Photolytic reaction of chromium and molybdenum carbene complexes with imines : Synthesis of cepham, oxapenam, and oxacepham derivatives

A variety of substituted β-lactams, including a cepham analog, were synthesized by the photochemical reaction of [(methoxy)(methyl)carbene]chromium complexes with substituted imines. Oxazines and oxazolines were inert towards chromium carbene complexes. Oxazines were converted to bicyclic β-lactams by the photolytic reaction of molybdenum carbene complexes. Oxazolines were considerably less reactive and produced only low yields of β-lactam product and an equivalent amount of the corresponding oxazinone, incorporating two (MeO)(Me)C(CO) groups.     

CH3--MoF], [CH2=MoHF], and [CH[triple bond]MoH2F] formed by reaction of laser-ablated molybdenum atoms with methyl fluoride: persistent photoreversible interconversion through alpha-hydrogen migration and agostic interaction

Simple molybdenum methyl, carbene, and carbyne complexes, [CH3--MoF], [CH2=MoHF], and [CH[triple chemical bond]MoH(2)F], were formed by the reaction of laser-ablated molybdenum atoms with methyl fluoride and isolated in an argon matrix. These molecules provide a persistent photoreversible system through alpha-hydrogen migration between the carbon and metal atoms: The methyl and carbene complexes are produced by applying UV irradiation (240-380 nm) while the carbyne complex is depleted, and the process reverses on irradiation with visible light (lambda>420 nm). An absorption at 589.3 cm(-1) is attributed to the Mo--F stretching mode of [CH3--MoF], which is in fact the most stable of the plausible products. Density functional theory calculations show that one of the alpha-hydrogen atoms of the carbene complex is considerably bent toward the metal atom (angle-spherical HCMo=84.5 degrees ), which provides evidence of a strong agostic interaction in the triplet ground state. The calculated C[triple chemical bond]Mo bond length in the carbyne is in the range of triple-bond values in methylidyne complexes.     

The Photochemical Reaction of Vinylaziridines and Vinylazetidines with Chromium(0) and Molybdenum(0) (Fischer) Carbene Complexes

The [5+2] and [6+2] cycloaddition reactions of vinylaziridines and vinylazetidines with ketenes generated photochemically from chromium(0) and molybdenum(0) Fischer carbene complexes have been investigated. These processes constitute a straightforward and efficient route to azepanones and azocinones, respectively. The peculiar electronic properties of the metalated ketenes allow for the introduction of electron‐rich substituents in the final cycloadducts, a difficult task using conventional organic chemistry procedures. The versatility of the process is demonstrated by using Cr⁰ Fischer bis(carbene) complexes as metalated bis(ketene) precursors. These species produce tethered bis(azepanone)s in a single step under mild reaction conditions. Density functional theory calculations point to a stepwise reaction pathway through the initial nucleophilic attack of the nitrogen atom of the aziridine on the metalated ketene, followed by ring closure of the zwitterionic intermediate formed.     

Synthesis and cyclic voltammetry of some new metal carbene substituted cyclopentadienyliron half sandwich complexes

The syntheses of a number of cyclopentadienylcarbene iron half sandwich complexes are described. In addition to their spectroscopic characterization an X-ray crystal structure analysis is provided. The complexes prepared include chromium, molybdenum, and tungsten carbene complexes. The dicarbonyliron unit is substituted by benzyl, butyl or trimethylsilylmethyl groups. In two cases tetrametallic dimeric complexes connected by a propyl chain are presented. The reaction of the formylcyclopentadienyliron complex with a chromium carbene complex provided a vinylogous representative. Cyclic voltammetry was performed, and the data obtained are discussed in comparison to similar ferrocene based complexes.     

Synthesis and X-ray structure of an unexpected bidentate allene-aminocarbene complex of tungsten

This contribution describes the results obtained from the alkylation reaction upon base-induced of propargyl bromide at nitrogen of Ferrocenyl-N-pentylaminocarbene of tungsten, chromium and molybdenum. This led to unexpected highly strained Fischer-type carbene complexes (5a-c) containing a bidentate allene-aminocarbene ligand. This is the first report of the η²-allene Fischer carbene complexes. The structure of 5a was confirmed by an X-ray diffraction analysis.     

