Valence isomerization in dendrimers by photo-induced electron transfer and energy transfer from the dendrimer backbone to the core

A series of poly(aryl ether) dendrimers with a norbornadiene (NBD) group attaching to the core (Gn-NBD), generations 1–4, were synthesized and characterized, and their photophysical and photochemical properties were examined. The fluorescence of the dendrimer backbone is quenched by the norbornadiene group as a result of the electron transfer and energy transfer from the dendrimer backbone to the norbornadiene group in Gn-NBD. Selective excitation of the dendrimer backbone results in an isomerization of the norbornadiene group to the quadricyclane (QC) group. The intramolecular electron transfer and energy transfer efficiencies are ca. 0.93, 0.73, 0.54, 0.30 in dichloromethane, and ca. 0.90, 0.70, 0.55, 0.34 in tetrahydrofuran for generations 1–4, respectively, with the rate constant ca. 10¹⁰ s⁻¹. The light-harvesting ability of these dendritic molecules is demonstrated by the enhanced valence isomerization rate of NBD to QC with increasing generation.     

Quantum chemical study of the mechanism of catalytic [2+2+2] cycloaddition of acrylic acid esters to norbornadiene in the presence of nickel(0) complexes

The mechanism of the [2+2+2] cycloaddition of ethyl acrylate to norbornadiene (NBD) catalyzed by Ni⁰ complexes was modeled in terms of the density functional theory (DFT) at the PBE level. The formation of the first C—C bond between the coordinated NBD and ethyl acrylate molecules is the rate-determining step of the process. The low stereoselectivity of the reaction is due to the close matching of the activation barriers for the formation of exo- and endo-cycloadducts (25.6 and 24.9 kcal mol^?1, respectively).     

An unusual way of the reaction of PI3 with norbornene and norbornadiene

The iodination of norbornene and norbornadiene with phosphorus triiodide in the presence and in the absence of weak electrophiles (arylsulfenamides orN-chloramines) and with elemental iodine was studied. Depending on the reaction conditions, the reactions resulted in mono- or diiodides of the norbornene and norbornadiene series. All the compounds obtained were characterized by¹H and¹³C NMR spectra.     

Quantum chemical modeling of the cis-trans isomerization of the allyl ligand in Ni(η3-C3H5)2 in the presence of norbornadiene

Quantum chemical (DFT/PBE) modeling of the cis-trans isomerization of the allyl ligand in bis(η³-allyl)nickel in the presence of norbornadiene revealed that the type of coordination of the norbornadiene ligand affects the energy parameters of its isomerization. The Gibbs energies of activation of the rate-limiting step for different isomerization pathways range from 23.7 to 27.8 kcal mol^?1.     

Quantum-chemical study on the mechanism of catalytic dimerization of norbornadiene in the presence of hydride nickel(I) complex

[2+2]-Cyclodimerization of norbornadiene to pentacyclic products with exo-trans(cis)-exo structure in the presence of model catalytically active η⁴-norbornadiene nickel hydride complex was simulated at the DFT/PBE level of theory. According to the calculations, the rate-determining step in the cyclodimerization process is reductive elimination of the metallacycle whose decomposition yields norbornadiene dimer. The formation of cis-dimer is unfavorable for both kinetic and thermodynamic reasons.     

Chemical synthesis with metal Atoms. Cyclodimerization of norbornadiene via nickela-cyclopentane intermediates.

The catalytic cyclodimerization of norbornadiene by reaction with nickel atoms has been re-investigated. Nickela-cyclopentane derivatives are formed in the presence of α,α′-dipyridyl, α,α′-dipyridyl-exo-trans-endo-3-nickela-pentacyclo[9.2.1.1^5,8 0^2,10.0^4,9]-pentadeca-6,12-diene being the major component. By contrast, the catalytic dimerization leads predominantly to the exo-trans-exo isomer of pentacyclo[8.2.1.1^4,7.0^2,9.0^3,8] tetradeca-5,11-diene. A norbornadiene cyclotrimer of exo-trans-exo-trans-exo structure is subsequently formed.     

Iridium and rhodium complexes with the planar chiral thioether ligands in asymmetric hydrogenation of ketones and imines

Rhodium complexes with the planar chiral phosphinoferrocenyl thioether ligands [Rh(P,SR)(diene)X] (R = Me, Bu^t, Ph, Bn, diene is cyclooctadiene (COD) or norbornadiene (NBD), X = Cl, BF₄) catalyze hydrogenation of ketones, imines, and heteroaromatic compounds; in the case of acetophenone, the enantioselectivity reached 60% ee. Similar iridium complexes demonstrate a good activity in the hydrogenation of imines, the maximal enantioselectivity in the case of N-phenyl-N-(1-phenylethylidene)amine was about 40% ee.     

