13C NMR spectroscopy of heteroaromatic compounds: A quantitative measure of electron deficiency and excessiveness

A quantitative measure, ^πΔ, for expressing the π-deficiency and π-excessiveness of heteroaromatic compounds has been developed through ¹³C NMR chemical shift studies. An example which correlates electrophilic reactivity in π-excessive systems with ^πΔ of these compounds is given. The following decreasing order of π-excessiveness and increasing π-deficiency of a number of heteroaromatic systems has been established (numbers in parentheses correspond to ^πΔ): tellurophene (1.83), selenophene (1.43), thiophene (1.35), furan (1.31), benzene (1.00), pyrazine (1.00) pyridine (0.89), pyrimidine (0.77), pyridazine (0.71), 1,2,4-triazine (0.64). These correlations also allow the prediction of ¹³C chemical shifts in substituted heteroaromatic compounds, where the ratio of the difference between a given substituted heterocyclic compound and its parent, and the identically substituted benzene derivative, has been determined.     

Catalytic intermolecular Pauson-Khand-type reaction: strong directing effect of pyridylsilyl and pyrimidylsilyl groups and isolation of Ru complexes relevant to catalytic reaction

Some circumstantial evidence for the directing effect of the 2-pyridylsilyl group in the Ru-catalyzed intermolecular Pauson-Khand-type reaction (PKR) of alkenyl(2-pyridyl)silane, alkyne, and carbon monoxide has been provided. Most importantly, we have succeeded in isolating several monometallic Ru complexes relevant to the catalytic reaction: Ru(vinylsilane)(CO)(3) complexes and ruthenacyclopentene. While the stoichimetric reaction of the Ru(vinylsilane)(CO)(3) complex with an alkyne led to the formation of the corresponding cyclopentenone (PKR product) at 100 degrees C, the ruthenacyclopentene intermediate was quantitatively produced at 50 degrees C. This complex was also converted to a cyclopentenone upon heating at 100 degrees C. Moreover, it was also found that the Ru(vinylsilane)(CO)(3) complex and ruthenacyclopentene serve as catalysts in intermolecular PKR.     

Rh(I)-catalyzed carbonylative cyclization reactions of alkynes with 2-bromophenylboronic acids leading to indenones

The Rh-catalyzed reaction of alkynes with 2-bromophenylboronic acids involves carbonylative cyclization to give indenones. The key steps in the reaction involve the addition of an arylrhodium(I) species to an alkyne and the oxidative addition of C-Br bonds on the adjacent phenyl ring to give vinylrhodium(I) species II. The regioselectivity depends on both the electronic and the steric nature of the substituents on the alkynes. A bulky group and an electron-withdrawing group favor the -position of indenones. In the case of silyl- or ester-substituted alkynes, the regioselectivity is extremely high. The selectivity increases in the order SiMe3 > COOR > aryl > alkyl. The reaction of norbornene with 2-bromophenylboronic acids under 1 atm of CO gives the corresponding indanone derivative. The reaction of alkynes with 2-bromophenylboronic acids under nitrogen gives naphthalene derivatives, in which two molecules of alkynes are incorporated. A vinylrhodium complex similar to II can also be generated by a different route by employing 2-bromophenyl(trimethylsilyl)acetylene and arylboronic acids in the presence of Rh(I) complex as the catalyst, resulting in the formation of indenones. The reaction of 1-(2-bromophenyl)-hept-2-yn-1-one with PhB(OH)2 in the presence of Rh(I) complex also resulted in carbonylative cyclization to give an indan-1,3-dione derivative.     

Heteroarylation of 6-Aryl-2,3-dihydroimidazo[2,1-b]thiazole with N-(Ethoxycarbonyl)heteroaromatic Salts

The ethyl chloroformate salts of a variety of benzo-fused six membered π-deficient heteroaromatics, including quinoline, isoquinoline, 4-chloroquinoline, 3-bromoquinoline, phthalazine, and quinazoline, reacted with 6-aryl-2,3-dihydroimidazo[2,1-b]thiazole at the 5-position. The dihydroheteroaromatic adducts were oxidized by o-chloranil, sulfur, or electrochemical methodology to form the 5-heteroaromatic-6-aryl-2,3-dihydroimidazo[2,1-b]thiazoles, 10-15 . In each example, the regiochemistry of addition to the heteroaromatic ring was established.     