Übergangsmetall-carben-komplexe: CXXXVIII. Neue übertragungsreaktionen von carbenliganden zur darstellung von gold-carben-komplexen

Chloroauric acid reacts with pentacarbonyl[(dimethylamino)ethoxycarbene]chromium(0) (I) to give trichloro[(dimethylamino)ethoxycarbene]gold(III) (IV), and with pentacarbonyl{(dimethylamino)[methoxy(phenyl)methyleneamino]carbene} complexes of molybdenum(0) (II) and tungsten(0) (III) to give chloro{(dimethylamino)[methoxy(phenyl)methyleneamino]carbene}gold(I) (VII) and trichloro{(dimethylamino)[methoxy(phenyl)methyleneamino]carbene}gold(III) (VIII). IV and VIII react with boron tribromide to give tribromo[(dimethylamino)ethoxycarbene]gold(III) (V) and tribromo{(dimethylamino)[methoxy(phenyl)methyleneamino]carbene}gold(III) (IX), which react with boron triiodide to yield the triiodogold complexes [(dimethylamino)ethoxycarbene]triiodogold(III) (VI) and {dimethylamino[methoxy(phenyl)methyleneamino]carbene} triiodogold(III) (X).     

Cycloaddition reactions of alkoxy alkynyl Fischer carbene complexes with o-quinodimethanes (oQDMs)

[reaction: see text] The reaction of o-quinodimethanes (oQDMs) with alkoxy alkynyl Fischer carbene complexes is highly dependent on the carbene complex. Thus, for arylalkynyl carbene complexes, the initial [4 + 2]-cycloadduct evolves opening a new entry to the benzo[b]fluorene skeleton, which is present in many natural products. However, for alkenylalkynyl carbene complexes, the reaction takes place through the double bond, instead of the triple bond, in an unprecedented fashion, leading to new functionalized alkynyl carbene complexes.     

Pd-catalyzed inter- and intramolecular carbene transfer from group 6 metal-carbene complexes.

The use of group 6 metal-carbene complexes in inter- and intramolecular carbene transfer reactions has been studied. Thus, pentacarbonyl[(aryl)(methoxy)carbene]chromium(0) and tungsten complexes, 10, efficiently dimerize at room temperature in the presence of diverse Pd(0) and Pd(II)/Et(3)N catalysts. The effect of additives (PPh(3), AsPh(3), or SbPh(3)) on the nature and the isomeric ratio of the reaction products is negligible. The nature of the reaction products is more catalyst-dependent for metal carbenes 12 bearing alkyl groups attached to the carbene carbon. In these cases, either carbene ligand dimerization or beta-hydrogen elimination reactions are observed, depending on the catalyst. The carbene ligand dimerization reaction can be used to prepare conjugated polyenes, including those having metal moieties at both ends of the polyene system, as well as enediyne derivatives. The intramolecular carbene ligand dimerization of chromium bis-carbene complexes 28 and 30 allows the preparation of mono- and bicyclic derivatives, with ring sizes from six to nine members. For bis-carbene derivatives the beta-hydrogen elimination reaction is inhibited, provided that both metal centers are tethered by an o-xylylene group. Other alkyl complexes 32 form new mononuclear carbene complexes 37 or decompose to complex reaction mixtures. The results obtained in these reactions may be explained by transmetalation from Cr(0) to Pd(0) and the intermediacy of Pd-carbene complexes. Aminocarbene-chromium(0) complexes 15, need harsher reaction conditions to transfer the carbene ligand, and this transfer occurs only in the presence of deactivated olefins. The corresponding insertion/hydrolysis products 48 resulted in these cases. A catalytic cycle involving transmetalation from a chromacyclobutane to a palladacyclobutane is proposed to explain these results.     

First crystallographic elucidation of a high-valent molybdenum oxo N-heterocyclic carbene complex [CpMo(VI)O2(IBz)]2[Mo6O19

A high-valent molybdenum NHC dioxo complex [CpMoO(2)(IBz)](2)[Mo(6)O(19)] (IBz = 1,3-dibenzylimidazol-2-ylidene) has been isolated in its hexamolybdate form and crystallographically elucidated to show a Mo-C(NHC) bond of 2.173(4) ?. (NHC = N-heterocyclic carbene).     

