Chromium Tricarbonyl Research Articles

Planar chiral arene chromium(0) complexes: potential ligands for asymmetric catalysis

Planar chiral (η6-arene) tricarbonyl chromium(0) complexes are well-studied compounds in synthetic organic chemistry. However, the number of investigations concerned with their application in asymmetric catalysis is rather low. This review aims to summarise the main approaches toward the stereoselective synthesis of planar chiral derivatives of (η6-benzene) tricarbonyl chromium(0) and to present recent applications of such complexes as ligands in enantioselective catalytic processes.

Tricarbonyl (η6 arene) chromium(0) complex derived from 8-phenylmenthol chiral auxiliary

(—)-8-Phenylmenthol was prepared and converted to the corresponding tricarbonyl (η6-arene) chromium(0) complex. Derivitization as the acrylate ester (1) and subsequent X-ray analysis revealed a 1:1 mix of s-cis and s-trans acrylate in the unit cell, and geometric proximity to support a through-space π stacking interaction in the case of the s-trans isomer. The dihedral angle between the best planes through the chromium-bound aryl ring and the acrylate group is 19.9° for the s-trans isomer and 34.4° for the s-cis isomer. Crystal data for (1): C22H26O5Cr, monoclinic, P21 (No. 4), a = 10.269(1), b = 10.482(1), c = 19.787(2), β = 95.85(1), Z = 4, and Dcalc = 1.32 g cm−3.

The asymmetric synthesis of phosphorus- and sulfur-containing tricarbonyl(η6-arene)chromium complexes using the chiral base approach

The use of a simple chiral lithium amide base 2 enables the asymmetric transformation of tricarbonyl[η6-(diphenylphosphinoyl)benzene]chromium(0) 12 into the corresponding ortho-silylated complex in up to 86% ee. A tin derivative was prepared similarly and was then used to prepare other derivatives via reduction to the corresponding phosphine, followed by transmetallation–electrophilic quench. In the case of tricarbonyl(η6-1,3-dihydroisobenzothiophene)chromium(0) the chiral base 2 was ineffective, and it was necessary to use a bis-lithium amide base 9 to effect asymmetric substitution in high ee (up to 95%). Decomplexation gave the corresponding chiral sulfides in highly enantiomerically enriched form. In all cases the absolute stereochemistry of the products was derived by conducting X-ray structure determinations on selected examples.

New planar chiral P,N-ligands containing tricarbonyl(arene)chromium for enantioselective asymmetric hydroboration of styrenes

Optically active (1,2-disubstituted arene) chromium tricarbonyl complexes 2A– 2D with a pyridyl and a phosphorus group in the two ortho benzylic positions have been stereoselectively synthesized from a commercially available (+)-(4,6- O-benzylidene)methyl-α- d-glucopyranoside. These chromium complexes have been used as chiral ligands in the preparation of rhodium catalysts for the hydroboration of styrene derivatives. High enantioselectivities were observed in the hydroboration of vinylarenes.

Synthesis, Characterization, and Some Reactions of the Tricarbonylchromium Complexes of 1,3-Indandione and Ninhydrin

The syntheses and characterization of the tricarbonylchromium complexes of 1,3-indandione and ninhydrin are described. As the direct complexation proved unsuccessful, the acetal route was tried. Although some complexes were obtained, complex deacetalization did not work in the case of ninhydrin. Finally, the oxidation of tricarbonyl(η6-1,3-indandione)chromium(0) with dimethyldioxirane gave the ninhydrin complex in 75% yield in equilibrium with the indantrione complex. The latter reacts in hetero Diels-Alder reactions with dienes with diastereoselective formation of spiro anellated pyran derivatives.

