Chromium Carbene Complexes Research Articles

Asymmetric Dearomatization of the Furan Ring Promoted by Conjugate Organolithium Addition to (Menthyloxy)(3‐furyl)carbene Complexes of Chromium.

The sequential low-temperature addition reaction of an organolithium compound and methyl triflate to (menthyloxy)(3-furyl)carbene complexes of chromium and tungsten proceeded with excellent regioselectivity (1,4-addition) and diastereoselectivity (2,3-trans disposition of the nucleophile and electrophile groups) to afford new 2,3-disubstituted (2,3-dihydro-3-furyl)carbene complexes. In addition, a high degree of diastereofacial selectivity was achieved by employing alkenyllithium compounds. After detachment of both the metal fragment and the chiral auxiliary group, trisubstituted 2,3-dihydrofuran derivatives containing a quaternary stereogenic center at the C3 position were obtained. The characterization, including X-ray crystallography, of a novel type of stable four-membered chelate (eta(2)-alkene)tetracarbonylcarbene complex of chromium is also reported.

Synthesis and selective silylation of ω-hydroxy functionalized (η 2-alkenyl)carbene complexes of chromium(0) and tungsten(0) Part 11. The chemistry of metallacyclic alkenylcarbene complexes

Tungsten(0) carbene complexes of the type (OC) 5WC(NMeCH 2CHCHCH 2OH)R 2 (R=Me: 2a; R=Ph: 2b) were generated by aminolysis of (OC) 5WC(OMe)R with cis-NHMeCH 2CHCHCH 2OH. Like their Cr-congeners 1, complexes 2 exist at room temperature as mixtures of Z- and E-isomers with regard to the C–N bond. The metallacyclic complexes (OC) 4WC(η 2-NMeCH 2CHCHCH 2OH)R ( 4) were obtained in good yields upon photo-decarbonylation of 2. Deprotonation/silylation of the complexes (OC) 4MC(η 2-NMeCH 2CHCHCH 2OH)Me (M=Cr: 3a; M=W: 4a) with one equivalent of n BuLi/Me 3SiCl gave (OC) 4MC(η 2-NMeCH 2CHCHCH 2OSiMe 3)CH 3 (M=Cr: 5; M=W: 6), whereas with two equivalents of n BuLi/Me 3SiCl complexes (OC) 4MC(η 2-NMeCH 2CHCHCH 2OSiMe 3)CH 2SiMe 3 (M=Cr: 7; M=W: 8) were formed. Hydrolysis of the latter yielded selectively (OC) 4MC(η 2-NMeCH 2CHCHCH 2OH)CH 2SiMe 3 (M=Cr: 9; M=W: 10). The complexes 1– 10 were analyzed in solution by one- and two-dimensional NMR spectroscopy ( 1H, 13C, 29Si, 1H/ 1H COSY, 1H/ 1H NOESY, 13C/ 1H HETCOR).

High Diastereoselective Michael and Aldol Additions of Fischer‐Type Alkyl(hydrazino)carbene Complexes: Synthesis of New Hydrazides.

Abstract The addition of the enolate, generated from the tetracarbonylethyl(hydrazino)carbene chromium complex 6 , to achiral enones and aldehydes gave the corresponding Michael and aldol adducts in very high chemical yield and d.e.. Some of the new δ-keto and β-hydroxy hydrazino carbene complexes have been oxidised to give the corresponding hydrazides in high yield. A spectroscopic study to establish the geometry of the enolate generated from 6 has also been performed.

New Fischer Carbene Complexes of Rhodium(I): Preparation and 2-Cyclopentenone Ring Synthesis by Annelation to Alkynes

A new type of metal carbene complexes of group 9, specifically a cationic Fischer carbene of rhodium(I), has been synthesized from chromium carbene complexes via double transfer of carbene and CO ligands. These complexes reveal a different reactivity than other transition metal carbenes, including their chromium precursors, toward neutral and electron-poor alkynes, giving selectively polysubstituted cyclopentenones.

Asymmetric dearomatization of the furan ring promoted by conjugate organolithium addition to (menthyloxy)(3-furyl)carbene complexes of chromium.

The sequential low-temperature addition reaction of an organolithium compound and methyl triflate to (menthyloxy)(3-furyl)carbene complexes of chromium and tungsten proceeded with excellent regioselectivity (1,4-addition) and diastereoselectivity (2,3-trans disposition of the nucleophile and electrophile groups) to afford new 2,3-disubstituted (2,3-dihydro-3-furyl)carbene complexes. In addition, a high degree of diastereofacial selectivity was achieved by employing alkenyllithium compounds. After detachment of both the metal fragment and the chiral auxiliary group, trisubstituted 2,3-dihydrofuran derivatives containing a quaternary stereogenic center at the C3 position were obtained. The characterization, including X-ray crystallography, of a novel type of stable four-membered chelate (eta(2)-alkene)tetracarbonylcarbene complex of chromium is also reported.

Amine‐Catalyzed Coupling of Aldehydes and Ketenes Derived from Fischer Carbene Complexes: Formation of β‐Lactones and Enol Ethers.

AbstractFor Abstract see ChemInform Abstract in Full Text.

High diastereoselective Michael and aldol additions of Fischer-type alkyl(hydrazino)carbene complexes: synthesis of new hydrazides

The addition of the enolate, generated from the tetracarbonylethyl(hydrazino)carbene chromium complex 6, to achiral enones and aldehydes gave the corresponding Michael and aldol adducts in very high chemical yield and d.e.. Some of the new δ-keto and β-hydroxy hydrazino carbene complexes have been oxidised to give the corresponding hydrazides in high yield. A spectroscopic study to establish the geometry of the enolate generated from 6 has also been performed.