Evidence for Carbene Intermediates in Isocyanide Homologation by Aluminium(I)

The C−C bond formation between C1 molecules plays an important role in chemistry as manifested by the Fischer–Tropsch (FT) process. Serving as models for the FT process, we report here the reactions between a neutral Al^I complex (^MeNacNac)Al ( 1 , ^MeNacNac=HC[(CMe)(NDipp)]₂, Dipp=2,6-diisopropylphenyl) and various isocyanides. The step-by-step coupling mechanism was studied in detail by low-temperature NMR monitoring, isotopic labeling, as well as quantum chemical calculations. Three different products were isolated in reaction of 1 with the sterically encumbered 2,6-bis(benzhydryl)-4-Me-phenyl isocyanide (BhpNC). These products substantiate carbene intermediates. The reaction between 1 and adamantyl isocyanide (AdNC) generated a trimerization product, and a corresponding carbene intermediate could be trapped in the form of a molybdenum(0) complex. Tri-, tetra-, and even pentamerization products were isolated with the sterically less congested phenyl and p-methoxyphenyl isocyanides (PhNC and PMPNC) with concurrent construction of quinoline or indole heterocycles. Overall, this study provides evidence for carbene intermediates in FT-type chemistry of aluminium(I) and isocyanides.     

Computational and experimental studies on the mechanism of the photochemical carbonylation of group 6 Fischer carbene complexes

The photocarbonylation reaction of Group 6 Fischer carbene complexes has been studied by DFT and experimental procedures. The process occurs by intersystem crossing (ISC) from the lowest excited singlet state (S1) to the lowest triplet state (T1), the latter structure being decisive for the outcome of the reaction. Methylenepentacarbonylchromium(0) complexes, alkoxypentacarbonylchromium(0)carbene complexes, and alkoxyphosphinetetracarbonylchromium(0) carbene complexes have coordinatively unsaturated chromacyclopropanone T1 structures with a biradical character. The evolution of the metallacyclopropanone species occurs by a jump (spin inversion) to the S(0) hypersurface by coordination of a molecule of the solvent, leading to ketene-derived products in the presence of ketenophiles or reverting to the starting carbene complex in their absence. The T1 excited states obtained from methylenephosphinetetracarbonylchromium(0) complexes and pentacarbonyltungsten(0)carbene complexes are unable to produce the carbonylation. The reaction with ketenophiles is favored in coordinating solvents, which has been tested experimentally in the reaction of alkoxypentacarbonylchromium(0) complexes and imines.     

Generation and reactivity of simple chloro(aryl)carbenes within the cavities of nonacidic zeolites

A number of para-substituted chloro(aryl)carbenes are generated within the cavities of a series of dry alkali metal cation-exchanged zeolites (LiY, NaY, KY, RbY, and CsY) upon laser flash photolysis of the corresponding diazirine precursor. The absolute reactivity of the chloro(aryl)carbene is found to be strongly dependent on both the nature of the electron-donating and -withdrawing properties of the aryl substituent and the nature of the zeolite charge-balancing cations. The results strongly suggest that two opposing mechanisms for capture of the carbene can occur depending on whether the zeolite framework behaves as a nucleophilic reagent or an electrophilic reagent in its reaction with the carbene center. Hammett relationships for the decay of the carbene as a function of aryl substituent and zeolite counterion versus the sigma+ substituent parameter support a change in mechanism as the carbene center toggles between being electron poor and electron rich. For the electron-poor chloro(4-nitrophenyl)carbene, a framework adduct is proposed upon reaction of the nucleophilic [Si-O-Al]- bridge with the carbene center, and for the electron-rich chloro(4-methoxyphenyl)carbene, an adduct with the tight Li+ cation is proposed.     

Density functional computations of the cyclopropanation of ethene catalyzed by iron (II) carbene complexes Cp(CO)(L)FeCHR,L = CO,PMe3, R = Me,OMe, ph,CO2Me

Density functional theory has been used to study the Fe‐catalyzed cyclopropanation of Fe‐carbene complexes with ethene. All the intermediates and transition states were optimized completely at the B3LYP/6‐31+G(d,p) level. Calculation results confirm that the cyclopropanation of Fe‐carbene complexes with ethene involves the two reaction paths I and II . In the reaction path I , the double bond of ethene attacks directly on the carbene carbon of Fe‐carbene complexes to generate the cyclopropane. In the reaction path II , ethene substitution for PMe₃ or CO in the Fe‐carbene complexes leads to the complexes M2 ; and the attack of one carbon of ethene on the carbene carbon results in the complexes M3 with a Fe? C? C? C four‐membered ring, and then generates the cyclopropane via the elimination reaction. For Fe‐carbene complexes A , C , D , E , and H , the main reaction mode is the reaction path I ; for Fe‐carbene complexes B , F , and G , the main reaction mode is the reaction path II . © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Fluorescence and crystal structures of new mercury(II) macrocyclic N-heterocyclic carbene complexes with ether chains

Polyerther-linked mercury(II) carbene complexes are synthesized by the reaction of carbene precursores with mercury(II) acetate. The binuclear carbene complexes with the chloride ion linkage are easily prepared by the reaction of mononuclear mercury carbene complexes with tetrabutylammoium chloride. The crystal structures reveal that the fluorescence of mercury complexes are related to the coordination mode of the ligands.     

