Showing papers on "Double bond published in 2009"

Molecular Double-Bond Covalent Radii for Elements Li–E112

Abstract: The previous systems of triple-bond and single-bond self-consistent, additive covalent radii, R(AB)=r(A)+ r(B), are completed with a fit for σ2π2 double-bonds.The primary bond lengths, R, are taken from experimental or theoretical data corresponding to chosen group valencies. All r(E) values are obtained from the same, self-consistent fit. Many of the calculated primary data came from ECH2 and HECH2 models. Homonuclear LEEL, formaldehyde-type Group 14–Group 16 and open-shell, X3 Σ Group-16 dimer data are included. The standard deviation for the 316 included data points is 3 pm.

Direct Conversion of Alcohols to Acetals and H2 Catalyzed by an Acridine-Based Ruthenium Pincer Complex

Abstract: The crystallographically characterized ruthenium complex RuHCl(A-iPr-PNP)(CO) (1) [A-iPr-PNP = 4,5-bis-(diisopropylphosphinomethyl)acridine], which bears a nonplanar acridine moiety, catalyzes in a neutral medium the transformation of primary alcohols to the corresponding acetals with the liberation of H2. In the presence of base, complex 1 catalyzes the dehydrogenative coupling of alcohols to form esters. Acetal formation may involve hemiacetal dehydration to form an enol ether followed by alcohol addition to the double bond.

Reconsideration of the excited-state double proton transfer (ESDPT) in 2-aminopyridine/acid systems: role of the intermolecular hydrogen bonding in excited states.

Abstract: In the present work, the excited-state double proton transfer (ESDPT) in 2-aminopyridine (2AP)/acid systems has been reconsidered using the combined experimental and theoretical methods. The steady-state absorption and fluorescence spectra of 2AP in different acids, such as formic acid, acetic acid, propionic acid, etc. have been measured. We demonstrated for the first time that the ESDPT reaction can take place between 2AP and all of these acids due to the formation of the intermolecular double hydrogen bonds. Furthermore, the vitally important role of the intermolecular double hydrogen bonds between 2AP and acids for ESDPT reaction has also been confirmed by the disappearance of ESDPT when we add the polar acetonitrile to the 2AP/acids systems. This may be due to that the respective polar solvation of 2AP and acids by the acetonitrile solvent disrupts the formation of intermolecular double hydrogen bonds between 2AP and acids. Moreover, the intermolecular double hydrogen bonds are demonstrated to be significantly strengthened in the electronic excited state of 2AP/acid systems using the time-dependent density functional theory (TDDFT) method. The ESDPT reaction is facilitated by the electronic excited-state hydrogen bond strengthening. In addition, potential energy curves of the electronic excited state along the proton transfer coordinate are also calculated by the TDDFT method. The stepwise mechanism of the ESDPT reaction in the 2AP/acid systems is theoretically reconfirmed, and the concerted mechanism is theoretically excluded. At the same time, the sequence of the double proton transfers is theoretically clarified for the first time using the potential energy curves calculated by TDDFT method.

Reactions of an Intramolecular Frustrated Lewis Pair with Unsaturated Substrates: Evidence for a Concerted Olefin Addition Reaction

Abstract: The intramolecular frustrated Lewis pair (mesityl)(2)P-CH(2)-CH(2)-B(C(6)F(5))(2) was generated in situ by HB(C(6)F(5))(2) hydroboration of dimesitylvinylphosphine. The compound reacts with 1-pentyne by C-H bond cleavage. It undergoes a 1,2-addition to the carbonyl group of trans-cinnamic aldehyde to yield a zwitterionic six-membered heterocycle by B-O and P-C bond formation. The Lewis pair regioselectively adds to the electron-rich C=C double bond of ethyl vinyl ether, and it selectively undergoes an exo-cis-2,3-addition to norbornene. A combined experimental/theoretical study suggests that this reaction takes place in an asynchronous concerted fashion with the B-C bond being formed in slight preference to the P-C bond. The addition products were characterized by X-ray crystal structure analyses.

