonyl complexes of arenes. For most of the compounds, the rules of molar volume additivity with respect to the bonds and groups increments hold. Molecules of bis(2-substituted benzimidazol-1-yl)methanes exist in the form of conrotatory comformers with aryl fragments being out of the plane of bridging NCN bond angle. Strong linear correlations between the molar volume and molar refraction have been revealed within the studied compounds classes. In the case of chromium tricarbonyl complexes of arenes, the coordination bond polarity has increased with growing π-donor ability of the ligand, the molar volume increment of Cr(CO)₃ has increased as well, due to transfer of π-electron density from the ligand. The simplification of dipole moment and Kerr constant determination has been demonstrated.     

Asymmetric synthesis of axially chiral benzamides and anilides utilizing planar chiral arene chromium complexes

Optically active axially chiral 2,6-disubstituted benzamides and anilides were stereoselectively prepared by utilizing planar chiral (arene)chromium complexes. Nucleophilic addition to enantiomerically pure planar chiral tricarbonyl(N,N-diethyl-2-methyl-6-formyl- (or 6-acyl)benzamide)chromium complex gave axially chiral 2-methyl-6-substituted N,N-diethyl benzamide chromium complexes with high selectivity. An alternative method for the preparation of axial chiral benzamides or anilides is an enantiotopic lithiation at the benzylic methyl of prochiral tricarbonylchromium complexes of N,N-diethyl-2,6-dimethylbenzamide and N-methyl-N-acyl-2,6-dimethylaniline with a chiral lithium amide followed by electrophilic substitution. The resulting axially chiral chromium-complexed benzamides and anilides were oxidized in air to give chromium-free axially chiral benzamides and anilides in enantiomerically enriched form without axial bond rotation at room temperature.     

The tricarbonylchromium template for stereocontrol in radical reactions of arenes

Chromium tricarbonyl complexed aryl aldeyhydes and ketones underwent Sm(II)-promoted radical lactone formation in the presence of alpha,beta-unsaturated esters to produce diastereomerically pure lactones in good yields. The completely diastereoselective lactone formation involves capture of the benzylic ketyl radical by the ester anti to the chromium tricarbonyl moiety. The relative stereochemistry of the lactone and chromium tricarbonyl moieties was proven by X-ray crystallography and supports the proposed mechanism. Enantiopure chromium tricarbonyl complexed arenes afforded single enantiomers when subjected to Sm(II)-promoted radical lactone formation condiditions. The enantio- and diastereomerically pure chromium tricarbonyl complexed lactones were subsequently treated with BF3.OEt2 to generate a mixture of diastereomers via Lewis acid promoted chromium tricabonyl directed cationic rearrangement. The diastereomers were separated and individually decomplexed with I2 to afford both of the corresponding chromium-free enantiomerically pure lactones starting from a single enantiomerically pure chromium tricarbonyl complex.     

Heterogeneous and homogeneous hydrogenation of styrene and stilbene chromium tricarbonyl complexes

Heterogeneous hydrogenation of the styrene and stilbene chromium tricarbonyl complexes by molecular hydrogen on skeletal nickel and palladium on carbon as catalysts was studied. As compared to styrene and stilbene, their arene chromium tricarbonyl analogs are hydrogenated considerably more slowly, which is related, most likely, to strong adsorption of the π-complexes on the catalyst surface. For the homogeneous hydrogenation of these complexes using a H₂PtCl₆-SnCl₂-LiBr system, styrene and η⁶-styrene chromium tricarbonyl are reduced with a high rate, whereas stilbene and its chromium tricarbonyl complex are hydrogenated very slowly. A possibility of reduction of the unsaturated arene chromium tricarbonyl complexes by sodium borohydride in the presence of cobalt(II) chloride as a catalyst was shown. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 44–47, January, 2007.     

Organometallic radical-initiated cleavage of the metal chelate and thiazole rings in a cyclopentadienylchromium complex

Extensive ring cleavage in the mercaptobenzothiazole chromium complex CpCr(CO)2(SCSN(C6H4)) (2), initiated by cyclopentadienyl chromium tricarbonyl, produced novel polynuclear complexes 3-5.     

