bluuebunny/crossref_metadata_2025 huggingface dataset:
New potential dendritic cores: Selective chemistry of dipentaerythritol
Authors: Al-Mughaid Hussein , Grindley T Bruce , Robertson Katherine N , Cameron T Stanley
Published: July 2003

Methods were developed to convert dipentaerythritol efficiently into di-O-benzylidene, di-O-cyclohexylidene, di-O-1-ethylpropylidene, and di-O-methylene diacetals. The known di-O-benzylidene acetal, considered to be a single isomer, was shown to be close to a statistical mixture of the cis,cis-, cis,trans-, and trans,trans-isomers. Structures were established by a combination of NMR spectroscopy and X-ray crystallography. Reaction of the di-O-benzylidene acetals with benzyl chloride and potassium hydroxide surprisingly gave good to excellent yields of the monobenzyl ethers. An explanation for this observation was advanced. The dibenzyl derivatives of the di-O-benzylidene acetals were formed by reaction of their dianions with benzyl bromide in DMF. Selective reduction of the di-O-benzylidene acetals with triethylsilane and boron trifluoride etherate gave 2',6'-di-O-benzyldipentaerythritol. A crown ether was formed in modest yield (12%) by reaction of the dianion of the di-O-methylene acetal with α,α'-dibromo-m-xylene in DMF. Key words: dipentaerythritol, pentaerythritol, acetals, dendrimer, selective benzylation, crown ether.
Regioselectivity of Meisenheimer complexation in reaction of oxygen-centred nucleophiles with picryl aryl ethers: Polar vs. SET mechanisms
Authors: Buncel Erwin , Dust Julian M , Manderville Richard A , Tarkka Richard M
Published: July 2003

Picryl alkyl ethers react with hydroxide and methoxide ions to give regioisomeric Meisenheimer (anionic σ-) adducts; the C-3 adduct is kinetically favoured and the C-1 adduct is thermodynamically favoured (K3T1 behaviour). In the current 400 MHz NMR spectroscopic study of the reactions of two picryl aryl ethers, picryl phenyl ether (PicOPh, 1) and picryl mesityl ether (PicOMes, 2), the charge localized nucleophiles OHand MeOdisplayed the same K3 regioselectivity as found with picryl alkyl ethers; attachment at C-1 leads to SNAr displacement of the aryloxide. In contrast, phenoxide (PhO) and the sterically demanding 2,4,6-trimethylphenoxide (mesitoxide, MesO) react with 1 and 2 to form the C-1 O-adduct as the product of kinetic control (i.e., K1 behaviour). These reactions were studied at low temperature (40°C in acetonitrile-d3:dimethoxyethane-d101:1) and as a function of increasing temperature (40°C to ambient). On the thermodynamic side, the C-1 PhOO-adduct of 1 is also the more stable of the possible phenoxide O-adducts; it shows T1 regioselectivity within the manifold of O-adducts (K1T1), but the C-3 C-adduct (via para-attack of PhO) is the ultimate thermodynamic product. The C-1 O-adducts formed by MesOwith 1 or 2 give way with time (or temperature increase) in favour of their C-3 regioisomers or a C-1,3-O-diadduct. Mesitoxide, therefore, displays K1T3 regioselectivity. Stereoelectronic stabilization is discussed as a factor influencing T1 regioselectivity in O-adduct formation. Frontier molecular orbital (FMO) interactions between the HOMO of the nucleophile and the LUMO of the picryl ether may play a role in the K1 preference of aryloxides. An alternative argument is presented based on a single electron (radical) transfer (SET) pathway for the aryloxide nucleophiles rather than the polar (SNAr) pathway for hydroxide and methoxide. The SET pathway also predicts a kinetic preference for C-1, as the C-1 position is of higher spin density than C-3 in the radical anion of the picryl ether and thus should be the preferred site for coupling by the aryloxide radical.Key words: anionic Meisenheimer adducts, regioselectivity, kineticthermodynamic control, FMO, stereoelectronic stabilization, single electron transfer (SET).
Variation of the ease of α-sulfonyl carbanion formation with the orientation of different β-substituents: Experimental evidence for the generality of negative hyperconjugation as an important substituent effect
Authors: King James F , Li Manqing , Cheng Allan Zijun , Dave Vinod , Payne Nicholas C
Published: July 2003

Following up on our previous observation that the rate of formation of a β-alkoxy-substituted α-sulfonyl carbanion depends on the stereochemistry of the alkoxy group, we have found similar behaviour when the β-substituent is R2N, RS, or R3N+. With each substituent, the variation of kN (defined by kN = (kexch)X /(kexch)model) is consistent with an equation of the form log kN = a + b cos2θ, where θ is the H-C-C-X torsion angle. We propose that the a term describes the polar (field plus inductive) effect and the b term the negative hyperconjugative effect of the substituent; we show how the variations in a and b may be readily accommodated within this framework. Some features of the trialkylammonio group previously postulated in the literature are discussed in the light of our results.Key words: negative hyperconjugation, substituent effects, stereoelectronic factors, sulfonyl carbanions, anomeric effect.
Photocycloaddition of cyclohex-2-enones to acrylonitrile
Authors: Meyer Lars , Alouane Nacira , Schmidt Kerstin , Margaretha Paul
Published: July 2003