N-heterocyclic carbene-catalyzed hydroacylation of unactivated double bonds

Abstract: An intramolecular N-heterocyclic carbene (NHC)-catalyzed hydroacylation of unactivated double bonds is reported. Systematic variation of the catalyst structure revealed an N-mesitylthiazolylidene annulated with a seven-membered ring to be especially reactive. This NHC enables a unique C−C bond-forming reaction to afford substituted chroman-4-ones in moderate to excellent yields, even ones containing all-carbon quaternary centers.

Divalent carbon(0) compounds

Abstract: Quantum chemical studies show that there is a class of carbon compounds with the general formular CL 2 where the carbon atom retains its four valence electrons as two lone pairs. The C-L bonds come from L → C donor-acceptor interactions where L is a strong σ-donor. Divalent C(0) compounds (carbones) are conceptually different from divalent C(II) compounds (carbenes) and tetravalent carbon compounds, but the bonding situation in a real molecule may be intermediate between the three archetypes. There are molecules like tetraaminoallenes which may be described in terms of two double bonds (R 2 N) 2 C=C=C(NR 2 ) 2 where the extraordinary donor strength of the dicoordinated carbon atom comes only to the fore through the interactions with protons and Lewis acids. They may be considered as "hidden divalent C(0) compounds". The donor strength of divalent C(0) molecules has been investigated by calculations of the binding energies with protons and with main-group Lewis acids and the bond dissociation energies (BDEs) of transition-metal complexes.

Palladium‐Catalyzed Oxidative Intermolecular Difunctionalization of Terminal Alkenes with Organostannanes and Molecular Oxygen

Abstract: A cationic palladium complex catalyzes the title transformations, which are thought to proceed via a pi-allyl or pi-benzyl intermediate. The regioselectivity of the reaction (1,2- or 1,1-difunctionalization) depends on the type of terminal double bond (conjugated or nonconjugated) in the substrate (see scheme) and appears to be controlled by the relative rates of beta-hydride elimination and transmetalation. DMA=dimethylacetamide, Tf=triflyl.

Easy Access to Isoquinolines and Tetrahydroquinolines from Ketoximes and Alkynes via Rhodium-Catalyzed C−H Bond Activation

Abstract: Described herein is a convenient and highly regioselective synthesis of substituted isoquinoline derivatives from various aromatic ketoximes and alkynes via a one-pot, rhodium-catalyzed C−H bond activation. In addition, tetrahydroquinoline derivatives are formed in good yields from 2-arylidene-1-cyclohexanone oximes possessing an exocyclic double bond and from tetrahydroxanthone oximes. A possible mechanism is proposed that involves chelation-assisted C−H activation via oxidative addition of Rh(I) to an ortho-C−H bond, insertion of the alkyne, reductive elimination, intramolecular electrocyclization, and aromatization. This mechanism is supported by isolation of the ortho-alkenylation products 7p and 7q. Also described herein is an example of an iridium-catalyzed activation of an sp3 C−H bond.

The U=C double bond: synthesis and study of uranium nucleophilic carbene complexes.

Abstract: Treatment of U(BH4)4 with 1 or 3 equiv of Li2(SCS)·1.5Et2O, 1, afforded the actinide carbene complexes U(μ-SCS)3[U(BH4)3]2 (4) and U(μ-SCS)3[Li(Et2O)]2 (6), respectively [SCS = (Ph2P = S)2C]. In THF, complex 4 was transformed into the mononuclear derivative (SCS)U(BH4)2(THF)2 (5). The multiple bond character of the uranium−carbon bond was first revealed by the X-ray crystal structures of the three complexes. The U═C bond in these complexes present a nucleophilic character, as shown by their reaction with a carbonyl derivative. Finally, DFT calculations prove the involvement of both 5f and 6d orbitals in both the σ and the π U−C bonds.

A germanium(II) hydride as an effective reagent for hydrogermylation reactions.

