Mechanistic study on the coupling reaction of aryl bromides with arylboronic acids catalyzed by (iminophosphine)palladium(0) complexes. Detection of a palladium(II) intermediate with a coordinated boron anion

The complexes [Pd(eta2-dmfu)(P-N)] [P-N = 2-(PPh2)C6H4-1-CH=NR, R = C(6)H(4)OMe-4; CHMe2; C6H3Me2-2,6; C6H3(CHMe2)-2,6] react with an excess of BrC6H4R1-4 (R1= CF3; Me) yielding the oxidative addition products [PdBr(C6H4R1-4)(P-N)] at different rates depending on R [C6H4OMe-4 > C6H3(CHMe2)-2,6 > CHMe2 approximately C6H3Me2-2,6] and R1 (CF3> Me). In the presence of K2CO3 and activated olefins (ol = dmfu, fn), the latter compounds react with an excess of 4-R2C6H4B(OH)2 (R2= H, Me, OMe, Cl) to give [Pd(eta2-ol)(P-N)] and the corresponding biaryl through transmetallation and fast reductive elimination. The transmetallation proceeds via a palladium(II) intermediate with an O-bonded boron anion, the formation of which is markedly retarded by increasing the bulkiness of R. The intermediate was isolated for R = CHMe2, R1 = CF3 and R2= H. The boron anion is formulated as a diphenylborinate anion associated with phenylboronic acid and/or as a phenylboronate anion associated with diphenylborinic acid. In general, the oxidative addition proceeds at a lower rate than transmetallation and represents the rate-determining-step in the coupling reaction of aryl bromides with arylboronic acids catalyzed by [Pd(eta2-dmfu)(P-N)].     

Redox properties of C6S8(n-) and C3S5(n-) (n = 0, 1, 2): stable radicals and unusual structural properties for C-S-S-C bonds

The new anionic carbon sulfides C6S10(2-) and C12S16(2-) are described and crystallographically characterized. The C12S16(2-) anion consists of two C6S8 units connected by an exceptionally long (2.157(12) A) S-S bond. In solution, C12S16(2-) exists in equilibrium with the radical C6S8(-*). The equilibrium constant for radical formation (293 K, THF) is 1.2 x 10(-4) M, as determined by optical spectroscopy at varying concentrations. Radical formation occurs through scission of the S-S bond. On the basis of variable temperature EPR spectra, the thermodynamic parameters of this process are DeltaH = +51.5 +/- 0.5 kJ x mol(-1) and DeltaS = +110 +/- 3 J x mol(-1) x K(-1). C6S10(2-) is an oxidation product of C3S5(2-) and consists of two C3S5 units connected by an S-S bond. The S-S bond length (2.135(4) A) is long, and the CS-SC torsion angle is unusually acute (52.1 degrees ), which is attributed to an attractive interaction between C3S2 rings. The oxidation of (Me4N)2C3S5 occurs at -0.90 V vs Fc+/Fc in MeCN, being further oxidized at -0.22 V. The similarity of the cyclic voltammogram of (Me4N)2C6S10 to that of (Me4N)2C3S5 indicates that C6S10(2-) is the initial oxidation product of C3S5(2-).     

Two metal centers bridging two C60 cages as a wide passage for efficient interfullerene electronic interaction

The reaction of Ir4(CO)8(PMe3)4 with excess C60 in refluxing 1,2-dichlorobenzene, followed by treatment by CNR (R = CH2C6H5) at 70 degrees C, affords a fullerene-metal sandwich complex Ir4(CO)3(mu4-CH)(PMe3)2(mu-PMe2)(CNR)(mu-eta2,eta2-C60)(mu4-eta1,eta1,eta2,eta2-C60) (1), which exhibits an interesting structural feature of two metal atoms bridging the two C60 centers as well as the first example of a mu4-eta1,eta1,eta2,eta2-C60 bonding mode. Compound 1 has been characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction study. A cyclic voltammetry study reveals strong electronic communication between the two C60 centers in 1, which is due to the presence of a wide channel of two metal centers between the two C60 cages for efficient electronic interaction.     

