Arene Tricarbonylchromium Research Articles

Solvent and Structural Effects on Ultrafast Chelation Dynamics of Arene Chromium Tricarbonyl Sulfide Derivatives

The chelation dynamics of three new [Cr{eta6-C6H5C(O)R}(CO)3] complexes, 1 [R = CH2(SCH3)], 2 [R = CH(SCH3)2], and 3 [R = C(SCH3)3], has been investigated on the picosecond to millisecond time scales by UV pump/IR probe transient absorption spectroscopy following photodissociation of CO in room temperature n-heptane, tetrahydrofuran (THF), and acetonitrile. In n-heptane, UV irradiation of 1, 2, or 3 dissociates CO to initially yield a Cr-S chelate (in which the pendant sulfide moiety is coordinated to the metal center) and a transient Cr-heptane solvate in approximately 1:2, 1:2, and 2:1 ratios, respectively. The Cr-heptane solvate is unstable and converts to the Cr-S chelate within 30 ns in each case. Irradiation of 2 or 3 in THF yields both the Cr-S chelate and Cr-THF solvate in approximately 1:3 and 1:1 ratios, respectively. The Cr-THF solvate converts to the Cr-S chelate on the second or longer time scale. All three complexes appear to yield the Cr-NCCH3 solvate exclusively within 50 ps following irradiation in acetonitrile. The solvent effect on chelation is in striking contrast to that previously reported for the analogous RCpMn(CO)3 derivatives, 4-6. In acetonitrile, only chelation is observed for the Mn series and only solvent coordination is observed for the Cr series, but in heptane both chelation and solvent coordination are observed in both series.

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.

A Versatile Approach to Planar Chiral Monophosphane Ligands

The enantioselective reaction of a chiral bisamide and an electrophile with benzyl ether complexes of arene tricarbonyl chromium(0) was used to introduce the asymmetry into a synthesis of novel enantiopure planar chiral ligands. Elaboration of the resulting ether complexes [η 6 -4-t-BuC 6 H 4 CHE 1 (OR)]Cr(CO)3 via a diastereoselective ortholithiation/chlorophosphane quench protocol led to the synthesis of planar chiral monophosphane complexes with high enantiomeric purity (typically 96-98% ee) for assessment in the palladium-catalyzed asymmetric hydrosilylation of styrene. The identity of benzylic substituent E 1 (installed using the chiral base reaction) was found to have a significant effect on the enantioselectivity of resulting catalysts. Removal of the tricarbonylchromium(O) unit from the structurally optimized phosphane ligand was accomplished using a high-yielding borane protection-aerial oxidation-deprotection strategy.

Radical Additions to (η6-Styrene) Chromium Tricarbonyl

[reaction: see text] Alkyl radical addition reactions to styrene chromium tricarbonyl can be accomplished using alkyl halides and tris(trimethylsilyl)silane in the presence of AIBN in refluxing benzene. The ketyl generated from acetone with SmI2 in THF/HMPA also underwent successful addition. These addition reactions are believed to proceed through intermediates in which the radical is interacting with an adjacent arene chromium tricarbonyl functionality and do not appear to be complicated by polymerization.

Diastereoselective hydrogenation of folic acid esters with the Daniphos ligand

Folic acid dimethylester benzenesulfonate was hydrogenated homogeneously in a rhodium-catalyzed diastereoselective reaction employing a set of the previously published planar-chiral “Daniphos” ligands, which are based on an arene chromium tricarbonyl scaffold. Diastereoselectivities of up to 42% de were achieved, almost matching the benchmark ligand BINAP. An X-ray structure of the most successful ligand P( i-Pr 2)/PPh 2 is presented and discussed.