Exo-andendo-Tricarbonyl[(4b,5,6,7,8,8a-η)-cis-N-methyl-2,3,4,4a,9,9a-hexahydro-1H-carbazole]chromium(0)

Acid-mediated hydride reduction of tricarbonyl(η 6 -N-methyl-1,2,3,4-tetrahydrocarbazole)chromium(0) affords either the cis - fused exo - hexahydrocarbazole-chromium(0) complex {[Cr(C 13 H 17 N)(CO) 3 ], (I)} exclusively, or a separable mixture of (I) and the endoisomer {[Cr(C 13 H 17 N)(CO) 3 ], (II)}, depending upon the choice of hydride donor. The conformations of the hexahydrocarbazole systems differ in the orientation of the indoline moiety with respect to the saturated hexahydrocarbazole rings. The isolation of the exo-isomer is unusual, as this complex arises via reaction at the sterically more hindered endo face of the coordinated ligand.

Addition and Ring Expansion Reactions of Tricarbonyl(1,2-dioxobenzocyclobutene)chromium(0) with Carbon Nucleophiles – Unexpected Formation of Benzocycloheptene Derivatives and the First Head-to-Head Coupling of Two Methoxyallene Molecules

Addition of carbon nucleophiles to (benzocyclobutenedione)tricarbonylchromium(0) (4) results in the formation of exo mono- and diadducts as well as 1,2-diketones as the consequence of proximal ring-opening reactions. In one case the unexpected formation of benzocycloheptenedione complexes is observed. Treatment of 4 with an excess of 1-ethoxy-1-lithioethene gives the product of a dianionic oxy-Cope rearrangement followed by an intramolecular aldol addition. This is also the case with lithiated methoxyallene, and as the result the first head-to-head coupling product 12 of two methoxyallene molecules is isolated in good yield. 12 is used as a diene in Diels-Alder cycloadditions, and its molecular structure is compared to that of the similar molecule 13, lacking the two exo methylene substitutents.

Total Synthesis of (−)-Macrocarpal C. Stereoselective Coupling Reaction with a Novel Hexasubstituted Benzene Cr(CO)3 Complex as a Biomimetic Chiral Benzyl Cation Equivalent

The first total synthesis of (−)-macrocarpal C (3) is described. The synthesis features a highly stereoselective coupling reaction of silyldienol ether 6 with biomimetic benzyl cation species (R)- and (S)-B, which were generated from novel hexasubstituted benzene chromium tricarbonyl complexes (R)- and (S)-17. At the final step of the total synthesis, we developed the tris-O-demethylation of macrocarpal C trimethyl ether 34 under basic conditions using lithium p-thiocresolate. Moreover, spectroscopic evidence shows that the synthetic (−)-3 is identical to natural macrocarpal G (4).

Influence of the Tricarbonyl Chromium Complexation of Cyclic Alcohols and of the Planar Chirality on the Kinetic Resolution by Candida Cylindracea Lipase

Tricarbonylchromium complexes of 1-indanol, 1-tetralol, 2-tetralol and the uncomplexed counterparts were kinetically resolved by asymmetric esterification with the Candida cylindracea lipase. We observed a beneficial effect of the tricarbonyl chromium group on the enantioselectivity and we have studied the implication of the relative positions of the tricarbonylchromium group and the alcohol function.

Ricochet inter-ring haptotropic rearrangement of σ-methyl-( η5-indenyl) chromium tricarbonyls. Experimental kinetic and theoretical DFT study

σ-Methyl-( η 5-indenyl) chromium tricarbonyl ( III) rearranges quantitatively into η 6-1-endo-methylindene) chromium tricarbonyl ( IV) in C 6D 6 solution at 30–60°C. Methyl group attachment to the positions 2 or 3 of indenyl ligand in ( III) has no influence on the activation parameters of this ricochet inter-ring haptotropic rearrangement ( ΔG #=23.6 kcal mol −1; ΔH #=18.9±0.2 kcal mol −1; ΔS #=−18.6±0.2 cal K −1 mol −1). ( IV) undergoes further irreversible isomerization at 60–120° into ( ν 6-3-methylindene) chromium tricarbonyl ( V) with a higher activation barrier ( ΔG #=28.5±0.1 kcal mol −1) via two consecutive [1,5]-sigmatropic hydrogen shifts. The mechanisms of both rearrangements have been studied in detail using density functional theory (DFT) calculations with extended basis sets. Calculations show that the rearrangement ( III) → ( IV) proceeds in two steps. Methyl group migration from chromium into position 1 of the indenyl ligand is the rate-determining step leading to the formation of the 16-electron intermediate ( VII). The calculated activation barrier ( E a=19.6 kcal mol −1) is in good agreement with the experimental one. Further rearrangement ( VII) → ( V) proceeds via a trimethylenemethane-type transition state ( XVIII) with an activation barrier 11.8 kcal mol −1. The coordination of the chromium tricarbonyl group at the six-membered ring has only minor influence on the kinetic parameters of the hydrogen [1,5]-sigmatropic shift in indene.

