Showing papers on "Intramolecular force published in 2008"
TL;DR: The relevant mechanistic insights that enabled synthetic advances and distinguished the resulting transformations from other methods are emphasized and researchers will better understand the details of the aforementioned Rh-catalyzed C-H bond functionalization reactions, resulting in the design of more efficient and robust catalysts, expanded substrate scope, and new transformations.
Abstract: Nitrogen heterocycles are present in many compounds of enormous practical importance, ranging from pharmaceutical agents and biological probes to electroactive materials. Direct functionalization of nitrogen heterocycles through C−H bond activation constitutes a powerful means of regioselectively introducing a variety of substituents with diverse functional groups onto the heterocycle scaffold. Working together, our two groups have developed a family of Rh-catalyzed heterocycle alkylation and arylation reactions that are notable for their high level of functional-group compatibility. This Account describes our work in this area, emphasizing the relevant mechanistic insights that enabled synthetic advances and distinguished the resulting transformations from other methods. We initially discovered an intramolecular Rh-catalyzed C-2 alkylation of azoles by alkenyl groups. That reaction provided access to a number of di-, tri-, and tetracyclic azole derivatives. We then developed conditions that exploited mic...
860 citations
TL;DR: Pd(II)-catalyzed intramolecular amination of sp2 and sp3 C-H bonds are developed using a combination of CuCl2 and AgOAc as the oxidant, providing practical access to a wide range of beta-, gamma-, and delta-lactams.
Abstract: Pd(II)-catalyzed intramolecular amination of sp2 and sp3 C-H bonds are developed using a combination of CuCl2 and AgOAc as the oxidant. The reaction protocol tolerates the presence of a double bond in the substrates. This catalytic reaction provides practical access to a wide range of beta-, gamma-, and delta-lactams.
427 citations
370 citations
TL;DR: The first ansa-aminoborane N-TMPN-CH2C6H4B(C6F5)2 which is able to reversibly activate H2 through an intramolecular mechanism is synthesized and is employed in the catalytic reduction of nonsterically demanding imines and enamines under mild conditions to give the corresponding amines in high yields.
Abstract: The first ansa-aminoborane N-TMPN-CH2C6H4B(C6F5)2 (where TMPNH is 2,2,6,6-tetramethylpiperidinyl) which is able to reversibly activate H2 through an intramolecular mechanism is synthesized This new substance makes use of the concept of molecular tweezers where the active N and B centers are located close to each other so that one H2 molecule can fit in this void and be activated Because of the fixed geometry of this ansa-ammonium-borate it forms a short N−H···H−B dihydrogen bond of 178 A as determined by X-ray analysis Therefore, the bound hydrogen can be released above 100 °C In addition, the short H···H contact and the N−H···H (154°) and B−H···H (125°) angles show that the dihydrogen interaction in N-TMPNH-CH2C6H4BH(C6F5)2 is partially covalent in nature As a basis for discussing the mechanism, quantum chemical calculations are performed and it is found that the energy needed for splitting H2 can arise from the Coulomb attraction between the resulting ionic fragments, or “Coulomb pays for Heitler−
332 citations
TL;DR: The use of pivalic acid as the reaction solvent, instead of acetic acid, results in greater reproducibility, higher yields, and broader scope, and the use of electron-rich diarylamines as illustrated in the synthesis of three naturally occurring carbazole products.
Abstract: New reaction conditions for intramolecular palladium(II)-catalyzed oxidative carbon-carbon bond formation under air are described. The use of pivalic acid as the reaction solvent, instead of acetic acid, results in greater reproducibility, higher yields, and broader scope. This includes the use of electron-rich diarylamines as illustrated in the synthesis of three naturally occurring carbazole products: Murrayafoline A, Mukonine, and Clausenine. A variety of side products have also been isolated, casting light on competing reaction pathways and revealing new reactivity with palladium(II) catalysis.
329 citations
TL;DR: The time‐dependent density functional theory (TDDFT) method was carried out to investigate the hydrogen‐bonded intramolecular charge‐transfer (ICT) excited state of 4‐dimethylaminobenzonitrile (DMABN) in methanol (MeOH) solvent and demonstrated that the intermolecular hydrogen bond C≡N···HO is significantly strengthened in the TICT state.