Carbon-carbon reductive elimination from homoleptic uranium(IV) alkyls induced by redox-active ligands

The synthesis, characterization, and reactivity of the homoleptic uranium(IV) alkyls U(CH(2)C(6)H(5))(4) (1-Ph), U(CH(2)-p-CH(3)C(6)H(4))(4) (1-p-Me), and U(CH(2)-m-(CH(3))(2)C(6)H(3))(4) (1-m-Me(2)) are reported. The addition of 4 equiv of K(CH(2)Ar) (Ar = Ph, p-CH(3)C(6)H(4), m-(CH(3))(2)C(6)H(3)) to UCl(4) at -108 °C produces 1-Ph in good yields and 1-p-Me and 1-m-Me(2) in moderate yields. Further characterization of 1-Ph by X-ray crystallography confirmed η(4)-coordination of each benzyl ligand to the uranium center. Magnetic studies produced an effective magnetic moment of 2.60 μ(B) at 23 °C, which is consistent with a tetravalent uranium 5f(2) electronic configuration. Addition of 1 equiv of the redox-active α-diimine (Mes)DAB(Me) ((Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr]; Ar = 2,4,6-trimethylphenyl (Mes)) to 1-Ph results in reductive elimination of 1 equiv of bibenzyl (PhCH(2)CH(2)Ph), affording ((Mes)DAB(Me))U(CH(2)C(6)H(5))(2) (2-Ph). Treating an equimolar mixture of 1-Ph and 1-Ph-d(28) with (Mes)DAB(Me) forms the products from monomolecular reductive elimination, 2-Ph, 2-Ph-d(14), bibenzyl, and bibenzyl-d(14). This is confirmed by (1)H NMR spectroscopy and GC/MS analysis of both organometallic and organic products. Addition of 1 equiv of 1,2-bis(dimethylphosphino)ethane (dmpe) to 1-Ph results in formation of the previously synthesized (dmpe)U(CH(2)C(6)H(5))(4) (3-Ph), indicating the redox-innocent chelating phosphine stabilizes the uranium center in 3-Ph and prevents reductive elimination of bibenzyl. Full characterization for 3-Ph, including X-ray crystallography, is reported.     

Reactivity of secondary metallocene alkyls and the question of dormant sites in catalytic alkene polymerization

The reactivity of [rac-(C2H4(1-indenyl)2)Zr(n-butyl)][MeB(C6F5)3] (4), [rac-(C2H4(1-indenyl)2)Zr(sec-butyl)][MeB(C6F5)3] (5), and [rac-(C2H4(1-indenyl)2)Zr(polypropenyl)][MeB(C6F5)3] with propene, ethene, and hydrogen was studied by low-temperature (<-40 degrees C) 1H and 13C NMR spectroscopy in toluene solutions. In contrast with previous suggestions that 2 degrees zirconium alkyl species such as 5 are dormant sites, these measurements demonstrate reactivity of 2 degrees zirconium alkyls with propene and ethene comparable to the 1 degrees zirconium alkyl species 4 and [rac-(C2H4(1-indenyl)2)Zr(polypropenyl)][MeB(C6F5)3]. Because 2,1-insertion of propene is an infrequent event, these results preclude significant accumulation of catalyst in the form of 2 degrees zirconium alkyls for this metallocene and counterion. The reactivity of 5 with hydrogen is at least 2 orders of magnitude faster than other 1 degrees zirconium alkyls. Such high reactivity accounts for the puzzlingly high fraction of butyl end groups in prior hydrooligomerization studies and implies that catalyst responsivity to H2 as a molecular weight control agent correlates with the regioselectivity of the catalyst.     

Assessment of the in vivo genotoxicity of new lead compounds to treat sickle cell disease

The compounds 1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)methyl nitrate (C1), (1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)ethyl nitrate (C2), 3-(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)benzyl nitrate (C3), 4-(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)-N-hydroxy-benzenesulfonamide (C4), 4-(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)benzyl nitrate (C5), and 2-[4-(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)phenyl]ethyl nitrate (C6) were evaluated with a micronucleus test using mouse peripheral blood to identify new candidate drugs for the treatment of sickle cell disease (SCD) that are safer than hydroxyurea. The compounds induced an average frequency of micronucleated reticulocytes (MNRET) of less than six per 1,000 cells at 12.5, 25, 50, and 100 mg/kg, whereas hydroxyurea induced an average MNRET frequency of 7.8, 9.8, 15, and 33.7 per 1000 cells respectively, at the same concentrations. Compounds C1-C6 are new non-genotoxic in vivo candidate drugs for the treatment of SCD symptoms.     

