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).     

Reaktionen von komplexliganden: XXVII. Cycloaddition von carbonyl-carben-komplexen mit alkinen: cyclobutenon-bildung versus carbenanellierung

Upon reaction with tolan pentacarbonyl[1,3-difluorophenyl(methoxy)carbene]-chromium undergoes a [2 + 1 + 1]cycloaddition to give the cyclobutenone skeleton while the 2,6-dimethyl(phenyl)carbene analogue leads to the carbene annulation product.     

Reaktionen von komplexliganden : XIX. Einschiebung von inaminen in chrom-alkenylcarben- und chrom-aminocarber-bindungen

1-Diethylaminopropyne reacts with pentacarbonyl[methoxy(2,2-diphenylethenyl)carbene]chromium in a stereoselective way to give pentacarbonyl-[diethylamino-E-(2-methoxy-1-methyl-4,4-diphenyl-1,3-butadienyl)carbene]-chromium via insertion of the alkyne into the metal-carbene bond. The reaction of the ynamine with pentacarbonyl[methylamino(phenyl)carbene]chromium results in insertion of the alkyne and loss of carbon monoxide to give cis-tetracarbonyl[3-aza-1-methyl-2-phenyl-2-butenyl)diethylaminocarbene]-chromium. Its structure was established by oxidative degradation in an aqueous medium to give 2-benzoyl-N,N-diethylpropanamide and finally confirmed by an X-ray analysis. The new compounds are characterized spectroscopically.     

Einschiebung von inaminen in metall—carbenkohlenstoff-bindungen

Pentacarbonyl[methoxy(methyl)carbene]chromium(0) and pentacarbonyl[methoxy(phenyl)carbene]chromium(0) react in hexane at room temperature with diethylaminoethyne and l-diethylamino-1-propyne to give $\text{1}\text{1}$ compounds. These are identified as insertion products of the alkyne entity into the metal—carbene bond by spectroscopic methods.     

Diastereoselective synthesis of three-, five-, six-, and seven-membered rings from Fischer carbene complexes and 4-unsubstituted 1-amino-1,3-dienes

Fischer carbene complexes react with 4-unsubstituted 1-amino-1,3-dienes to give different carbocyclization products depending on the nature of the carbene complex and on the substitution pattern of the aminodiene. Thus, the reaction of arylcarbene chromium complexes and 1-aminodienes diastereoselectively affords cyclopropane derivatives by means of a formal [2+1] carbocyclization reaction. In particular, pentacarbonyl[(2-furyl)(methoxy)methylene]chromium complex furnishes formal [4+1] carbocyclization products. Starting from beta-substituted alkenylcarbene complexes, formal [4+1] reactions occur and cyclopentenamine derivatives are diastereoselectively formed. However, when the alpha,beta-disubstituted alkenylcarbene complex pentacarbonyl[(5,6-dihydro-2H-pyran-2-yl)(methoxy)methylene]tungsten is used, the outcome of the reaction depends on the substitution on the carbon atom at the 3-position of the aminodiene, generating the [3+2] or [4+3]-cyclization products if the substituent is or is not a hydrogen atom, respectively. Finally, when the reaction is performed with alkynylcarbene complexes, benzaldehyde derivatives are obtained if the triple-bond substituent is a phenyl group or indene derivatives if the group is an alkenyl moiety.     

Basicity of nucleophilic carbenes in aqueous and nonaqueous solvents-theoretical predictions

The complete basis set method CBS-QB3 was used in conjunction with the CPCM solvation model to predict both the absolute and relative pKa's of 12 nucleophilic carbenes in dimethyl sulfoxide (DMSO), acetonitrile (MeCN), and water. Average absolute pKa values in DMSO ranged from 14.4 +/- 0.16 for 3-methylthiazol-2-ylidene (12) to 27.9 +/- 0.23 in the case of bis(dimethylamino)carbene (11), while values in MeCN were determined to be between 25.7 +/- 0.16 (12) and 39.1 +/- 0.25 (11). Relative pKa calculations yielded similar results. Calculations in aqueous solution gave pKa's between 21.2 +/- 0.2 (12) and 34.0 +/- 0.3 (11). Excellent agreement between calculated and experimental pKa's was obtained for the few cases where experimental numbers are available, confirming that this theoretical approach may be used to calculate highly accurate pKa values.     

