1,3-Dipolar cycloaddition reaction of nitrone η6-(arene)chromium tricarbonyl complexes with styrene and η6-(styrene)chromium tricarbonyl

The products of 1,3-dipolar cycloaddition of the nitrone-type η⁶-(arene)chromium tricarbonyl complexes (CO)₃CrC₆H₅CH=N⁺(O^?)R, where R = Me, Ph, Bu^t, with styrene and η⁶-(styrene)chromium tricarbonyl were obtained and characterized by a combination of physicochemical methods. This type of reactions proceeded with very high regio- and stereoselectivity to exclusively form cis-2,3,5-tri-substituted isoxazolidines.     

Synthesis of new η6-(arene) chromium tricarbonyl complexes of isoxazolidines by 1,3-dipolar cycloaddition

The 1,3-dipolar cycloaddition products of C,N-disubstituted nitrones and different in situ prepared mono-N-substituted nitrones to η⁶-(styrene) chromium tricarbonyl and η⁶-(ethyl cynnamate) chromium tricarbonyl were synthesized and characterized by HPLC, IR, ¹H NMR spectroscopy, and mass spectrometry. The resulted isoxazolidines were produced with a full regioselectivity and high stereoselectivity reached 100% in most cases.     

The rate of substitution from η6-arene ruthenium(II) complexes

Transition Metal Chemistry - The rate and mechanism of substitution in the Ru(II) complexes (C1–C6) by thiourea nucleophiles was studied at pH 2 and rate constants measured as a function of...     

(η6-arene)tricarbonyl chromium complexes in organic synthesis 1,5-remote diastereoselection

1,5-Remote diastereoselection on the acyclic systems is achieved by diastereoselective Cr(CO)₃ complexation, chirality transfer in acyclic system and following stereoselective alkylation at the benzylic position.     

Opportunities offered by chiral η⁶-arene/N-arylsulfonyl-diamine-RuII catalysts in the asymmetric transfer hydrogenation of ketones and imines

Methods for the asymmetric transfer hydrogenation (ATH) of ketones and imines are still being intensively studied and developed. Of foremost interest is the use of Noyori's [RuCl(η?-arene)(N-TsDPEN)] complexes in the presence of a hydrogen donor (i-PrOH, formic acid). These complexes have found numerous practical applications and have been extensively modified. The resulting derivatives have been heterogenized, used in ATH in water or ionic liquids and even some attempts have been made to approach the properties of biocatalysts. Therefore, an appropriate modification of the catalyst that suits the specific requirements for the reaction conditions is very often readily available. The mechanism of the reaction has also been explored to a great extent. Model substrates, acetophenone (a ketone) and 6,7-dimethoxy-1-methyl-3,4-dihydroisoquinoline (an imine), are both reduced by this Ru catalytic system with almost perfect selectivity. However, in each case the major product is a different enantiomer (S- for an alcohol, R- for an amine when the S,S-catalyst is used), which demanded an in-depth mechanistic investigation. Full-scale molecular modelling of this system enabled us to visualize the plausible 3D structures of the transition states, allowing the proposition of a viable explanation of previous experimental findings.     

Oxidation-promoted synthesis of ferrocenyl planar chiral rhodium(iii) complexes for C–H functionalization catalysis

The chemical oxidation of rhodium(i) complexes [Rh(L)(COD)][BF4], where L is a ferrocenyl phosphine/N-heterocyclic carbene ligand, with 2 equiv. of a triaryl-aminium salt [(4-BrC6H4)3N][BF4] in acetonitrile gave planar chiral, air-stable [Rh(L–H)(MeCN)3][BF4]2 complexes where the ferrocene (C5H4CH2ImR or C5H4CH2BImCH2Mes) ring has been C–H activated at the position 2 in good to excellent yields. An important reactivity difference between our complexes and the ubiquitous [Cp*Rh(MeCN)3]X2 complex has been observed in the Grignard-type arylation of 4-nitrobenzaldehyde.     