Elektrophile Addition des Thiomethyl-Kations an η-Cyclopentadienyl(ethoxymethylcarben)-(nitrosyl)(trimethylphosphin)molybdän

Carbonyl(η⁵-cyclopentadienyl)(ethoxymethylcarbene)(nitrosyl)molybdenum reacts with trimethylphosphine with substitution of the carbonyl ligand. Subsequent electrophilic addition of the methylthio cation SCH₃⁺ at the nucleophilic carbene carbon atom affords a new cationic molybdathia-cyclopropane system.     

Efficient route to 1,3-Di-N-imidazolylbenzene. A comparison of monodentate vs bidentate carbenes in Pd-catalyzed cross coupling

A new bis(carbene) ligand architecture has been developed and was evaluated in the Suzuki-Miyaura cross-coupling reaction of various aryl halides with phenylboronic acid. Several new bis(carbene) ligands were tested in different carbene:Pd ratios. Pd(OAc)(2) and Pd(2)(dba)(3) were compared for efficiency as a Pd source. It was found that the Pd(OAc)(2)/bis(carbene) system formed a catalyst for the activation of chlorobenzene. [reaction: see text]     

A tantalum(V) carbene complex: formation of a carbene-bis(phenoxide) ligand by sequential proton and hydride abstraction

Proton and subsequent hydride abstraction from the bis(phenoxide) ligand of the trimethyltantalum(V) complex affords a cationic tantalum(V) carbene complex, in which two phenoxide groups are linked to the carbene center. The electrophilic nature of the carbene functionality is demonstrated by the reaction with PPh3.     

Diamino‐Substituted Carbene Transfer between Transition Metal Ions

Diamino‐carbene ligand transfer between various metalions is studied, particularly with Pd (II) to Rh(I), Rh(I) to Au(I). Reactions of various carbene complexes with AgPF₆ result in the cleavage of the M=C bond to give the protonated carbene species, imidazolidin‐2‐ylidinium salt, indicating the presence of free carbene ligand in the reaction medium. When the carbene transfer process was carried out in tetrahydrofuran, the polymerization of tetrahydrofuran occurred.     

Inter- and innermolecular reactions of chloro(phenyl)carbene

Supramolecular photolyses of 3-chloro-3-phenyl-3H-diazirine (8) were performed within cyclodextrin (CyD) hosts to determine whether these toroidal inclusion compounds could alter the reactivity of the ensuing carbene reaction intermediate, chloro(phenyl)carbene (9). Remarkably, no intramolecular products stemming from carbene 9 could be detected. Instead, modified CyDs were formed via so-called innermolecular reactions. Hence, diazirine 8 was photolyzed in various conventional solvents to gauge the intermolecular reactivity of carbene 9. Relevant results were used to rationalize the CyD innermolecular reaction products.     

Chemiluminescence from arylcarbene oxidation: Phenylchlorocarbene and (2-chlorophenyl)carbene

Chemiluminescence is observed in the thermal reaction of phenylchlorocarbene or (2-chlorophenyl)carbene and O₂, matrix-isolated in Ar. The chemiluminescence spectra closely match the phosphorescence of the corresponding carbonyl compounds. The reactivity of both carbenes towards O₂ is very different. Singlet carbene phenylchlorocarbene reacts thermally only slowly with O₂ up to 60 K. The oxidation products phenylchloroformate, benzoyl chloride and O(³P) are mainly formed photochemically on irradiation of the diazirine precursor. Triplet carbene (2-chlorophenyl)carbene reacts readily with O₂ at cryogenic temperatures to give mostly 2-chlorobenzaldehyde-O-oxide. The carbonyl-O-oxide is photochemically easily cleaved to give 2-chlorobenzaldehyde and O(³P). The reaction step leading to carbonyl compounds in their excited states is in both carbene oxidations the recombination of the free carbene and O(³P).