Donor-(π-bridge)-azinium as D-π-A+ one-dimensional and D-π-A(+)-π-D multidimensional V-shaped chromophores

Heteroaromatic cations reacted with N-heteroarylacetylenes under Sonogashira conditions to allow easy access to potential single donor D-π-A(+) and V-shaped D-π-A(+)-π-D chromophores, where the acceptor moiety A is the π-deficient pyridinium cation and the donor moiety is represented by different π-excessive N-heterocycles. The β hyperpolarizabilities were measured using hyper-Rayleigh scattering experiments and the experimental data are supported by a theoretical analysis that combines a variety of computational procedures, including Density Functional Theory (DFT) and correlated Hartree-Fock-based methods (RCIS(D)).     

Density functional studies on the Pauson--Khand reaction.

The Pauson--Khand reaction represents a one-step Co(2)(CO)(8)-catalyzed synthesis of cyclopentenone through [2 + 2 + 1] assembly of one molecule each of alkene, alkyne, and carbon monoxide. Density functional studies (B3LYP/631LAN) on the reaction pathway of the Pauson--Khand (PK) reaction reported here for the first time provides valuable information on the structures and energetics of various intermediates and transition states. The PK reaction consists of olefin insertion, CO insertion, and reductive elimination steps. The olefin insertion step was found to be an irreversible step that determines the stereo- and regiochemistry of the overall reaction. The following steps are low activation energy processes and reversible. The bond-forming events occur only on one of the two metal atoms, while the second metal atom not only acts as an anchor that fixes the metal cluster to the organic substrate but also exerts electronic influences on the reaction at the first atom.     

Ligand effects on rates and regioselectivities of Rh(I)-catalyzed (5 + 2) cycloadditions: a computational study of cyclooctadiene and dinaphthocyclooctatetraene as ligands

The first theoretical study on the effects of ligands on the mechanism, reactivities, and regioselectivities of Rh(I)-catalyzed (5 + 2) cycloadditions of vinylcyclopropanes (VCPs) and alkynes has been performed using density functional theory (DFT) calculations. Highly efficient and selective intermolecular (5 + 2) cycloadditions of VCPs and alkynes have been achieved recently using two novel rhodium catalysts, [Rh(dnCOT)](+)SbF(6)(-) and [Rh(COD)](+)SbF(6)(-), which provide superior reactivities and regioselectivities relative to that of the previously reported [Rh(CO)(2)Cl](2) catalyst. Computationally, the high reactivities of the dnCOT and COD ligands are attributed to the steric repulsions that destabilize the Rh-product complex, the catalyst resting state in the catalytic cycle. The regioselectivities of reactions with various alkynes and different Rh catalysts are investigated, and a predictive model is provided that describes substrate-substrate and ligand-substrate steric repulsions, electronic effects, and noncovalent π/π and C-H/π interactions. In the reactions with dnCOT or COD ligands, the first new C-C bond is formed proximal to the bulky substituent on the alkyne to avoid ligand-substrate steric repulsions. This regioselectivity is reversed either by employing the smaller [Rh(CO)(2)Cl](2) catalyst to diminish the ligand-substrate repulsions or by using aryl alkynes, for which the ligand-substrate interactions become stabilizing due to π/π and C-H/π dispersion interactions. Electron-withdrawing groups on the alkyne prefer to be proximal to the first new C-C bond to maximize metal-substrate back-bonding interactions. These steric, electronic, and dispersion effects can all be utilized in designing new ligands to provide regiochemical control over product formation with high selectivities. The computational studies reveal the potential of employing the dnCOT family of ligands to achieve unique regiochemical control due to the steric influences and dispersion interactions associated with the rigid aryl substituents on the ligand.     

Selective synthesis of β-alkylpyrroles

β-Alkylpyrroles are key structural motifs found in many natural products and biologically active compounds as well as functional organic materials. For this reason, synthetic chemists continue to be interested in construction of the framework of β-alkylpyrroles. Due to sufficient aromaticity and π-excessive nature of pyrroles, a straightforward approach to β-alkylpyrroles should be electrophilic aromatic substitution (S(E)Ar) toward the pyrrole ring. However, since a primary nucleophilic site of pyrroles is an α-position, some "trick" is required to direct incoming alkyl electrophiles toward a β-position. This Concept article focuses on presenting previous efforts that have been devoted to the synthesis of β-alkylpyrroles, mainly through the S(E)Ar route.     