Abstract: The time-dependent density functional theory (TDDFT) method was carried out to investigate the hydrogen-bonded intramolecular charge-transfer (ICT) excited state of 4-dimethylaminobenzonitrile (DMABN) in methanol (MeOH) solvent. We demonstrated that the intermolecular hydrogen bond C[triple bond]N...H-O formed between DMABN and MeOH can induce the C[triple bond]N stretching mode shift to the blue in both the ground state and the twisted intramolecular charge-transfer (TICT) state of DMABN. Therefore, the two components at 2091 and 2109 cm(-1) observed in the time-resolved infrared (TRIR) absorption spectra of DMABN in MeOH solvent were reassigned in this work. The hydrogen-bonded TICT state should correspond to the blue-side component at 2109 cm(-1), whereas not the red-side component at 2091 cm(-1) designated in the previous study. It was also demonstrated that the intermolecular hydrogen bond C[triple bond]N...H-O is significantly strengthened in the TICT state. The intermolecular hydrogen bond strengthening in the TICT state can facilitate the deactivation of the excited state via internal conversion (IC), and thus account for the fluorescence quenching of DMABN in protic solvents. Furthermore, the dynamic equilibrium of these electronically excited states is explained by the hydrogen bond strengthening in the TICT state.
303 citations
TL;DR: With intrinsic advantages resulting from its intramolecular signal transduction architecture, the ASP design holds promising potential for future applications, such as biochip and in situ imaging, which require reusability, excellent stability, prompt response, and high sensitivity.
Abstract: A novel aptamer-based molecular probe design employing intramolecular signal transduction is demonstrated. The probe is composed of three elements: an aptamer, a short, partially cDNA sequence, and a PEG linker conjugating the aptamer with the short DNA strand. We have termed this aptamer probe an "aptamer switch probe", or ASP. The ASP design utilizes both a fluorophore and a quencher which are respectively modified at the two termini of the ASP. In the absence of the target molecule, the short DNA will hybridize with the aptamer, keeping the fluorophore and quencher in close proximity, thus switching off the fluorescence. However, when the ASP meets its target, the binding between the aptamer and the target molecule will disturb the intramolecular DNA hybridization, move the quencher away from the fluorophore, and, in effect, switch on the fluorescence. Both ATP and human alpha-thrombin aptamers were engineered to demonstrate this design, and both showed that fluorescence enhancement could be quantitatively mediated by the addition of various amounts of target molecules. Both of these ASPs presented excellent selectivity and prompt response toward their targets. With intrinsic advantages resulting from its intramolecular signal transduction architecture, the ASP design holds promising potential for future applications, such as biochip and in situ imaging, which require reusability, excellent stability, prompt response, and high sensitivity.
290 citations
TL;DR: In this paper, the double-peaked structure in the vibrational spectrum of hydrogen-bonded interfacial water molecules originates from vibrational coupling between the stretch and bending overtone, rather than from structural effects.
Abstract: Using the surface-specific vibrational technique of vibrational sum-frequency generation, we reveal that the double-peaked structure in the vibrational spectrum of hydrogen-bonded interfacial water molecules originates from vibrational coupling between the stretch and bending overtone, rather than from structural effects. This is demonstrated by isotopic dilution experiments, which reveal a smooth transition from two peaks to one peak, as D2O is converted into HDO. Our results show that the water interface is structurally more homogeneous than previously thought.
285 citations
TL;DR: This finding suggests the possibility of creating a catalytic cycle that could oxidize two water molecules bound to 2 along the reverse pathway in Scheme 1 (1-3H!1-2H! 1- H!1), eventually forming 3 by photochemical release of O2 and a phosphinate ligand.