The photocycloaddition of cyclohex-2-enones 1a, 1b, and 5a5c to acrylonitrile (2) in benzene has been studied. The reactions proceed with moderate regioselectivity and with preferential formation of diastereomeric mixtures of endo- and exo-5-oxobicyclo[4.2.0]octane-7-carbonitriles 3, 4, and 68. Very similar relative rates of conversion to products are observed in the photocycloadditions of 1b, 5a, and 5b to 2 and methacrylonitrile (9), respectively. In contrast, 5,5-dimethylcyclohex-2-enone (1b) forms cycloadducts with 2,3-dimethylbut-2-ene (12) as efficiently as with the unsaturated nitriles, but the 2,5,5-trialkylcyclohex-2-enones 5a and 5b do not react with this alkene at measurable rates of conversion. A dual path mechanism is presented that involves either exciplex formation between triplet-excited cyclohexenones and electron-rich alkenes or radical addition of the former to electron-deficient alkenes bearing a substituent, exercising a resonance effect.Key words: [2+2]-photocycloaddition, cyclobutane, cyclobutanecarbonitrile, radical addition to unsaturated nitriles.
Dimethylplatinum(IV) chemistry: Stannyl, hydride, hydroxide, and aqua complexes
Authors: Prokopchuk Ernest M , Puddephatt Richard J
Published: July 2003

The dimethylplatinum(II) complex [PtMe2(Me3TACN)] (Me3TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane) reacts easily with Me3SnCl, Me2SnCl2, or HX (X = CF3CO2, CF3SO3, BF4) to give the thermally stable cationic dimethyl platinum(IV) complexes [Pt(SnMe3)Me2(Me3TACN)]+, [Pt(SnMe2Cl)Me2(Me3TACN)]+, or [PtHMe2(Me3TACN)]+. The complexes [PtMe2(Me3TACN)] and [PtHMe2(Me3TACN)]+are oxidized by moist air to give the hydroxo complex [PtMe2(OH)(Me3TACN)]+, which can then be protonated reversibly to form the aqua complex [PtMe2(OH2) (Me3TACN)]2+. The structures of the hydroxo complex [PtMe2(OH)(Me3TACN)]+, as both the BF[Formula: see text]or CF3SO[Formula: see text]salt, and of the mixed hydroxo, aqua complex [PtMe2(OH)(Me3TACN)][PtMe2(OH2)(Me3TACN)][BF4]3have been determined and the complexes are shown to display interesting hydrogen bonding. Key words: platinum, oxidation, organometallic, tin, hydride.
Photo aldol reactions with 5-methoxyoxazoles: Highly regio- and diastereoselective synthesis of α-amino β-hydroxy carboxylic acid derivatives
Authors: Griesbeck Axel G , Bondock Samir
Published: July 2003

A versatile route to derivatives of α-amino β-hydroxy carboxylic acids, either with tertiary (from a corresponding glycine equivalent) or a quaternary α-carbon center is described. The key reaction is the cycloaddition of electronically excited carbonyl compounds (aromatic and aliphatic aldehydes, respectively) to oxazoles. To make the products hydrolytically more labile, a methoxy substituent at position C-5 was introduced leading to the formation of cycloadducts with an orthoester substructure. The photocycloaddition, either with triplet excited (aromatic) carbonyls or with singlet excited (aliphatic) carbonyls, led to the formation of mixtures of endo- and exo-diastereoisomers with moderate to very high exo-selectivities. The 4-unsubstituted 5-methoxyoxazole 1 (glycine equivalent) gave the [2 + 2]-adducts 3a3f with aldehydes 2a2f in high yields and excellent diastereoselectivities. Hydrolysis of these compounds resulted in the α-amino β-hydroxy esters 4a4f with preferred erythro (S*,S*) configuration. As an extension of this process, 4-alkylated 5-methoxyoxazoles 5a5f were applied as alkene components and the corresponding cycloadducts with benzaldehyde 6a6f were obtained. Again, the exo-diastereoisomers were formed as the major products in diastereoselectivities from 73:27 (exo:endo) up to >95:5. Hydrolysis of these adducts resulted in the formation of α-amino β-hydroxy esters 7a7f with like (S*,S*) configuration of the major diastereoisomers.Key words: photo aldol reaction, oxazoles, oxetanes, photocycloaddition, amino acids, hydroxy acids.
The photochemistry of trans-1,4,4,4-tetraphenyl-but-2-en-1-one: A highly efficient aryl migration (type B) enone photorearrangement
Authors: Scheffer John R , Vishnumurthy Kodumuru
Published: July 2003

Photolysis of trans-1,4,4,4-tetraphenylbut-2-en-1-one (3) in acetonitrile or benzene leads to transcis isomerization (7) along with rearrangement to trans-1-benzoyl-2,2,3-triphenylcyclopropane (8). Formation of the latter product represents a new example of the aryl migration (type B) enone photorearrangement reaction first reported by Zimmerman and co-workers for 4,4-diphenylcyclohex-2-en-1-one (1). The quantum yield in the case of enone 3 (0.4) is approximately 10 times greater than that for 4,4,-diphenylcyclohex-2-en-1-one, a result that is ascribed to steric acceleration of phenyl migration from the triphenylmethyl group plus greater resonance stabilization of the intermediate biradical.Key words: photochemistry, mechanism, rearrangement, aryl migration, enone, di-π-methane.
Photogeneration of a diene template for surface DielsAlder reactions: Photoenolization of an ortho-methyl-benzophenone-modified Au cluster
Authors: Kell Arnold J , Montcalm Christopher C , Workentin Mark S
Published: July 2003