Organoyttrium-catalyzed sequential Cyclization/Silylation reactions of nitrogen-heteroaromatic dienes demonstrating "Aryl-Directed" regioselectivity

The reaction of 1-allyl-2-vinyl-1H-pyrroles and 1-allyl-2-vinyl-1H-indoles with arylsilanes in the presence of catalytic [Cp(TMS)(2)Y(&mgr;-Me)](2) leads to highly selective cyclization/silylation events. In this process the active catalyst for the reaction, "Cp(TMS)(2)YH", undergoes initial olefin insertion at the vinyl group. Even isopropenyl substituents on the heteroaromatics react in preference to less sterically encumbered allyl groups. Furthermore, the observed regioselectivity reflects an "aryl-directed" process, whereby the more highly substituted secondary or tertiary organometallic is initially generated. This intermediate undergoes cyclization onto the remaining alkene and subsequent silylation by a sigma-bond metathesis reaction, affording the observed products.     

Highly diastereoselective aziridination of imines with trimethylsilyldiazomethane. Subsequent silyl substitution with electrophiles, ring opening, and metalation of C-silylaziridines--a cornucopia of highly selective transformations

Treatment of a range of N-sulfonyl (Ts and SES) imines derived from aromatic, heteroaromatic, aliphatic, and unsaturated aldehydes with trimethylsilydiazomethane gave C-silylaziridines in good yield (32-83%) and with high diastereoselectivity in favor of the cis product (80:20-100:0). In contrast, an alpha-imino ester gave predominantly the trans-aziridine (89:11) in high yield (91%). The synthetic potential of C-silylaziridines was investigated. Treatment with F(-) (tetrabutylammonium triphenyldifluorosilicate was used) in the presence of aldehydes gave the alpha-hydroxyaziridines in high yield and high diastereoselectivity (86:14-98:2) for the newly created stereogenic center. Complete retention of configuration was observed in the substitution of the silyl group with electrophiles in all cases. Trapping with deuterium (using CDCl(3) as electrophile) was also successful, but trapping with phosphate [using ClP(O)(OPh)(2)] and acetate (using Ac(2)O) was unsuccessful. In these latter cases ring opening by chloride and acetate, respectively, was observed. Further ring-opening reactions were effected using azide and thiolate nucleophiles and in all cases complete regioselectivity in favor of attack at the silyl-bearing carbon occurred. Complete regioselectivity was also observed in the carbonylative ring expansion using Co(2)(CO)(8) to give a beta-lactam. Treatment of cis-1-tosyl-2-phenyl/butyl-3-trimethylsilylaziridines with n-BuLi and subsequent quenching with MeI followed completely different pathways, depending on the 2-substituent. In the case of the 2-phenylaziridine, metalation was initiated alpha to the phenyl group and led finally to a fused tricyclic adduct with four stereogenic centers as a single diastereoisomer. In the case of the 2-butylaziridine, metalation occurred alpha to the silyl group and led to a trisubstituted silylaziridine, probably via an azirine intermediate.     

一维链状配合物[Cd(FA)(pyim)(H2O)]的合成、晶体结构和荧光性质

A one-dimensional zigzag chain complex of Cd(FA)(pyim)(H₂O) (1) (H₂FA=4,4′-(hexafluoroisopropylid-ene)bis(benzoic acid) and pyim=2-(2-pyridyl)imidazole) was hydrothermal synthesized from Cd(NO₃)₂·4H₂O, 4,4′-(hexafluoroisopropylidene)bis(benzoic acid) and 2-(2-pyridyl)imidazole. Single-crystal X-ray analysis revealed that the zigzag chains of 1 are linked into three-dimensional supramolecular networks by both O-H…O hydrogen-bonds and π…π stacking interactions. The crystal is of monoclinic, space group P2₁/n with a=0.856 34(10) nm, b=1.020 49(12) nm, c=2.809 0(3) nm, β=98.115(2)°, V=2.430 2(5) nm³, D_c=1.820 g·cm^-3, Z=4, F(000)=1 320, Goof=1.009, R₁=0.047 0, wR₂=0.115 5. In addition, complex 1 exhibits strong photoluminescent emission at room temperature. CCDC: 733837.     