Abstract: The creation of efficient catalysts for splitting water into H2 and O2 is one of the greatest challenges for chemists working on the production of renewable fuel. The water oxidizing center (WOC) within photosynthetic organisms is the only natural system able to efficiently photooxidize water using visible light, and is thus a blueprint for catalyst design. One of the atomic structural models of the WOC derived from X-ray diffraction involves a “cubelike” core comprised of a {CaMn3O4} unit tethered to a fourth manganese atom through one or two bridging oxo units. A few nonbiological tetramanganese complex mimics of this site have been prepared that contain an incomplete or distorted cubic {Mn4Ox} core [4–7] or are part of a larger Mnx–oxo lattice. [4] However, none of these have shown activity towards water oxidation. We have previously synthesized a prototypical molecular manganese–oxo cube [Mn4O4] n+ in a family of “cubane” complexes [Mn4O4L6], where L is a diarylphosphinate ligand (p-R-C6H4)2PO2 (R=H, alkyl, OMe). The diphenylphosphinate complex (1, R=H, Figure 1) assembles spontaneously from manganese(II) and permanganate salts in high yield in non-aqueous solvents. The release of O2 by the {Mn4O4} 6+ core in 1 was shown to be possible on thermodynamic grounds, but cannot take place because of the rigidity of the core arising from the six diarylphosphinate ligands, which bridge pairs of manganese atoms on the six cube faces. The assembly of 1 is also driven by intramolecular van der Waals forces that attract three aryl rings from adjacent phosphinate ligands. The cubic core in 1 is a much stronger oxidant than any known dimanganese complex with {Mn2O2} 3+ cores. Cubane 1 abstracts hydrogen atoms from various organic substrates by breaking O H and N H bonds with dissociation energies greater than 390 kJmol . Titrations of 1 against compounds containing either amine or phenol groups reach an end point after the abstraction of four successive hydrogen atoms, yielding two water molecules (from corner oxo groups) plus [L6Mn4O2], the so-called “pinned butterfly” complex 2 (Scheme 1). {Mn4O4} cubane complexes are unique in releasing an O2 molecule upon photoexcitation of the Mn !O charge transfer band, which reaches a maximum at 350 nm. This process, which occurs with high quantum efficiency only in the gas phase, involves the core oxygen atoms and is triggered by ejection of one phosphinate ligand, thereby generating the [L5Mn4O2] + “butterfly” complex 3 (Scheme 1). In contrast, noncuboidal manganese molecular complexes possessing {Mn2O}, {Mn2O2}, and {Mn3O6} cores in the Mn or Mn oxidation states fail to release O2, but instead photodecompose into multiple fragments. Thus, O2 release is favored by complexes with a {Mn4O4} cubane core. The composition of the butterfly complexes 2 and 3 differs only by one phosphinate ligand (Scheme 1). This finding suggests the possibility of creating a catalytic cycle that could oxidize two water molecules bound to 2 along the reverse pathway in Scheme 1 (1-3H!1-2H!1-H!1), eventually forming 3 by photochemical release of O2 and a phosphinate ligand. Thus far it has proved impossible to realize a catalytic cycle, as in Scheme 1, because O2 is not photodissociated from 1 or 1 (the one-electron oxidized cubane) in condensed phases. This was attributed to a large activation barrier for O2 release when all the phosphinate ligands remain ligated or re-ligate by fast geminate recombination. Figure 1. X-ray crystal structure of 1.
272 citations
TL;DR: This method features the use of a novel catalytic system consisting of 10 mol % of Pd(II), 50 mol% of Cu(I), and 2 equiv of Bu4NBr that produced variously substituted benzothiazoles in high yields with good functional group tolerance.
258 citations
TL;DR: In this article, the authors highlight the synthesis of heterocycles via cascadereactions that involve the activation of an alkyne using carbophilic Lewis acids, and discuss the functionalization of existing Heterocycles.