Abstract: Herein we report on the reactivity of the stable germanium(II) hydride LGeH (L = CH{(CMe)(2,6-iPr(2)C(6)H(3)N)}(2)) (2), which contains a low-valent germanium atom. 2 is prepared from the corresponding germanium(II) chloride LGeCl (1) using H(3)Al x NMe(3) or K[HB(iBu)(3)] in toluene. The reaction of 2 with carbon dioxide in toluene at room temperature affords a germanium(II) ester of formic acid, LGe-O-C(O)H (3), which is formed by insertion of the carbon dioxide into the germylene hydrogen bond. 2 also reacts with alkynes at room temperature to give the first germanium(II)-substituted alkenes (4, 5, and 6). These two reaction types have in common the fact that the hydrogen and germylene from LGeH are transferred to an unsaturated bond: the carbon-oxygen double bond (C=O) in the former case and the carbon-carbon triple bond (C[triple bond]C) in the latter. Moreover, the reaction of 2 with elemental sulfur in toluene at room temperature leads to the germanium dithiocarboxylic acid analogue LGe(S)SH (7). Compound 7 is formed by the unprecedented insertion of elemental sulfur into the germylene hydrogen bond and oxidative addition of elemental sulfur to the germanium(II) atom. This leads to the formal conversion of the GeH hydride to a SH proton. Compounds 3-7 were investigated by microanalysis, multinuclear NMR spectroscopy, and single-crystal X-ray structural analyses.

Theoretical insights on methylbenzene side-chain growth in ZSM-5 zeolites for methanol-to-olefin conversion.

Abstract: The key step in the conversion of methane to polyolefins is the catalytic conversion of methanol to light olefins. The most recent formulations of a reaction mechanism for this process are based on the idea of a complex hydrocarbon-pool network, in which certain organic species in the zeolite pores are methylated and from which light olefins are eliminated. Two major mechanisms have been proposed to date-the paring mechanism and the side-chain mechanism-recently joined by a third, the alkene mechanism. Recently we succeeded in simulating a full catalytic cycle for the first of these in ZSM-5, with inclusion of the zeolite framework and contents. In this paper, we will investigate crucial reaction steps of the second proposal (the side-chain route) using both small and large zeolite cluster models of ZSM-5. The deprotonation step, which forms an exocyclic double bond, depends crucially on the number and positioning of the other methyl groups but also on steric effects that are typical for the zeolite lattice. Because of steric considerations, we find exocyclic bond formation in the ortho position to the geminal methyl group to be more favourable than exocyclic bond formation in the para position. The side-chain growth proceeds relatively easily but the major bottleneck is identified as subsequent de-alkylation to produce ethene. These results suggest that the current formulation of the side-chain route in ZSM-5 may actually be a deactivating route to coke precursors rather than an active ethene-producing hydrocarbon-pool route. Other routes may be operating in alternative zeotype materials like the silico-aluminophosphate SAPO-34.

Utilization of N-X bonds in the synthesis of N-heterocycles

Abstract: Nitrogen-containing heterocycles—such as aziridines, pyrrolidines, piperidines, and oxazolines—frequently show up as substructures in natural products. In addition, some of these species show potent biological activities. Therefore, researchers would like to develop practical and convenient methods for constructing these heterocycles. Among the available methods, the transfer of N1 units to organic molecules, especially olefins, is a versatile method for the synthesis of N-heterocycles. This Account reviews some of our recent work on the synthesis of N-heterocycles using the N−X bond. A nitrogen−halogen bond bearing an electron-withdrawing group on the nitrogen can be converted to a halonium ion. In the presence of C−C double bonds, these species produce three-membered cyclic halonium intermediates, which can be strong electrophiles and can produce stereocontrolled products. N-Halosuccinimides are representative sources of halonium ions, and the nitrogen of succinimide is rarely used in organic synthesis....