H-Ge bond activation by tungsten carbonyls: An experimental and theoretical study

The activation of the Ge-H bond and the formation of several hydride complexes, characterized by high-field resonances, have been detected during the ¹H NMR spectroscopy monitoring of the photochemical reaction of Et₃GeH and Et₂GeH₂ with W(CO)₆ and the norbornadiene complex [W(CO)₄(η⁴-nbd)]. The activation of the Ge-H bond of triethylgermane in the photochemical reactions of tungsten(0) complexes has been applied in the hydrogermylation of norbornadiene (nbd), which leads to the formation of endo-triethylgermylnorbornene as the major product. The complex [{W(μ-η²-H-GeEt₂)(CO)₄}₂] has been fully characterized by NMR spectroscopy and by a single-crystal X-ray diffraction study. Evidence for the hydride ligand of the W(μ-η²-H-GeEt₂) group has been provided by ¹H NMR spectroscopy (δ = −9.02, ¹J_H-W = 31 Hz) and by DFT calculations. A DFT study of the structural properties and ¹H NMR chemical shifts of several possible intermediate σ and hydride complexes formed during the photochemical reaction of W(CO)₆ and Et₂GeH₂ has been performed.     

Radiation-induced Cis to Trans isomerization of poly(n-butylpropiolate) prepared with a [Rh(norbornadiene)Cl]2 complex as a stereospecific catalyst

Normal alkylpropiolate, i.e., n-butylpropiolate: HCCCOOnC₄H₉ was polymerized with [Rh(norbornadiene)Cl]₂ catalyst in methanol solvent to produce poly(n-butylpropiolate), (PBPA) polymer called polyacetylene esters in relatively high yields. The obtained polymer was characterized in detail using ¹HNMR, gel permeation chromatography (GPC), diffuse reflective UV methods. Consequently, the data showed that the resulting polymers selectively have cis-transoid form. We also found that γ-ray irradiation of the cis isomer resulted in isomerization to the trans isomer even at room temperature in air to generate the extended trans sequences which can stabilize mobile unpaired electrons as π-radicals called solitons.     

Rhodium catalyzed hydroformylation of 2-phenylsulfonylbicyclo[2.2.1] alkenes: effect of the phenylsulfonyl group

The preliminary results of the hydroformylation of 2-phenylsulfonyl substituted norbornene and norbornadiene derivatives catalyzed by the unmodified Rh(CO)₂acac system are presented. The reaction, occurring under standard oxo conditions, gives polyfunctionalized exo norbornene- and exo norbornanecarboxaldehydes. The effect of the phenylsulfonyl group has been evaluated: it has been found that the steric properties of the sulfonyl substituent, more than the electronic ones, influence the regioselectivity of the process.     

The predictable rearrangement of tetracyclic C8H9 cations

The homo-1,4 adduct obtained by addition of dichlorocarbene to norbornadiene on reaction with diethylzinc in ether solution gives 2-chloro-3-ethyltetracyclo[3.3.0.0^2,8.0^4,6]octane (46.5%), 2-chlorobicyclo[3.2.1]octa-2,6-diene (5.8%) and its 4-ethyl derivative (47.7%). The exo and endo monochloro derivatives (obtained from the homo-1,4 adduct) on reaction with diethylzinc, are inert for the former, whereas the endo isomer reacts as before, undergoing reductive dechlorination with rearrangement. The C₈H₉ cations involved behave as predicted by MINDO/3 calculations.     

Determination of the absolute configuration of two αvβ6 integrin inhibitors for the treatment of idiopathic pulmonary fibrosis and investigations on the asymmetric 1,4-addition of arylboronic acids to crotonate esters bearing a C4-oxygen substituent

The absolute configuration of two novel α_vβ₆ integrin inhibitors was established via degradation to the corresponding C3-aryl substituted butyrolactone. The configuration of the resulting lactones was established by asymmetric synthesis using 1,4-addition of arylboronic acids to butenolide, catalysed by bis(norbornadiene)rhodium (I) tetrafluoroborate in the presence of (R)-BINAP, and confirmed by X-ray crystallography. Studies on arylboronic acid conjugate additions to acyclic crotonate esters bearing a γ-oxygen substituent are also reported. Three Rh catalysts were investigated and the one giving the highest enantioselectivity was bis(norbornadiene)rhodium (I) tetrafluoroborate.     

Activation of the Ge-H bond of Et3GeH in photochemical reaction with molybdenum(0) carbonyl complexes and hydrogermylation of norbornadiene

The germane intermediate σ-complexes, characterized by high-field resonances in the region from −6 to −8 ppm, have been detected during the ¹H NMR spectroscopy monitoring of the photochemical reaction of Et₃GeH with Mo(CO)₆, [Mo(CO)₄(η⁴-cod)], and [Mo(CO)₄(η⁴-nbd)] in the NMR tube. The activation of the Ge-H bond of germane in photochemical reaction of the norbornadiene (nbd) complex [Mo(CO)₄(η⁴-nbd)] has been applied in the hydrogermylation of norbornadiene, which leads to the formation of triethylgermylnorbornene.     

The vibrational spectra of complexes of norbornadiene with some group VIII metals

The infrared and Raman spectra of norbornadiene complexes of Pd, Pt, Rh and Fe have been studied. An assignment of the normal modes is given and the ligand vibrations in the complex are compared with those for “free” norbornadiene. The ν(CC) stretching frequency of coordinated norbornadiene is from 1575 cm^?1 to over 1400 cm^?1.The strength of the metal—ligand bond increases in the series Pd < Pt < Rh and for halogen complexes in the series chloride < bromide < iodide. ESCA spectroscopy data obtained are in agreement with these conclusions.     