Abstract: This review highlights the synthesis of heterocycles via cascadereactions that involve the activation of an alkyne using carbophilicLewis acids. Primarily guided by the type of reactivity evolvingfrom the alkyne activation, such key steps are categorized as theaddition of simple heteroatom nucleophiles, intramolecular carboalkoxylations,addition of carbonyl nucleophiles, rearrangement of propargylicesters, and enyne cycloisomerizations. Additionally, the functionalizationof existing heterocycles is discussed. 1 Introduction 2 Functionalization of Existing Heterocycles 3 Synthesis of Heterocycles through Cyclization Reactions 3.1 Intramolecular Addition of Simple Heteroatom Nucleophiles 3.2 Intramolecular Carboalkoxylations and Carboaminations 3.3 Carbonyls and Imines as Nucleophiles 3.4 Propargylic Esters 3.5 Enyne Cycloisomerizations 4 Miscellaneous Reactions 5 Conclusions
TL;DR: The development of a new method for the assembly of unsymmetrical carbazole product can be controlled by the design of the biaryl amide substrate, and the method is compatible with a variety of functional groups.
Abstract: The development of a new method for the assembly of unsymmetrical carbazoles is reported. The strategy involves the selective intramolecular functionalization of an arene C-H bond and the formation of a new arene C-N bond. The substitution pattern of the carbazole product can be controlled by the design of the biaryl amide substrate, and the method is compatible with a variety of functional groups. The utility of the new protocol was demonstrated by the concise synthesis of three natural products from commercially available materials.
TL;DR: The enforced folding and folding-assisted cyclization of oligomers have provided a predictable strategy for developing crescent, helical, and cyclic structures containing nanosized voids that are mostly associated with the tertiary and quaternary structures of proteins.
Abstract: This Account reviews the progress made by us on creating porous molecular crescents, helices, and macrocycles based on aromatic oligoamides. Inspired by natural pore- or cavity-containing secondary structures, work described in this Account stemmed from the development of foldamers consisting of benzene rings linked by secondary amide groups. Highly stable, three-center intramolecular hydrogen bonds involving the amide linkages are incorporated into these aromatic oligoamides, which, along with meta-linked benzene units that introduce curvatures into the corresponding backbones, leads to tape-like, curved backbones. Depending on their chain lengths, aromatic oligoamides that fold into crescent and helical conformations have been obtained. Combining results from modeling and experimentally measured data indicates that the folding of these oligomers is readily predictable, determined by the localized intramolecular three-center H-bonds and is independent of side-chain substitution. As a result, a variety of...
TL;DR: The concept of electronic coupling from theories of intramolecular electron transfer is extended and applied in the scattering theory (Landauer) formalism, which yields a simple sum over independent channels that is used to interpret and explain the unusual features of junction transport through cross-conjugated molecules and the differences among benzene rings substituted at the ortho, meta, or para positions.
Abstract: Theory and experiment examining electron transfer through molecules bound to electrodes are increasingly focused on quantities that are conceptually far removed from current chemical understanding. This presents challenges both for the design of interesting molecules for these devices and for the interpretation of experimental data by traditional chemical mechanisms. Here, the concept of electronic coupling from theories of intramolecular electron transfer is extended and applied in the scattering theory (Landauer) formalism. This yields a simple sum over independent channels, that is then used to interpret and explain the unusual features of junction transport through cross-conjugated molecules and the differences among benzene rings substituted at the ortho, meta, or para positions.
TL;DR: The first catalytic direct alkylation of allylic C-H bonds via Pd(II)-catalysis is described in the absence of base, and polysubstituted cyclic compounds can be constructed by the intramolecular direct allylicAlkylation.
Abstract: The first catalytic direct alkylation of allylic C−H bonds via Pd(II)-catalysis is described in the absence of base. Polysubstituted cyclic compounds can also be constructed by the intramolecular direct allylic alkylation.
TL;DR: The discovery that indoles could be generated from azidoacrylates 2 through exposure to catalytic amounts of rhodium(II) perfluorobutyrate and the combination of this potential method with the earlier one would enable the r Rhodium-catalyzed synthesis of indoles by the formation of either the aryl C N bond or the creation of the vinyl C N Bond.