A series of monolayer-protected clusters (MPCs) modified with a photoreactive [4-(11-mercaptoundecyl)-phenyl](2-methylphenyl)methanone (1) moiety have been prepared where 1 is co-absorbed to the MPC surface with dodecanethiol, octadecanethiol, or 11-mercaptoundecanoic acid methyl ester. Upon irradiation the MPC-anchored 1 reacts efficiently through its triplet excited states, yielding 1,4-biradicals that collapse to synthetically useful, long-lived photodienol intermediates, which can be efficiently trapped in DielsAlder type chemistry by dienophiles  namely, dimethyl acetylenedicarboxylate (DMAD). In all cases the DielsAlder trapping of the dienol occurred efficiently resulting in >60% conversion to the DielsAlder adduct. This indicates that the local environment surrounding 1 did not influence its ability to react via the DielsAlder reaction; however, the reaction could not be taken to completion. The inability to react completely is attributed to 1 binding to distinct sites on the MPC core; there are edge, vertice, and terrace sites. Selective population of these specific sites and the subsequent irradiations show that MPCs with 1 anchored predominantly at edge and vertice sites results in an extent of reaction of 85 ± 3%, whereas selectively populating the terrace sites results in an extent of reaction of 36 ± 2%. These results suggest that 1 anchored to edge and vertice sites is more reactive to the DielsAlder reaction than that involving terrace sites.Key words: monolayer protected cluster, site selective reactivity, DielsAlder, photochemistry.
Application of time-resolved linear dichroism spectroscopy: Intensity borrowing in charge transfer complex absorption spectra
Authors: Levy Dustin , Arnold Bradley R
Published: July 2003

Time-resolved linear dichroism spectroscopy has been used to study the influence of solvent on the charge transfer complex formed between hexamethylbenzene and 1,2,4,5-tetracyanobenzene. It was shown that cyano-substituted solvents induce a 1500 cm1 increase in the charge transfer transition energies relative to those observed in chlorinated solvents. Furthermore, the angle between the charge transfer absorption transition moments and the photochemically produced radical anion absorption transition moment, after relaxation, has been measured for this complex in several solvents. A simple model was used to correlate the angles measured using time-resolved linear dichroism spectroscopy with the extent of localized excitation mixed into the charge transfer transitions. These measurements reveal that different charge transfer transitions borrow intensity from the localized excitation to different extents. By using different excitation wavelengths, the partitioning of the borrowed intensity among the charge transfer transitions of this complex could be evaluated for the first time.Key words: 1,2,4,5-tetracyanobenzene, hexamethylbenzene, donoracceptor complex, photoinduced electron transfer, photoselection.
Potentiometric titration of metal ions in methanol
Authors: Gibson Graham , Neverov Alexei A , Brown R S
Published: July 2003

The potentiometric titrations of nine lanthanideIII triflates, ZnII and CuII triflate, and the NiII, CoII, MgII, and TiIV perchlorates were obtained in methanol to determine the titration constants (defined as the sspH at which the [OCH3]/[Mx+]t ratios are 0.5 and 1.5) as well as the apparent sspKa values for deprotonation of the metal-bound solvent molecules. The titrations were performed under various conditions with and without added salts as electrolytes, and the variations in the titration constants are discussed. In selected cases (La3+, Zn2+) the titration profiles were analyzed using a complex fitting program to obtain information about the species present in solution.Key words: potentiometric titration, methanol, sspH, metal ion, lanthanides, apparent sspKa.
An ab initio and AIM study on the decomposition of phosphite ozonides
Authors: Langeland Jeff L , Werstiuk Nick H
Published: July 2003

DFT calculations at the Becke3PW91/631+G(d) level of theory provided optimized geometries, transition states, and wave functions suitable for the study of the reactivity and molecular structure with Atoms-in-molecules (AIM) of phosphite ozonide complexes. These calculations also provided activation energies for the extrusion of singlet oxygen from the ozonides, which occurs in a concerted manner. The molecular species investigated were trimethyl phosphite ozonide (1), triphenyl phosphite ozonide (2), trifluoromethyl phosphite ozonide (3), trifluoroethyl phosphite ozonide (4), 4-ethyl-1-phospha-2,6,7-trioxabicyclo[2.2.2]octane ozonide (5), 1-phospha-2,6,7-trioxabicyclo[2.2.2]octane ozonide (6), 1-phospha-2,8,9-trioxadamantane ozonide (7), and propylene phenyl phosphite ozonide (8). Single-point calculations at the Becke3PW91/6311++G(d,p) level on the geometries obtained from the lower level theory yielded activation energies of 15.1 and 16.4 kcal mol1 for the nonconstrained complexes 1 and 2, respectively. These values differed from the electronegative trifluoro derivatives 3 and 4, which had much higher barriers of 23.5 and 20.8 kcal mol1, respectively. The activation energies of the bicyclic complexes 57 were significantly higher than 1 and 2 and comparable to 3 and 4, ranging from 23 to 26 kcal mol1. An intermediate barrier of 20.5 kcal mol1 was computed for 8. AIMPAC studies showed no direct correlation between the AIM atomic charges on the phosphorus or oxygen atoms of the ozonide ring with the ease of decomposition of 18 to singlet oxygen and the corresponding phosphate.Key words: phosphite ozonide complexes, decomposition, DFT methods, AIM, activation energy.
Generation of 6-methylene-2,4-cyclohexadienylidene ketene by flash photolysis of benzocyclobutenone in aqueous solution and study of the reactions of this ketene in that medium
Authors: Chiang Y , Kresge A J , Zhan H -Q
Published: July 2003

Flash photolysis of benzocyclobutenone in aqueous solution produced a transient species with a microsecond lifetime whose rates of decay were measured in perchloric acid, sodium hydroxide, and buffer solutions over the acidity range [H+] 1 × 1013  100 M. This produced a rate profile, isotope effects, and buffer behaviour typical of ketene reactions, and that, together with product identification, served to identify this transient as 6-methylene-2,4-cyclohexadienylidene ketene, formed by electrocyclic opening of the four-membered ring of benzocyclobutenone. Comparison of rates of reaction of this ketene with those of its saturated analog, pentamethyleneketene, produced some expected as well as some unexpected results. Key words: cyclobutenone chemistry, electrocyclic ring opening, ketene hydration, rate profile, solvent isotope effects.
An unusual abnormal Wolff rearrangement
Authors: Zimmerman Howard E , Wang Pengfei
Published: July 2003