A study of the negative ion states of selected cyclodienes by electron transmission spectroscopy

Electron transmission spectroscopy is employed to locate sharp variations in the total cross sections for electrons scattered from several cyclic compounds containing two carboncarbon bonds. For each molecule, structure is observed which we associate with the temporary occupation of the two low-lying, normally unfilled, π^* orbitals by impacting electrons. Electron affinities are reported for 1,5-cyclooctadiene, 1,4-and1,3-cyclohexadiene, norbornadiene and also cyclohexene, propene, and cis-butene.     

Reactions of areneselenenamides with alkenes in the presence of phosphorus(V) and sulfur(IV) oxyhalides. New synthesis of β-haloalkyl selenides

A new procedure was proposed for activation of areneselenenamides with phosphorus(V) and sulfur(IV) oxyhalides. According to the ¹H, ¹³C, and ³¹P NMR data, areneselenenamide reacts with phosphorus oxyhalide to form intermediate adduct in which the phosphorus atom is coordinated at the selenium. Areneselenenamides activated by phosphorus(V) oxyhalides react with alkenes (norbornene and norbornadiene) with high trans-stereoselectivity. Their reactions with terminal olefins are regioselective, and they lead to preferential formation of the corresponding Markovnikov adducts.     

The synthesis of new framework structures containing rigidly-linked pyrimidines suitable for uracil and cytosine elaboration

Fused norbornenes and tricyclo[4.2.2.1^2,5]undec-3,7,9-trienes containing 2,4-dichloropyrimidine and 2,4-dimethoxy pyrimidine units are coupled with norbornadiene, benzonorbornadienes and dimethoxynaphthaleno norbornadienes using s-tetrazines or ACE coupling techniques to afford framework structures where different geometrical interrelationships can be attained between the chromophores.     

[6+2]Cycloadditions catalyzed by titanium complexes

The Ziegler catalyst TiCl₄-Et₂AlCl and the arenetitanium(II) complex (η6-C₆H₆)Ti(II)(AlCl₄)₂ induce [6 + 2]cycloaddition reactions of cycloheptatriene with dienes and acetylenes. Addition to 1,3-butadiene affords 7 - endo - vinyl - bicyclo[4.2.1]nona - 2,4 - diene (main product) and bicyclo[4.4.1]- undeca - 2,4,8 - triene, a product of [6+4]cycloaddition. Isoprene reacts similarly, yielding mainly 7- endo - isopropenyl - bicyclo[4.2.1]nona - 2,4 - diene. 2,3 - Dimethyl - 1,3 - butadiene gives 8,9dimethylbicyclo [4.4.1]undeca - 2,4,8 - triene, a product of [6 + 4]cycloaddition, while [6 + 2]cross-adducts are minor products. The reaction of cycloheptatriene with norbornadiene gives mainly hexacyclo[6.5.1.0^2,7.0^3,12.^6,10.0^9,13]tetradec - 4 - ene via [6+2]cycloaddition followed by intramolecular Diels-Alder reaction. As a by-product, pentacyclo[7.5.0.0^2,7.0^3,5.0⁴⁸]tetradeca - 10,12 - diene is formed by a [2+2+2]mechanism. Addition of cycloheptatriene to diphenylacetylene and bis - (tri- methylsilyl)acetylene furnishes sustituted bicyclo[4.2.1]nona - 2,4,7 - trienes. Alkenes, E,E-2,4 - hexadiene and 1,3 - cyclooctadiene are unreactive. The [6+2]cycloaddition is made possible by coordination of cycloheptatriene to titanium, which changes the symmetry of the frontier orbitals in the triene. The reactivity of the trienophile is also enhanced by coordination to the catalyst.     

Catalytic syntheses of polycyclic compounds based on norbornadiene in the presence of nickel complexes: VI. Heterogenized catalyst for isomerization reactions of norbornene derivatives

The kinetics and mechanism of the thermal izomerization of 5-exo-methyl-tricyclo[3.2.1.0^5,7]octene-2 into 2-methyl-tetracyclo[3.2.1.0^2,7.0^4,6]octane are reported. The use of catalysts based on nickel compounds is demonstrated. As compared to the other catalytic systems, the heterogenized nickel catalyst obtained by thermal treatment of nickel norbornadiene complexes shows much better performance characteristics (selectivity, specific activity, and service life).     

Catalytic Syntheses of Polycyclic Compounds Based on Norbornadiene in the Presence of Nickel Complexes: IV. Problems of Regioselectivity in Norbornadiene Codimerization with Activated Olefins

Cyclocodimerization of norbornadiene with acrylates in the presence of bis(³-allyl)nickel is studied. The regio- and stereoselectivities of cycloaddition depend on the catalyst composition, temperature, reactant ratio, and the nature of acrylate. Conditions excluding the side direction (norbornadiene homodimerization) are selected. A mechanism that explains the main regularities of codimerization is proposed.