Photolysis and Thermolysis of N-Alkoxycarbonyl-α-diazoamide Acetals

In the photoysis and thermolysis of 1-phenyl-2,2-dimethoxy-2-(N-alkoxycarbonylamino)-diazoethanes the presence of the N-alkoxyearbonylamino substituent is of fundamental importance. In both cases the carbene intermediate, through intramolecular addition to carbonyl oxygen, gives, as main products, oxazole derivatives. In the thermolysis, especially at relatively low temperature (160°), the diazoamide acetals in part cyclize to form triazole derivatives.     

Die Gasphasen-Flussthermolyse von 1-Isobutenyl- und 2-Methylphenyl-alkinyl-ketonen. Eine Synthese von Methylenomycin B

The Gas-Flow Thermolysis of 1-Isobutenyl Alkynyl and 2-Methylphenyl Alkynyl Ketones. Synthesis of Methylenomycin B The gas-flow thermolysis of 1-isobutenyl alkynyl or 2-methylphenyl alkynyl ketones were found to lead to phenols and cyclopentenones or to naphthols and indanones, respectively. These conversions involve two cyclization processes so far unknown with α-alkynones; they are interpreted as intramolecular additions of an allylic or a benzylic C,H bond to a triple bond which may occur in two directions. In addition, the cyclopentenones formed by the α-alkynone cyclization, a known carbene process yielding 5-rings, were also found. The available evidence ruled out a carbene process yielding 6-rings. The addition process yielding 5-rings was applied to a short (but low yield) synthesis of methylenomycin B .     

Thermolytic removal of t-butyloxycarbonyl (BOC) protecting group on indoles and pyrroles

The t-Butyloxycarbonyl (BOC) group on indoles and pyrroles can be removed cleanly and in high yield by simple thermolysis: no acid, no base, no solvent is required.     

Central-to-axial chirality transfer in the benzannulation reaction of optically pure Fischer carbene complexes in the synthesis of allocolchicinoids

A method for the synthesis of allocolchicinoids is explored that involves the benzannulation reaction of Fischer chromium carbene complexes with alkynes. The benzannulation reaction is employed to install the aromatic C-ring via the reaction of an α,β-unsaturated carbene complex in which the carbene complex is attached to a seven-membered ring that is to become the B-ring of the allocolchicinoids. Two different regioisomeric series can be accessed depending on which position the carbene complex is on the seven-membered ring. A key issue that is addressed is the stereochemistry of the newly formed axis of chirality that results from a stereo-relay from an existing chiral center on the seven-membered ring at the position destined to be C(7) in the allocolchicinoids. The level of stereochemistry is dependent on the position of the carbene complex on the seven-membered ring. A mechanism is proposed to account for this stereochemical dependence and to account for the observed effects of temperature and solvent on the stereoselectivity. Finally, the benzannulation reactions of optically pure complexes are examined and quite surprisingly one, but not both, of the diastereomeric products is racemized. The racemization can be prevented with the proper choice of solvent and temperature. A mechanism is proposed to account for the racemization of only one of the diastereomers of the product that involves the intermediacy of an o-quinone methide chromium tricarbonyl complex.     

Determinations cinetiques par analyse microcalorimetrique differentielle. XV. Thermolyse de peroxydes de benzoyle p,p′-disubstitués en solution dans l'ether de diphenyle

Differential scanning microcalorimetric and chemical studies of the thermolysis of some p,p′-disubstituted benzoyl peroxides have been performed in diphenyl oxide as solvent. They have shown that the decomposition proceeds only via the homolysis of the peroxide bond; neither free radical induced nor autocatalytic processes occur. The activation parameters of the thermolysis of the peroxides have the same values as those found when propiophenone is used as solvent. Under these conditions, the free enthalpy of activation, ΔG^≠, is the intrinsic measure of the thermal stability of benzoyl peroxide and its p,p′-disubstituted derivatives.     