The Metal‐Conjugated Peptide Nucleic Acid Challenge

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis and Characterization of Organometallic Ionic Liquids and a Heterometallic Carbene Complex Containing the Chromium Tricarbonyl Fragment

Direct reaction between [Cr(CO)6] and arenes with ionic substituents affords the corresponding arene-chromium tricarbonyl complexes, [Cr(CO)3(arene)], in only modest yield. In contrast, these complexes can be prepared in pure form in excellent yield from the reaction of [Cr(CO)3(η6-C6H5CH2Br)] with, for example, N-methylimidazole. The structures of [Cr(CO)3(η6-C6H5CH2MIM)]Br (MIM = 3-methylimidazolium), [Cr(CO)3(η6-C6H5CH2MMIM)]Br (MMIM = 2,3-dimethylimidazolium), and [Cr(CO)3(η6-C6H5CH2NMe2(CH3)2OH)]Br have been established by X-ray diffraction analysis. Subsequent exchange of the bromide anion for Tf2N- affords new organometallic salts with melting points below 70 degrees C. Reaction of the bromide salts include tosylation of [Cr(CO)3(η6-C6H5CH2NMe2(CH3)2OH)]Br to afford [Cr(CO)3(η6-C6H5CH2NMe2(CH2)2OTs)]Br and the formation of the heterometallic carbene complex [Ru(η6-p-cymene)Cl2{C4H5N2CH2Ph-η6-Cr(CO)3}]. Both compounds were characterized in the solid state by X-ray diffraction.

The Metal-Conjugated Peptide Nucleic Acid Challenge

This Account describes the advances in the chemistry of metal-conjugated peptide nucleic acids (PNAs) obtained in our laboratories over the last three years. Our interest in bioorganometallic chemistry began several years ago and led to new PNA research, including the design, synthesis and study of the properties of metal-conjugated PNAs with the aim of providing new molecules with suitable spectroscopic or electrochemical markers that may be ­useful in biomedical diagnostics. We describe three classes of ­metal-conjugated PNA monomers: Fischer-type carbene complexes (FTC), arene chromium tricarbonyl complexes (ACT) and ferro­cene (Fc), all of which are inserted on the α-carbon of the glycine unit in order to provide chiral substrates as well as the chemical and spectroscopic properties of the adducts. In addition, we have obtained a lysine-containing PNA 10-mer conjugated to a FTCC, in which the presence of the organometallic unit seems to stabilise the duplex with a complementary DNA strand. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) studies of mono- and tris-Fc labelled tyrosine PNA monomers have shown constantly reversible signals with high currents; the almost tripled current intensity of the tris-Fc monomer allows a detection limit of 10-8.

Highly enantioselective catalytic asymmetric ring opening reaction employing the Daniphos ligand

A set of the recently published planar-chiral Daniphos diphosphine ligands, based on an arene chromium tricarbonyl scaffold, has been applied to the rhodium-catalyzed asymmetric ring opening (ARO) reaction of 1,4-dihydro-1,4-epoxynaphthalene with methanol as the nucleophile. Enantioselectivities of up to 97.5% ee at satisfactory conversions have been obtained. The most successful ligand showed to be a PPh 2/P( t-Bu) 2-substituted derivative. An X-ray structure of this ligand is presented and discussed.

Selective functionalization of crown ethers via arene chromium tricarbonyl complexes

The crown ethers dibenzo-16-crown-4 and dibenzo-18-crown-5 and a diaryl polyether were complexed by the chromium tricarbonyl group for the purpose of selective functionalization. This complexation did indeed permit exclusive functionalization of the complexed ring. CHO and CH 2OH functionalities were introduced ortho to the ether group. It was noted that the nature of the two ether chains had a strong influence on the regioselectivity of the functionalization, which occurred preferentially on the side with the polyether chain. Photochemical decomplexation produced functionalized organic crown ethers.

Vicinal Stereocontrol During Nucleophilic Addition to Arene Chromium Tricarbonyl Complexes: Formal Synthesis of (.+‐.)‐erythro Juvabione.

[reaction: see text] Vicinal stereocontrol during nucleophilic addition of tert-butyl lithiopropionate to eta(6)-anisole chromium tricarbonyl complexes with differing para substituents has been studied. Excellent vicinal double stereoinduction (>99:1) was observed when the para substituent was Si(CH(3))(3), and this has been applied to a stereoselective formal synthesis of (+/-)-erythro Juvabione. Asymmetric synthesis by chiral auxiliary directed nucleophilic addition is also discussed.

