Tungsten Carbene Complexes Research Articles

Covalent binding of a Fischer-type metal carbene in ordered mesoporous MCM-41-functionalized silica

The synthesis of ordered mesoporous silicas has received extensive attention due to their ordered arrangement, large surface area, and chemical stability. The pore size and pore volume of these materials make them suitable potential matrices for hosting. Here, we show that the covalent grafting of the Fischer carbene complex of tungsten [(CO) 5W C(ph)OCH 2CH 3] onto the silanol groups in inner walls of ordered mesoporous MCM-41 silica has been successfully accomplished. The introduction of the carbene complex is made by following two distinct anchoring procedures. The first one goes through the reaction of a mesoporous silica functionalized with 3-aminopropyltriethoxy silane (APTES) and a tungsten carbene, while in the second approach a precursor previously synthesized by reacting APTES with the tungsten carbene is then anchored via a direct bond with the silanol groups of the interior mesoporous pore channels of silica. XRD, N 2 adsorption/desorption, and TEM analysis provide strong evidence that the mesoporous support structure retains its long range ordering after the grafting process, despite a significant reduction of its specific surface area, pore volume and pore size. The chemical bonding of the tungsten carbene complex on the pore channels is also clearly confirmed by means of FTIR and solid-state NMR spectroscopy.

4+2] Cyclization reactions of chiral Cβ-substituted Fischer alkenyl carbene complexes: efficient synthesis of enantiopure cyclohexenone and norbornene derivatives

Chiral alkenyl carbene complexes of tungsten(0) 1 and 2, readily available from the chiral pool, undergo the [4+2] cycloaddition with the Danishefsky diene to provide enantiopure 4-alkenyl-2-cyclohexenone adducts 5 and 7 with high stereoselectivity after treatment of the primary cycloadducts 4 and 6 with TBAF. Cyclopentadiene also cycloadds to carbenes 1 and 2 affording the expected norbornene metal carbene complexes 10 and 12 with remarkable diastereo and face selectivity. Oxidative removal of the metal pentacarbonyl fragment leads to the ester derivatives 11 and 13. The X-ray structure analysis of two cycloadducts derived from carbenes 1 and 2 has been performed.

Biscarbene complexes from the reactions of O-ethyl lactim and 1-alkynyl Fischer carbene complexes of chromium and tungsten

Reactions of O-ethyl lactim ∼(CH 2) 3–N C(OEt)∼ with 1-alkynyl Fischer carbene complexes (OC) 5M C(OEt)C CPh (M = Cr, W) afforded biscarbene complexes with an azabicyclo[3.2.0]heptene core. Under a nitrogen atmosphere, the resultant chromium biscarbene complex gave a SiO 2-promoted rearrangement complex in 86% yield. Thermal annelation of the rearrangement product followed by hydrolysis over SiO 2 formed a cyclopentenone derivative. A cyclopentadiene derivative was obtained as a rare example of intermediate Fischer carbene species, suggesting a possible reaction pathway for the thermal annelation of the rearrangement complex. Stepwise oxidation of the chromium biscarbene complexes with an azabicyclo[3.2.0]heptene core with pyridine- N-oxide (PNO) under mild controlled conditions generated partially and completely demetalated carbonyl products, respectively. The key carbene complexes and demetalated compounds were structurally characterized by X-ray crystallographic studies.

The Photochemical Reactivity of the “Photo‐Inert” Tungsten (Fischer) Carbene Complexes

The light fantastic: Tungsten(0) Fischer carbene complexes, previously considered to be photo-inert, produce, by adequately choosing the ligands, three classes of photoproducts (see scheme). Thus, irradiation of tetracarbonyltungsten(0) carbene complexes 1 leads to syn-metalated imines 2, amino complexes 3, and α-amino esters 4. Reasonable reaction pathways are proposed for all these processes.

The Photochemical Reactivity of the “Photo‐Inert” Tungsten (Fischer) Carbene Complexes

Überraschende Reaktivität: Wolfram(0)-Fischer-Carbenkomplexe, die bislang als photochemisch inert galten, liefern im Falle passend gewählter Liganden bei der Bestrahlung drei Klassen von Produkten (siehe Schema). So entstehen aus den Tetracarbonylwolfram(0)-Carbenkomplexen 1 syn-metallierte Imine 2, Aminokomplexe 3 und α-Aminosäureester 4. Für alle Prozesse werden plausible Reaktionswege vorgeschlagen. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2006/z501590_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Reaction of α,β-unsaturated Fischer carbene complexes with allyl alkoxide