New 1,2,3-triazole ligands through click reactions and their palladium and platinum complexes

Abstract: The new ligands, 1-(4-isopropyl phenyl)-4-(2-pyridyl)-1,2,3-triazole, 1 and 1-(mesityl)-4-(2-pyridyl)-1,2,3-triazole, 2 were prepared by the reactions of the respective azides with 2-ethynylpyridine following the “click method”. These ligands together with the reported ligands 1-(phenyl)-4-(2-pyridyl)-1,2,3-triazole, 3 and 1-(benzyl)-4-(2-pyridyl)-1,2,3-triazole, 4 were reacted with palladium and platinum precursors to give mononuclear cis-dichloropalladium and platinum complexes containing the triazole ligands. Structural characterisation of the free ligand 3 shows that the central N–N bond in the triazole ring has double bond character and hence is best described as an “azo-like” N–N double bond. The pyridine ring in 3 has an almost “anti” conformation with respect to the central triazole ring. The metal centers bind to the ligands through the pyridine N and a triazole N atom. The metal–N(triazole) distances are shorter than the metal–N(pyridine) distances. Cyclic voltammograms of the ligands show reduction processes that appear at extreme negative potentials. Coordination of metal centers induces huge anodic shifts of the reduction potentials due to σ-polarisation by the metal centers. UV/Vis spectra of the ligands and complexes are also discussed. The properties of such chelating triazole ligands towards palladium and platinum centers is being compared and contrasted to the widely used 2,2′-bipyridine ligand.

Double Bond Isomerization/Enantioselective Aza-Petasis−Ferrier Rearrangement Sequence as an Efficient Entry to Anti- and Enantioenriched β-Amino Aldehydes

Abstract: Highly anti- and enantioselective synthesis of β-amino aldehydes having an aliphatic substituent at the β-position was accomplished by a combination of two catalytic reactions, that is, an initial Ni(II) complex-catalyzed isomerization of a double bond followed by a chiral phosphoric acid catalyzed aza-Petasis−Ferrier rearrangement, using hemiaminal allyl ethers as the initial substrate. The chiral phosphoric acid also functioned as an efficient resolving catalyst of racemic hemiaminal vinyl ethers.

The gas-phase ozonolysis of β-caryophyllene (C15H24). Part I: an experimental study

Abstract: The gas phase reaction of ozone with β-caryophyllene was investigated in a static glass reactor at 750 Torr and 296 K under various experimental conditions. The reactants and gas phase products were monitored by FTIR-spectroscopy and proton-transfer-reaction mass spectrometry (PTR-MS). Aerosol formation was monitored with a scanning mobility particle sizer (SMPS) and particulate products analysed by liquid chromatography/mass spectrometry (HPLC-MS). The different reactivity of the two double bonds in β-caryophyllene was probed by experiments with different ratios of reactants. An average rate coefficient at 295 K for the first-generation products was determined as 1.1 × 10−16 cm3 molecule−1 s−1. Using cyclohexane as scavenger, an OH-radical yield of (10.4 ± 2.3)% was determined for the ozonolysis of the more reactive internal double bond, whereas the average OH-radical yield for the ozonolysis of the first-generation products was found to be (16.4 ± 3.6)%. Measured gas phase products are CO, CO2 and HCHO with average yields of (2.0 ± 1.8)%, (3.8 ± 2.8)% and (7.7 ± 4.0)%, respectively for the more reactive internal double bond and (5.5 ± 4.8)%, (8.2 ± 2.8)% and (60 ± 6)%, respectively from ozonolysis of the less reactive double bond of the first-generation products. The residual FTIR spectra indicate the formation of an internal secondary ozonide of β-caryophyllene. From experiments using HCOOH as a Criegee intermediate (CI) scavenger, it was concluded that at least 60% of the formed CI are collisionally stabilized. The aerosol yield in the ozonolysis of β-caryophyllene was estimated from the measured particle size distributions. In the absence of a CI scavenger the yield ranged between 5 and 24%, depending on the aerosol mass. The yield increases with addition of water vapour or with higher concentrations of formic acid. In the presence of HCHO, lower aerosol yields were observed. This suggests that HCOOH adds to a Criegee intermediate to form a low-volatility compound responsible for aerosol formation. The underlying reaction mechanisms are discussed and compared with the results from the accompanying theoretical paper.