(Phenalene)chromium Tricarbonyl Complexes. Synthesis, Structure, and Thermal Rearrangements

The reaction of phenalene with Cr(CO)3Py3/BF3·OEt2 gives a mixture of two isomeric complexes: tricarbonyl(6a,7−9,9a,9b-η6-phenalene)chromium (1) and tricarbonyl (3a,4−6,6a,9b-η6-phenalene) chromium (2). Deprotonation of the mixture of 1 and 2 followed by the treatment with MeI, n-BuI, or D2O results in the formation of 1-exo-R-(1) complexes 3−5 ((3) R = Me, (4) R = n-Bu, (5) R = D). The molecular geometry of the complex 3 was characterized by a single-crystal X-ray diffraction study. Heating of 5 and 3 in toluene or C6F6 at temperatures 90−110 °C leads for 5 to deuterium label redistribution over the positions 1-exo, 1-endo, and 3 by means of sigmatropic shifts of exo- and endo-hydrogens in nonaromatic ring and inter-ring migrations of chromium tricarbonyl group, whereas for 3 the methyl label distributes over the positions 1-exo and 3 via the same processes resulting in the formation of structurally similar isomeric complexes 6−8 (6 = 1-exo-methyl-(2); 7 = 3-methyl-(2); 8 = 3-methyl-(1)), respectively, ...

Tetracyclic Arenes by Benzannulation of Tricyclic Carbene Complexes of Chromium with Alkynes: Chemo-, Regio-, and Stereoselectivity

The regio- and stereochemical selectivity of benzannulation reactions is investigated. The tricarbonyl chromium complexes 4–6 along with three tetracyclic arenes were obtained regioselectively and, in case of 5, stereoselectively from the reactions of the tricyclic carbene complexes 1–3 with 1-hexyne.

Enantioselective synthesis and use in catalytic asymmetric addition of diethylzinc to benzaldehyde of new chiral cyclic hydroxyamino ferrocene and chromium complexes: influence of the complexation on the enantioselectivity

The optically active (arene)chromium complexes (+)-( S,1 S)- 7– 13, (+)-( S,1 S)- 20 and ferrocenyl amino alcohol (+)-( S,1 S)- 14– 17 were synthesized from enantiomerically pure tricarbonyl(1-indanone)chromium (ee>98), tricarbonyl(1-tetralone)chromium (ee>99) and 1,2-(α-oxotetramethylene)ferrocene (ee>98). These compounds were used as chiral catalysts in the asymmetric addition of diethylzinc to benzaldehyde: 1-phenylpropanol has been obtained in up to 70% enantiomeric excess. The influence of the metallic group on the enantioselectivities has been studied.

Use of a bis-lithium amide base in asymmetric metallation of tricarbonyl(η 6-1,3-dihydroisobenzothiophene)chromium(0)

Enantiomerically enriched tricarbonyl(η 6-arene)chromium complexes derived from tricarbonyl(η 6-1,3-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.

