Reactions Of Fischer Carbene Complexes Research Articles

ChemInform Abstract: Intermolecular and Regioselective Access to Polysubstituted Benzo‐ and Dihydrobenzo[c]azepine Derivatives: Modulating the Reactivity of Group 6 Non‐Heteroatom‐Stabilized Alkynyl Carbene Complexes.

AbstractA three‐component reaction of Fischer carbene complexes (I), lithium acetylides and benzaldehyde imines offers an efficient approach to stable tungsten containing benzazepinium zwitterions.

Rational Synthesis for All All‐Homocalixarenes

Calixarenes and related compounds are established scaffolds useful as ligands and very popular as building blocks for molecular design. At the same time, the use of these compounds is virtually always defined by the ability of researchers to prepare them efficiently in useful quantities, and in a controlled fashion. As an example, calix[4]arenes, calix[6]arenes, and calix[8]arenes gained popularity very quickly after Gutsche et al. developed and perfected onestep methods for their preparation. Fitting geometry, namely appropriate ring size and dominant conformation, are of paramount importance in most macrocycle applications, yet the overwhelming majority of publications still only report modifications of the three above-mentioned easily prepared calixarene cores. Homocalixarenes have been a subject of scientific interest for a couple of decades. Whereas calixarenes have only one carbon atom between the aromatic rings, the presence of extra linkers between the aromatic rings allows the tuning of the desired cavity size, symmetry, and conformational mobility of the molecule. All-homocalixarenes have been defined by Vcgtle and co-workers as those in which all aromatic rings are separated by two carbon atoms. All-bis(homocalixarene)s are those separated by three carbon atoms, All-tris(homocalixarene)s separated by four carbon atoms, etc. We use the term homocalixarenes to refer to all calixarenes which have tether lengths greater than one carbon atom for at least one of the tethers and all-homocalixarenes for all calixarenes in which each of the tethers is greater than one carbon atom. The most direct method for the synthesis of all-homocalixarenes is the M ller–Rcscheisen cyclization which gives mixtures of calixarenes in low yields. All-homocalixarenes have also been made in a multistep synthesis by sulfur extrusion upon thermolysis of the corresponding sulfone. Several methods have been used in the synthesis of all-bis(homocalixarene)s, including alkylation of bis(bromomethyl) benzenes with tosylmethyl isocyanide and diethyl malonate, a Claisen rearrangement, and aryllithium/alkyl halide coupling. Larger all-homocalixarenes (n 4) have been made by the reaction of a bis(benzyllithium) with a,w-dihalides, and by the trimerization of a Fischer carbene complex. Homocalixarenes with heteroatoms in the linkers are much more widely studied compared to those with all-carbon linkers. The most popular of these are the bis(homooxacalix[3]arene)s which have enjoyed significant attention from researchers only after the discovery of a direct method for their preparation. Earlier reports from our laboratory have shown that the benzannulation reaction of Fischer carbene complexes can be used to synthesize paracyclophanes with various tether lengths. This chemistry was extended to the synthesis of calix[4]arenes, having various substituents, in a highly regiocontrolled triple annulation, and later extended to chiral calix[4]arenes. The purpose of the present work is to test the viability of this method to serve as a very general approach to all-homocalixarenes, having tethers with any number of desired carbon atoms, on a gram scale. One can envisage that both the homocalix[4]arenes 3 and homocalix[3]arenes 5 can be prepared by the triple annulation method (Scheme 1) involving the formation of two

Palladium‐Catalyzed Amidation of N‐Tosylhydrazones with Isocyanides

Aminocarbonylation of N-tosylhydrazones: The direct formation of ketenimines from carbenes and isocyanides is limited to the reaction of Fischer carbene complexes or some specially stabilized carbenes with isocyanides. A Pd-catalyzed amidation of N-tosylhydrazones with isocyanides via a ketenimine intermediate in the presence of water is described. The method offers a general way to synthesize amides from carbonyl compounds through one-carbon extension (see scheme).

Reaction of Fischer Alkynyl Carbene Complexes with Fluorenone Imines: Mechanistic Studies

The reaction of Fischer carbene complexes with imines to yield fluorenylidene-pyrroline molecular switches has been explored. A combined experimental and theoretical study was carried out to understand the reaction mechanism. Experimental and theoretical data are in good agreement, which allows delimiting the reaction outcome. Nucleophilic attack of the imine to alkyne is the first step in the reaction path and the one with a higher energy barrier. The most stable intermediate was isolated from the reaction mixture, thus supporting the proposed mechanism. The different reaction outcome when using fluorenone or benzophenone imines was investigated with diverse imine moieties. The effect of substitution in both imine and carbene complexes was also explored in order to increase the reaction’s scope.