An unusual example of the abnormal Wolff rearrangement was studied. The bicyclic structure of the reactant and intermediates precluded some mechanistic possibilities of this unusual Abnormal Wolff Rearrangement. The study involved the photochemical rearrangement of diazodehydronorbornone bearing phenyl and 2-pyridyl groups at C-7. A number of examples of interesting stereoselectivity were encountered. The mechanism was investigated by use of ab initio QST type computations which located a unique transition structure species accounting for the reaction mechanism.Key words: abnormal-Wolff, carbene, rearrangement, QM computations, reaction mechanism.
The synthesis and structural characterization of linear and macrocyclic bis(dinitrosyliron) complexes supported by bis(phosphine) bridging ligands
Authors: Li Lijuan , Reginato Nada , Urschey Michael , Stradiotto Mark , Liarakos John D
Published: July 2003

Reactions involving Fe(NO)2(CO)2 and the bis(phosphine) ligands bis(diphenylphosphino)methane (DPPM), bis(diphenylphosphino)acetylene (DPPA), 1,6-bis(diphenylphosphino)hexane (DPPH), and 1,4-bis(diphenyl phosphino)benzene (DPPB) have been examined. From these reactions, the mononuclear complex, Fe(κ1-DPPM)(NO)2(CO) 3, linear dinuclear species of the type Fe2(µ-L)(NO)4(CO)2 (L = Ph2PCH2PPh2 4, Ph2PC[Formula: see text]CPPh2 5, Ph2PCH2(CH3)4CH2PPh2 6, and Ph2P(p-C6H4)PPh2 7), and macrocyclic dinuclear species of the type Fe2(µ-L)2(NO)4 (L = Ph2PCH2PPh2 8 and Ph2PC[Formula: see text]CPPh2 9) were isolated and spectroscopically characterized. For 4, 5, 8, and 9, the solid-state molecular structures of the products were determined by use of single-crystal X-ray diffraction techniques. Key words: dinitrosyliron, iron nitrosyls, dinuclear macrocycles, bis(phosphine) complexes.
Alkali metal ion controlled product selectivity during photorearrangements of 1-naphthyl phenyl acylates and dibenzyl ketones within zeolites
Authors: Warrier M , Kaanumalle Lakshmi S , Ramamurthy V
Published: July 2003

Photochemical behaviors of 1-naphthyl phenyl acylates and dibenzyl ketones included in zeolites have been compared. 1-Naphthyl phenyl acylates while in solution produce eight photoproducts; within NaY it gives a single product. The selectivity is attributed to the restriction brought on the mobility of the primary radical pair by the alkali metal ions present in zeolites. Photochemistry of dibenzyl ketones within NaY reveals that the intersystem crossing in caged radical pairs could be influenced by the heavy alkali metal ions. Structures of complexes among Li+ion and the guest 1-naphthyl phenyl acetates and dibenzyl ketone computed at the B3LYP level have been useful to understand the origin of the observed product selectivity within zeolites.Key words: photo-Fries reaction, zeolites, cationπ interaction, spin-orbit coupling, heavy atom effect.
Compounds of chromium(VI): The pyridine  chromic anhydride complex, benzimidazolinium dichromate, and three 2-alkyl-1H-benzimida -zolinium dichromates
Authors: Cameron T Stanley , Clyburne Jason AC , Dubey Pramod K , Grossert J Stuart , Ramaiah K , Ramanatham J , Sereda Sergei V
Published: July 2003

Pyridine, when allowed to react with chromic anhydride under strictly anhydrous conditions, gives the known, very air-sensitive, crystalline 2:1 Lewis acidbase complex 1. The crystal structure has now been successfully determined. When benzimidazole and three 2-alkyl-1H-benzimidazoles were treated with chromium trioxide in aqueous acetic acid, crystalline dichromate salts (25) were readily formed. These salts consist of dichromate anions linked to the cations by hydrogen bonds of the type NH···O. The chromium atoms have distorted tetrahedral environments, with the CrO distances being typical for dichromate anions. In the cases of the 2-methyl and 2-ethyl salts, the anions are disordered about a centre of inversion. The hydrogen bonding arrangements are discussed and the structures are compared with other oxygenated chromium(VI) species. The dichromate salts are useful selective oxidants for a range of primary and secondary alcohols; examples of these reactions are reported.Key words: chromium(VI), pyridine  chromium trioxide complex, benzimidazolinium dichromate salts, controlled oxidation of alcohols.
Acrylonitrile (AN)Cu9(100) interfaces: Electron distribution and nature of bonded interactions
Authors: Mitrasinovic Petar M
Published: July 2003

There is a fundamental interest in the investigation of the interfacial interactions and charge migration processes between organic molecules and metallic surfaces from a theoretical standpoint. Quantum mechanical (QM) concepts of bonding are contrasted, and the vital importance of using combined QM methods to explore the nature of the interfacial interactions is established. At the one-electron level, the charge distribution and nature of bonded interactions at the ANCu9(100) (neutral and charged (1)) interfaces are investigated by both the Becke (B)  Vosko (V)  Wilk (W)  Nusair (N)/DZVP density functional theory (DFT) method and the MP2/631+G* strategy within the conceptual framework provided by natural bond orbital (NBO)  natural atomic orbital (NAO) population analysis and Atoms-In-Molecules (AIM) theory. By this approach, the interfacial interactions are given physical definitions free of any assumptions and are visualized by using the topological features of the total electron density. A natural link between the electron density on the one side and the shapes (not energies) of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) on the other side is clarified. The question of whether the spatial extents of the HOMO and LUMO resemble the corresponding spatial maps of the negative (charge locally concentrated) and positive (charge locally depleted) Laplacian of the total electron density in [ANCu9(100)]1 is addressed.Key words: ANCu9(100) interfaces, NBONAO population, electron distribution, AIM, bonded interactions.
Chemistry of photogenerated α-hydroxy-p-nitrobenzyl carbanions in aqueous solution: protonation vs. disproportionation
Authors: Morrison James , Wan Peter , Corrie John ET , Munasinghe V Ranjit
Published: July 2003