Übergangsmetall-Komplexe des 1,1,3,3-Tetrakis(dimethylamino)-1λ5,3λ5-diphosphets

Transition Metal Complexes of 1,1,3,3-Tetrakis(dimethylamino)-1λ⁵,3λ⁵-Diphosphete 1,1,3,3-Tetrakis(dimethylamino)-1λ⁵,3aλ⁵-diphosphete, 1 , reacts with W(CO)₆ to yield the isomeric complexes {1,1,3,3-tetrakis(dimethylamino)-1λ⁵,3λ⁵-diphosphete}(pentacarbonyl)tungsten 4 and {1,1,3,3-tetrakis(dimethylamino)-1,4-dihydro-1λ⁵,3λ⁵-[1,3]diphosphetium}(pentacarbonyl)tungsten 5 . With Cr(CO)₆ the complex {1,1,3,3-tetrakis(dimethylamino)-1λ⁵,3λ⁵-diphosphete}(pentacarbonyl)chromium 6 is formed. From the reaction products of Fe₃(CO)₁₂ and Fe₂(CO)₉ with 1 the complex {1,1,3,3-tetrakis(dimethylamino)-1λ⁵,3λ⁵-diphosphete}(pentacarbonyl)iron 7 could be isolated. Properties, nmr, ir and mass spectra of the new compounds are reported. 6 and 7 were characterized by X-ray structure determinations.     

Organometallic complexes containing the bridging group N,N′-o-phenylenebis(salicylideneimine); crystal and molecular structure of μ-N,N′-o-phenylenebis(salicylideneiminato)bis(2-C,N-acetophenoneoximepalladium(II))

The reactions of the conjugate base of (methylmethoxycarbene)pentacarbonylchromium(0) with epoxides and with methyl bromoacetate give alkylated carbene complexes which cannot be obtained by previous synthetic routes.     

Reaktionen von carben-komplexen mit tetramethylcyanoguanidin

Tetramethylcyanoguanidine, NCNC(NMe₂)₂, reacts with pentacarbonyl-[methoxy(phenyl)carbene]chromium, (CO)₅Cr[C(Ph)OMe], via substitution of the carbene ligand to give pentacarbonyl(tetramethylcyanoguanidine)chromium. X-ray structure analysis shows that the CrNCN fragment is nearly linear in the crystal, and that the CNC angle is 122.9(1)°. In solution rapid syn-anti isomerization at the NC double bond occurs. The reaction of tetramethylcyanoguanidine with pentacarbonyl[acetoxy(phenyl)carbene]-chromium and -tungsten, (CO)₅M-[C(Ph)OCO(O)Me] (M = Cr, W), yields (CO)₅M[C(PHNC(NMe₂)₂], where the acetoxy group of the carbene ligand is exchanged for the NC(NMe₂)₂ group of the cyanoguanidine.     

Photochromic dihetarylethenes. 13. Optimization of conditions for the acylation of 2,5-dimethylthiophene with squaric acid dichloride

The conditions for acylation of 2,5-dimethylthiophene with squaric acid dichloride were optimized, and 3,4-bis(2,5-dimethylthiophen-3-yl)cyclobut-3-ene-1,2-dione was obtained in good yield. X-ray diffraction analysis demonstrated that the by-product has the structure of 1a,1b-dichloro-5-(2,5-dimethylthiophen-3-yl)-3-hydroxy-4,5a-dimethyl-1b,4a,5,5a-tetrahydro-1aH-1-thiacyclopropa[a]pentalen-2-one.     

