Ni(0)/ZnCl(2)-promoted coupling of enones and enynes. Domino process via formation of three C-C bonds and cleavage of one C-C bond

Domino coupling of enones and enynes, which proceeds via the formation of three carbon-carbon bonds and the cleavage of one carbon-carbon bond, was developed using a Ni(0) and ZnCl2 system.     

Matrix infrared spectroscopic studies of the MH-C2H3 and MH2-C2H2 intermediates in the reactions of ethylene with laser-ablated group 5 metal atoms

Reactions of ethylene with laser-ablated group 5 metal atoms in excess argon have been carried out during codeposition at 8 K, and the matrix infrared spectra of intermediate products have been investigated. Oxidative C-H insertion of the transition metal into a C-H bond occurs and beta-hydrogen transfer follows to form the dihydrido complexes (MH2-C2H2). In the Ta spectra, the dihydrido complex is the primary product, whereas the Nb and V spectra reveal absorptions from both the insertion (MH-C2H3) and dihydrido complexes. The insertion and dihydrido complexes identified here are in fact the reaction intermediates in the hydrogen elimination of ethylene proposed in previous reaction dynamics studies. Calculations also show that the higher oxidation-state complex becomes more stable relative to the insertion product going down the group 5 family.     

Acrylate formation via metal-assisted C-C coupling between CO2 and C2H4: reaction mechanism as revealed from density functional calculations

The reaction path for the formation of a binuclear hydrido-acrylate complex in a CO(2)-C(2)H(4) coupling process is explored in detail by locating the key intermediates and transition states on model potential energy surfaces derived from density functional calculations on realistic models. The formation of the new C-C bond is shown to take place via oxidative coupling of coordinated CO(2) and C(2)H(4) ligands resulting in a metalla-lactone intermediate, which can rearrange to an agostic species allowing for a beta-hydrogen shift process. The overall reaction is predicted to be clearly exothermic with all intermediates lying below the reactants in energy, and the highest barrier steps correspond to C-C coupling and beta-hydrogen transfer. The phosphine ligands are found to play an important role in various phases of the reaction as their dissociation controls the coordination of CO(2), the formation of the agostic intermediate, and the dimerization process; furthermore, their presence facilitates the oxidative coupling by supplying electrons to the metal center. Our results provide a theoretical support for the reaction mechanism proposed from experimental observations. The effect of the solvent medium on the relative energy of reaction intermediates and transition states is examined and found important in order to predict reliable energetics.     

Competition between C-C and C-H Activation in Reactions of Neutral Nickel Atom with Cycloalkanes （n = 3-7）

A theoretical investigation of the reaction mechanisms for C-H and C-C bond activation processes in the reaction of Ni with cycloalkanes C,,H2. （n = 3-7） is carried out. For the Ni ＋ CnH2, （n = 3, 4） reactions, the major and minor reaction channels involve C-C and C-H bond activations, respectively, whereas Ni atom prefers the attacking of C-H bond over the C-C bond in CnH2n （n = 5=7）. The results are in good agreement with the experimental study. In all cases, intermediates and transition states along the reaction paths of interest are characterized, It is found that both the C-H and C-C bond activation processes are proposed to proceed in a one-step manner via one transition state. The overall C-H and C-C bond activation processes are exothermic and involve low energy barriers, thus transition metal atom Ni is a good mediator for the activity of cycloalkanes CnH2n （n = 3 -7）.     

Mechanism of C[bond]H bond activation/C[bond]C bond formation reaction between diazo compound and alkane catalyzed by dirhodium tetracarboxylate

The B3LYP density functional studies on the dirhodium tetracarboxylate-catalyzed C-H bond activation/C-C bond formation reaction of a diazo compound with an alkane revealed the energetics and the geometry of important intermediates and transition states in the catalytic cycle. The reaction is initiated by complexation between the rhodium catalyst and the diazo compound. Driven by the back-donation from the Rh 4d(xz) orbital to the C[bond]N sigma*-orbital, nitrogen extrusion takes place to afford a rhodium[bond]carbene complex. The carbene carbon of the complex is strongly electrophilic because of its vacant 2p orbital. The C[bond]H activation/C[bond]C formation proceeds in a single step through a three-centered hydride transfer-like transition state with a small activation energy. Only one of the two rhodium atoms works as a carbene binding site throughout the reaction, and the other rhodium atom assists the C[bond]H insertion reaction. The second Rh atom acts as a mobile ligand for the first one to enhance the electrophilicity of the carbene moiety and to facilitate the cleavage of the rhodium[bond]carbon bond. The calculations reproduce experimental data including the activation enthalpy of the nitrogen extrusion, the kinetic isotope effect of the C[bond]H insertion, and the reactivity order of the C[bond]H bond.     