Quantum chemical study of the structures and dynamic behavior of tricarbonyl complexes of Group 6 metals (Cr,Mo, W) with polyaromatic hydrocarbons using the density functional theory

The quantum chemical study of the mechanism was performed for tricarbonyl η⁶-complexes of coronene I-M and kekulene II-M (M = Cr, Mo, W) by the density functional method. The activation barriers of η⁶,η⁶-interring haptotropic rearrangements (IHR), being the migration of the metaltricarbonyl group M(CO)₃ from one six-membered aromatic ring to another, were determined. The processes of η⁶,η⁶-IHR in the metal tricarbonyl complexes with relatively high polycyclic aromatic hydrocarbons (PAH) I and II occur with close energy barriers (ΔG^≠ ≈ 20—25 kcal mol^–1), which are lower than the barriers (ΔG^≠ ~ 30 kcal mol^–1) of similar transformations measured or calculated earlier for the chromium tricarbonyl complexes of naphthalene and its derivatives and other PAH. For the molybdenum tricarbonyl complexes the activation barriers of η⁶,η⁶-IHR decrease additionally by ~ 5 kcal mol^–1 compared to those for the chromium tricarbonyl complexes, whereas for the tungsten tricarbonyl complexes they increase again and become approximately equal to the activation barriers of similar chromium tricarbonyl complexes. All stationary states on the potential energy surface determining the mechanism of η⁶,η⁶-IHR are characterized by a decrease in hapticity compared to the initial and final complexes.     

Addition of carbon nucleophiles to arene-chromium complexes

The electrophilic reactivity of arenes coordinated to the chromium tricarbonyl unit has been developed into several distinct methods for coupling carbon nucleophiles with aromatic rings. Addition of the nucleophile produces stable η⁵-cyclohexadienyl chromium complexes which can be oxidized to induce loss of the endo hydrogen and the metal, overall nucleophilic substitution for hydrogen. Alternatively, the intermediate can be protonated and the resulting cyclohexa-1,3-diene can be detached from the chromium, effecting nucleophilic addition with reduction of one double bond. If a halogen (F, Cl) is present as a ring substituent, and if the nucleophile can migrate about the arene ligand, then loss of halide can occur parallel with classical nucleophilic aromatic substitution for halogen in electron-deficient haloarenes.With substituted arenes, the regioselectivity of addition becomes important and is often very high. Particularly useful are strong resonance donor substituents (RO-, R₂N-, F-) where selectivity for meta attack is high. Indole provides an excellent example of selective activation, as the six-membered ring complexes selectively and is then susceptible to nucleophilic substitution, predominantly at the 4 and 7 positions.Substitution for halogen is a somewhat limited process and depends upon the nature of the nucleophile. Very reactive nucleophiles add to unsubstituted positions and are often slow to isomerize to the ipso position from which loss of halide can occur.     

Complexation diastereoselective d''alcools benzyliques par le chrome hexacarbonyle. Difference de reactivite des diastereoisomeres en milieu acide.

Reaction of ortho alkyl or alkoxy phenyl carbinol with Cr(CO)₆ affords arene chromium tricarbonyl complexes with 38 to 86% of diastereoisomeric excess. The behavior of the two diastereoisomers shows a striking difference. Equimolar mixture of (RR,SS) and (RS,SR) diastereoisomers treated with concentrated sulfuric acid and methanol gives (RS,SR) ether while (RR,SS) alcool remains unchanged.     

Use of a bis-lithium amide base in asymmetric metallation of tricarbonyl(η6-1,3-dihydroisobenzothiophene)chromium(0)

Enantiomerically enriched tricarbonyl(η⁶-arene)chromium complexes derived from tricarbonyl(η⁶-1,3-dihydroisobenzothiophene)chromium(0) can be obtained in up to 95% ee by means of an enantioselective metallation reaction using the bis-lithium amide base 8.     

Theoretical DFT studies of chromium tricarbonyl complexes with polycyclic aromatic ligands