Optically enriched homo-binuclear Fischer chromium carbene complexes with planar chiral arene chromium complexes gave α-allyl β-arylpropionates up to 97% ee by reaction with allyl alkoxide and subsequent photo-oxidative demetalation. The chiral hetero-binuclear tungsten carbene complexes afforded anti α-allyl β-hydroxy β-arylpropionates as a major product up to 92/8 dr by the same reaction sequence. High diastereoselectivity in these reactions is contributed to the planar chirality of the arene chromium complex, even though the reaction was carried out under vigorous basic media. The reaction products, α-allyl β-arylpropionates were derived by 1,3-M(CO) 5 shift and subsequent [3,3]-sigmatropic rearrangement. Also, the corresponding chromium-uncomplexed α,β-unsaturated Fischer carbene complexes afforded α-allyl β-arylpropionates under the same conditions. Formation of β-allyl β-arylpropionates via 1,2-M(CO) 5 shift followed by [3,4]-sigmatropic rearrangement was not observed in both reactions of chromium-coordinated and the corresponding chromium-uncoordinated α,β-unsaturated Fischer carbene complexes with allyl alkoxide in the presence of base.

Synthesis of trinuclear silicon-, germanium-, and tin-containing tungsten carbene complexes [(ButO)2(Cl)2W=CH]2EPh2 (E = Si, Ge, or Sn). Crystal structure of [(ButO)2(Cl)2W=CH]2SiPh2 complex

New trinuclear organosilicon, organogermanium, and organotin-containing tungsten carbene complexes Ph2E[CH=WCl2(OBut)2]2 (E = Si, Ge, or Sn) were synthesized by the reaction of the trinuclear carbyne complexes Ph2E[C≡W(OBut)3]2 with HCl. The tin-containing carbene complex is thermally unstable and was identified in solution by 1H and 13C NMR spectroscopy. The silicon-and germanium-tungsten carbene complexes were isolated in high yields as individual crystals and were characterized by elemental analysis, IR spectroscopy, and 1H and 13C NMR spectroscopy. The structure of the silicon-containing complex Ph2Si[CH=WCl2(OBut)2]2 was established by X-ray diffraction.

The electrochemical activity of heteroatom-stabilized Fischer-type carbene complexes

A cyclic voltammetry study of some alkoxy, amino and hydrazino carbene complexes of chromium and tungsten has been performed, affording for the first time a detailed overview on the electrochemical activity of this useful and versatile compound class, focusing on the modulating effects of both the carbene metal and substituents. The interest for the above results is also related to the possibility of using Fischer carbenes as new electrochemical active probes for labeling ‘peptide nucleic acids’, which are DNA analogues, for diagnostic purposes.

Synthesis and some properties of 14 group element-containing alkylidene complexes of molybdenum and tungsten

Alkylidene complex of molybdenum Ph 3Si–CH Mo(NAr)(OCMe 2CF 3) 2 ( 1) ( Ar = 2 , 6 - Pr 2 i C 6 H 3 ) has been prepared by the reaction of Ph 3SiCH CH 2 with known alkylidene compounds Alkyl-CH Mo(NAr)(OCMe 2CF 3) 2 (Alkyl = Bu t , PhMe 2C). According to X-ray diffraction studies the geometry of the Mo atom in ( 1) can be described as a distorted tetrahedron. Reactions of heteroelement-containing imido alkyl compounds of molybdenum and tungsten (ArN) 2M(CH 2EMe 3) 2 ( Ar = 2 , 6 - Pr 2 i C 6 H 3 ; M = Mo. W; E = Si, Ge, Sn) with triflic acid were shown to yield a complex mixture of products among which carbene complexes have not been observed. The formation of carbene complexes of tungsten in low yields was observed by 1H NMR spectroscopy in the reactions of (ArN) 2W(CH 2EMe 3) 2 (E = Si, Ge, Sn) with hydrogen chloride in THF- d 8. Catalytic properties of heteroelement-containing alkylidene complexes of molybdenum R 3E–CH Mo(NAr)(OCMe 2CF 3) 2 (E = Si, Ge; R = Me, Ph) and also hydrocarbon analogs Alkyl-CH Mo(NAr)(OCMe 2CF 3) 2 (Alkyl = Bu t , PhMe 2C) in metathesis of 1-hexene were investigated. The catalytic activity of alkylidene complexes was shown to depend essentially on the nature of substituent bonded to carbene carbon atom.