Chiral diphosphine ligands based on an arene chromium tricarbonyl scaffold: a modular approach to asymmetric hydrogenation

A planar-chiral diphosphine ligand-class based on an arene chromium tricarbonyl backbone has been devised featuring a modular architecture that allows for the synthesis of a broad ligand library. The substituents on the donor phosphorus atoms contain aliphatic, alicyclic and aromatic groups of different kinds. A ( R, R)-2,5-dimethylphospholane moiety is also included. That library has been subdued to a ligand profiling by employment in a number of homogeneous enantioselective hydrogenations including substrates with C C, C N and C O double bonds. These reactions are described and the results are evaluated and discussed.

Enantioselective Synthesis by Lithiation to Generate Planar or Axial Chirality

AbstractFor Abstract see ChemInform Abstract in Full Text.

Investigations into the hydrogenation of diolefins and prochiral olefins employing the “Daniphos”-type ligandsDedicated to the memory of Professor Noel McAuliffe.

A number of enantiopure, planar chiral diphosphines of the Daniphos-type have been synthesized in good chemical yields and excellent stereochemical outcome according to a modular, straightforward synthetic method based on an arene–chromium–tricarbonyl scaffold. They were readily converted to their rhodium complexes, two of which have been characterized by X-ray crystallography. Their catalytic performance has been assessed by carrying out hydrogenations of dienes and prochiral olefins, taking the Josiphos ligand as a reference system for comparison. Interesting influences of the substitution patterns and solvents were found. Reaction rates of the hydrogenations of diolefins were also found to be strongly dependent on the respective substituents.

Electron Transfer Driven Transformations of Transition Metal π‐Complexes: Samarium(II) Iodide Mediated Coupling of Fluoroarene‐Cr(CO)3 Complexes with Ketones.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stereocontrolled [3 + 2] Annulations with Arene Chromium Tricarbonyl Complexes: Construction of Spirocyclic Compounds Related to Fredericamycin A.

AbstractFor Abstract see ChemInform Abstract in Full Text.

An approach to serrulatane diterpenes via endo-selective conjugate nucleophilic addition to arene-Cr(CO)3 complexes.

[formula: see text] Starting from a nonracemic planar-chiral arene tricarbonyl chromium complex, the serrulatane diterpenoid (+)-20-methoxy-serrulat-14-en-7,8-diol was synthesized in a highly stereoselective fashion. The key step of the synthesis is an endo-selective conjugate nucleophilic addition of lithio-methylphenyl sulfone to a 1-ethylidene-tetralin-Cr(CO)3 derivative. By employing different substrates and nucleophiles it was shown that the surprising and rather general endo selectivity must result from a unique complex induced proximity effect under participation of the Cr(CO)3 moiety.

Stereocontrolled [3 + 2] Annulations with Arene Chromium Tricarbonyl Complexes: Construction of Spirocyclic Compounds Related to Fredericamycin A

[reaction: see text] An efficient method has been developed for the stereocontrolled construction of polycyclic and spirocyclic compounds, including the spirocyclic core of the antitumor agent fredericamycin A. The strategy involves a one-pot aldol addition/Brook rearrangement/cyclization sequence beginning from arene chromium tricarbonyl complexes and can formally be described as a [3 + 2] annulation.

Application of chiral lithium amide base chemistry to the synthesis of planar chiral (arene)tricarbonylchromium(0) complexes

Asymmetric functionalisation of tricarbonylchromium(0) complexes of benzyl alkyl ethers using a dilithiated chiral amide base is the key step in a short and versatile synthesis of planar chiral (arene)tricarbonylchromium(0) complexes. The synthesis of four complexes of potential use in asymmetric catalysis is described herein together with the first demonstration that the asymmetric functionalisation mediated by this chiral base may be used to introduce phosphine substituents.

Synthesis and biological evaluation of new antimalarial isonitriles related to marine diterpenoids

By exploiting the chemistry of arene tricarbonylchromium(0) complexes, some analogues of antimalarial marine isonitrile diterpenoids were synthesized in a completely diastereoselective fashion and their activity against the malaria parasite Plasmodium falciparum was assessed in an in vitro assay.