Copper(I) and copper(II) complexes possessing cross-linked imidazole-phenol ligands: structures and dioxygen reactivity

Catalytic reduction of O2 to H2O, and coupling to membrane proton translocation, occurs at the heterobinuclear heme a3-CuB active site of cytochrome c oxidase. One of the CuB ligated histidines is cross-linked to a neighboring tyrosine (C-N bond; tyrosine C6 and histidine epsilon-nitrogen), and the protic residue of this cross-linked His-Tyr moiety is proposed to participate as both an electron and a proton donor in the catalytic dioxygen reduction event. To provide insight into the chemistry of such a moiety, we have synthesized and characterized tetra- and tridentate pyridylalkylamine chelate ligands {LN4OR and LN3OR (R = H or Me)}, which include an imidazole-phenol (or anisole) cross-link and their copper(I/II) complexes. [CuI(LN4OH)]B(C6F5)4 (1) reacts with dioxygen at -80 degrees C in THF, forming an unstable trans-mu-1,2-peroxodicopper(II)complex, which subsequently converts to a dimeric copper(II)-phenolate complex [{Cu(LN4O-)}2](B(C6F5)4)2 (5a). The close analogue [CuI(LN4OMe)]B(C6F5)4 (3) binds dioxygen reversibly at -80 degrees C in tetrahydrofuran. Stopped-flow kinetics of the reaction [CuI(LN3OH)]ClO4 (2) with O2 in CH2Cl2 indicate a steady formation of the purple dimeric product [{Cu(LN3O-)}2](ClO4)2 (5b), which has been analyzed in the temperature range from -40 to +20 degrees C, DeltaH = -9.6 (6) kJ mol-1, DeltaS = -168 (2) J mol-1 K-1 (k(-40 degrees C) = 1.05(4) x 106 and k(+20 degrees C) = 4.6(2) x 105 M-2 s-1). The X-ray crystal structures of 1, [CuII(LN3OH)(MeOH)(OClO3-)](ClO4) (4), 5a, and 5b are reported.     

PNP pincer osmium polyhydrides for catalytic dehydrogenation of primary alcohols

This paper reports the synthesis, structure, and properties of a series of PNP pincer complexes of osmium OsH(3)Cl[HN(C(2)H(4)P(i)Pr(2))(2)] (1), OsH(3)[N(C(2)H(4)P(i)Pr(2))(2)] (2), OsH(4)[HN(C(2)H(4)P(i)Pr(2))(2)] (3), and OsH(2)(PMe(3))[HN(C(2)H(4)P(i)Pr(2))(2)] (4). The tetrahydride 3 operates as an efficient catalyst at 0.1 mol% loading for the reactions of amination and dehydrogenative coupling of primary alcohols, producing secondary amines and symmetrical esters, respectively. The catalyst 3 is distinguished by outstanding stability, and it can be used in an aqueous environment at temperatures as high as 200 °C.     

Regioselective nucleophilic addition of triphenylphosphine to the nitrosylruthenium alkynyl complexes having a hydrotris(pyrazol-1-yl)borate: formation of phosphonio-alkenyl, alkynyl, and allenyl species