Palladium-catalyzed cascade reaction of alpha,beta-unsaturated sulfones with aryl iodides

Unlike traditionally used acyclic 1,2-disubstituted alkenes, the reaction of alpha,beta-unsaturated phenyl sulfones with aryl iodides under Heck reaction conditions (Pd(OAc)(2) as catalyst, Ag(2)CO(3) as base in DMF at 120 (0)C) takes place mainly by a cascade process, involving one unit of the alkene and three units of the aryl iodide, to afford a substituted 9-phenylsulfonyl-9,10-dihydrophenanthrene. The dominant formation of this 3:1 coupling product, instead of the Heck trisubstituted olefin, shows that aromatic C-H bond activation processes can compete with the usually fast syn beta-hydrogen elimination step in the Heck arylation of an acyclic olefin. The structural scope of this palladium-catalyzed cascade arylation of alpha,beta-unsaturated sulfones has proved to be wide with regard to substitution at the beta-position (alkyl, aryl, or alkenyl substitution), substitution at the sulfone unit (alkyl or phenyl sulfones), and configuration at the CdoublebondC bond (trans or cis). Moreover, although less favored than in the case of the arylation of alpha,beta-unsaturated sulfones, similarly substituted 9,10-dihydrophenanthrenes have also been obtained in the case of alpha,beta-unsaturated phosphine oxides and alpha,beta-unsaturated phosphonate esters. A Pd(0)-Pd(II)-Pd(IV) mechanistic pathway involving the successive formation of highly electrophilic sigma-alkylpalladium intermediates and palladacycles is proposed for this multicomponent arylation.     

Theoretical prediction on the reactivity of the Co-mediated intramolecular Pauson-Khand reaction for constructing bicyclo-skeletons in natural products

This work performed a theoretical investigation to explore the mechanism and reactivity of the Co-mediated intramolecular Pauson-Khand reaction for constructing bicyclo-skeletons.     

Ruthenium-catalyzed cross-coupling of 7-Azabenzonorbornadienes with alkynes. an entry to 3a,9b-dihydrobenzo[g]indoles

Electron-rich half-sandwich ruthenium complex CpRuI(PPh3)2, generated in situ, catalyzed the coupling reaction of 7-azabenzonorbornadienes with alkynes to form 3a,9b-dihydrobenzo[g]indoles. This transformation involves the cleavage of one C-N bond of the bicyclic alkene and formation of two (C-C and C-N) bonds at the acetylenic carbons. The scope and limitations of the reaction are addressed according to the substitution patterns of the alkyne and of the substituent at the nitrogen atom of the azabenzonorbornadiene.     

Mechanism of the intramolecular hydroamination of alkenes catalyzed by neutral indenyltitanium complexes: a DFT study

Three mechanistic pathways for the [Ind(2)TiMe(2)]-catalyzed intramolecular hydroamination of alkenes have been investigated by employing density functional theory calculations on the possible intermediates and transition states. The results indicate that the reaction cycle proceeds via a Ti-imido-amido complex as the catalytically active species. However, at the moment, the question as to whether this imido-amido complex is involved in a [2+2]-cycloaddition with the alkene or a newly proposed insertion of the alkene into a Ti--N single bond cannot be answered; the calculated barriers of both the insertion mechanism and the [2+2]-cycloaddition mechanism are similar (143 vs. 136 kJ mol(-1)), and both pathways are in accordance with the experimentally observed rate law (first-order dependence on the aminoalkene concentration). Interestingly, the newly proposed insertion mechanism that takes place by an insertion of the alkene moiety into the Ti--N single bond of an imido-amido complex seems to be much more likely than a mechanism that involves an alkene insertion into a Ti--N single bond of a corresponding trisamide. The latter mechanism, which has been proposed in analogy to rare-earth-metal-catalyzed hydroamination reactions, can be ruled out for two reasons: a surprisingly high activation barrier (164 kJ mol(-1)) and the fact that the rate-limiting insertion step is independent of the aminoalkene concentration. This is in sharp contrast to the experimental findings for indenyltitanium catalysts.     

