Chromium Carbene Complexes Research Articles

Synthesis of 2-pyranylidene or (2-furyl)carbene–chromium complexes from conjugated enyne carbonyl compounds with Cr(CO) 5(THF)

The reaction of β-ethynyl α,β-unsaturated esters or amides with Cr(CO) 5(L) (L=Et 2O, THF), derived from chromium hexacarbonyl, gives 2-pyranylidene–chromium complexes in good yields. The conjugated ene-carbonyl-vinylidene complex is a key intermediate to these Fischer-type oxacarbene complexes. In contrast, β-ethynyl α,β-unsaturated ketones react with Cr(CO) 5(THF) to give (2-furyl)carbene complexes via nucleophilic attack of the carbonyl oxygen atom to the intermediate π-alkyne–chromium complexes. Some chemical transformation, such as [4+2] type cyclization of 2-pyranylidene complexes with alkynes and aldehyde formation from (2-furyl)carbene complexes, has also been described.

2- and 3-haloalkoxy fischer carbene complexes of chromium as synthons for either hydroxycyclopropanation or oxaspirocyclopropanation of alkenes.

The thermal reaction of 2-haloethoxy- and 3-chloropropoxy(alkenyl)carbene complexes of chromium with electronically neutral alkenes furnished diastereoselectively the corresponding 1-haloalkoxy-l-vinylcyclopropanes that, subjected to subsequent halogen-lithium exchange reactions, provided vinylcyclopropanols or compounds containing the spiro[cyclopropane-tetrahydrofuran/tetrahydropyran] structure, depending on the nature of both the halogen atom and the lithiating reagent. The hydroxycyclopropanation reaction can be efficiently achieved in a one-pot fashion.

Symmetric and Non-Symmetric Dinuclear Alkenyl-Bridged Carbene Complexes of Chromium and Tungsten

Treatment of (CO)5W=C(NMe2)CH=CH−Ar−(OH)(H)C≡CH (1a−b) [a: −Ar− =1,4-C6H4−; b: −Ar− = 2,5-C4H2S−] with W(CO)5(THF) in methanol yields the nonsymmetrical bis(alkenylcarbene)-bridged ditungsten complexes (CO)5W=C(NMe2)CH=CH−Ar−CH=CH(OMe)C=W(CO)5 (3a−b). Similarly, the symmetrical bis(methoxycarbene)ditungsten derivative [(CO)5W=C(OMe)CH=CH−]2(1,4-C6H4−) (4a) is obtained in a low yield from the precursor (CO)5W=C(OMe)CH=CH−C6H4−(OH)(H)C≡CH (2a). Complexes 3a and 3b undergo aminolysis reactions with diamines such as piperazine and ethylenediamine to produce the corresponding bis[alkenyl(amino)carbene]ditungsten complexes 5a−b and 6a−b. The reaction of 1a with Cr(CO)5(THF) affords the heterobimetallic (W−Cr) bis(carbene) complex (CO)5W=C(NMe2)CH=CH−Ar−CH=CH(OMe)C=Cr(CO)5 (3′a). This complex, as well as the bis(methoxycarbene) derivative 4′a, can also be obtained by treatment of (CO)5W=C(XR)CH=CH−C6H4−CHO [1c: XR = NMe2; 2c: XR = OMe] with (CO)5Cr=C(OMe)CH2−Li+. All the new compounds have been characterized by analytical and spectroscopic methods, and the single-crystal structures of 3a, 4a−b, and 4′a have been established.

Chelated η2-alkene- and η3-allyl-carbene complexes of late transition metals: structure–reactivity relations and preparative use

Abstract Chelated alkene–carbene complexes of Group VI metals typically undergo intramolecular [2+2]-cycloaddition reactions, whereas the iron analogues and related (η3-allyl)–ironcarbene complexes show a more diverse behaviour. They are amenable to reactions at the carbene carbon atom, at the terminus of the π-ligand and even at the tether depending on intrinsic stereoelectronic parameters as well as external factors like solvent polarity and ‘softness’ of attacking nucleophiles. Cascades with extensive reorganization of the organic ligand leading to oligo-enynes, δ-lactones and cyclopentenones may ensue. They are discussed together with some new findings allowing a generalized mechanistic description. The chemical and configurational stabilities of planar-chiral alkene–carbene and allyl–carbene complexes of iron, chromium, manganese and cobalt are compared.