Diastereoselective Synthesis of Five‐ and Seven‐Membered Rings by [2+2+1], [3+2], [3+2+2], and [4+3] Carbocyclization Reactions of β‐Substituted (Alkenyl)(methoxy)carbene Complexes with Methyl Ketone Lithium Enolates

beta-Substituted alkenylcarbene complexes react with methyl ketone lithium enolates to give different carbocyclization products depending on the structure of the lithium enolate, on the metal of the carbene complex, and on the reaction media. Thus, the reactions of aryl and alkyl methyl ketone lithium enolates with beta-substituted alkenyl chromium and tungsten carbene complexes in diethyl ether afford 1,3-cyclopentanediol derivatives derived from a formal [2+2+1] carbocyclization reaction. However, the lithium enolates of acetone and tungsten complexes furnish formal [3+2+2] carbocyclization products. In the case of alkynyl methyl ketone lithium enolates, competitive formal [2+2+1] and [3+2] carbocyclization reactions occur and 1,3-cyclopentanediol and 3-cyclopentenol derivatives are formed. Conversely, alkenyl methyl ketone lithium enolates react with alkenylcarbene complexes under the same reaction conditions to form 2-cycloheptenone derivatives by a formal [4+3] carbocyclization reaction. Finally, when the reaction was performed in the presence of a coordinating medium, the [3+2] carbocyclization pattern was observed independently of the nature of the methyl ketone lithium enolate used.

Mass Spectrometric Detection and Fragmentation Patterns of Synthetically Useful Chromium and Tungsten Carbene Complexes

A novel method for the detection of synthetically useful pentacarbonyl chromium and tungsten carbene complexes by electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry is presented which utilizes the addition of CN- anions to the electrophilic carbene carbon. This method is nondestructive, provided the ionization proceeds under soft conditions. The scope and limitations of this method are examined by utilizing different types of carbene complexes. Depending on the α-CH acidity of the carbene complexes, [M − H]- anions can be observed either in competition with the CN- adducts or as the only ions, even in the presence of cyanide. Secondary aminocarbene complexes exclusively form [M − H]- anions by deprotonation of the NH moiety. The ions under study have been mass-selected and subjected to collision-induced decay (CID) experiments in order to examine their fragmentation reactions. CO loss is not the only observable process. Most prominently, 1,2-eliminations and a retro-aldol reaction within a sugar carbene complex compete with decarbonylation and thus have rather low activation barriers. Some reactions can be attributed to coordinatively unsaturated chromium atoms formed through preceding CO losses.

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.

Improved approaches and structures of new ferrocenyl carbene complexes of chromium, tungsten, and molybdenum

Ferrocenyllithium reacts with M(CO) 6 (M = Cr, W, Mo) in THF to give, after alkylation at oxygen, the corresponding carbene complexes 3a– c in good yield. Complexes 3a, b were characterized by X-ray analysis. These complexes react with pentylamine to give the corresponding aminocarbene complexes 7a– c and with allylamine to give, in the case of chromium and tungsten, the corresponding and expected aminonocarbene complexes 8a, b, and for molybdenum, complex 9c in which the double bond is already coordinated to the metal. 8a, b could be converted in 9a, b in excellent yield. The structure of 9a could be confirmed by an X-ray analysis. Alkylations at nitrogen could be carried on complex 9c as well as on complexes 9a, b.

Stereoselective [3,3]‐Sigmatropic Rearrangement Promoted by the Metal [1,3]‐Shift of Binuclear Fischer Carbene Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Binuclear biscarbene complexes of furan

Carbene complexes of chromium and tungsten with a bridging furan substituent were synthesized from lithiated furan precursors and metal hexacarbonyls. The binuclear biscarbene complexes [(CO)5M{C(OEt)-C4H2O-C(OEt)}M'(CO)5](M = M'= Cr (3), W (4)) were obtained as well as the corresponding monocarbene complexes [M{C(OEt)-C(4)H3O}(CO)5](M = Cr (1), W (2)). A method of protecting the carbene moiety during the metal acylate stage was used to increase not only the yields of the binuclear Fischer biscarbene complexes 3 and 4 but to establish a method to synthesize analogous mixed heterobinuclear carbene complexes (M = W, M'= Cr (5)) in high yields. The binuclear biscarbene complexes 3 and 5 were reacted with 3-hexyne and yielded the corresponding benzannulated monocarbene complexes [M{C(OEt)-C14H17O3}(CO)5](M = Cr (6), W(7)). Complex 5 reacted regioselectively with the benzannulation reaction occurring at the chromium-carbene centre. The major products from refluxing 3 in the presence of [Pd(PPh3)4] were a monocarbene-ester complex [Cr{C(OEt)-C4H2O-C(O)OEt}(CO)5](8), the 2,5-diester of furan (9) and a carbene-carbene coupled olefin EtOC(O)-C4H2O-C(OEt)=C(OEt)-C4H2O-C(O)OEt (10). X-Ray structure analysis of 4 and 6 confirmed the molecular structures of the compounds in the solid state and aspects of electron conjugation between the transition metals and the furan substituents in the carbene ligands were investigated.