Mechanistic insights into the rhodium-catalyzed intramolecular ketone hydroacylation.

Abstract: [Rh((R)-DTBM-SEGPHOS)]BF4 catalyzes the intramolecular hydroacylation of ketones to afford seven-membered lactones in large enantiomeric excess. Herein, we present a combined experimental and theoretical study to elucidate the mechanism and origin of selectivity in this C−H bond activation process. Evidence is presented for a mechanistic pathway involving three key steps: (1) rhodium(I) oxidative addition into the aldehyde C−H bond, (2) insertion of the ketone C═O double bond into the rhodium hydride, and (3) C−O bond-forming reductive elimination. Kinetic isotope effects and Hammett plot studies support that ketone insertion is the turnover-limiting step. Detailed kinetic experiments were performed using both 1,3-bis(diphenylphosphino)propane (dppp) and (R)-DTBM-SEGPHOS as ligands. With dppp, the keto-aldehyde substrate assists in dissociating a dimeric precatalyst 8 and binds an active monomeric catalyst 9. With [Rh((R)-DTBM-SEGPHOS)]BF4, there is no induction period and both substrate and product inhib...

Experimental and density functional studies on 4-(4-cyanobenzylideneamino)antipyrine

Abstract: 4-(4-cyanobenzylideneamino)antipyrine (CBAP) has been synthesized and characterized by elemental analysis, FT-IR, UV-VIS and X-ray single crystal diffraction techniques. Crystallographic study reveals that the compound adopts trans configuration about the Schiff base imine double bond. The substituted p-cyanophenyl ring indirectly linked to the pyrazoline ring by the C=N double bond is almost coplanar with the pyrazole ring, whereas the phenyl ring directly attached to the pyrazoline ring forms an effective dihedral angle. Density functional calculations have been carried out to optimize and to characterize the title compound by using B3LYP method at 6-31G(d) basis set. The calculated results show that the optimized geometry can well reproduce the crystal structural parameters and the theoretical vibrational frequencies show good agreement with experimental values. On the basis of theoretical vibrational analyses, the thermodynamic properties (standard heat capacities, standard entropies, and standard ent...

Cobalt Catalyzed Functionalization of Unactivated Alkenes: Regioselective Reductive C−C Bond Forming Reactions

Abstract: The cobalt catalyzed hydroaldoximation and hydrocyanooximation of unactivated alkenes is reported. Secondary and tertiary aldoximes and oximonitriles are synthesized with excellent regioselectivity under mild conditions, and conversion of the products to valuable intermediates is documented. The reactions expand the arsenal of reductive carbon−carbon bond forming reactions as well as regioselective functionalizations of unactivated double bonds.

Gold complexes as catalysts: Chemoselective hydrogenation of nitroarenes

Abstract: Nitro groups on different compounds – containing double bonds, carbonyl, nitrile or halide groups – have been successfully hydrogenated with well-defined homogeneous gold and palladium complexes as catalysts using a batch reactor under low H2 pressure. Gold complexes show high chemoselectivity towards reduction of the nitro group at near-complete conversion of the substrate. The corresponding amino derivatives are isolated in high yields.

Oligomerization and autocatalysis of NH2BH2 with ammonia-borane.

Abstract: The reactivity of NH2BH2 in the presence of ammonia−borane (AB) is investigated using ab initio CCSD(T) simulations to answer the following three questions: How do AB and NH2BH2 react? How do aminoborane species oligomerize apart from catalytic centers? Can the formation of experimentally observed products, especially cyclic N2B2H7−NH2BH3, be explained through the kinetics of NH2BH2 oligomerization in the presence of AB? AB is shown to react with NH2BH2 by the addition of NH3−BH3 across the N═B double bond, generating linear NH3BH2NH2BH3. This species decomposes by surmounting a reasonable barrier to produce two NH2BH2 and H2. The generation of additional NH2BH2 from NH2BH2 and AB provides a pathway for autocatalytic NH2BH2 production. The important intermediates along the oligomerization pathway include cyclic (NH2BH2)2 and linear NH3BH2NH2BH3, both of which have been observed experimentally. Calculations show cyclic N2B2H7−NH2BH3, an aminoborane analogue of ethylcyclobutane, to be the kinetically prefer...