Enantioselective formal synthesis of (−)-ovalicin using quinic acid as a chiral template

The key intermediate 18 for the synthesis of (−)-ovalicin was synthesized using (−)-quinic acid as the chiral source, through a series of stereocontrolled and efficient chemical reactions, thus establishing a new, formal synthesis of the natural target. The featuring spirocyclic epoxide function has been installed by internal Williamson ether synthesis using the functionalities originally present at C-1 of (−)-quinic acid after the appropriate adjustments required to introduce the necessary functionality at C-2.     

Recent topics on catalytic transformations of aromatic molecules via η6-arene transition metal complexes

The π-complexation of an arene to a transition metal center delivers many useful reactivities to the arene moiety. Although synthetic methodologies that take advantage of such π-coordination have mostly been developed as stoichiometric processes, there have been considerable recent advances in the catalytic transformations of aromatic molecules through their activation in the form of transition metal η⁶-arene complexes. These advances include the π-coordination catalyzed transformations of aryl-heteroatom and side-chain CH and CC bonds and the palladium-catalyzed CH functionalization in pre-formed transition metal η⁶-arene complexes. This digest paper aims to provide a concise view of these recent advances in the area of transition metal η⁶-arene complexes.     

Ring-closing metathesis within chromium-coordination sphere: Facile access to phosphine-chelate (π-arene)chromium complexes

A variety of phosphine-chelate (π-arene)chromium complexes were prepared in good to excellent yields by the ring-closing metathesis reaction of [η⁶-(ω-alkenyl)benzene][(ω-alkenyl)phosphine]chromium(0) dicarbonyl complexes catalyzed by the Grubbs’ ruthenium-carbene complexes.     

Stereoselective hetero Diels–Alder reactions of chiral tricarbonyl (η6-benzaldehyde)chromium complexes

The ZnCl₂-promoted cycloaddition of a series of enantiopure ortho-substituted benzaldehyde–Cr(CO)₃ complexes and Danishefsky's diene gave the corresponding 2-aryl pyranones in good yields and complete enantiospecificity. Some of the mechanistic aspects of the cycloaddition were investigated and the reaction extended to different dienes.     

Diastereoisomeric (η6-arene)ruthenium(II) chiral Schiff base complexes: crystal structure of a triphenylphosphine adduct

Reaction of [(η⁶-p-cymene)RuCl(L*)] with AgClO₄ in Me₂CO gives a perchlorate complex which on subsequent treatment with PPh₃, γ-picoline or Cl⁻ yields adducts showing that there can be retention as well as inversion of configuration at the metal centre. The (R)_Ru,(S_C absolute configurations of the chiral centres in the triphenylphosphine adduct have been established by an X-ray diffraction study [HL*, (S-α-methylbenzylsalicylaldimine]. The CD spectral study reveals that there is an inversion of configuration during formation of the PPh₃ adduct.     

Planar chiral (η5-cyclohexadienyl)- and (η6-arene)-tricarbonylmanganese complexes: synthetic routes and application

Planar chiral arenetricarbonylchromium complexes have been intensively investigated and they have been applied as valuable building blocks for asymmetric synthesis and as ligands for asymmetric catalysis. In contrast, in the field of the isoelectronic cationic [(η(6)-arene)Mn(CO)(3)](+) complexes, until these last 10 years, very few studies were published involving nonracemic planar chiral cationic complexes and their potential applications, certainly because of the difficult access to enantiopure starting material. In 2009, however, the discovery of the first resolution of such compounds opened a new area for their application in the field of organic as well as of organometallic enantioselective syntheses. We felt it important to write a review on this subject to give an up-to-date summary of the methodologies used to prepare enantiomerically pure planar chiral neutral [(η(5)-cyclohexadienyl)Mn(CO)(3)] and cationic [(η(6)-arene)Mn(CO)(3)](+) complexes as well as their potential in enantioselective synthesis.     

Asymmetric synthesis of α-amino acids using polymer-supported Cinchona alkaloid-derived ammonium salts as chiral phase-transfer catalysts

Cinchonidine and cinchonine have been N-alkylated with Merrifield resin and employed as chiral phase-transfer catalysts for the enantioselective alkylation of enolates from N-(diphenylmethylene)glycine esters with activated electrophiles (up to 90% ee). The use of the polymer-supported cinchonidine ammonium salt afforded the corresponding (S)-isomers, whereas the (R)-isomers were obtained using related cinchonine-supported polymers.     