A nitrosylruthenium alkynyl complex of TpRuCl(C[triple bond]CPh)(NO)(1a) was reacted with PPh3 in the presence of HBF4.Et2O at room temperature to give a beta-phosphonio-alkenyl complex (E)-[TpRuCl{CH=C(PPh3)Ph}(NO)]BF4(2.BF4). On the other hand, for gamma-hydroxyalkynyl complexes TpRuCl{C[triple bond]CC(R)2OH}(NO)(R = Me (1b), Ph (1c), H (1d)), similar treatments with PPh3 were found to give gamma-phosphonio-alkynyl [TpRuCl{C[triple bond]CC(Me)2PPh3}(NO)]BF4(3.BF4),alpha-phosphonio-allenyl [TpRuCl{C(PPh3)=C=CPh2}(NO)]BF4(4.BF4), and a novel product of gamma-hydroxy-beta-phosphonio-alkenyl (E)-[TpRuCl{CH=C(PPh3)CH2OH}(NO)]BF4(5.BF4), respectively. Dominant factors for the selectivity in affording 3-5 were associated with the steric congestion and electronic properties at the gamma-carbons, along with those around the metal fragment. From the bis(alkynyl) complex TpRu(C[triple bond]CPh)2(NO)6, a bis(beta-phosphonio-alkenyl)(E,E)-[TpRu{CH=C(PPh3)Ph}2(NO)](BF4)2{7.(BF4)2} was produced at room temperature. However, similar reactions at 0 degrees C gave an alkynyl beta-phosphonio-alkenyl complex (E)-[TpRu(C[triple bondCPh){CH=C(PPh3)Ph}(NO)]BF4(8.BF4) as a sole product, of which additional hydration in the presence of HBF4.Et2O afforded a [small beta]-phosphonio-alkenyl ketonyl (E)-[TpRu{CH2C(O)Ph}{CH=C(PPh3)Ph}(NO)]BF(.9BF4). Five complexes, 2-5 and 7 were crystallographically characterized.     

石墨烯掺杂LiFePO4电极材料的合成及其电化学性能

采用水热辅助法合成石墨烯改性的LiFePO4多孔微球电极材料.并对材料进行了X射线衍射(XRD),扫描电子显微镜(SEM),透射电子显微镜(TEM),傅里叶变换红外(FT-IR)光谱,充放电等表征.从结果可以看出在2 mol·L-1LiNO3电解液体系中单纯包碳的LiFePO4微球在1C、50C倍率时的比容量分别为137、64 mAh·g-1,而石墨烯改性的LiFePO4微球的比容量分别为141、105 mAh·g-1,表现出较好的倍率特性.恒流循环充放电测试60次后两种材料容量保持率分别为70.2％、83.7％.说明掺杂石墨烯构成的三维导电网络能明显改善LiFePO4的电化学性能.     

Electronic transitions of CsC2, CsC2-, and CsC4 in neon matrixes

The anions of CsC2 and CsC4 produced by sputtering a graphite surface with Cs+ were mass-selected and trapped in neon matrixes at 6 K. The electronic absorption spectra of CsC2 and CsC4, obtained by photodetachment of electrons from the anions, were measured subsequently and reveal strong absorptions in the visible range, which resemble the known band systems of C2- and C4-, respectively. The origin band of CsC2 (500.4 nm) and CsC4 (442.6 nm) is shifted by approximately 1100 or by approximately 700 cm(-1) to the blue from the position of the 0(0)0 band of C2- or C4-. The observed system of CsC2 is assigned to the 2B2-X 2A1 electronic transition of the T-shaped form. The CsC4 spectrum is consistent with a C4- chain slightly perturbed by the Cs atom. The oscillator strength of the observed electronic transition of CsC2 and CsC4 is an order of magnitude larger than for the respective carbon anions. CsC2- has a weak electronic transition, assigned to 1B2-X 1A1 in the C2v form, with origin band at 516.5 nm.     

Dynamic Ring-chain Equilibrium of Nucleophilic Thiol-yne “Click” Polyaddition for Recyclable Poly(dithioacetal)s

We report a dynamic polymerization system based on the reversible nucleophilic Michael polyaddition of activated alkynes and dithiols. Four poly(dithioacetal)s(P1-P4) were prepared via the base-catalyzed thiol-yne "click" polyaddition of two dithiols(1,4-butanedithiol(4 S) and 1,5-pentanedithiol(5 S)) and two alkynones(3-butyn-2-one(Y1) and 1-phenyl-2-propyn-1-one(Y2)) at high concentrations. We systematically investigated the base-catalyzed polymerization of 4 S and Y1(for polymer P1) under different conditions, and found that this polymerization was a highly concentration-dependent dynamic system: polymer P1 was formed at high concentration, while seven-membered dithioacetal, 1-(1,3-dithiepan-2-yl) propan-2-one(C1), was obtained at low concentration. The polymerization of 4 S and Y2(for polymer P4)displayed similar polymerization behavior, generating 2-(1,3-dithiepan-2-yl)-1-phenylethanone(C4) at low concentration. On the contrary,polymer P2(from Y1 and 5 S) was exclusively obtained with no formation of eight-membered dithioacetal. The polymerizations of Y1 with 1,2-ethanedithiol(2 S) and 1,3-dimercaptopropane(3 S) only afforded corresponding five-and six-membered dithioacetals, 1-(1,3-dithiolan-2-yl)propan-2-one(C2) and 1-(1,3-dithian-2-yl) propan-2-one(C3). This dynamic behavior of P1 and P4 was attributed to the concentrationdependent retro-Michael addition reaction between a thiol and a β-sulfido-α,β-unsaturated carbonyl compound catalyzed by bases.Furthermore, polymers P1 and P4 could be depolymerized into C1 and C4 in yields of 58% and 95%, respectively. The ring-opening polymerization of C1 at high concentration could successfully regenerate polymer P1. Thus, a new type of closed-loop recyclable poly(dithioacetal)s was developed.     