Abstract: The development of new transition-metal-catalyzed methods for selective functionalization of carbon–hydrogen bonds continues to be an active area of research. Whereas many transition-metal complexes exhibit activity, rhodium(II) dimers are well established to react with a-diazo compounds or sulfonyliminoiodinanes 6] to access metal carbenoids or nitrenoids, which can functionalize proximal aliphatic C H bonds to form new C C bonds or C N bonds in a stereoselective manner. Transition-metal-mediated formation of new carbon–nitrogen bonds from vinyl or aryl C H bonds, however, is much less common. Azides can be employed in the amination of aromatic or vinyl C H bonds. Thermolysis or photolysis of azides produces nitrenes, 9] which react with proximal C H bonds to form N-heterocycles. Nitrenes, however, are highly reactive and can decompose into a variety of byproducts, including amines, azobenzenes, or tars. Whereas metalmediated nitrogen atom transfer reactions from azides are well-known to attenuate this extreme nitrene reactivity, dirhodium(II) carboxylates have been rarely employed to catalyze these processes despite their proven utility in other related atom transfer reactions. Since azides are readily available, 17] their use in new transition-metal-mediated methods that create new C N bonds is highly appealing. We recently discovered that indoles could be generated from azidoacrylates 2 through exposure to catalytic amounts of rhodium(II) perfluorobutyrate. Whereas this reaction exhibited a broad substrate scope, it required an a-azidomethylacetate, which restricted product formation to 2-indolecarboxylate esters. Achievement of indole synthesis from aryl azides 3 through rhodium-catalyzed vinyl C H bond amination would address this limitation as a broader range of aryl azides are readily available from commercial starting materials in two mild, functional group tolerant steps: palladium-catalyzed Suzuki cross-coupling of 2-bromoanilines and subsequent diazo transfer would produce 3. The combination of this potential method with our earlier one would enable the rhodium-catalyzed synthesis of indoles by the formation of either the aryl C N bond (from azidoacrylates) or the creation of the vinyl C N bond (from aryl azides; Scheme 1). As the thermal variant of this reaction, the Sundberg indole synthesis, requires heating of the poten-
TL;DR: Investigations are described on the scope and mechanism as well as preliminary results on the asymmetric version of the reaction to provide novel access to chiral quaternary stereocenters.
Abstract: A catalyst system derived from nickel and cocatalytic AlMe2Cl effects the intramolecular arylcyanation of alkenes. The reaction takes place in an exclusive exo-dig manner to give a wide range of nitriles having a benzylic quaternary carbon in good yields. Detailed investigations are described on the scope and mechanism as well as preliminary results on the asymmetric version of the reaction to provide novel access to chiral quaternary stereocenters.
TL;DR: The collected knowledge about intramolecular kinetic isotope effects associated with the activation of C–H(D) bonds of methane can be used to distinguish the nature of the bond activation as a mere hydrogen-abstraction, a metal-assisted mechanism or more complex reactions such as formation of insertion intermediates or σ-bond metathesis.
Abstract: Motivated by the search for ways of a more efficient usage of the large, unexploited resources of methane, recent progress in the gas-phase activation of methane by ligated transition-metal ions is discussed. Mass spectrometric experiments demonstrate that the ligands can crucially influence both reactivity and selectivity of transition-metal cations in bond-activation processes, and the most reactive species derive from combinations of transition metals with the electronegative elements fluorine, oxygen, and chlorine. Furthermore, the collected knowledge about intramolecular kinetic isotope effects associated with the activation of C-H(D) bonds of methane can be used to distinguish the nature of the bond activation as a mere hydrogen-abstraction, a metal-assisted mechanism or more complex reactions such as formation of insertion intermediates or sigma-bond metathesis.
TL;DR: DFT calculations support a stepwise mechanism for the cycloaddition by the initial formation of an anti-cyclopropyl gold(I)-carbene followed by its opening to form a carbocation stabilized by a pi interaction with the aryl ring, which undergoes a Friedel-Crafts-type reaction.