Composing NLO Chromophore as a Puzzle: Electrochemistry-based Approach to Design and Effectiveness

A series of D-π-A, D-π-D′-π-A, D-π-A′-π-A nonlinear optical chromophores with vinylene π-electron bridges or bridges with π-deficient/π-excessive heterocyclic moieties along with the corresponding precursors D-vinylene, D-π-D′, D′-π-A, D-π-A′ and A′-π-A are synthesized and studied both experimentally and computationally. The effect of the heterocycle in the π-electron bridge on the oxidation/reduction potentials and the energy gap (ΔE^el) is investigated in detail. The properties of the D-π-A′(D′)-π-A chromophores are shown to correlate with those of building blocks: the oxidation potential is determined by the D-vinylene, and the reduction potential is determined by A′(D′)-π-A truncated compounds. The contribution of the acceptor to the oxidation potential of chromophores in comparison with those of the precursors was estimated and analyzed in terms of electronic communication between the end groups. A good correlation between the ΔE^el and the chromophores’ first hyperpolarizability is revealed.     

First C-C bond formation in the Pauson-Khand reaction: Influence of carbon-carbon triple bond polarization on regiochemistry: A density functional theory study

The influence of carbon-carbon triple bond polarization on the regiochemistry of the Pauson-Khand reaction has been studied with the B3LYP functional. The regiochemistry determining step of this reaction, i.e., olefin insertion leading to cobaltacycle formation, has been examined with ethylene as the olefin and propyne, methyl 2-butynoate, and methyl propiolate as the acetylenes. From this study, it has been concluded that, in absence of overwhelming steric effects of an acetylene substituent, the regiochemistry is influenced by the polarization of the acetylenic bond, which arises from the different substituents. The initial C-C bond is preferentially formed with the acetylenic carbon that has the greater electron density: with propyne, this leads to a cyclopentenone having the methyl group in the α-position; with methyl 2-butynoate, to a cyclopentenone with the CO₂Me in the β-position; with methyl propiolate, which is virtually unpolarized in the complex, to a cyclopentenone with the CO₂Me in the α-position (a result of steric effects). These theoretical results are concordant with those observed experimentally with norbornene. The question of axial versus equatorial reactive positions for the coordinated olefin is also addressed and a kinetic simulation is presented.     

Ruthenium‐Catalyzed [2+2+2] Cycloaddition of 1,6‐Enynes and Unactivated Alkynes: Access to Ring‐Fused Cyclohexadienes

The [2+2+2] intermolecular carbocyclization reactions between 1,6‐enynes and alkynes catalyzed by [RuCl(cod)(Cp*)] (cod=1,5‐cyclooctadiene, Cp*=pentamethylcyclopentadienyl) are reported to provide bicyclohexa‐1,3‐dienes. The presented reaction conditions are compatible with internal and terminal alkynes and the chemo‐ and regioselectivity issues are controlled by the presence of substituents at the propargyl carbon center of the alkyne(s) partner(s).     

Electronically excited states of membrane fluorescent probe 4-dimethylaminochalcone. Results of quantum chemical calculations

Quantum-chemical calculations of ground and excited states for membrane fluorescent probe 4-dimethylaminochalcone (DMAC) in vacuum were performed. Optimized geometries and dipole moments for lowest-lying singlet and triplet states were obtained. The nature of these electronic transitions and the relaxation path in the excited states were determined; changes in geometry and charge distribution were assessed. It was shown that in vacuum the lowest existed level is of (n, π*) nature, and the closest to it is the level of (π, π*) nature; the energy gap between them is narrow. This led to an effective (1)(π, π*) →(1)(n, π*) relaxation. After photoexcitation the molecule undergoes significant transformations, including changes in bond orders, pyramidalization angle of the dimethylamino group, and planarity of the molecule. Its dipole moment rises from 5.5 Debye in the ground state to 17.1 Debye in the (1)(π, π*) state, and then falls to 2 Debye in the (1)(n, π*) state. The excited (1)(n, π*) state is a short living state; it has a high probability of intersystem crossing into the (3)(π, π*) triplet state. This relaxation path explains the low quantum yield of DMAC fluorescence in non-polar media. It is possible that (3)(π, π*) is responsible for observed DMAC phosphorescence.     