Fluorescent sensors for Hg(2+) in micelles: a new approach that transforms an ON-OFF into an OFF-ON response as a function of the lipophilicity of the receptor

A new approach to the use of micelles in the fluorescent sensing of metal cations is proposed and applied to the case of Hg(2+). We demonstrate how it is possible to transform a system from an ON-OFF to an OFF-ON sensor by changing the length of the chain used to lipophilise a ligand that resides inside TritonX-100 micelles together with pyrene as the fluorophore. Three tetrathia-monoaza macrocyclic ligands have been synthesised with the same ring but functionalised on the nitrogen atom with a methyl (C1-NS4), an n-butyl (C4-NS4) or an n-dodecyl (C12-NS4) chain. The three ligands have been fully characterised in water containing TritonX-100 micelles by means of potentiometric titrations and their apparent protonation and complexation constants with Hg(2+) were determined. On the basis of the distribution diagrams obtained, the more lipophilic C12-NS4 has been developed as an ON-OFF fluorescent sensor for mercury: working at pH<4, in the absence of Hg(2+) the ligand is inside the micelles, protonated and non-quenching, while on addition of mercury the [C12-NS4Hg](2+) complex forms which remains inside the micelles and is quenching. On the other hand, the ligand of intermediate chain length, C4-NS4, can be used to obtain an OFF-ON sensor at 7.07.0 the ligand is unprotonated, it stays inside the micelles and is quenching, while addition of Hg(2+) in the 7.0-9.5 pH range results in the formation of [C4-NS4Hg](2+), which is hydrophilic enough to leave the micelles and to be released into the bulk solution where it is no longer capable of quenching pyrene fluorescence. Additional studies on C1-NS4, C3-NS4 and C8-NS4 indicate that the optimal chain length to observe this OFF-ON behaviour is C(3)-C(4).     

Nucleophilic Addition of Amines to N-Aryl-substituted Pyrrolin-2-ones

The nucleophilic addition of n-butyl- and benzylamines to 1-(4-nitrophenyl)-5H-pyrrolin-5-one and 1-(4-sulfamoylphenyl)-5H-pyrrolin-2-one at 50°C in an excess of the amines with the formation of N-substituted amides of 3-alkyl(benzyl)amino-4-(4-R-anilino)butyric acids was investigated. The N-substituted amides of 3-arylamino-4-hydroxybutyric and 4-hydroxy-2-butenoic acids were synthesized from 2(5H)-furanone and aromatic amines (1:3) at 180°C. 4-Alkylamino-1-(4-nitrophenyl)pyrrolid-2-ones were obtained in the reaction of 1-(4-nitrophenyl)-5-pyrrolin-2-one with ammonia or aliphatic, alicyclic, and aromatic amines (1:3, 90°C, in DMF).     