Alkyne Competition in the Benzannulation Reaction with Chromium Carbene Complexes

The benzannulation reaction of Fischer carbene complexes is investigated under conditions where the reaction of the carbene complex is occurring in the presence of two different alkynes. A series of competition experiments are examined where the effects of various structural factors are explored by pitting 10 different carbene complexes with 11 different alkynes. Terminal alkynes will react selectively over internal alkynes in all cases examined including both aryl and alkenyl complexes. Aryl carbene complexes with methoxy substituents do not give quite as high selectivity for terminal alkynes over internal alkynes ( approximately 95:5) as do isopropoxy substituents (>99:1), whereas most alkenyl complexes give high selectivity with both substituents (>99:1). Competition experiments between two different terminal alkynes or between two different internal alkynes did not result in anything more than very modest selectivities at best ( approximately 2:1). Excellent selectivities were realized between two different terminal acetylenes if one of the terminal acetylene was protected with a trimethylsilyl group. Finally, it was demonstrated that the high selectivities between terminal and internal alkynes can be utilized in the reaction with molecules that contain both types of alkyne functions.

On the Mechanism of Cyclization of 5-Hexenylchromate Intermediates in the Reactions of Fischer Carbene Complexes with a Lithium Enolate and Allylmagnesium Bromide

The mechanisms for the evolution of pentacarbonyl-5-hexenylchromate complexes, unsubstituted and methyl substituted at C2, formed from a pentacarbonyl(alkoxy)carbene complex of chromium, the corresponding ketone lithium enolate, and allylmagnesium bromide, were theoretically investigated by using DFT (Density Functional Theory) at the B3PW91/6-31G* level (LANL2DZ for Cr and Br) taking into account the effect of THF solvent through the PCM model (Polarizable Continuum Model). Methyl substitution at C2 provokes a shortening of about 5 degrees in the C1-C2-C3 angle that favors the formation of the pentacyclic product. Also, the presence of this methyl substituent at C2 sterically disfavors the formation of the hexacyclic product. Thus, our results yield the hexacyclic system as the most favored product for the evolution of the unsubstituted alkylpentacarbonylchromate complex, and the pentacyclic product in the case of the substituted system, in good agreement with the experimental findings. The stereochemistry of the products experimentally observed is determined at the transition state for the migration of the Cr(CO)(5) fragment from C1 to C6 and the conformational rearrangement of the C1-C6 skeleton. Amine molecules, present in the reaction medium, can play a catalytic role by assisting the 1,2-H migration in the last step for the formation of hexacyclic products.

Carbene Transfer Reactions from Chromium(0) to Gold(I): Synthesis and Reactivity of New Fischer-Type Gold(I) Alkenyl Carbene Complexes

The synthesis of new cationic alkenyl Fischer-type gold(I) complexes through a carbene transfer reaction of chromium and tungsten alkenyl Fischer carbene complexes with LAu1Cl complexes in the presence of AgSbF 6 is reported. The stability of the resulting gold(I) carbene complexes highly depends on the nature of the ligand (L). We have used unstable gold(I) carbene complexes for developing some catalytic reactions of Fischer carbene complexes. © 2009 American Chemical Society.

Coupling of Fischer carbene complexes with conjugated enediynes featuring radical traps: Novel structure and reactivity features of chromium complexed arene diradical species

The reaction of Fischer carbene complexes with conjugated enediynes that feature a pendant alkene group has been examined. The reaction proceeds through carbene–alkyne coupling to generate an enyne–ketene intermediate. This intermediate then undergoes Moore cyclization to generate a chromium complexed arene diradical, which then undergoes cyclization with the pendant alkene group. The radical cyclization prefers the 6-endo mode unless radical-stabilizing groups are present to favor the 5-exo mode. The intermediate diradical species were evaluated computationally in both the singlet and triplet configurations. Arene triplet diradicals feature minimal spin density at oxygen and delocalization to chromium. The 6-endo cyclization product was kinetically and thermodynamically favored.

ChemInform Abstract: Thermal and Photochemical Reactions of Fischer Carbene Complexes with Trialkylsilyl‐Substituted Alkynes.

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Reactions of [Aryloxy(phenyl)carbene]pentacarbonylchromium(0) Complexes with Thiolate Ions. Decreasing Reactivity with Increasing Basicity of the Nucleophile

A kinetic study of the reactions of thiolate ions with three Fischer-type [aryloxy(phenyl)carbene]pentacarbonyl chromium(0) complexes in 50% MeCN-50% water (v/v) is reported. Brønsted plots of the second-order rate constants are biphasic with an initial steep rise for weakly basic thiolate ions (beta(nuc) approximately equal to 1.0) followed by a slightly descending leg with a negative slope (beta(nuc) approximately equal to -0.2) for strongly basic thiolate ions. This indicates a change from rate-limiting leaving group departure at low pK(RSH)(a) to rate-limiting nucleophilic attachment at high pK(RSH)(a). The negative beta(nuc) values result from a combination of minimal progress of C-S bond formation at the transition state and the requirement for partial desolvation of the nucleophile before it enters the transition state. Possible factors that may affect the degree of bond formation in reactions of Fischer carbene complexes as well as reactions of other unsaturated electrophiles with thiolate ions are discussed.