Nickel(0)-mediated [3+2+2] and [2+2+2+1] cyclization reactions of chromium Fischer carbene complexes and alkynes.

Dramatic changes in Fischer carbene reactivity occur in the presence of [Ni(cod)2]. Thus, instead of Dötz [3+2+1] benzannulation giving substituted phenols, alkenyl carbene complexes incorporate two equivalents of alkyne in a [3+2+2] cyclization, while simple chromium carbene complexes undergo a [2+2+2+1] cyclization (see scheme). These cyclizations are regio- and diastereoselective. cod=1,5-cyclooctadiene.

Amine-catalyzed coupling of aldehydes and ketenes derived from Fischer carbene complexes: formation of beta-lactones and enol ethers.

Aldehydes react with ketenes generated from photolysis of Fischer chromium carbene complexes to generate either beta-lactones or enol ethers resulting from decarboxylation of beta-lactones. The reaction is catalyzed by tertiary amines and can occur with diastereoselectivity greater than 20:1 with DMAP as the catalyst.

Construction of [4.3.1]Propellanes via Molybdenum Fischer Carbene Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A Simple Approach toward Enantiomerically Pure Fischer Carbene Complexes of Chromium and Molybdenum: Chiral Modification of the Metal Fragment

Replacing two carbonyl ligands of the common pentacarbonyl fragment by the suitable chiral biphosphite ligand 2 allows for the novel synthesis of stable chiral Fischer carbene complexes of molybdenum and chromium. In the case of the phenyl molybdenum complex 1, this exchange reaction yields complex fac-3, which can be thermally isomerized to mer-3. The thermal reaction exchange of methyl, phenyl, and alkenyl chromium carbene complexes 4a-c with ligand 2 affords directly the complexes mer-5a-c. Two of the novel complexes, fac-3 and mer-5a, have been characterized in the solid state.

Fischer Chromium Carbene Complexes as Nucleophiles in Palladium‐Catalyzed Allylic Substitution Reactions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

New Rearrangement and Fragmentation Processes of Group 6 Alkoxyalkynyl (Fischer) Carbene Complexes Induced by Aromatic Diamines

The rearrangement of chromium(0) carbene complexes 4 to isocyanides 5 occurs either thermally or by SiO2 catalysis through 1,2-diaminobenzene addition followed by breakage of the bicyclic carbene intermediate 11. Complexes 4 with 1,8-diaminonaphthalene lead in very smooth conditions to 2-substituted perimidines 9 with good yields through an unprecedented cleavage of the carbon skeleton of the metal complex in a formal retro-Aumann reaction.

Construction of [4.3.1]propellanes via molybdenum fischer carbene complexes.

Treatment of molybdenum Fischer carbene complexes with 6-methylene-7-octen-1-yne derivatives at 40 degrees C generates substituted tricyclo[4.3.1.0(1,6)]deca-2,4-dienes in good yield. Pentacarbonyl(butylmethoxycarbene)molybdenum(0) afforded the highest cyclization yields (54%), while the analogous chromium carbene complex gave no reaction. The range of dienyne substrates that participate in this reaction is explored and its mechanism is analyzed and discussed.

Fischer chromium carbene complexes as nucleophiles in palladium-catalyzed allylic substitution reactions

The Pd-catalyzed reaction of carbanions derived from chromium aminocarbene complexes with allylic acetates and carbonates smoothly affords the corresponding allyl-substituted aminocarbenes. On the contrary, the same reaction of the anion derived from pentacarbonyl[(methoxy)methylcarbene]chromium(0) with cinnamyl acetate affords 5-methoxy-1-phenylhexa-1,5-diene, the product of 1-methoxyethen-1-yl group transfer.

ChemInform Abstract: Synthesis of 2‐Pyranylidene or (2‐Furyl)carbene—Chromium Complexes from Conjugated Enyne Carbonyl Compounds with Cr(CO)5(THF).

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Coupling of ferrocenyl chromium carbene complex with cyclobutenediones

The coupling of ferrocenyl chromium carbene complex with cyclobutenediones leads to ferrocenyl-substituted 5-alkylidenefuranones and 4-cyclopentene-1,3-diones, methyl ferrocenoate and acetylferrocene in varying amounts. The scope and limitations of these processes are investigated. In comparison with the phenyl analog, ferrocenyl chromium carbene complex has been found to be less reactive. This is also supported by PM3 calculations.

Cross metathesis reactions using γ,δ-unsaturated chromium carbene complexes

Cross metathesis reactions of γ,δ-unsaturated carbene complexes have been explored. Successful cross metathesis was observed in cases involving electron neutral alkenes and allylic ether/ester derivatives. Attempted metathesis reactions using ethyl acrylate were not competitive with cyclopropanation reactions.

ChemInform Abstract: 2‐ and 3‐Haloalkoxy Fischer Carbene Complexes of Chromium as Synthons for Either Hydroxycyclopropanation or Oxaspirocyclopropanation of Alkenes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Cyclopropanation of alkenes mediated by novel chiral Fischer carbene complexes

The cyclopropanation reaction using chiral-at-metal Fischer carbene complexes is examined. Racemic molybdenum and chromium carbene complexes fac- and mer-1 and mer-2, respectively, undergo cycloaddition to acrylonitrile, methyl acrylate and 1-hexene to give cylopropanes 3 and 4 with trans/cis ratio up to 4.7:1. The cyclopropanation of acrylonitrile with the enantiopure carbene complex (+)-mer-1 resulted in a modest asymmetric induction of 2:1 for the trans- isomer 3a.