Stereoselective Synthesis of 3-Substituted 4-(Formyloxy)-2-azetidinones by the Unusual Baeyer-Villiger Reaction of beta-Lactam Aldehydes. Scope and Synthetic Applications

The Baeyer-Villiger oxidation of 4-formyl-beta-lactams 1with m-CPBA gave 4-(formyloxy) beta-lactams 2 in a simple, efficient, and totally stereoselective process. This reaction is one of the scarce examples of the preferred migration of a carbon moiety in an aliphatic aldehyde. The influence of the substituents at N1 and C3 of the four-membered ring in the Baeyer-Villiger rearrangement has been studied. Thus, alkyl, alkenyl, aryl, and alkyloxy 3-substituted-1-(p-anisyl)-2-azetidinones 1 form exclusively 4-(formyloxy) beta-lactams 2. Amide or acetoxy substituents at C3 of the four-membered ring produce mixtures of 4-(formyloxy) beta-lactams 2and 4-carboxy beta-lactams 5. The exclusive formation of carboxy derivatives is observed sometimes for 1-alkyl-substituted-2-azetidinones 1. 4-(Formyloxy) beta-lactams 2 are suitable starting materials to prepare different 4-unsubstituted beta-lactams 9 using beta-hydroxy amides 8 as isolable intermediates. The overall transformation 4-formyl-2-azetidinone to 4-unsubstituted beta-lactam is an easy and convenient stereoselective route to these interesting types of compounds.     

Ab initio/GIAO-CCSD(T) study of structures, energies, and 13C NMR chemical shifts of C4H7(+) and C5H9(+) ions: relative stability and dynamic aspects of the cyclopropylcarbinyl vs bicyclobutonium ions

The structures and energies of the carbocations C 4H 7 (+) and C 5H 9 (+) were calculated using the ab initio method. The (13)C NMR chemical shifts of the carbocations were calculated using the GIAO-CCSD(T) method. The pisigma-delocalized bisected cyclopropylcarbinyl cation, 1 and nonclassical bicyclobutonium ion, 2 were found to be the minima for C 4H 7 (+) at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level the structure 2 is 0.4 kcal/mol more stable than the structure 1. The (13)C NMR chemical shifts of 1 and 2 were calculated by the GIAO-CCSD(T) method. Based on relative energies and (13)C NMR chemical shift calculations, an equilibrium involving the 1 and 2 in superacid solutions is most likely responsible for the experimentally observed (13)C NMR chemical shifts, with the latter as the predominant equilibrating species. The alpha-methylcyclopropylcarbinyl cation, 4, and nonclassical bicyclobutonium ion, 5, were found to be the minima for C 5H 9 (+) at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level ion 5 is 5.9 kcal/mol more stable than the structure 4. The calculated (13)C NMR chemical shifts of 5 agree rather well with the experimental values of C 5H 9 (+).     

HPLC and 1H-NMR Studies of Alkaline Hydrolysis of Some 7-(Oxyiminoacyl)cephalosporins

Alkaline hydrolysis (pH 10.5) of the three 7-(oxyiminoacyl)cephalosporins 1a–c (cefuroxime, ceftazidime, and ceftriaxone) was studied at 37° using HPLC and ¹H-NMR techniques. The 7-epicephalosporin 2 , the 3-methylidene compound 3 , and the 6-epimer 4 of the 3-methylidene compound 3 were identified for each cephalosporin as the major degradation products under the conditions used; ceftazidime ( 1b ) yielded also the Δ²-isomer 5b (Scheme 1). A kinetic scheme was developed to account for the production of these compounds, and the different kinetic constants involved in the process were calculated. The experimental results show that the presence of a pyridinio group at position C–C(3) favours the appearance of the Δ²-isomer, which was detected mainly in cephalosporins bearing an ester function at C(4). The presence of an oxyimino group at C? CONH? C(7) facilitates epimerization at C(7) (→ 2 ), whereas that of an electron-withdrawing group at C? C(3) results in a increased formation constant for the 3-methylidene compound 3 . The 3-methylidene compounds 3a–c produced by the three cephalosporins on cleavage of the β-lactam ring all underwent epimerization at C(6) to yield the corresponding 6-epimer 4 .     