This paper presents a theoretical analysis of the structures of tricarbonyl chromium complexes of carbo- and heterocyclic polyaromatic ligands (PAL) and the mechanisms of interring haptotropic rearrangement in such complexes performed by using density functional theory (DFT) with the nonempirically constructed PBE functional and extended split basis sets. The reaction paths were calculated for interring haptotropic rearrangements and rotations of the metalcarbonyl fragment in the regioisomeric complexes. The structures and energy characteristics of stationary points of the systems were determined. The migration of the Cr(CO)₃ group was shown to occur at the periphery of the ligand via transition states with the structure of η³-allylic or η⁴-trimethylmethane complexes. Calculated geometries of the complexes and the activation barriers were in a close agreement with the experimental data.The reaction of η⁶-tricarbonylchromium complexes of PAL with n-BuLi (lithiation) was also studied by the DFT. The kinetic and thermodynamic factors that control the direction and selectivity of metallation were calculated for the model η⁶-biphenylenetricarbonylchromium complex. Both approaches indicate that lithiation occurs exclusively at the aromatic ring bonded to the transition metal, which agrees with the experimental data. The selectivity inside this ring is governed by a thermodynamic factor. The solvation effects were simulated for the lithium salt of the model η⁶-naphthalenechromium tricarbonyl complex in which lithium is localized at the α(1)-position of coordinated ring. The simulation showed the most stable coordination of the lithium atom with two THF molecules. Addition of extra THF molecules is thermodynamically unfavorable. The tricarbonylchromium complexes of naphthalene, biphenyl, biphenylene and dibenzothiophene calculated relative energies for all solvated by two THF molecules lithium salts indicate that the difference in energies ΔE ? 1 kcal mol^?1 corresponds to the experimentally observed absence of selectivity, while the difference more than 2.5  kcal mol^?1 corresponds to the selectivity of the reaction. No additional coordination of lithium to heteroatom was observed for the sulfur-containing dibenzothiophene complex. Similar calculation shows that double metallation in the dibenzothiophene complex occurs at positions 1 and 4. The developed approach enables one to predict the direction and selectivity of metallation reactions of transition metal complexes with different arenes and thus to synthesize labeled complexes for the investigation of degenerate IRHR.     

Characterization of electronic properties of the Cr(CO)3 group in chromium, tricarbonyl [3-[(η-aryl)methylene]-Z-1(3H)-isobenzofuranones] using spectral and theoretical methods

The IR, ¹³C- and ¹⁷O-NMR spectral characteristics of the Cr(CO)₃ group in a series of eight chromium, tricarbonyl[3-[(η⁶-aryl)methylene]-Z-1(3H)-isobenzofuranones] were correlated mutually as well as with theoretical data obtained by optimized MMX force-field and EHT calculations. The net charges on the carbon and oxygen atoms of the CO group and their differences were found as the most appropriate quantitative characteristics for the electronic properties of the Cr(CO)₃ group. Using the results of the previously reported linear correlations, the electron-withdrawing effect of the PhCr(CO)₃ moiety, weakened by the back-donation effect, was estimated as σ0.45 on the scale of Hammett substituent constants.     

Quantum chemical simulation of 1,3-dipolar cycloaddition of nitrones to alkenes and their (η6-arene)(tricarbonyl)-chromium complexes

Charge distribution and frontier orbital energies of styrene, C,N-diphenylnitrone, and their (arene)-(tricarbonyl)chromium complexes were calculated by quantum chemical methods. The difference in the HOMO and LUMO energies of the chromium complexes was found to be smaller than in the free ligands, and the reactions with (arene)(tricarbonyl)chromium complexes were characterized by higher rate and selectivity.     

1H NMR conformational analysis of disubstituted arene tricarbonyl chromium complexes

The positions of conformational equilibrium in disubstituted arene chromium tricarbonyl complexes, i.e. alkylanisole chromium tricarbonyl complexes and alkylbenzoate chromium tricarbonyl complexes, have been determined by ¹H NMR spectroscopy. It has been found that the donor effect is the dominant factor in determining the preferred conformation.     

Bicyclo[4.2.0]octa-1,3,5-triene: 2-Mono- and 2,5-Disubstituted Derivatives via Highly Regioselective Lithiation of Its Cr(CO)3 Complex and via Reductive Silylation/Oxidation