Pd-catalyzed borylative polycyclization of enediynes to allylboronates

Pd-catalyzed dicyclization of 6-ene-1,11-diynes with bis(pinacolato)diboron affords synthetically useful allylboronates under smooth conditions. Two new C-C and one C-B bonds are stereospecifically formed in a single operation. The stereochemical outcome depends on the starting alkene configuration. The reaction is general and has been applied to differently substituted enediynes. Isolation of intermediate 1,3-dienes suggests a regioselective beta-hydrogen elimination along the reaction pathway.     

pi-Acidic alkene ligand effects in Pd-catalysed cross-coupling processes: exploiting the interaction of dibenzylidene acetone (dba) and related ligands with Pd(0) and Pd(II)

pi-Acidic alkene (olefin) ligands positively influence Pd-catalysed cross-coupling processes, interacting with both palladium(0) and palladium(ii) species, in some cases stabilising key catalytic intermediates. Rates of oxidative addition and reductive elimination are both affected. In certain cases, beta-hydrogen elimination can be slowed down by pi-acidic alkenes, which opens up new reaction pathways (e.g. interception of sigma-alkylpalladium(ii) species by appropriate nucleophiles). pi-Acidic alkene ligands can act independently or in a synergistic fashion with another two-electron donor ligand (e.g. amine, phosphine or N-heterocyclic carbene). The purpose of this perspective article is to highlight the impressive results that can be obtained using pi-acidic alkene ligands, with a particular focus on dibenzylidene acetone (dba) derivatives. Other types of alkene ligands, e.g. macrocyclic alkenes, are also discussed.     

C-C and C-H bond activation reactions in N-heterocyclic carbene complexes of ruthenium

Thermolysis of Ru(PPh3)3(CO)H2 with the N-heterocyclic carbene bis(1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene) (IMes) results in C-C activation of an Ar-CH3 bond in one of the mesityl rings of the carbene ligand. Upon addition of IMes to Ru(PPh3)3(CO)H2 at room temperature in the presence of an alkene, C-H bond activation is observed instead. The thermodynamics of these C-C and C-H cleavage reactions have been probed using density functional theory.     

Ion/molecule reactions of isomeric bromobutene radical cations with ammonia

The ion/molecule reaction of the radical cations of three isomeric bromobutenes (2-bromobut-2-ene 1, 1-bromobut-2-ene 2, 4-bromobut-1-ene 3) with ammonia were studied by Fourier transform ion cyclotron resonance spectrometry to reveal the effect of a different position of the bromo substituent relative to the C-C double bond. Further, the reaction pathways of the ion/molecule reactions were analyzed by theoretical calculations at the level B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d). All three bromobutene radical cations 1(.+) to 3(.+) react efficiently with NH(3). The reactions of 1(.+) carrying the halogen substituent at the double bond follow the pattern observed earlier for other ionized vinylic halogenoalkenes. The major reaction corresponds to proton transfer to NH(3) as to be expected from the high acidity of but-2-ene radical cations exposing six acidic H atoms at allylic positions. The other, still important, reaction of 1(.+) is substitution of the Br substituent by NH(3). Although the radical cations 2(.+) and 3(.+) are expected to be as acidic as 1(.+), proton transfer is the minor reaction pathway of these radical cations. Instead, 2(.+) displays bomo substitution as the major reaction. It is suggested that the mechanism of this reaction is analogous to S(N)2' of nucleophilic allylic substitution. Substitution of Br is not efficient for the reactions of 3(.+)-the two major reactions correspond to C-C bond cleavage of the two possible beta-distonic ammonium ions which are generated by the addition of NH(3) to the ionized double bond of 3. This observation, as well as the results obtained for 1(.+) and 2(.+), emphasize the role of the fast and very exothermic addition of a nucleophile to the ionized double bond for the ion/molecule reactions of alkene radical cations. Clearly the energetically-excited distonic ion arising from the addition fragments unimolecularly by energetically accessible pathways. In the case of a halogene subsituent (except F) at the vinylic or allylic position, this is loss of thesubsituent. In the case of remote halogeno substituents, this is C-C bond cleavage adjacent to the radical site of the distonic ion.     

Control of up to five stereocenters in a cascade reaction: synthesis of highly functionalized five-membered rings

Mukaiyama-Aldol and Prins reactions have been identified as highly efficient methods in C-C bond formation since they were discovered several decades ago. Since both reactions gave the same common intermediate, oxocarbonium, we intend to combine the two reactions into a domino process, which means the formation of multiple C-C bonds and stereogenic centers in one pot without isolation of any intermediates. We envisage that the domino reactions involving the Mukaiyama-Aldol reaction of silyl enol ether and acetal treated with Lewis acid (TiBr4) will produce an oxonium intermediate which upon trapping by an alkene functionality in an intramolecular Prins cyclization will generate the cyclopentyl ring system.     