Electron delocalization in acyclic and N-heterocyclic carbenes and their complexes: a combined experimental and theoretical charge-density study

Combined experimental and theoretical charge-density studies on free and metal-coordinated N-heterocyclic carbenes have been performed to investigate the extent of electron delocalization in these remarkable species. Tracing the orientation of the major axis of the bond ellipticity (the least negative curvature in the electron density distribution) along the complete bond paths distinguishes unambiguously between fully delocalized systems and those with interrupted cyclic electron delocalization. Evaluation of charge-density-based properties such as atomic quadrupole moments serves as a direct and quantitative measure of the extent of pi-electron delocalization and reveals consistency between theory and experiment. A detailed topological analysis of theoretical charge densities for two benchmark carbene systems, viz., 1,2-dimethylpyrazol-3-ylidene 1a and 1,3-dimethylimidazol-2-ylidene 2a, and their corresponding stable chromium pentacarbonyl complexes 1 and 2, highlights the advantages of charge-density-based criteria to analyze such complex electronic situations. Thus, 1a and 2a display a different extent of electron delocalization; yet nearly identical p(pi) occupations at the carbene center are computed for 1a and 2a. However, atomic quadrupoles Q(zz) - the charge-density analogues of p(pi) occupation - reveal faithfully the electronic differences in 1a and 2a and demonstrate the sensitivity of charge-density-based properties to the bonding situation. The acyclic aminocarbene (iPr(2)N)(2)CCr(CO)(4) has also been studied, and the high barrier to rotation about the C-N bond is shown not to arise solely from p(pi)-p(pi) bonding.     

Concerted vs stepwise mechanism in 1,3-dipolar cycloaddition of nitrone to ethene, cyclobutadiene, and benzocyclobutadiene. A computational study

The problem of competition between concerted and stepwise diradical mechanisms in 1,3-dipolar cycloadditions was addressed by studying the reaction between nitrone and ethene with DFT (R(U)B3LYP/6-31G) and post HF methods. According to calculations this reaction should take place via the concerted cycloaddition path. The stepwise process is a viable but not competitive alternative. The R(U)B3LYP/6-31G study was extended to the reaction of the same 1, 3-dipole with cyclobutadiene and benzocyclobutadiene. The very reactive antiaromatic cyclobutadiene has an electronic structure that is particularly disposed to promote stepwise diradical pathways. Calculations suggest that its reaction with nitrone represents a borderline case in which the stepwise process can compete with the concerted one on similar footing. Attenuation of the antiaromatic character of the dipolarophile, i.e., on passing from cyclobutadiene to benzocyclobutadiene, causes the concerted 1,3-dipolar cycloaddition to become once again prevalent over the two-step path. Thus, our results suggest that, in 1,3-dipolar cycloadditions that involve normal dipolarophiles, the concerted path (Huisgen's mechanism) should clearly overwhelm its stepwise diradical (Firestone's mechanism) counterpart.     

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.     

Solution Structure of One-Electron Reduced and Oxidized Molecules with Twisted Donor and Acceptor Moieties

The solution structures of the radical anion and the radical cation of the donor-acceptor molecules 3,4-di(1,3-dithiol-2-ylidene)-4-phenylbut-1-ene-1,1-dicarbonitrile ( 1 ) and 3,4-di(1,3-dithiol-2-ylidene)-4-phenylbut-1-ene-1,1,2-tricarbonitrile ( 2 ) are discussed based on cyclovoltammetric and ESR/ENDOR measurements. It is shown that the spin population of the radical anions is limited to the di- and tricyanoethene moiety and the coplanar 1,3-dithiole at C(3), whereas that of the radical cations resides mainly inside the two 1,3-dithiole rings. The energies of the long-wave bands in the electronic-absorption spectra of 1 and 2 correspond to the differences between the oxidation and reduction potentials and thus point to a charge-transfer character of these transitions.     