Synthesis and crystal structures of carbene complexes with conjugated thiophene units

Binuclear carbene complexes of chromium and tungsten containing a conjugated thiophene spacer, dithieno[3,2-b:2′,3′-d]thienylene (TTT), were synthesized according to the Fischer method for the synthesis of carbene complexes. Biscarbene complexes of the type [(CO)5M{C(OEt)TTTC(OEt)}M(CO)5] (M=Cr, 2; W, 5) were afforded together with the corresponding monocarbene complexes [M{C(OEt)TTTH}(CO)5] (M=Cr, 1; W, 4) and decomposition products [M{C(OEt)TTTC(O)OEt}(CO)5] (M=Cr, 3; W, 6), formed in the reaction of the biscarbene complexes with trace amounts of oxygen. X-ray crystallographic determinations were performed on compounds 1, 2 and 6 to assess the influence of the different substituents in the C9 position of the TTT carbene complexes on the structural features of the condensed rings. The crystallographic data of these three complexes are reported in this paper.

ChemInform Abstract: Acylamino Chromium Carbene Complexes: Direct Carbonyl Insertion, Formation of Muenchnones, and Trapping with Dipolarophiles.

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ChemInform Abstract: Benzannulation and Cyclopentannulation Reactions of Electron Poor Amino-Stabilized Alkenyl Fischer Carbene Complexes.

Abstract The alkenyl(amino)carbene complexes of chromium and tungsten bearing an electron-withdrawing group at the Cβ-position 5–6 were prepared by condensation of methyl(amino)carbene complexes 1–2 and esters of glyoxylic acid. The complexes 5–6 were readily α-metallated with n-BuLi affording the deuteriated metal carbene complexes 7–8. The chromium complexes 5, 7 display a remarkable reactivity toward different types of alkynes. These complexes reacted with both unactivated alkynes and terminal electron-poor alkynes to give exclusively the phenol products 11–13 via the benzannulation reaction (Dotz reaction). In contrast, the reaction of carbene complexes 5 with internal electron-poor alkynes yielded solely the cyclopentadiene derivatives 15 via the cyclopentannulation reaction. The tetracarbonyl carbene intermediates 16 were isolated and fully characterized.

Carbohydrate-Modified Fused Pyranosylidene Complexes via Radical Addition of Epoxides to Unsaturated Metal Carbenes

Alkenyl and alkynyl carbene complexes of chromium and tungsten undergo a diastereoselective cyclization upon reaction with epoxides in presence of [Cp 2TiCl] 2. trans-Fused tetrahydropyranosylidene complexes are obtained from the radical addition of cyclohexene oxide to monosaccharide-derived vinylcarbene tungsten complexes. The benzyl-protected 1,2-anhydro-α- d-glucopyranose adds to styryl and alkynyl carbene complexes to give bicyclic trans-fused tetrahydro- and dihydropyranosylidene complexes as well as the acyclic addition products.

Diastereoselective Intermolecular Cyclopropanation of Simple Alkenes by Fischer Alkenyl and Heteroaryl Carbene Complexes of Chromium: Scope and Limitations

The intermolecular cyclopropanation of simple alkyl-substituted alkenes with Fischer methoxycarbene complexes under thermal conditions is reported. The scope and limitations of this [2 + 1] cycloaddition with respect to the nature of the carbene complex, substitution pattern of the electron neutral olefin, and functional group tolerance in the electron neutral olefin are described in detail. This methodology provides functionalized cyclopropanes in good to excellent yields and generally with high diastereoselectivity. A mechanism involving the initial formation of a chelated tetracarbonyl complex intermediate is proposed to account for the observed results.