Abstract: The cyclizations of enynes substituted at the alkyne gives products of formal [4+2] cyclization with Au(I) catalysts. 1,8-Dien-3-ynes cyclize by a 5-exo-dig pathway to form hydrindanes. 1,6-Enynes with an aryl ring at the alkyne give 2,3,9,9a-tetrahydro-1H-cyclopenta[b]naphthalenes by a 5-exo-dig cyclization followed by a Friedel−Crafts-type ring expansion. A 6-endo-dig cyclization is also observed in some cases as a minor process, although in a few cases, this is the major cyclization pathway. In addition to cationic gold complexes bearing bulky biphenyl phosphines, a gold complex with tris(2,6-di-tert-butylphenyl)phosphite is exceptionally reactive as a catalyst for this reaction. This cyclization can also be carried out very efficiently with heating under microwave irradiation. DFT calculations support a stepwise mechanism for the cycloaddition by the initial formation of an anti-cyclopropyl gold(I)-carbene, followed by its opening to form a carbocation stabilized by a π interaction with the aryl ring,...
TL;DR: This is the first report of an enantioselective intramolecular alkene aminooxygenation process, catalyzed by copper(II) triflate, complexed with (4S,5R)-Bis-Phbox, providing new chiral methyleneoxy-functionalized dihydroindolines and pyrrolidines.
Abstract: The copper-catalyzed enantioselective intramolecular aminooxygenation of alkenes is reported herein. This is the first report of an enantioselective intramolecular alkene aminooxygenation process. N-Arylsulfonyl-2-allylanilines and 4-pentenylarylsulfonamides cyclize in high yield and with good enantioselectivity, providing new chiral methyleneoxy-functionalized dihydroindolines and pyrrolidines. Tetramethylaminopyridyl radical (TEMPO) serves as both the source of the oxygen and the stoichiometric oxidant. These reactions are catalyzed by copper(II) triflate, complexed with (4S,5R)-Bis-Phbox. The unprotected aminoalcohols can be obtained by sequential dissolving metal reductions of the N−S and O−N bonds.
TL;DR: It is demonstrated that S( 2) fluorescence can be explained by the calculated energy gap between the S(2) and S(1) states of these molecules.
Abstract: Experimental and theoretical methods were used to study newly synthesized thiophene-pi-cojugated donor-acceptor compounds, which were found to exhibit efficient intramolecular charge-transfer emission in polar solvents with relatively large Stokes shifts and strong solvatochromism. To gain insight into the solvatochromic behavior of these compounds, the dependence of the spectra on solvent polarity was studied on the basis of Lippert-Mataga models. We found that intramolecular charge transfer in these donor-acceptor systems is significantly dependent on the electron-with-drawing substituents at the thienyl 2-position. The dependence of the absorption and emission spectra of these compounds in methanol on the concentration of trifluoroacetic acid was used to confirm intramolecular charge-tranfer emission. Moreover, the calculated absorption and emission energies, which are in accordance with the experimental values, suggested that fluorescence can be emitted from different geometric confirmations. In addition, a novel S-2 fluorescence phenomenon for some of these compounds was also be observed. The fluorescence excitation spectra were used to confirm the S-2 fluorescence. We demonstrate that S-2 fluorescence can be explained by the calculated energy gap between the S-2 and S-1 states of these molecules. Furthermore, nonlinear optical behavior of the thiophene-pi-conjugated compound with diethylcyanomethylphosphonate substituents was predicted in theory.
TL;DR: A convenient gold(III)-catalyzed synthesis of azepines from the intermolecular annulation of propargyl esters and alpha,beta-unsaturated imines is reported.
Abstract: A convenient gold(III)-catalyzed synthesis of azepines from the intermolecular annulation of propargyl esters and α,β-unsaturated imines is reported (19 examples, 55−95% yield). This formal [4 + 3]-cycloaddition reaction is proposed to proceed via a stepwise process involving intramolecular trapping of an allyl−gold intermediate.
TL;DR: This method allows the generation of two new C-C bonds and one new quaternary carbon stereogenic center in a single synthetic step, converting readily available benzonitrile substrates into 1-1-disubstituted indanes in 49-85% yield and 92-97% ee.