Photoinitiated polymerization of methacrylates comprising phenyl moieties

Investigation of photopolymerization kinetics of 4-(4-methacryloyloxyphenyl)-butan-2-one (1) in comparison with 2-phenoxyethyl methacrylate (2) and phenyl methacrylate (3) using a UV-LED emitting at 395 nm shows significantly faster polymerization of 1 compared to both 2 and 3 at 40°C. Vitrification affects photopolymerization kinetics of all methacrylates under investigation. Interestingly, quantitative final conversion is observed during photoinitiated polymerization of 1 and 2 whereas 3 shows limited conversion at about 80%. Furthermore, higher degree of polymerization is obtained by photoinitiated polymerization of 1 compared to 2 and 3. This shows that the 3-oxobutyl substituent at the phenyl ring of 1 significantly affects both polymerization kinetics and final conversion of the photoinitiated polymerization. Moreover, an additional higher molecular weight fraction is observed in case of polymerization of 1 at 85°C that is above the glass transition temperature of the polymer formed during photoinitiated polymerization. As a thermal polymerization at 85°C in the absence of light results in a high molecular weight polymer as well, an additional thermal process may be discussed as reason for the higher molecular weight polymer fraction in case of the photopolymer made at 85°C.     

Haptotropic rearrangement in tricarbonylchromium complexes of 2-aminobiphenyl and 4-aminobiphenyl

The para-aminobiphenyl compound [(η(6)-C(6)H(5))(C(6)H(4)-4-NH(2))]Cr(CO)(3) (1) has an arene-phenyl dihedral angle of 38.01(6)°, as determined by single-crystal X-ray crystallography, and 34.7(11)°, as determined by DFT calculations. It undergoes haptotropic rearrangement at 140 °C in solution to form [(η(6)-C(6)H(4)-4-NH(2))(C(6)H(5))]Cr(CO)(3) (2), even though previous reports have suggested that such rearrangements should not be observed in compounds with arene-phenyl dihedral angles greater than 22°. NMR analysis gave a rate constant of k = 5.0 × 10(-5) s(-1) for the rearrangement of 1 to 2. The ortho-substituted analog [(η(6)-C(6)H(5))(C(6)H(4)-2-NH(2))]Cr(CO)(3) (3) has an arene-phenyl dihedral angle of 67.70(7)°, as determined by single-crystal X-ray crystallography, and 51.9(10)°, as determined by DFT calculations. Surprisingly, even though it displays a more extreme canting of arene rings, 3 rearranges to [(η(6)-C(6)H(4)-2-NH(2))(C(6)H(5))]Cr(CO)(3) (4) at 140 °C in solution with a rate constant of k = 2.6 × 10(-4) s(-1). This approximately five-fold rate enhancement likely results from the ortho-amino group providing intramolecular stabilization for intermediates formed during the rearrangement.     

DFT study of the fragmentation mechanism of uracil RNA base

The fragmentation process of the uracil RNA base has been investigated via DFT calculations in order to assign fragments to the ionisation mass spectrum obtained after dissociation induced by collision experiments. The analysis of the electronic distribution and geometry parameters of the cation allows selection of several bonds that may be cleaved and lead to the formation of various fragments. Differences are observed in the electronic behaviour of the bond breaking as well as the energy required for the cleavage. It is reported that N(3)-C(4) and N(1)-C(2) bonds are more easily cleaved than the C(5)-C(6) bond, since the corresponding energy barriers amount to ΔG = +1.627, +1.710, +5.459 eV, respectively, which makes the C(5)-C(6) bond cleavage almost prohibited. Among all possible formed fragments, the formation of the OCN(+) fragment for the peak at m/z = 42 Da is excluded because of an intermediate that was not observed experimentally and too a large free energy barrier. Based on the required free energy, it is observed that two fragment derivatives: C(2)H(4)N(+) and C(2)H(2)O˙(+) may be formed, with a small preference for C(2)H(4)N(+). This latter product is not formed through a retro Diels Alder reaction in contrast to C(2)H(2)O˙(+). The following sequence is proposed for the peak at 42 Da: C(2)H(4)N(+) (from N(1)-C(2), C(4)-C(5) cleavages) > C(2)H(2)O˙(+) (from N(3)-C(4), N(1)-C(2) and C(5)-C(6) cleavages) > C(2)H(4)N(+) (from N(1)-C(2), N(3)-C(4) and C(4)-C(5)) > C(2)H(2)O˙(+) (from C(5)-C(6), N(1)-C(2) and N(3)-C(4) cleavages) > NCO(+) (from N(1)-C(2), C(4)-C(5) and N(3)-C(4) cleavages). Finally the peak at 28 Da is assigned to CNH(2)(+) derivatives that can be formed through two different paths, the easiest one requiring 5.4 eV.