Incorporation of chromium carbenes in a silica matrix by sol-gel processing: application to aminolysis of alkoxycarbene complexes

Chromium carbene complexes have been immobilized in a silica matrix by sol-gel processing based on the polycondensation of (trialkoxy)silylethylphosphane ligands and tetraalkoxysilanes. The microstructure of the material obtained depends on the gelation conditions. In situ gelation of alkoxy- or aminocarbene complexes with tetramethoxysilane (TMOS) affords mesoporous materials with a homogeneous distribution of the metal complex. The metal carbene moiety is accessible for small substrates as demonstrated for the aminolysis of the incorporated methoxy(phenyl)carbene complex which slows down with increasing bulk of the amine.

Acylamino Chromium Carbene Complexes: Direct Carbonyl Insertion, Formation of Münchnones, and Trapping with Dipolarophiles.

Acylamino Chromium Carbene Complexes: Direct Carbonyl Insertion, Formation of Münchnones, and Trapping with Dipolarophiles.

Benzannulation and Cyclopentannulation Reactions of Electron-Poor Amino-Stabilized Alkenyl Fischer Carbene Complexes

The alkenyl(amino)carbene complexes of chromium and tungsten bearing an electron-withdrawing group at the Cβ-position 5–6 were prepared by condensation of methyl(amino)carbene complexes 1–2 and esters of glyoxylic acid. The complexes 5–6 were readily α-metallated with n-BuLi affording the deuteriated metal carbene complexes 7–8. The chromium complexes 5, 7 display a remarkable reactivity toward different types of alkynes. These complexes reacted with both unactivated alkynes and terminal electron-poor alkynes to give exclusively the phenol products 11–13 via the benzannulation reaction (Dötz reaction). In contrast, the reaction of carbene complexes 5 with internal electron-poor alkynes yielded solely the cyclopentadiene derivatives 15 via the cyclopentannulation reaction. The tetracarbonyl carbene intermediates 16 were isolated and fully characterized.

A Major Breakthrough in the Use of Alkoxycarbene Complexes of Chromium and Tungsten for the Synthesis of Elaborate Organic Compounds: Dihydropyridine Induced Reductions and Cascade Insertion Reactions

Alkoxy (alkyl)carbene complexes of tungsten and chromium react with dihydropyridine and N-methyldihydropyridine to give respectively pyridinium ylid complexes and N-methylpyridinium tungstates and chromates. These two types of complexes can be used, for the former, as cyclopropanation reagents, for the latter, as unprecedented initiators of cascade multiinsertions of olefins, alkynes and CO. These insertions lead to elaborate polycyclic compounds.

Annulation With and Without CO-Insertion: Striking a Balance in Reactivity of Chromium Carbene Complexes with Chalcogen-Stabilized Iron Cluster Appendage

Two new sets of products [(CO)6Fe2EE′{μ-indanone}] (2a: E, E′=Se; 2b: E=S, E′=Se) and [(CO)6Fe2EE′{μ-(2-ethoxy, α-naphthol)}] (3a: E, E′=Se; 3b: E=S, E′=Se; 3c: E=S, E′=Te) have been isolated from the thermolysis of the cluster-bound Fischer carbene complex, [(CO)6Fe2EE′{μ-C(Ph)C–C(OEt)Cr(CO)5}], 1a–c (1a: E, E′=Se; 1b: E=S, E′=Se; 1c: E=S, E′=Te) in THF. The products are formed by annulation pathway with and without the intermediacy of CO insertion step, from the same substrate. They have been characterized by IR and 1H, 13C, 77Se and 125Te NMR spectroscopy and the structural types were unequivocally established by crystallographic analysis of compounds 2a and 3c.

Synthesis of aminocyclopentitols from chromium carbene complex derived aminocyclobutanones.

Functionalized cyclopentenones were synthesized by the diazomethane ring expansion of cyclobutanones, produced by the photochemical reaction of vinyl oxazolidinones with chromium carbene complexes.

The Reaction of Chromium Alkenyl Carbene Complexes with Alkenyl Oxazolines: A Diastereoselective [3 + 2] Annulation Route to Cyclopentanones

Pentacarbonyl(alkenylmethoxy)carbene chromium complexes 1 undergo regio- and diastereoselective formal [3 + 2] cycloaddition to electron-deficient alkenes, e.g. alkenyl oxazoline derivatives 2, affording cyclopentanoids 3 and 4, the cis isomer 3 being the major stereoisomer. The cyclopropane intermediates 5/6 were isolated, under appropriate reaction conditions, and further converted into 3 and 4. The cycloadduct 3b was efficiently elaborated into the 3,4-disubstituted cyclopentanones 10 and 11.