Synthesis and structural characterization of (pyrazolyl)alkenyl Fischer carbene complexes of chromium and tungsten

Michael addition of substituted pyrazoles 2 to 1-alkynyl Fischer carbene complexes (CO) 5M C(OEt)(C CPh) ( 1) ( a, M = Cr and b M = W) afforded (pyrazolyl)alkenyl Fischer carbene complexes (CO) 5M C(OEt)(CH C(R 1R 2R 3pz)Ph) (R 1R 2R 3pz = pyrazolyl) 3 (M = Cr) and 4 (M = W), respectively, with an exclusive ( E)-configuration in mild to excellent yields. The reaction of 1a and 3,5-dimethylpyrazole ( 2b) was monitored to demonstrate the formation and decomposition of complex 3b by 1H NMR measurements in CDCl 3 at 23 °C. Complexes 3 and 4 were characterized with 1H, 13C{ 1H} NMR, IR spectroscopies and elemental analysis. When the substituted pyrazoles were 3-methylpyrazole ( 2a) and 3,5-di- tert-butylpyrazole ( 2d), molecular structures of the corresponding (pyrazolyl)alkenyl Fischer carbene complexes 3a and 4d were characterized by X-ray crystallographic study.

Chemoselective Stepwise Demetalation of Unusually Stable Fischer Biscarbene Complexes by Domino [4+2]/[2+2] Cycloaddition of 2-Isopropenyl-2-oxazoline to 1-Alkynyl Fischer Carbene Complexes of Chromium and Tungsten

Domino [4+2]/[2+2] cycloaddition of 2-isopropenyl-2-oxazoline 2 to 1-alkynyl fischer carbene complexes (co)(5)m=c(oet)c=cph 1 (a, m = cr; b, w) in a 1:2 molar ratio afforded unusually stable biscarbene complexes 3a and 3b containing a novel four-, five-, and six-membered tricyclic core in 99.6% and 45.2% yields, respectively. chain-opening beta-aminoalkenyl monocarbene complex 4b and beta-amidoalkenyl monocarbene complex 5b of tungsten were also isolated from the cycloaddition upon treatment of the reaction mixture of 1b and 2 on silica gel. partial and full oxidation of 3a,b with pyridine n-oxide underwent efficient chemoselective stepwise demetalation to afford the corresponding monocarbene complexes 6a,b and organic diester 7, respectively, under mild conditions. the x-ray crystallographic study revealed the presence of a four-, five-, and six-membered tricyclic core in compounds 3, 6, and 7, and the methyl and oxazolindinyl groups derived from oxazoline 2 are positioned syn with respect to the azabicyclo[4.2.0]octadiene bicyclic moiety, which is oriented in the opposite direction. x-ray crystal structural data are reported for the bis- and monocarbene complexes 3a, 5b, and 6b as well as for diester 7.

1,4- to 1,5-Homologation of Alkene−Carbene and Alkyne−Carbene Complexes of Chromium and Tungsten: The Clue to Reactivity

The carbene carbon of 1,4-alkene and -alkyne carbene complexes of chromium and tungsten (1 and 2) reacts with nucleophiles such as hydrides originating from dihydropyridine and methyllithium to give upon CO and alkyne (or alkene) insertions polycyclic lactones 6 and 7, cyclopentanones 10 and 11, and cyclopentanols 12.

Stereoselective Synthesis of Pentacarbonyl(3-oxa-2-bicyclo[3.1.0]hexylidene)- and Pentacarbonyl(cyclopropylmethoxymethylene)tungsten Compounds on the Route to Cyclopropane-γ-lactones and -carboxylates

The diastereoselective synthesis of pentacarbonyl-3-oxa-2-bicyclo[3.1.0]hexylidene- and pentacarbonyl(cyclopropylmethoxymethylene)tungsten compounds 4 and 9 by the reaction of lithiated oxazolinyloxiranes 2 and 8 and Fischer carbene tungsten complexes 3 is described. A mechanism for the formation of 4 and 9 is reported as well as their oxidation to cyclopropane-gamma-lactone 13 and -carboxylate 14.

Stereoselective [3,3]-Sigmatropic Rearrangement Promoted by the Metal [1,3]-Shift of Binuclear Fischer Carbene Complexes

Reaction of chiral homobinuclear Fischer chromium carbene complexes with allyl alcohol in the presence of NaH and the following oxidative demetalation gave alpha-allyl esters in up to 97% ee via [3,3]-sigmatropic rearrangement reaction promoted by the metal 1,3-shift. On the other hand, chiral heterobinuclear tungsten carbene complexes with arene chromium complexes afforded alpha-allyl-beta-hydroxy esters as a major product in up to 92/8 dr by the same reaction sequence.