Etude electrochimique de la labilite des complexes paramagnetiques η5-cyclopentadienyle η6-arene fer(I). Reaction d'echange de l'arene par le cyclopentadiene

The decomposition of neutral electrogenerated η⁵-cyclopentadienyl-η⁶-arene iron(I) is investigated. The cyclic voltammetry shows that the cationic precursors η⁵-CpFe^II+-η⁶-Ar are reduced in a one-electron transfer step followed by a chemical step which involves coordination of solvent molecules. The increasing stability of η⁵-C₅H₅Fe^I-η⁶-C₆H₆ follows the order of solvent used: acetonitrile or pyridine < dimethylformamide < acetone or methylene chloride. In acetonitrile, the fast replacement of C₆H₆ by donor ligands occurs via a postulated intermediate, the 17-electron complex η⁵-C⁵H⁵Fe^I(CH₃CN)₂. Without donor ligands decomposition of η⁵-CpFe^I-η⁶-Ar leads to the corresponding ferrocenes, Cp₂Fe^II. In the presence of cyclopentadiene the electrochemical activation allows replacement of the arene group of η⁵-CpFe^II+-η⁶-Ar by a cyclopentadienyl ligand.     

Studies on characterization, telomerase inhibitory properties and G-quadruplex binding of η6-arene ruthenium complexes with 1,10-phenanthroline-derived ligands

Two arene ruthenium complexes [Ru(η(6)-C(6)H(6))(p-MOPIP)Cl](+)1 and [Ru(η(6)-C(6)H(6))(p-CFPIP)Cl](+)2, where p-MOPIP = 2-(4-methoxyphenyl)-imidazo[4,5f][1,10] phenanthroline and p-CFPIP = 2-(4-trifluoromethylphenyl)-imidazo[4,5f][1,10] phenanthroline, were prepared and the interactions of these compounds with DNA oligomers 5'-G3(T2AG3)3-3'(HTG21) have been studied by UV-vis and circular dichroism (CD) spectroscopy, gel mobility shift assay, fluorescence resonance energy transfer (FRET) melting assay, polymerase chain reaction (PCR) stop assay and telomeric repeat amplification protocol (TRAP) assay. The results show that both complexes can induce the stabilization of quadruplex DNA but complex 1 is a better G-quadruplex binder than complex 2. The two ruthenium complexes tested led to an inhibition of the enzyme telomerase and complex 1 was the significantly better inhibitor. A novel visual method has been developed for making a distinction between G-quadruplex DNA and double DNA by our Ru complexes binding hemin to form the hemin-G-quadruplex DNAzyme. Furthermore, in vitro cytotoxicity studies showed complex 1 exhibited quite potent antitumor activities and the greatest inhibitory selectivity against cancer cell lines.     

Enantioselective synthesis of (S)-α-methylphenylalanine using (S)-BINOLAMs as new phase-transfer catalysts

A series of (S)-bis(aminomethyl)binaphthols [(S)-BINOLAMs] 4 have been prepared and used as catalysts in the enantioselective C-alkylation reaction of the aldimine Schiff bases of alanine esters 5 under solid–liquid phase-transfer catalysis (PTC) conditions employing NaOH as base in toluene at room temperature. (S)-3,3′-Bis[(diethylamino)methyl]-2,2′-dihydroxy-1,1′-binaphthalene 4a gave the best e.e.s. (S)-α-Methylphenylalanine 7 was isolated, after hydrolysis of the iminoester, in 85% yield with an e.e. of 68%.     

Enantioselective addition of diethylzinc to aldehydes using novel axially chiral 2-aminomethyl-1-(2′-hydroxyphenyl)naphthalene catalysts

The synthesis of atropo-enantiomerically pure aminomethyl and hydroxymethyl substituted biaryls derivatives M-2 and M-3 (and, optionally, their enantiomers), by dynamic kinetic resolution of a racemic lactone precursor, is described. Their application as chiral catalysts in the asymmetric addition of diethylzinc to aldehydes leads to excellent enantiomeric ratios of up to 99:1 and high chemical yields.     

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.