Electrochemical Study of Tricarbonyl(η6-cyclooctatetraene)metal(0) Complexes of the Group 6 Elements

Abstract Tricarbonyl(η6-cyclooctatetraene)metal(0) complexes of the group 6 elements were prepared by using the procedures described in the literature with some minor modifications and identi­fied by IR and NMR spectroscopy. Their electrochemical behavior was studied by using cyclic voltammetry in dichloromethane solution containing 0.1 M tetrabutylammonium tetrafiuorob-orate as supporting electrolyte. Their oxidation and reduction potentials were measured and discussed in terms of the frontier energy levels in connection with the UV-Visible electronic ab­ sorption spectral data. In order to elucidate the mechanism of electrooxidation of the complexes, constant potential electrolysis was performed for one representative example, tricarbonyl(η6-cyclooctatetraene)chromium(O). The IR monitoring of the reaction showed that it is gradually converted to hexacarbonylchromium(O) upon electrolysis at constant potential.

Vibrational Overtone Spectroscopy of Cycloheptatriene Chromium Tricarbonyl, Benzene Chromium Tricarbonyl, and Corresponding Hydrocarbon Ligands

The vibrational overtone spectra of gaseous cycloheptatriene chromium tricarbonyl (CHTCT) and benzene chromium tricarbonyl (BCT) were recorded at the third overtone region using photoacoustic spectroscopy and compared to the spectra of the nonbonded ligands, cycloheptatriene (CHT) and benzene. Analysis of the spectra leads to the conclusion that the metal−ligand bonding lengthens the CH bond by 0.007 Å in the cycloheptatriene complex and by 0.003 Å in the benzene complex. The spectrum of BCT exhibits an additional transition which is absent in the spectrum of free benzene. The experimental peaks are assigned to the pure CH overtones and combination bands. The structural information obtained from the spectra is discussed and compared to the results of gas-phase electron diffraction and microwave spectroscopic studies. Analysis of experimental data indicates that μ6-bonding is more favorable for cycloheptatriene chromium tricarbonyl.

Synthesis of Planar Chiral (1,2-Disubstituted arene)chromium Tricarbonyl Compounds and Their Application in Asymmetric Hydroboration

Optically active (1,2-disubstituted arene)chromium tricarbonyl complexes with a diamine and a phosphorus group in the two ortho benzylic positions were stereoselectively synthesized from a commercially available (+)-(4,6-O-benzylidene)methyl-α-D-glucopyranoside. These chromium complexes have been used as chiral ligands in the preparation of rhodium catalysts for the hydroboration of styrene derivatives. Moderate enantioselectivities were observed in the hydroboration of vinylarenes.

Intramolecular Pd Catalyzed Carbocyclization Reactions with Planar Chiral Arene Tricarbonyl Chromium Complexes

ortho-Butenylchlorobenzene tricarbonylchromium complexes undergo intramolecular Pd catalyzed cyclizations (Heck-reactions) without decomplexation and/or alkene isomerization. The diastereoselectivity of the alkene carbopalladation is governed by the planar chirality of the complex.

Reduction of tricarbonyl( η6-Indole)chromium(0) complexes

Tricarbonyl( η 6-indole)chromium(0) complexes substituted at the C2 and/or C3 positions have been reduced to the corresponding indoline complexes by the action of a hydride donor in trifluoroacetic acid. NaBH 3CN and NaBH 4 were found to be superior to Et 3SiH as hydride sources. In general, reductions involving the use of NaBH 3CN were found to occur with greater stereoselectivity as compared to reductions using NaBH 4. With certain (indole)chromium complexes, this stereoselectivity was a result of preferential hydride addition from the endo face, as revealed by single crystal X-ray analysis.

Studies on enantiotopic differentiation in the horse liver alcohol dehydrogenase catalysed reduction of chiral tricarbonyl(η 6-benzaldehyde)chromium complexes

Horse liver alcohol dehydrogenase (HLADH) catalysed reduction of a series of ortho- and meta-substituted benzaldehyde chromium tricarbonyl complexes has been studied. The kinetic resolution proceeded with variable enantioselectivity and the extent of chiral discrimination strongly depended on the nature and the position of the substituent on the aromatic ring. Computer graphics were used to examine the docking of benzaldehyde complexes into the HLADH active site and to explain the different enantiopreference found along the ortho series.