Abstract: The enantioselective, intramolecular arylcyanation of unactivated olefins via C–CN bond activation has been accomplished using a Ni(0) catalyst and BPh3 co-catalyst. High enantioselectivities are achieved using TangPHOS as a chiral ligand. This method allows generation of two new C–C bonds and one new quaternary carbon stereogenic center in a single synthetic step, converting readily available benzonitrile substrates into 1,1-disubstituted indanes in 49–85% yield and 92–97% ee.
22 Jan 2008
TL;DR: In this article, the effect of intramolecular charge transfer on the geometries and the vibrational modes contributing to the linear electro-optic effect of the organic NLO material was analyzed.
Abstract: A comprehensive investigation on the intramolecular charge transfer (ICT) of an efficient π-conjugated potential push–pull NLO chromophore, 4-[ N , N -dimethylamino]-4′-nitro stilbene (DANS), from a strong electron-donor group (dimethylamino- N (CH 3 ) 2 ) to a strong electron-acceptor group (nitro-NO 2 ) through the π-conjugated bridge ( trans -stilbene) has been carried out from their vibrational spectra. The NIR FT-Raman and FT-IR spectra supported by the density functional theory (DFT) quantum chemical computations have been employed to analyze the effects of intramolecular charge transfer on the geometries and the vibrational modes contributing to the linear electro-optic effect of the organic NLO material. It has been observed that the changes in the endocyclic and exocyclic angles result from the charge-transfer interaction of the phenyl ring and the amino group in the electron-donor side of the NLO chromophore. The strongest vibrational modes contributing to the electro-optic effect have been identified and examined from the concurrent IR and Raman activation of ν (C C/C–C) mode, ring C C stretching modes, in-plane deformation modes, nitro modes and the umbrella mode of methyl groups. Furthermore, the splitting of the vinyl stretching modes and the electronic effects such as hyperconjugation and backdonation on the methyl hydrogen atoms causing the decrease of stretching frequencies and infrared intensities have also been analyzed in detail. The effect of frontier orbitals transition of electron density transfer and the influence of planarity between the phenyl rings of the stilbene moiety on the first hyperpolarizability have also been discussed.
TL;DR: Investigation of asymmetric [3 + 2] cycloadditions revealed that several kinds of optically active substituted pyrrolidine derivatives containing contiguous stereogenic tertiary and quaternary carbon centers were obtained with high diastereo- and enantioselectivities.
Abstract: Catalytic asymmetric 1,4-addition and [3 + 2] cycloaddition reactions using chiral calcium species prepared from calcium isopropoxide and chiral bisoxazoline ligands have been developed. Glycine Schiff bases reacted with acrylic esters to afford 1,4-addition products, glutamic acid derivatives, in high yields with high enantioselectivities. During the investigation of the 1,4-addition reactions, we unexpectedly found that a [3 + 2] cycloaddition occurred in the reactions with crotonate derivatives, affording substituted pyrrolidine derivatives in high yields with high enantioselectivities. On the basis of this finding, we investigated asymmetric [3 + 2] cycloadditions, and it was revealed that several kinds of optically active substituted pyrrolidine derivatives containing contiguous stereogenic tertiary and quaternary carbon centers were obtained with high diastereo- and enantioselectivities. In addition, optically active pyrrolidine cores of hepatitis C virus RNA-dependent polymerase inhibitors and potential effective antiviral agents have been synthesized using this [3 + 2] cycloaddition reaction. NMR spectroscopic analysis and observation of nonamplification of enantioselectivity in nonlinear effect experiments suggested that a monomeric calcium species with an anionic ligand was formed as an active catalyst. A stepwise mechanism of the [3 + 2] cycloaddition, consisting of 1,4-addition and successive intramolecular Mannich-type reaction was suggested. Furthermore, modification of the Schiff base structure resulted in a modification of the reaction course from a [3 + 2] cycloaddition to a 1,4-addition, affording 3-substituted glutamic acid derivatives with high diasterero- and enantioselectivities.
TL;DR: The N,C-chelate ligands in 1 and 2 were found to play a key role in promoting this unusual and reversible photo-thermal isomerization process on a tetrahedral boron center.