The photochemistry of p-nitrobenzyl derivatives 610 has been studied in aqueous solution as a function of pH, using product analysis, UVvis spectrophotometry, and laser flash photolysis (LFP). The compounds were chosen with the aim of further exploring the propensity of these systems to give rise to α-hydroxy-p-nitrobenzyl carbanions on photolysis, and to study their mechanisms of subsequent reaction. α-Hydroxy-substituted carbanions are anions that cannot be readily formed using thermal routes but which are believed to have some interesting chemistry. Three methods were employed for photogenerating these carbanions: (i) decarboxylation; (ii) retro-Aldol reaction; and (iii) carbon acid deprotonation. All three methods proved to be successful using the p-nitrobenzyl chromophore. Photogenerated α-hydroxy-p-nitrobenzyl carbanions react via disproportionation, giving rise to oxidized and reduced products; simple protonation of the anion was undetectable.Key words: photodecarboxylation, nitrobenzyl carbanions, photoredox, nitroaromatic compounds, excited-state carbon acids.
Metallocene-catalyzed olefin polymerizations using triphenylcyclopropenium tetrakis(pentafluorophenyl)borate as the activator
Authors: Li Huiying , Neckers Douglas C
Published: July 2003

Triphenylcyclopropenium (TPCP) tetrakis(pentafluorophenyl)borate activates bis(cyclopentadienyl)dimethyl titanium resulting in a highly reactive initiating system for the polymerization of styrene. In contrast to triphenylmethyl tetrakis(pentafluorophenyl)borate which is quite active in the absence of the metallocene, TPCP borate shows no activity for styrene polymerization in the absence of bis(cyclopentadienyl)dimethyl titanium. TPCP is the most efficient activator in the carbonium ion  borate class. We propose, based on 1H NMR evidence that reaction of Cp2TiMe2 and TPCP borate leads to the formation of the cationic Ti complex [Cp2TiMe]+B(C6F5)4. Evidence for the latter is also provided by UVvis spectroscopy in that we found a bathochromic shift of the Cp2TiMe2 LMCT absorption band from 361 to 482 nm in CH2Cl2 and 487 nm in toluene, respectively. Thermal decomposition of the cationic complex [Cp2TiMe]+B(C6F5)4 leads to less activity. The systems are good catalysts for ethylene polymerization as well, but are less active when using propylene. A conventional ZieglerNatta coordination polymerization mechanism accounts for ethylene and propylene polymerization while a carbocationic polymerization mechanism is proposed for styrene.Key words: triphenylcyclopropenium tetrakis(pentafluorophenyl)borate, bis(cyclopentadienyl)dimethyl titanium, activator, olefin polymerization.
TiO2-photocatalyzed reactions of some benzylic donors
Authors: Cermenati Laura , Fagnoni Maurizio , Albini Angelo
Published: July 2003

TiO2-photocatalyzed oxidation of toluene (1a), benzyltrimethylsilane (1b), and 4-methoxybenzyltrimethylsilane (1c) has been carried out in acetonitrile under oxygen, under nitrogen, and in the presence of electrophilic alkenes under various conditions (using Ag2SO4 as electron acceptor, adding 2.5% H2O, changing solvent to CH2Cl2). Benzyl radicals, formed via electron transfer and fragmentation, are trapped. A good material balance is often obtained. The overall efficiency of the process depends on the donor Eox, on the rate of fragmentation of the radical cation, and on the acceptor present (Ag+ is an efficient oxidant, an electrophilic alkene a poor one, O2 is intermediate). With ring-unsubstituted benzyl derivatives 1a and 1b, oxidative fragmentation occurs mainly close to the catalyst surface. The benzyl radicals form at a high local concentration and give benzaldehyde under O2, bibenzyl under N2 and dibenzylated derivatives by attack on the alkenes (acrylonitrile, fumaronitrile, maleic acid). In this case, using CH2Cl2O2 enhances the yield of benzaldehyde. With methoxylated 1c, however, the radical cation migrates into the solution before fragmentation and, therefore, the free benzyl radical is formed. This radical in part is oxidized to the cation, giving a considerable amount of benzylacetamide (or of the alcohol with water), and in part is trapped by the alkenes. The last reaction is less efficient in this case and yields monobenzyl derivatives.Key words: photocatalysis, oxidation, alkylation.
An ab initio molecular orbital study of the geometry of the dicationic Wallach rearrangement intermediate
Authors: Cox Robin A , Fung David YK , Csizmadia Imre G , Buncel Erwin
Published: July 2003

Ab initio calculations have been performed on several different structures for the dicationic intermediate proposed for the Wallach rearrangement of aromatic azoxy compounds to hydroxy-substituted azo systems in strongly acidic media. For the unsubstituted parent compound azoxybenzene, these calculations reveal that the preferred structure for the intermediate is planar, as previously assumed, but bent rather than linear as we have formulated it. The presence of two methyl groups at the para positions of both aromatic rings does not change this situation, but six methyl groups at all para and ortho ring positions lead to a different preferred structure  still bent but with the two aromatic rings now at 90° to one another rather than being coplanar  undoubtedly due to steric interference between the ortho methyl groups. In all the cases examined the two positive charges reside primarily in the aromatic rings rather than on the nitrogens, which are sp2 hybridized and still have their lone pairs. The overall structures can best be regarded as two six-electron π systems joined together, with little communication between the two rings. For the most part the calculations are in good agreement with experimental observations. Recent calculations on other possible reaction intermediates by other groups are also discussed.Key words: Ab initio calculations, Wallach rearrangement, azoxyarenes, reaction intermediate, acid catalysis, intermediate structure, reaction mechanism.
Benzotriazolium ylides: Experimental and theoretical study on the tautomeric equilibrium of monosubstituted benzotriazolium ylides
Authors: Miron Neculai Doru , Woisel Patrice , Surpateanu Georgiana G , Vergoten Gérard , Depature Ludovic , Surpateanu Gheorghe
Published: July 2003