Synthesis and intramolecular reactions of trans-cyclohexyl-1,2-bisacrylate

Photocycloaddition of dimethyl cyclobut-1-ene-1,2-dicarboxylate (1) with cyclohexene (7) afforded two photoadducts 8 and 9 in 44% and 28% yields, respectively. Spontaneous thermal isomerization of 8 gave (4Z,10Z)-dimethyl cyclodeca-4,10-diene-1,4-dicarboxylate (10), which subsequently isomerized to produce trans-1,2-cyclohexanebis-alpha-acrylic acid dimethyl ester 11. Hydride reduction of the bisacrylate 11 gave the trans-octahydro-1H-inden-2-ols 12a and 15 via a novel, stereoselective, intramolecular reaction. Reaction of the bisacrylate 11 with methyllithium afforded the bis-tertiary alcohol 16. In contrast, lithium dimethylcuprate reacted with the bisacrylate 11 to give the trans-hexahydro-1H-inden-2-one 17 in high yield via a novel, stereoselective, intramolecular reaction.     

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.     

Five-Membered 2,3-dioxoheterocycles: XCV. Recyclization of 4-benzoyl-5-phenylfuran-2,3-dione at the action of substituted 1,3,3-trimethyl-2-azaspiro[4.5]dec-1-enes. Crystal and molecular structure of substituted 5-(2-azaspiro[4.5]dec-1-ylidene)cyclopent-3-ene-1,2-dione

4-Benzoyl-5-phenylfuran-2,3-dione reacts with 2′,5′,5′-trimethyl-4′,5′-dihydro-4H-spiro[naphthalene-1,3′-pyrrol]-4-one and 8-(2-methoxy-5-methylphenyl)-1,3,3,9-tetramethyl-2-azaspiro[4.5]deca-1,7-dien-6-one with the formation of (Z)-3-benzoyl-5-(5′,5′-dimethyl-4-oxo-4H-spiro[naphthalene-1,3′-pyrrolidin]-2′-ylidene)-4-phenylcyclopent-3-ene-1,2-dione, whose structure was proved by XRD analysis, and of (Z)-3-benzoyl-5-{8-(2-methoxy-5-methylphenyl)-3,3,9-trimethyl-6-oxo-2-azaspiro[4.5]dec-7-en-1-ylidene}-4-phenylcyclopent-3-ene-1,2-dione.     

Aromaticity of butalene and its homologues

[structure: see text] The aromaticity of a series of fused cyclobutadiene systems is discussed in terms of their resonance energies. While there is considerable variation in their resonance energies per pi electron, all members of the series are calculated to be antiaromatic, though to a lesser degree than the parent cyclobutadiene.     

Dichotomy within 1,4-addition of organolithium and Grignard reagents to α,β-unsaturated Fischer alkoxycarbenes: A new synthesis of Fischer carbenes

The reaction of organolithium and Grignard reagents with pentacarbonyl[(ethoxy)(2-phenylethenyl)carbene]chromium(0) gave, after the quenching of the initially formed product of the 1,4-addition, different products depending on the organometallic and quenching reagents used. The addition of organolithiums, followed by a work-up with acid (AcOH or HCl), afforded the corresponding carbene complexes. In contrast, quenching the reaction mixture with ethanol led to the stereoselective formation of (Z)-enol ethers. The usage of Grignard reagents led to the formation of the carbene complexes regardless of which quenching reagent was used.     

Organometallic dehydro[14] annulenes containing Vollhardt's cyclobutadiene: are CpCo-complexed cyclobutadienes more aromatic then benzene?

Pd-catalyzed coupling of 1,2-diethynyl-3,4-(bistrimethylsilyl)cyclobutadienecyclopentadienylcobalt to a series of 1-iodo-2-(trimethylsilylethynyl)benzenes and 1-chloro-4-trimethylsilylbut-1-ene-3-yne is followed by desilylation with potassium carbonate. Cu(OAc)(2)-promoted oxidative ring closure leads to dehydro[14]annulenes and dehydro[14]benzoannulenes fused to a cyclobutadiene(cyclopentadienylcobalt) complex. Five of these fused dehydroannulenes were structurally characterized. (1)H NMR spectroscopy of the organometallic dehydro[14]annulenes incorporating the (bistrimethylsilyl)cyclobutadiene(cyclopentadienylcobalt) unit suggested that the aromaticity of the fused cyclobutadiene complex might be stronger than that of benzene according to the ring-current criterion.