Anionic Reagents with Silicon‐Containing Double Bonds

Abstract: E=Si transfer: Anionic compounds capable of transferring a silicon-containing double bond are reviewed (see figure), particularly reagents with Si=Si moieties (Tip=2,4,6-iPr(3)C(6)H(2), M=Li, Na, K) and their applications towards main-group and transition-metal electrophiles, as well as their reactivity towards organic compounds. A few recently reported derivatives with Si=C (Ad=1-adamantyl) and Si=P moieties are included for completeness.Anionic compounds capable of transferring a silicon double bond are summarized following an introduction to the differences between alkenes and their heavier homologues. The main focus is on reagents with Si=Si moieties and their applications towards main-group and transition-metal electrophiles, as well as their reactivity towards organic compounds, but a few recently reported derivatives with Si=C and Si=P bonds are also included.

Physisorption and chemisorption of alkanes and alkenes in H-FAU: a combined ab initio–statistical thermodynamics study

Abstract: The sorption in H-FAU zeolite of C4-C12 n-alkanes, and linear and branched C2-C8 alkenes has been quantified up to 800 K by combining QM-Pot(MP2//B3LYP) with statistical thermodynamics calculations. The physisorption strength increases linearly with increasing carbon number by 8.5 kJ mol(-1) and does not depend on the detailed alkane or alkene structure. Van der Waals interactions are dominant in physisorption, but alkenes are additionally stabilized by 20 kJ mol(-1) by formation of a pi-complex. Protonation of an alkene leads to the formation of alkoxides, which are more stable than the physisorbed species. As for physisorption a linear relation between the chemisorption energy and the carbon number is obtained. Protonation energies are independent of the carbon number but depend on the type of CC double bond being protonated. The relative stability difference between the secondary and tertiary alkoxides is 15 kJ mol(-1) in favor of the former. Both physisorption and chemisorption are accompanied with entropy losses which increase linearly with the carbon number. A typical compensation effect is obtained: the stronger the stabilization of the sorbed species the more pronounced the entropy loss. For temperatures ranging from 0 to 800 K, all of the derived linear relations expressing the physisorption and/or chemisorption enthalpy and entropy of the alkanes and the alkanes as function of the carbon number are independent of temperature. A good agreement between calculated and experimental values for alkanes is obtained at 500 K.

Composition domains in monoterpene secondary organic aerosol

Abstract: The composition and structure of freshly formed oligomers in alpha- and beta- pinene SOA are studied with high performance mass spectrometry to provide insight into the SOA formation mechanism Van Krevelen plots (H:C ratio vs O:C ratio) are interpreted in the context of distinct structural domains that correspond to separate oligomer formation routes The domain containing most of the signal intensity encompasses elemental formulas that correspond to oligomerization reactions of intermediates and/or stable molecule monomers produced by ozonolysis of the precursor While oligomers involving reactive intermediates from the hydroperoxide channel dominate the product distribution, products are also observed that uniquely map to the stable Criegee intermediate and/ or combinations of stable molecule monomers A second domain encompasses molecules having lower H:C ratios but similar O:C ratios to the first domain Many of the products observed in this domain have double bond equivalents greater than the maximum number possible when forming dimers by standard reaction mechanisms and are interpreted in the context of repeated self-reactions of alkoxy/peroxy radicals A third domain encompasses molecules having very high H:C and O:C ratios consistent with polymerization of formaldehyde and/or acetaldehyde These domains remain distinguishable from experiment to experiment and among different extraction solvents (50/50 methanol-water, 50/50 acetonitrile-water,100% water)

Photoactivation mechanism of PAmCherry based on crystal structures of the protein in the dark and fluorescent states.