Ligand-controlled regioselectivity in the hydrothiolation of alkynes by rhodium N-heterocyclic carbene catalysts

Rh-N-heterocyclic carbene compounds [Rh(μ-Cl)(IPr)(η(2)-olefin)](2) and RhCl(IPr)(py)(η(2)-olefin) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene, py = pyridine, olefin = cyclooctene or ethylene) are highly active catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity switch from linear to 1-substituted vinyl sulfides was observed when mononuclear RhCl(IPr)(py)(η(2)-olefin) catalysts were used instead of dinuclear precursors. A complex interplay between electronic and steric effects exerted by IPr, pyridine, and hydride ligands accounts for the observed regioselectivity. Both IPr and pyridine ligands stabilize formation of square-pyramidal thiolate-hydride active species in which the encumbered and powerful electron-donor IPr ligand directs coordination of pyridine trans to it, consequently blocking access of the incoming alkyne in this position. Simultaneously, the higher trans director hydride ligand paves the way to a cis thiolate-alkyne disposition, favoring formation of 2,2-disubstituted metal-alkenyl species and subsequently the Markovnikov vinyl sulfides via alkenyl-hydride reductive elimination. DFT calculations support a plausible reaction pathway where migratory insertion of the alkyne into the rhodium-thiolate bond is the rate-determining step.     

Transition metal promoted reactions of unsaturated hydrocarbons : II. Insertion of bicyclic olefins into allylic-palladium and -platinum bonds

Addition of strained olefins, based on norbornene, norbornadiene,benzonorbornadiene or bicyclo [2.2.2] octene skeletons to π-allylic(hexafluoroacetylacetonato) palladium(II) complexes [(π-All)Pd(Hfacac)], gives “enyl” products derived from “insertion” of the olefin into the least substituted terminal allylicpalladium bond. The reaction involves an initial rapid and reversible formation of (gs-allyl)(π-olefin)Pd(Hfacac). The rate-determining step involves migration of a σ-allylic carbon atom from Pd to the coordinated olefin in a concerted cis—exo addition of PdC across the double bond. Remote electronegative substituents on the olefin do not affect the coordinative ability of the olefin towards Pd. They do however inhibit the migration of the σ-allylic ligand to the coordinated olefin. This observation is interpreted in terms of a small degree of polarization of the π-olefin—Pd bond in the transition state for the σ-allyl migration.     

Redox‐Neutral Manganese(I)‐Catalyzed C−H Activation: Traceless Directing Group Enabled Regioselective Annulation

A strategy is reported in which traceless directing groups (TDGs) are used to promote the redox‐neutral Mn^I‐catalyzed regioselective synthesis of N‐heterocycles. Alkyne coupling partners bearing a traceless directing group, which serves as both the chelator and internal oxidant, were used to control the regioselectivity of the annulation reactions. This operationally simple approach is highly effective with previously challenging unsymmetrical alkyne systems, including unbiased dialkyl alkynes, with perfect regioselectivity. The simple conditions and the ability to carry out synthesis on a gram scale underscore the usefulness of this method. The application of this strategy in the concise synthesis of the bioactive compound PK11209 and the pharmaceutical moxaverine is also described.     

Dissociation energetics of the phenol(+)⋯Ar(2) cluster ion: The role of π→H isomerization

The dissociation energetics in the phenol(+)?Ar(2)(2π) cluster ion have been investigated using photoionization efficiency and mass analyzed threshold ionization spectroscopy. The appearance energies for the loss of one and two Ar atoms are determined as ～210 and ～1115?cm(-1), respectively. The difference between the appearance energy for the first Ar ligand in phenol(+)?Ar(2)(2π) and the dissociation energy of the phenol(+)?Ar(π) dimer (535cm(-1)) is explained by the isomerization of one π-bound Ar ligand to the OH binding site (H-bond) upon ionization. The energy difference between phenol(+)?Ar(2)(2π) and phenol(+)?Ar(2)(H/π) could also be estimated to be around 325cm(-1), which corresponds roughly to the difference of the binding energy of a π-bound and H-bound Ar ligands. The binding energy of the H-bound Ar atom in phenol(+)?Ar(2)(H/π) is derived to be ～905cm(-1).     

Synthesis and properties of <Emphasis Type="Italic">trans</Emphasis>-2-[2-(2-hetaryl)vinyl]benzothiazoles

Electrophilic substitution reactions (formylation and acylation) in the series of 2-[2-(2-furyl)vinyl]- and 2-[2-(2-thienyl)vinyl]benzothiazoles leads to the corresponding derivatives at the α-position of the furan or thiophene ring. The presence of a vinylene bridge weakens deshielding effect of the benzothiazole fragment on π-excessive heterocycles, so that such compounds react at a higher rate and under milder conditions as compared to hetarylbenzothiazoles having no vinylene bridge.