The biphenacyl benzotriazolium salts 12 give a tautomeric equilibrium 13[Formula: see text]14 in the presence of triethylamine (TEA) or NaOH. The evaluation of this tautomeric equilibrium has been studied using a dynamic NMR analysis and a new synthetic procedure of disubstituted benzotriazolium ylides 15 and 16, which have in their structures a picryl fragment. This study also includes a theoretical analysis on the reactivity of salts 12ac and the thermodynamical stability of tautomeric forms 13 and 14 by AM1 and PM3 procedure methods.Key words: synthesis, salts, ylides, semiempirical calculations, kinetics.
Substituent effects in oxime radical cations. 1. Photosensitized reactions of acetophenone oximes
Authors: de Lijser HJ Peter , Kim Jason S , McGrorty Suzanne M , Ulloa Erin M
Published: July 2003

A variety of ortho-, meta-, and para-substituted (-H, -F, -Cl, -CF3, -CN (meta and para only), -CH3, -OCH3, and -NO2) acetophenone oximes were synthesized and studied using laser flash photolysis (LFP) and steady-state photolysis experiments in acetonitrile with chloranil as the photosensitizer. In addition, semi-empirical (AM1) calculations were performed on the neutral species, the radical cations, and the corresponding iminoxyl radicals. The data was analyzed in terms of the electrochemical peak potentials of the oximes, the quenching rates of triplet chloranil (LFP), the calculated ionization potentials, and the measured conversions of the oximes in the steady-state photolysis experiments. Photolysis of the oximes in the presence of chloranil results in the formation of the chloranil radical anion, which reacts rapidly with the oxime radical cation to form the semiquinone radical and an iminoxyl radical. Evidence for the formation of the chloranil radical anion and the semiquinone radical was obtained from LFP studies. The measured quenching rates from the LFP studies represent the rates of electron transfer from the oximes to triplet chloranil. This data was correlated to various radical and polar substituent constants. The Hammett studies suggest that steric, polar, and radical effects are important for ortho-substituted acetophenone oximes, polar effects are important for para-substituted oximes, and radical stabilization is more important than polar effects for the meta-substituted substrates. The calculated ionization potentials of the oximes show an excellent correlation with the measured quenching rates supporting the electron transfer pathway. On the basis of calculated charge densities, we conclude that the measured substituent effects are transition state effects rather than ground state effects. At this point all of the available data suggests that the conversion of the oximes is controlled by two energetically opposing reactions, namely oxidation of the neutral oxime, which is favorable for oximes with electron-donating substituents, and deprotonation of the oxime radical cation, which is favorable for oximes with electron-withdrawing substituents. The overall result is a reaction with little selectivity as far as substituent effects are concerned.Key words: oxime, radical cation, iminoxyl radical, electron transfer, substituent effect.
A generally applicable synthetic approach for heteroleptic thiolate complexes of bismuth
Authors: Agocs Lisa , Briand Glen G , Burford Neil , Eelman Melanie D , Aumeerally Nadia , MacKay Deanna , Robertson Katherine N , Cameron T Stanley
Published: July 2003

As part of a systematic development of bismuth coordination chemistry, we are exploiting the thermodynamic and hydrolytic stability of BiS bonds. A series of heteroleptic thiolate bismuth complexes have been isolated and characterized. The generally applicable synthetic methodology involves the use of hetero-bifunctional ligands containing a thiolate moiety as an anchor to facilitate coordinate interaction of weak donors (carbonyls, amines, hydroxyls) with bismuth. The bifunctional nature of the ligands is manifested in chelating roles. Important comparisons with established thiolate complexes of bismuth are discussed.Key words: bismuth, thiolate, heteroleptic, crystallography.
Intramolecular sensitization within steroids: Excited-state interaction of C-17 α and β carbonbromine bonds with a C-6 carbonyl group
Authors: Li Wen-Shan , Torun Lokman , Morrison Harry
Published: July 2003

The synthesis, photochemistry, and photophysics of 17α-bromo-3α-(triphenylsilyloxy)-5α-androstan-6-one (1) and 17β-bromo-3β-(triphenylsilyloxy)-5α-androstan-6-one (2) have been studied in aqueous tetrahydrofuran. The 17α-bromo isomer gives evidence (reduced ϕf, τ1, and ϕisc for the ketone) for interaction between the ketone and CBr moieties in the excited singlet state. Some photodehalogenation is also observed upon excitation of the ketone chromophore. This interaction seems to be absent or minimal for the 17β-bromo isomer.Key words: photodehalogenation, bromosteroid, ketosteroid, intramolecular singletsinglet energy transfer (ISSET).
Derivatives of tetraphenylmethane and tetraphenylsilane: Synthesis of new tetrahedral building blocks for molecular construction
Authors: Fournier Jean-Hugues , Wang Xin , Wuest James D
Published: July 2003

Useful derivatives of tetraphenylmethane and tetraphenylsilane have been synthesized by efficient methods that give crystalline products without chromatographic purification. New compounds include tetrakis(4-hydroxyphenyl)methane (21), tetrakis(4-formylphenyl)methane (22), tetrakis[(4-hydroxymethyl)phenyl]methane (23), tetrakis(4-bromophenyl)silane (24), tetrakis(4-iodophenyl)silane (25), tetrakis(4-hydroxyphenyl)silane (26), tetrakis[(4-hydroxymethyl)phenyl]silane (27), and tetrakis[(4-chloromethyl)phenyl]silane (28). These compounds are valuable precursors for the construction of complex molecules with tetrahedral geometries.Key words: organic synthesis, molecular and supramolecular design and construction, tetraphenylmethane, tetraphenyl silane, tetrahedral building blocks.
Photolabile calixarene-based rosette
Authors: Li Zaiguo , Chiu Huy , Kutateladze Andrei G
Published: July 2003