Abstract: Photoactivatable fluorescent proteins (PAFPs) are required for super-resolution imaging of live cells. Recently, the first red PAFP, PAmCherry1, was reported, which complements the photo-activatable GFP by providing a red super-resolution color. PAmCherry1 is originally “dark” but exhibits red fluorescence after UV-violet light irradiation. To define the structural basis of PAmCherry1 photoactivation, we determined its crystal structure in the dark and red fluorescent states at 1.50 A and 1.65 A, respectively. The non-coplanar structure of the chromophore in the dark PAmChery1 suggests the presence of an N-acylimine functionality and a single non-oxidized Cα-Cβ bond in the Tyr-67 side chain in the cyclized Met-66-Tyr-67-Gly-68 tripeptide. MS data of the chromophore-bearing peptide indicates the loss of 20 Da upon maturation, whereas tandem MS reveals the Cα–N bond in Met-66 is oxidized. These data indicate that PAmCherry1 in the dark state possesses the chromophore N-[(E)-(5-hydroxy-1H-imidazol-2-yl)methylidene]acetamide, which, to our knowledge, has not been previously observed in PAFPs. The photoactivated PAmCherry1 exhibits a non-coplanar anionic DsRed-like chromophore but in the trans configuration. Based on the crystallographic analysis, MS data, and biochemical analysis of the PAmCherry1 mutants, we propose the detailed photoactivation mechanism. In this mechanism, the excited-state PAmCherry1 chromophore acts as the oxidant to release CO2 molecule from Glu-215 via a Koble-like radical reaction. The Glu-215 decarboxylation directs the carbanion formation resulting in the oxidation of the Tyr-67 Cα-Cβ bond. The double bond extends the π-conjugation between the phenolic ring of Tyr-67, the imidazolone, and the N-acylimine, resulting in the red fluorescent chromophore.

Rh(II)-Catalyzed skeletal reorganization of 1,6- and 1,7-enynes through electrophilic activation of alkynes.

Abstract: The skeletal reorganization of 1,6- and 1,7-enynes leading to 1-vinylcycloalkenes using Rh(II) as a catalyst is reported. Two possible isomers of 1-vinylcycloalkenes, type I and type II, can be obtained, the ratio of which are highly dependent on the substitution pattern of the enynes used. Formation of type I compounds involves a single cleavage of a C-C double bond, the product of which is identical to that of enyne metathesis. In contrast, the formation of type II compounds involves the double cleavage of both the C-C double and triple bonds, which has an anomalous bond connection. The presence of both a phenyl group at the alkyne carbon and a methyl group at the internal alkene carbon facilitates the formation of type II compounds. The electronic and steric nature of the substituents on the aromatic ring also affects the ratio of type I and type II. The nature of a tether also has a significant effect on the course of the reaction. Experimental evidence for the intermediacy of a cyclopropyl rhodium carbenoid, a key intermediate in the skeletal reorganization of enynes, is also reported. In addition to the skeletal reorganization of enynes, Rh(II) complexes were found to have a high catalytic activity for some cycloisomerization reactions of alkyne derivatives, including the bicyclization of enynes to bicyclo[4.1.0]heptene derivatives and the cyclization of alkynylfurans to phenol derivatives.

Enantioselective intramolecular oxidative aminocarbonylation of alkenylureas catalyzed by palladium-spiro bis(isoxazoline) complexes.

Abstract: An enantioselective synthesis of tetrahydropyrrolo[1,2-c]pyrimidine-1,3-diones via a palladium-catalyzed intramolecular oxidative aminocarbonylation is described. The carbon−carbon double bond of alkenylurea substrates has been shown to react intramolecularly with a nitrogen nucleophile in the presence of a palladium catalyst under a carbon monoxide atmosphere. The use of a chiral spiro bis(isoxazoline) ligand (SPRIX) is essential to obtain the desired products in optically active forms. In comparison with the coordination ability of other known ligands, this peculiar character of SPRIX originates from two structural characteristics: low σ-donor ability of the isoxazoline coordination site and rigidity of the spiro skeleton.