Palladium clusters: Stoichiometric and catalytic reactions

The mechanistic aspects of organic reactions catalyzed with palladium clusters and stoichiometric reactions of carbonyl and carbene clusters are discussed. Palladium carbonyl carboxylates Pd₄(CO)₄ (OCOR)₄(RMe, CMe₃, Ph, CF₃, CCl₃) undergo thermolysis above 110–130°C, giving rise to CO₂, CO and diacyls. In solutions of aromatic compounds the insertion of carbon dioxide into the aromatic CH bond or activated CH bond of alkylaromatic compounds was observed in the course of the thermolysis. The decomposition of palladium carbene carboxylate cluster Pd₄(Ph₂C)₄(OAc)₄ at 80°C has been found to involve inner sphere carbene oxidation during which an oxygen atom is transferred from the carboxylate group to the carbene ligand. Analogously, the reaction of [Pd(OAc)₂PPh₃]₂ with formic acid, a reaction involving intermediate cluster formation, includes the transfer of an oxygen atom from the formate droup to the P atom of a phosphorus containing ligand, supposedly a diphenylphosphido bridging group. Positional and geometric α-alkene isomerization in aqueous PdCl²⁻₂ solution has been found to be catalyzed by palladium (I) complexes of type Pd₂Cl²⁻₄. Colloidal clusters containing more than 500 palladium atoms in the metal core, which are soluble in polar organic solvents, have been found to catalyze the oxidative reactions of alkenes, toluene, alcohols and formic acid. Alcohols bearing at least one hydrogen atom in α-position undergo dehydration under mild conditions in non-acidic solution containing a Pd, Mo octanuclear anionic cluster [Pd₄Mo₄(CO)₁₂Cp]²⁻. The reaction of benzyl alcohol gives rise to trans-stilbene. All these catalytic reactions can be rationalized within a scheme including the oxidative addition of the substrates across cluster metal-metal bond as a key step.     

The reaction of intermediate zirconocene-aryne complexes with CH bonds in the thermolysis of diarylzirconocenes

Upon thermolysis in aromatic hydrocarbon solvents diarylzirconocenes undergo successive replacement of their σ-bonded ligands by aryl groups from the solvent. Analysis of the product mixture by photochemical degradation to biphenyls reveals that intermediate aryne zirconocene complexes are formed which then react with the solvent with CH bond fission and formation of new ZrC bonds. A β-hydride elimination process can be excluded for the formation of the aryne complexes and it is suggested that they are formed via abstraction of an ortho proton from one of the aryl ligands by the other σ-bonded group.     

Synthesis of dimethyl substituted benzimidazoles containing cyclopropane fused onto five to eight membered [1,2-a]alicyclic rings and influence of methyl group substituents on cytotoxicity of benzimidazolequinones

Upon thermolysis 5,6-dimethyl-N-[(allyl, but-3-enyl, pent-4-enyl and hex-5-enyl-benzimidazol-2-yl)methylene]-(trans)-2,3-diphenylaziridin-1-amines (Eschenmoser hydrazones) form cyclopropane fused onto pyrrolo-, pyrido-, azepino- and azocino[1,2-a]benzimidazoles in 70, 50, 77 and 11% yield, respectively. The latter reaction also gave carbene insertion products. Dimethyl group substituents were found to significantly reduce the cytotoxicity of benzimidazolequinone towards human skin fibroblast cells.     

Polyisobutylene-supported N-heterocyclic carbene palladium catalysts

This paper describes how the nonpolar polymer polyisobutylene (PIB) can be used as a handle to prepare PIB-bound NHC ligands that are soluble in monophasic mixtures of mixed solvents but phase separable when such solvent systems are perturbed to be biphasic. The results here show that such PIB-bound NHC ligands can be used to synthesize useful palladium catalysts. In this paper, both PIB-bound analogs of an N,N′-bis(2,6-diisopropylphenyl) heterocyclic carbene and simpler N,N′-dialkyl heterocyclic carbene ligand were prepared and were successfully used to form palladium cross-coupling catalysts. The reactivity, recycling and reusability of these catalysts has been examined.     