Reactions of C5H5M (M = Fe, Ni) with substituted thiophenes

The ion molecule reactions between C₅H₅M⁺ (M = Fe, Ni) with some substituted thiophenes have been studied in an ion trap mass spectrometer. The reactions of halogen substituted thiophenes lead to the formation of a new C-C bond between the cyclopentadiene ring and the thiophene with the loss of a neutral HX. The reaction mechanism has been investigated by means of DFT calculations and it was found that the insertion of the metal atom in the C-X bond is the key step in the process.     

Multiple active intermediates in oxidation reaction catalyzed by synthetic heme-thiolate complex relevant to cytochrome p450

We have studied oxidation reactions using a synthetic heme-thiolate (SR complex) in order to ascertain the contributions of multiple intermediates derived from heme-thiolate to the oxygen atom transfer reaction to substrate. First, degradation of peroxyphenylacetic acid (PPAA) was examined in the presence of various substrates. The O-O bond cleavage mode of PPAA was clearly dependent on the reactivity of the substrate, and an easily oxidizable substrate enhanced heterolytic O-O bond cleavage. Second, competitive oxidations of cyclooctane and cyclooctene were carried out with various peroxybenzoic acids containing a series of substituents at the para-position as an oxygen source. The ratios of alkane hydroxylation rate/alkene epoxidation rate were dependent on the nature of the para-substituent of the oxidant. We conclude that substrate and oxidant interact with each other during the oxygen atom transfer reaction, that is, oxidation reaction occurs before O-O bond cleavage, even in the reaction catalyzed by heme-thiolate, which is considered to promote O-O bond cleavage. The results of an (18)O-incorporation study that is frequently performed to determine the active intermediates derived from iron porphyrins were consistent with this conclusion.     

Application of Rh(I)-catalyzed C-H bond activation to the ring opening of 2-cycloalkenones in the presence of amines

[reaction: see text]. Herein described is the application of the Rh(I)-catalyzed C-H bond activation to the ring-opening of 2-cycloalkenones in the presence of cyclohexylamine. This reaction includes the C-C double bond cleavage of 2-cycloalkenones through the conjugate addition of cyclohexylamine followed by the retro-Mannich-type fragmentation. The resulting ring-opened intermediates subsequently underwent either chelation-assisted hydroacylation to afford a ring-opened dicarbonyl compound or beta-alkylation via a ring contraction.     

Oxidative fragmentation of oxiranes to nitriles with hypervalent iodine(V) reagents in aqueous ammonia

Oxiranes undergo oxidative fragmentation when treated with hypervalent iodine(V) reagents particularly o-iodoxybenzoic acid in aqueous ammonia to give nitriles. The reaction goes via the formation of 1, 2-amino alcohols as intermediates followed by C-C bond cleavage.     

Catalytic addition of metallo-aldehyde enolates to ketones: a new C-C bond-forming hydrogenation

The first catalytic cross-aldolization of metallo-aldehyde enolates with ketone acceptors is enabled via hydrogenation of keto-enals with cationic rhodium catalysts. These results, in conjunction with prior studies involving the catalytic hydrogen-mediated reductive coupling of enones, dienes, and diynes with carbonyl acceptors, support the feasibility of developing a broad new class of catalytic C-C bond formations based on the electrophilic trapping of hydrogenation intermediates. [reaction: see text]     

Photochemistry of 2-(1-naphthyl)-2H-azirines in matrixes and in solutions: wavelength-dependent C-C and C-N bond cleavage of the azirine ring

The photochemistry of 3-methyl-2-(1-naphthyl)-2H-azirine (1a) was investigated by the direct observation of reactive intermediates in matrixes at 10 K and by the characterization of reaction products in solutions. As already reported, the photolysis of the azirine 1a with the short-wavelength light (>300 nm) caused the C-C bond cleavage of the 2H-azirine ring to produce the nitrile ylide 2. However, the products derived from the C-N bond cleavage were exclusively obtained in the irradiation of 1a with the long-wavelength light (366 nm) both in matrixes and in solutions. When 1a was irradiated in the presence of O(2) with the long-wavelength light, acetonitrile oxide (6) was produced through the capture of the biradical 4 generated by the C-N bond cleavage of 1a with O(2). An introduction of a nitro group into the naphthyl ring of 1a resulted in an acceleration of the decomposition in the long-wavelength irradiation and an extension of the wavelength region where the products derived from the C-N bond cleavage were selectively obtained. On the basis of molecular orbital calculations with the INDO/S method, the reason for the wavelength-dependent selective C-C and C-N bond cleavage of the azirine ring of 1a is discussed.