Abstract: A diboron compound with both 3-coordinate boron and 4-coordinate boron centers, (5-BMes2-2-ph-py)BMes2 (1) and its monoboron analogue, (2-ph-py)BMes2 (2) have been synthesized. Both compounds are luminescent but have a high sensitivity toward light. UV and ambient light cause both compounds to isomerize to 1a and 2a, respectively, via the formation of a C−C bond between a mesityl and the phenyl group, accompanied by a drastic color change from yellow or colorless to dark olive green or dark blue. The structures of 1a and 2a were established by 2D NMR experiments and geometry optimization by DFT calculations. Both 1a and 2a can thermally reverse back to 1 and 2 via the breaking of a C−C bond, with the activation barrier being 107 and 110 kJ/mol, respectively. The N,C-chelate ligands in 1 and 2 were found to play a key role in promoting this unusual and reversible photo–thermal isomerization process on a tetrahedral boron center. Reactions with oxygen molecules convert 1a and 2a to 5-BMes2-2-[(2-Mes)-ph]-py...
TL;DR: Kinetic studies reveal an unusual inverse dependence of the reaction rate on the concentration of the aminoalkene substrate, which can be accurately explained by a model in which the carbamate protecting group of the Aminalkene acts as a Brønsted base to remove free protons from the catalytic cycle and thereby inhibits the turnover-limiting protonolysis step.
Abstract: Mechanistic studies of the intramolecular hydroamination of unactivated aminoalkenes catalyzed by a dicationic [bis(diphenylphosphinomethyl)pyridine]palladium complex highlight the important role that protonolysis plays in this reaction. Coordination of the aminoalkene substrate to this complex activates the alkene toward intramolecular nucleophilic attack to form a dicationic palladium alkyl complex (6). A stable monocationic palladium alkyl complex (7) was isolated by in situ deprotonation of 6 with mild base, and its structure was confirmed by X-ray crystallography. Complex 7 reacted rapidly with a variety of strong acids to undergo protonolysis, resulting in formation of hydroamination product 3 and regenerating the active catalyst. Evidence that formation of the palladium alkyl complex is reversible under the catalytic conditions was obtained from observation of the protonolysis at low temperature. During the course of all catalytic reactions, the resting state of the catalyst was palladium alkyl com...
TL;DR: Single molecule electron transport measurements of two oligophenylenevinylene (OPV3) derivatives placed in a nanogap between gold or lead electrodes in a field effect transistor device change the transport mechanism from a coherent regime with finite zero-bias conductance to sequential tunneling and Coulomb blockade behavior.
Abstract: We report on single molecule electron transport measurements of two oligophenylenevinylene (OPV3) derivatives placed in a nanogap between gold (Au) or lead (Pb) electrodes in a field effect transistor device. Both derivatives contain thiol end groups that allow chemical binding to the electrodes. One derivative has additional methylene groups separating the thiols from the delocalized pi-electron system. The insertion of methylene groups changes the open state conductance by 3-4 orders of magnitude and changes the transport mechanism from a coherent regime with finite zero-bias conductance to sequential tunneling and Coulomb blockade behavior.
TL;DR: A phenomenon of aggregation-induced emission enhancement (AIEE): luminescence of polyacetylenes is dramatically boosted by aggregate formation, and the excited species of the polymers become longer-lived in the aggregates.
Abstract: Whereas chain aggregation commonly quenches light emission of conjugated polymers, we here report a phenomenon of aggregation-induced emission enhancement (AIEE): luminescence of polyacetylenes is dramatically boosted by aggregate formation. Upon photoexcitation, poly(1-phenyl-1-alkyne)s and poly(diphenylacetylene)s emit blue and green lights, respectively, in dilute THF solutions. The polymers become more emissive when their chains are induced to aggregate by adding water into their THF solutions. The polymer emissions are also enhanced by increasing concentration and decreasing temperature. Lifetime measurements reveal that the excited species of the polymers become longer-lived in the aggregates. Conformational simulations suggest that the polymer chains contain n = 3 repeat units that facilitate the formation of intramolecular excimers. The AIEE effects of the polymers are rationalized to be caused by the restrictions of their intramolecular rotations by the aggregate formation.