A model calix[4]arene-based rosette carrying two alternating photocleavable dithianyl-hydroxy-methyl moieties and two benzophenonecarboxylates was synthesized and shown to be capable of photoinduced fragmentation, with efficiency comparable to that of the externally sensitized parent dithianebenzaldehyde adducts.Key words: photoinduced electron transfer, photofragmentation, calixarene, dithianecarbonyl adducts.
The effect of central bond torsional mobility on the Rydberg state ring opening of alkylcyclobutenes
Authors: Cook Bruce H , Leigh William J
Published: July 2003

The stereochemistry of the π,R(3s) excited state ring opening of a series of bicyclic alkylcyclobutenes has been studied in hydrocarbon solution with 228 nm excitation. In these compounds, the C=C bond is shared between the cyclobutene ring and a five-, six-, or seven-membered ancillary ring, which has the effect of restricting the torsional mobility about the central CC bond in the isomeric diene products. It has previously been shown that monocyclic alkylcyclobutenes undergo stereospecific conrotatory ring opening upon excitation at the long wavelength edge of the π,R(3s) absorption band (228 nm), and nonstereospecific ring opening upon irradiation at shorter wavelengths (within the π,π* absorption band). Different behaviour is observed for the bicyclic systems studied in the present work. The bicyclo[3.2.0]hept-1-ene, bicyclo[4.2.0]oct-1-ene, and one of the bicyclo[5.2.0]non-1-ene derivatives yield nearly the same mixtures of E,E- and E,Z-diene isomers upon irradiation at 214 and 228 nm, with the product mixtures being heavily weighted in favor of the isomer(s) corresponding to disrotatory ring opening. The results may indicate that the stereochemical characteristics of the Rydberg-derived ring opening of alkylcyclobutenes depends on the ability of the molecule to twist about the "central" bond (i.e., the C=C bond in the cyclobutene) as ring opening proceeds. It is proposed that restricting the torsional mobility about the central bond activates internal conversion from the π,R(3s) to the π,π* potential energy surface, from which predominant disrotatory ring opening ensues.Key words: cyclobutene, Rydberg, ring opening, photopericyclic, electrocyclic.
Intramolecular excimer fluorescence from folded ground state rotamers of N,N'dimethyl-N,N'-dipyrenylurea protophanes
Authors: Lewis Frederick D , Kurth Todd L
Published: July 2003

The molecular structure, absorption, and fluorescence spectra of N,N'-dimethyl-N,N'-di-1-pyrenylurea and N,N'-dimethyl-N,N'-di-2-pyrenylurea have been investigated and compared to the properties of N,N,N'-trimethyl-N'-pyrenylurea model compounds. Di-1-pyrenylurea exists as a mixture of folded (E,E) syn- and anti-rotamers that interconvert via flipping of one of the pyrene rings to an unfolded (E,Z) rotamer geometry. The symmetric di-2-pyrenylurea exists as a single folded (E,E) conformation which is in equilibrium with a less-stable, unfolded (E,Z) rotamer. The absorption and fluorescence spectra of the dipyrenylureas at 77 K in a rigid glass are similar to those of monopyrenylurea model compounds. However, in fluid solution, the dipyrenylureas exhibit excimer fluorescence and very weak monomer fluorescence which have identical decay times. This behavior is attributed to fast rotational equilibrium between folded rotamers which exhibit excimer fluorescence and unfolded rotamers which exhibit monomer fluorescence. The behavior of the dipyrenylureas is compared with that of other systems that form intramolecular pyrene excimers and diarylureas.Key words: excited state rotamers, dipyrenylureas, pyrene excimers, ureaphane.
Oxidation of cyclohexane using a novel RuO2zeolite nanocomposite catalyst
Authors: Zhan Bi-Zeng , White Mary Anne , Pincock James A , Robertson Katherine N , Cameron T Stanley , Sham Tsun-Kong
Published: July 2003

We report the synthesis, using an organic-template-free hydrothermal crystallization method, and catalysis of a new type of nanocomposite material, 1.3 nm-sized RuO2 particles confined in faujasite zeolite. The zeolite-confined RuO2 composites were fully characterized with X-ray powder diffraction, Ru K-edge X-ray absorption, and high-resolution transmission electron microscopy. XRD and X-ray fluorescence analysis indicate that the framework is faujasite zeolite with a Si:Al ratio of 1.25. Ru K-edge X-ray absorption near-edge structures indicate that the ruthenium species in the zeolite is Ru(IV) with nearest-neighbor octahedral environments similar to hydrous RuO2, i.e., distorted "RuO6". The k2-weighted extended X-ray absorption fine structure indicates that the Ru(IV) species anchored in the zeolite likely form amorphous RuO2 with a 2D-chain structure, in which RuO6 units are connected together by two shared oxygen atoms. TEM shows that the particle size of RuO2 encapsulated inside the supercages of FAU is about 1.3 nm. The RuO2FAU composites display significant catalytic activity in the oxidation of cyclohexane with tBHP under mild (room temperature and 1 atm (1 atm [Formula: see text] 101.325 kPa)) conditions. The ketone and alcohol concentration can be as high as 0.26 mol L1 in 5 h with 48% peroxide efficiency. The catalyst is stable and reusable. Possible oxidation mechanisms are also discussed.Key words: nanocomposite, ruthenium oxide, catalysis, oxidation, zeolite.
Control of disaccharide conformation by π-stacking
Authors: Watts Jonathan , Jiménez-Barbero Jesús , Poveda Ana , Grindley T Bruce
Published: July 2003