Synthesis of monodisperse chromium nanoparticles from the thermolysis of a Fischer carbene complex

We successfully synthesized monodisperse chromium nanoparticles from the thermolysis of a Fischer carbene complex.     

Preparation of diphenylphosphinothioylphenyldiazomethane and its thermolysis and photolysis involving unusual sulfur migration reaction

Diphenylphosphinothioylphenyldiazomethane was prepared and its thermolysis and photolysis were carried out; the results indicated that these reactions proceed through formation of carbene, 1,2-sulfur migration, and 1,2-phenyl migration.     

Design and synthesis of new bidentate alkoxy-NHC ligands for enantioselective copper-catalyzed conjugate addition

A new family of chiral alkoxy-N-heterocyclic carbene (NHC) ligands has been designed for the enantioselective copper-catalyzed conjugate addition of dialkylzincs to enones. These new bidentate NHC ligands were synthesized in high overall yields using a five-step procedure starting from commercially available β-aminoalcohols. Influence of the temperature, base, solvent and copper source were studied in order to optimize the stereoselectivity of the addition. High reactivity and excellent enantioselectivity were obtained at ambient temperature with a range of cyclic enones and dialkylzinc. Addition to acyclic enones has also been studied.     

Preparation of organometallic uracil-analogue Fischer carbene complexes: Comparative study of conventional heating vs microwave irradiation

Mono and disubstituted ureas react with alkynyl Fischer carbene complexes to give mono and di N,N-substituted organometallic uracil analogues. An optimization of the process using different starting metal carbene complexes and variously substituted ureas under conventional heating (with and without solvent) and microwave irradiation techniques is reported. The synthesis of the metal-carbene analog of the commercially available dimethyl uracil is reported.     

Photochemistry of (η5)-C5H5)Mn(CO)2[C(C6H5)2]. Generation of the diphenylcarbene radical anion

The electronic absorption spectrum of (η⁵-C₅H₅)Mn(CO)₂[C(C₆H₅)₂]shows an intense maximum which is assigned to a MLCT transition in which the empty p_π orbital on the carbene carbon is populated. Upon irradiation of this band, the complex undergoes a decomposition with a disappearance quantum yield Φ = 0.10 ± 0.01 independent of solvent. In the CT excited state, the complex can be roughly described as containing d⁵ Mn^II and a diphenylcarbene radical anion ligand C(C₆H₅)₂^?. Due to the kinetic lability, the complex decomposes producing a Mn^II species and the free carbene radical anion, which then undergoes secondary reactions. In addition, small amounts of substitution product are observed. It is proposed that prior to total decomposition of the excited state, a radical pair (η⁵-C₅H₅)Mn(CO)₂S⁺/C(C₆H₅)₂^?forms (S = solvent). A back electron transfer from C(C₆H₅)₂^?to the labile cation competes with decomposition to produce the substituted complex and free carbene.     

Total Synthesis of (±)-Clovene. On the Stereoelectronic Requirements of the α-Alkynone Cyclisation

The synthesis of the sequiterpenoid tricyclic hydrocarbon (±)-cleovene ( 1 ) by application of the α-alkynone cyclisation is described. The starting bicyclic carboxylic acid 2 was obtained from ethyl 3-methyl-2-oxocyclohexane-1-carboxylate by modified known methods (24%) and converted to the α-alkynone 3 (86%). The thermolysis of 3 in the gas phase at 620° selectively produced the tricyclo[6.3.1.0^1,5] dodecenone 4 (80%) which was converted to 1 (37%) by conventional procedures. The selectivity of the α-alkynone cyclisation is discussed in terms of the stereoelectronic requirements (coplanarity factor) of the carbene insertion. In order to throw further light on the importance of this factor, the (1-adamantyl) alkynone 16 was synthesised from adamantane-1-carboxylic acid (78%) and subjected to thermolysis at 620°. Since this led to the tetracyclo[6.3.1.1.1^3,10.0^3,7]tridecenones 17 and 18 (together 72%), we conclude that the planar carbene insertion transition state, while preferred, is not a stringent requirement.     