Synthesis of Carbon-Linked Bis(chromium carbene) Complexes, Bis(azapenams), and Bis(dioxotetraazacyclotetradecadienes)

Carbon-linked bis(chromium carbene) complexes were synthesized by the bis-alkylation of 1,n-alkyl bis(triflates). Photolysis with N-protected imidazolines followed by deprotection and acid-catalyzed dimerization produced bis(dioxotetraazacyclododecadienes) linked by five- or six-carbon aliphatic chains.

Stereoselective, Oxidative C−C Bond Coupling of Naphthopyran Induced by DDQ: Stereocontrolled Total Synthesis of Deoxyfrenolicin

A formal total synthesis of pyranonaphthoquinone natural product deoxyfrenolicin 1 is described. The key step in the synthesis involves the use of stereoselective 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-induced C−C bond coupling of the naphthopyran 10 with allyltriphenyltin to give exclusively 1,3-trans naphthopyran derivative 23. The naphthopyran 10 was obtained via oxa-Pictet−Spengler cyclization of substituted naphthalene 22, which was derived by regioselective benzannulation of chromium carbene complex 12 with acetylene 18. This newly developed synthetic route employing a tandem benzannulation/oxa-Pictet−Spengler cyclization/DDQ-induced coupling strategy should also be applicable to the synthesis of other pyranonaphthoquinone natural products such as kalafungin 4 and nanaomycin 5.

An Efficient Synthesis of Bicyclo[3.3.0]oct-2-en-4-ones and 2-Azabicyclo[3.3.0]oct-7-en-6-ones via β-Amino-Substituted α,β-Unsaturated Fischer Carbene Complexes

The formal [3+2] cycloaddition of β-amino-substituted α,β-unsaturated Fischer carbenechromium complexes 2, which are easily prepared in a one-pot procedure from terminal alkynes 1 via an initially formed alkynylidene complex by a Michael-type addition of a secondary amine, with a variety of alkynes 3 afford the 1,2,5-trisubstituted 3-alkoxy-5-dialkylamino-1,3-cyclopentadienes 4 with an acetal-protected aldehyde or ketone carbonyl group in either the 5-substituent R1 or the N-substituent R2. Under acidic conditions, both the enol ether moiety in the 5-membered ring as well as the acetals in the side chains are hydrolyzed, and the resulting cyclopentenones 6 with carbonyl-group containing side-chains undergo facile intramolecular aldol reactions to give bicyclo[3.3.0]oct-2-en-4-ones 8 and 8-azabicyclo[3.3.0]oct-2-en-4-ones 12, respectively, in good to very good yields for most cases.

An Efficient Synthesis of Bicyclo[3.3.0]oct-2-en-4-ones and 2-Azabicyclo[3.3.0]oct-7-en-6-ones via β-Amino-Substituted α,β-Unsaturated Fischer Carbene Complexes

The formal [3+2] cycloaddition of β-amino-substituted α,β-unsaturated Fischer carbenechromium complexes 2, which are easily prepared in a one-pot procedure from terminal alkynes 1 via an initially formed alkynylidene complex by a Michael-type addition of a secondary amine, with a variety of alkynes 3 afford the 1,2,5-trisubstituted 3-alkoxy-5-dialkylamino-1,3-cyclopentadienes 4 with an acetal-protected aldehyde or ketone carbonyl group in either the 5-substituent R1 or the N-substituent R2. Under acidic conditions, both the enol ether moiety in the 5-membered ring as well as the acetals in the side chains are hydrolyzed, and the resulting cyclopentenones 6 with carbonyl-group containing side-chains undergo facile intramolecular aldol reactions to give bicyclo[3.3.0]oct-2-en-4-ones 8 and 8-azabicyclo[3.3.0]oct-2-en-4-ones 12, respectively, in good to very good yields for most cases.