The conformations of a series of derivatives of the disaccharide α-L-fucopyranosyl-(1[Formula: see text]3)-2-acetamido-2-deoxy-D-glucopyranoside, part of the Lex determinant, were studied by molecular modelling using the MM3* forcefield and by 1H NMR spectroscopy. Unusually shielded O-benzyl protons were observed in the 1H NMR spectrum of phenyl 2,3,4-tri-O-benzyl-α-L-fucopyranosyl-(1[Formula: see text]3)-2-deoxy-2-phthalimido-1-thio-α-D-glucopyranoside and assigned to the 2-O-benzyl group. This observation was explained by a shift in the population of the conformational mixture present about the glycosidic linkage from the positive Ψ region in the unsubstituted disaccharide to the negative Ψ region induced by π-stacking between the phthalimide and the 2-O-benzyl phenyl ring. The experimental nuclear Overhauser enhancements confirm the accuracy of the calculations.Key words: disaccharide, conformation, π-stacking, Lex determinant, NOE measurements, MM3 calculations.
UNIFAC calculation of thermodynamic properties of binary 1-chloroalkane + alkane and α,ω-dichloroalkane + alkane mixtures: Comparison with NittaChao and DISQUAC predictions
Authors: García J , López E R , Comuñas M JP , Lugo L , Fernández J
Published: July 2003

Data available in the literature for vaporliquid equilibria, activity coefficients at infinite dilution, and enthalpies of mixing for binary mixtures of 1-chloroalkanes or dichloroalkanes with alkanes are used to determine interaction parameters for three versions of the UNIFAC model  the Tassios et al., Larsen et al., and Gmehling et al. versions. The interaction parameters for chlorine and methyl or methylene groups are calculated using data for the thermodynamic properties of 1-chloroalkane + alkane mixtures. In the case of the Gmehling version, the geometrical parameters for chlorine are also determined. In addition, structure-dependent interaction parameters for α,ω-dichloroalkane + alkane mixtures are presented, taking into account the proximity effect. When the two chlorine atoms of the dichloroalkane are more separated, they become more independent, and the reported values of the interaction parameters approach those of 1-chloroalkane. For all of the properties studied the mean deviation obtained with the new parameter values is lower than that obtained with older values. The results for the thermodynamic properties obtained using the new parameters of the three versions of UNIFAC are compared with those of DISQUAC and NittaChao models. Key words: alkanes, chloroalkanes, DISQUAC, excess thermodynamic properties, proximity effect, UNIFAC.
Experimental values and ERAS model calculations for excess molar volumes and enthalpies of the ternary system 2-butanol + 1,3-dioxolane + cyclohexane
Authors: Gascón Ignacio , Martín Santiago , Giner Beatriz , Haro Marta , Lafuente Carlos
Published: July 2003

Densities and heats of mixing for the ternary system 2-butanol + 1,3-dioxolane + cyclohexane have been measured at 298.15 and 313.15 K. Excess molar volumes and excess molar enthalpies have been calculated from experimental data and fitted by the RedlichKister equation. The ERAS model has been used to calculate excess molar properties of the ternary mixture from parameters obtained from the constituent binary mixtures.Key words: excess properties, ERAS model.
Solubility of carbon dioxide in alkylcarbonates and lactones
Authors: Blanchard F , Carré B , Bonhomme F , Biensan P , Lemordant D
Published: July 2003

The solubility of carbon dioxide in γ-butyrolactone (BL), caprolactone (CL), propylene carbonate (PC), ethylene carbonate (EC), dimethylcarbonate (DMC), diethylcarbonate (DEC), and mixtures of these components has been determined at temperatures from 275 to 333 K at atmospheric pressure. The Henry's law constant (kH) for the dissolution of CO2 in these solvents has been deduced from the solubility data. The value of kH increases in the following order: DEC < DMC < PC < CL < BL < EC, which is identical to the order of the Hildebrand parameters (δ) of the corresponding solvents. The accuracy of classical theories for predicting gas solubilities in liquids has been examined. The best results are given by the VilcuPerisamu equation, which is derived from the ScatchardHildebrand theory but takes into account the polarizability of the solute and the permittivity of the solvent. A modified form of the Prausnitz and Shair equation is proposed to estimate the solubility of nonpolar molecules in strongly dipolar solvents. This equation fits the experimental data with improved precision.Key words: alkylcarbonate, lactone, carbon dioxide, liquidgas equilibrium, Henry's law, Hildebrand parameter.
A fibre-optic biosensor for detection of microbial contamination
Authors: Almadidy Amer , Watterson James , Piunno Paul AE , Foulds Inge V , Horgen Paul A , Krull Ulrich
Published: July 2003

A fibre-optic biosensor is described for detection of genomic target sequences from Escherichia coli. A small portion of the LacZ DNA sequence is the basis for selection of DNA probe molecules that are produced by automated nucleic acid synthesis on the surface of optical fibres. Fluorescent intercalating agents are used to report the presence of hybridization events with target strands. This work reviews the fundamental design criteria for development of nucleic acid biosensors and reports a preliminary exploration of the use of the biosensor for detection of sequences that mark the presence of E. coli. The research work includes consideration of the length of the strands and non-selective binding interactions that can potentially block the selective chemistry or create background signals. The biosensors were able to detect genomic targets from E. coli at a picomole level in a time of a few minutes, and dozens of cycles of use have been demonstrated. In a step towards the preparation of a completely self-contained sensor technology, a new intercalating dye known as SYBR 101 (Molecular Probes, Inc.) has been end-labelled to the LacZ nucleic acid probe, to examine whether dye tethered onto an oligonucleotide terminus could fluorimetrically transduce the formation of hybrids. The results obtained from experiments in solution indicate that the use of tethered dye provides fluorescence signals that are due to hybridization, and that this process is functional even in the presence of a high concentration of non-selective background DNA obtained from sonicated salmon sperm. Key words: biosensor, DNA, fibre optic, hybridization, fluorescence, pathogen, E. coli.