Dinuclear gold(I)-N-heterocyclic carbene complexes: Synthesis,characterization, and catalytic application for hydrohydrazidation of terminal alkynes

Dinuclear gold(I)-N-heterocyclic carbene complexes were developed for the hydrohydrazidation of terminal alkynes. The gold(I)-N-heterocyclic carbene complexes 2a-2b were synthesized in good yields from silver complexes synthesized in situ, which in turn were obtained from the corresponding imidazolium salts with Ag₂O in dichloromethane as a solvent. The new air-stable gold(I)-NHC complexes, 2a - 2b, were characterized using NMR spectroscopy, elemental analysis, infrared, and mass spectroscopy studies. The gold(I) complex 2a was characterized using X-ray crystallography. Bis-N-heterocyclic carbene–based gold(I) complexes 2a - 2b exhibited excellent catalytic activities for hydrohydrazidation of terminal alkynes yielding acylhydrazone derivatives. The working catalytic system can be used in gram-scale synthesis. In addition, the catalytic reaction mechanism of the hydrohydrazidation of terminal alkynes by gold(I)-NHC complex was studied in detail using density functional theory.     

Skattebol type rearrangement via α-bromotrimethylstannyl cyclopropane thermolysis. What is the mechanism?

Solution thermolysis of the epimers 5 gave rise to separate sets of products, including, for one, Skattebol-type rearrangement. However, neither epimer produced a free carbene.     

A novel chelating acid-assisted thermolysis procedure for preparation of tin oxide nanoparticles

Pure tin dioxide (SnO₂) nanoparticles were synthesized via thermolysis of tin phthalate and tin oxalate in the presence of oleic acid (OA) as solvent. Oleic acid (OA) was employed as an organic solvent, which can be applied to control particle growth and to stabilize the particles. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. The orthorhombic phase SnO₂ nanoparticles with average size about 12 nm were synthesized through thermolysis of tin phthalate in the presence of oleic acid.     

Oxidative addition of aryl chlorides to palladium N-heterocyclic carbene complexes and their role in catalytic arylamination

Density functional theory has been used to investigate various solvated species that may be formed from palladium bis N-heterocyclic carbene complexes, [Pd(cyclo-C{NRCH}₂)₂], (PdL₂) in benzene solution. Formation of an η²-arene complex is shown to stabilise a monocarbene species, PdL(η²-C₆H₅X), where the arene is either the solvent or a reacting aryl halide. Oxidative addition of an aryl chloride has been modelled, and the most likely transition state has been established as a PdL(arylchloride) species, with just one carbene ligand coordinated to the palladium. The catalytic cycle for aryl amination has been investigated and the oxidative addition of the aryl halide shown to be the rate determining step. Reductive elimination of the aryl amine has a lower activation energy. Oxidative addition of alkyl halides has been shown to be less favourable because of the absence of an unsaturated group, such as the aryl ring, to bond to the palladium.     

Chemistry of diazocarbonyl compounds: XXVII. Thermolysis and photolysis of diazirines, derivatives of 1,3-dioxane-4,6-dione

Photolysis of diazirines, 1,3-dioxane-4,6-dione derivatives, occurs in the presence of methanol or dimethyl sulfide as carbene traps without a formation of carbene intermediates. It was established for the first time that the thermolysis and photolysis of diazirines from the 1,3-dioxane-4,6-dione series led mainly to Wolff rearrangement with a subsequent formation of 5-oxo-1,3-dioxolane-4-carboxylic acids or their derivatives. The data obtained show that the α-oxodiazirines are the key intermediates in the photolysis and Wolff rearrangement of α-diazocarbonyl compounds.