Treatment of [η⁶-(bicyclo[4.2.0]octa-1,3–5 triene)]tricarbonylchromium(0) ( 2 ) with BuLi or lithium 2,2,6,6-tetramethylpiperidinide (TMPLi) gives rise to a highly regioselective deprotonation at C(2). Subsequent reaction with electrophiles (6 examples) gives [η⁶-(2-R-bicyclo[4.2.0]octa-1,3,5-triene)]tricarbonylchromiurn complexes 3 and 5 – 9 in moderate (R?I, 50%; R?CHO, 67%) to good (R?Me, D, SiMe₃, CO₂Me, > 80%) yield (Scheme 1). Analogous reactions with tricarbonyl (η⁶-indane)chromium ( 10 ) give mixtures of complexes substituted at C(4) and C(5) (Scheme 2). In 10 , deprotonation β to the ring junction is strongly favoured with the bulky base TMPLi. Double lithiation/electrophile additions to 2 give access to [η⁶-(2-R′-5-R″-bicyclo[4.2.0]octa-1,3,5-triene)]tri-carbonylchromium complexes (e.g. 13 (R′?R″?Me₃Si) and 14 (R′?Me₃Si, R″?CHO)) as single products. The Cr(CO)³ group can be easily removed by oxidation (I₂, Ce(IV), O₂/light; 2 examples each) to give the free arenes. Base-catalyzed (C_sF, DMF/D₂O) deuterodesilylation of 13 yields the [(2,5-²H₂)bicyclo[4.2.0]octa-1,3,5-triene]chromium complex 15 , and treatment of 2,5-bis(trimethylsilyl) compound 16 with CF₃COOD gives the 2,4-dideuterated 17 . Compound 16 is also accessible more directly via reductive silylation/oxidation of bicyclo[4.2.0]octa-1,3,5-triene ( 1 ). Stereoselective base-catalyzed (t-BuOK.) H/D exchange of the benzylic H-atoms. opposite to the Cr(CO)₃ moiety in 2 takes place rapidly in (D₆)DMSO, but benzylic functionalization via this route remains elusive.     

Asymmetric Synthesis of Axially Chiral Anilides by Enantiotopic Lithiation of Tricarbonyl(N-methyl-N-acyl-2,6-dimethylanilide)chromium Complex

Axially chiral N-methylanilides were synthesized by enantioselective lithiation of prochiral tricarbonyl(N-methyl-N-pivaloyl-2,6-dimethylaniline)chromium (1) with the lithium amide of the 4-methylpiperazinylethylamine derivative 13 followed by electrophilic quenching up to 97% ee in good yields. The resulting axially chiral chromium-complexd anilides 2 were oxidized under air to give the axially chiral anilides 14 in enantiomerically active form without axial bond rotation at room temperature.     

Kinetische und mechanistische untersuchungen von übergangsmetallkomplex-reaktionen : IX. Substitution von CO durch PPh3 in aminocarbinkomplexen: Präparative und kinetische untersuchungen

Pentacarbonyl(diethylaminocarbyne)chromium tetrafluoroborate, [(CO)₅₋ CrCNEt₂]BF₄ (I), reacts with PPh₃ with substitution of CO and formation of trans-tetracarbonyl(diethylaminocarbyne)triphenylphosphanechromium tetra-fluoroborate, trans-[PPh₃(CO)₄CrCNEt₂]BF₄ (III). Substitution of CO by PPh₃ in neutral trans-tetracarbonyl(halo)(diethylaminocarbyne)chromium complexes, trans-X(CO)₄CrCNEt₂ (IVa: X = Br, IVb: X = I), leads in a reversible reaction to the corresponding tricarbonyl complexes, mer-X(PPh₃)(CO)₃₋ CrNEt₂ (V), PPh₃ occupying the cis-position to the carbyne ligand. With PPh₃ in large excess both reactions follow a first-order rate law. This as well as the activation parameters (ΔH≠ = 104–113 kJ mol⁻¹, ΔS≠ = 64–71 J mol⁻¹ K⁻¹) indicate a dissociative mechanism.     

Synthesis and characterization of chiral iron carbonyl complexes from imine ligands with carbohydrates and amino acids as substituents

Imine ligands derived from 6-amino-6-desoxy-1,2,3-O-trimethyl--d-glucopyranose or from various amino acid esters react with Fe₂(CO)₉ to give chiral iron carbonyl complexes. Derivatives produced from benzaldehyde react via a C–H activation reaction in ortho-position with respect to the exocyclic imine substituent followed by an intramolecular hydrogen transfer reaction of the activated hydrogen towards the former imine carbon atom. The molecular structure of the diiron hexacarbonyl complexes of benzylideneamino-l-phenylalanine ethyl ester and benzylideneamino-l-methionine methyl ester were characterized by means of X-ray structure determinations. Imines produced from cinnamaldehyde upon reaction with Fe₂(CO)₉ produce mononuclear iron tricarbonyl complexes with the imine ligand being coordinated in a η⁴-fashion.