Thermal and photochemical reactions of Fischer carbene complexes with trialkylsilyl-substituted alkynes

Thermal reaction of Fischer carbene complexes with triisopropylsilyl (TIPS) substituted alkynes in benzene afforded TIPS-substituted vinylketenes or 2-TIPS-substituted cyclobutenones as major products while photochemical reaction of Fischer carbene complexes with trimethylsilyl (TMS) substituted alkynes in acetonitrile afforded 3-TMS-substituted cyclobutenones.

Extended Dötz‐Like Cyclization Reactions Towards the Synthesis of Eight‐Membered Ring‐Containing Polycycles: Scope and Theoretical Studies

The reaction between cyclobutene-containing dienyl Fischer carbene complexes (FCCs) and alkynes has been studied from both an experimental and a theoretical point of view. We have found that the reaction follows different pathways depending on the nature of the terminal double bond of the carbene complex. Thus, eight-membered carbocycles have been obtained from an extended Dötz-like reaction of dienyl FCCs and indolyl- or benzofuryl-substituted alkenyl FCCs. On the other hand, phenyl- and naphthyl-substituted alkenyl FCCs follow a typical benzannulation pathway giving rise to cyclohexadienones. DFT calculations have allowed us to formulate detailed mechanistic proposals and to provide an explanation for the formation of either the benzannulation products or the eight-membered carbocycles.

4 + 1]/[2 + 1] Cycloaddition Reactions of Fischer Carbene Complexes with α,β‐Unsaturated Ketones and Aldehydes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

4+1]/[2+1] Cycloaddition Reactions of Fischer Carbene Complexes with α,β-Unsaturated Ketones and Aldehydes

4+1]/[2+1] Cycloaddition Reactions of Fischer Carbene Complexes with α,β-Unsaturated Ketones and Aldehydes

4+1]/[2+1] Cycloaddition Reactions of Fischer Carbene Complexes with α,β‐Unsaturated Ketones and Aldehydes

4+1]/[2+1] Cycloaddition Reactions of Fischer Carbene Complexes with α,β‐Unsaturated Ketones and Aldehydes

Chromene Chromium Carbene Complexes in the Syntheses of Naphthopyran and Naphthopyrandione Units Present in Photochromic Materials and Biologically Active Natural Products

The carbene complex 5-(2,2-dimethyl-2H-chromene)methoxylmethylene chromium pentacarbonyl will undergo a benzannulation reaction with phenylacetylene, 1-pentyne, 3-hexyne, and trimethylsilylacetylene to give 7-hydroxy-10-methoxy-3H-naphtho[2.1-b]pyrans as the primary product. These compounds are difficult to obtain pure due to their sensitivity to air. If the benzannulation reaction is performed in conjunction with protection of the phenol function at C-7, then good to excellent yields of 7-alkoxy-10-methoxy-3H-naphtho[2.1-b]pyrans are afforded. If the 7-hydroxy products are captured by triflic anhydride, then the resulting aryl triflate can be used to access 3H-naphtho[2.1-b]pyrans bearing C-7 carbon substituents. The 7-hydroxy products can be oxidized to 3H-naphtho[2,1-b]pyran-7,10-diones which are stable. The chromenyl carbene complex reacts with 1,6-bis(triisopropylsilyl)-1,3,5-hexatriyne to give a 2,3-dihydro-2,2-dimethylbenzo[de]chromene, a product type that has not been seen before in the reaction of Fischer carbene complexes with alkynes. A mechanism is proposed for this process that involves alpha,beta-hydride elimination from a chromacyclobutane intermediate. Chromenyl tungsten complexes react with alkynes to give products that result from cyclization without CO insertion.

Facile and Versatile Annulation of the Imidazole Ring: Single and Sequential Cyclization Reactions of Fischer Carbene Complexes with 1,4‐Diazafulvenes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Stereoselective [4 + 1] Annulation Reactions with Silyl Vinylketenes Derived from Fischer Carbene Complexes

Stable silyl vinylketenes were prepared via the thermal reaction of Fischer carbene complexes with triisopropylsilyl- or tert-butyldimethylsilyl-substituted alkynes. The ability of these silyl vinylketenes to participate with carbenoid reagents in [4 + 1] annulation reactions was investigated. The best results were obtained with diazomethane and substituted diazomethane reagents, which provided cyclopentenone products in excellent yields and essentially complete stereoselectivity.

The First Synthesis of Cyclopropanone Acetals from the Reaction of Fischer Carbene Complexes with Ketene Acetals.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Role of Imino Group in Photochemical Reactions of Fischer Carbene Complexes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.