Transoid Isomers Research Articles

Is this how bromine spiropyran salt is converted to merocyanine under UV irradiation? A look through the prism of quantum chemical calculations.

The stability of merocyanine forms formed under UV irradiation of a solution of a spiropyran salt, in which an organic part acts as a cation and a compact bromide ion as an anion, their photophysical properties, and the formation mechanism are studied in this work using time-dependent density functional theory. Theoretical calculations show that TTC and CTT are the most stable open forms (the difference in stability energies is 10.5 and 12.0 kcal mol-1 relative to the formation energy of spiropyran, respectively). The simulated absorption bands in the UV spectrum of the merocyanine forms are observed both in the UV region at 308-366 nm and in the visible region at 544-757 nm due to n → π* and π → π* type transitions. We found that the isomerisation mechanism of spiropyran into merocyanine forms includes two key stages: the ring opening to form cisoid merocyanine forms (except unstable TCC) through conical intersection and their subsequent isomerisation to form stable transoid isomers. The length of the Cspiro-O bond is 1.97 Å and the C1'-C2'-C3'-C4' angle is 70° in the structure close to conical intersection. The stage that determines the rate of this process is the isomerisation between transoid forms, as in the case of transformation of open merocyanine forms into spiropyran.

Cyclic polyacetylene.

Here we demonstrate the synthesis of cyclic polyacetylene (c-PA), or [∞]annulene, via homogeneous tungsten-catalysed polymerization of acetylene. Unique to the cyclic structure and evidence for its topology, the c-PA contains >99% trans double bonds, even when synthesized at -94 °C. High activity with low catalyst loadings allows for the synthesis of temporarily soluble c-PA, thus opening the opportunity to derivatize the polymer in solution. Absolute evidence for the cyclic topology comes from atomic force microscopy images of bottlebrush derivatives generated from soluble c-PA. Now available in its cyclic form, initial characterization studies are presented to elucidate the topological differences compared with traditionally synthesized linear polyacetylene. One advantage to the synthesis of c-PA is the direct synthesis of the trans-transoid isomer. Low defect concentrations, low soliton concentration, and relatively high conjugation lengths are characteristics of c-PA. Efficient catalysis permits the rapid synthesis of lustrous flexible thin films of c-PA, and when doped with I2, they are highly conductive (398 (±76) Ω-1 cm-1).

Ullmann‐Type and Related Redox Reactions of Nitrosyl Molybdenum Complexes Bearing a Large‐Bite‐Angle Diphosphine

AbstractThe reactions of ArX (X = Cl and Br) with [Mo(NO)(P∩P)(NCMe)3][BArF4] P∩P = 2,2′‐bis(diphenylphosphanyl)diphenyl ether (DPEphos), BArF4 = tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate at 120 °C resulted in the formation of biphenyl (through CAr–CAr reductive homocoupling) and the dinuclear halide salts [Mo2(NO)2(P∩P)2(NCMe)2(μ‐X)2][BArF4]2 (X = Cl, 1; Br, 2). Complexes 1 and 2 potentially show cisoid (1c and 2c) and transoid (1t and 2t) regioisomerism with respect to the position of the NO ligand. The crystal‐structure determinations of 1 and 2 revealed the presence of the transoid isomers 1t and 2t and MI–MI bonding in both cases. A proposed mechanism for the formation of 1t and 2t involves reductive CAr–CAr coupling to form biphenyl from two Ph–MoII centers. In addition, the complexes Mo(NO)(P∩P)(CO)2Cl (3) and [Mo(NO)(mer‐κ3‐P,P,O‐DPEphos)Cl(PR3)] (R = Me, 4; Ph, 5) were obtained through the reductions of [Mo2(NO)2(P∩P)2Cl4(μ‐Cl)2].

Synthesis of the C10-C24-bis-spiroacetal core of 13-desmethyl spirolide C based on a sila-Stetter-acetalization process.

Synthesis of the bis-spiroacetal core of 13-desmethyl spirolide C has been completed based on a sila-Stetter-acetalization process. The acylsilane and enone partners in the Stetter reaction were prepared in 7 and 11 steps, respectively, from (S) and (R)-aspartic acid. The quaternary center at C19 in the enone moiety was controlled by relying on the Seebach's chiral self-reproduction method using an enantiopure (S)-lactic acid based dioxolanone. The final acid-catalyzed spiroacetalization provided the desired spiroacetal as a mixture of diastereoisomers in 13 linear steps. Whatever the conditions used, the non-natural transoid isomer was formed preferentially. However, both cisoid and transoid isomers were isolated pure and their structure assigned unambiguously through NMR spectroscopic studies.

Dinuclear Olefin and Alkyne Complexes of Platinum(II)

AbstractBis(olefin) complexes of Zeise's dimer type – [{PtCl2(cis‐MeHC=CHMe)}2] (2) or [{PtCl2(c‐Hex)}2] (3, c‐Hex = cyclohexene) – were prepared by treatment of K2[PtCl4] in the presence of SnCl2 or K[PtCl3(cis‐MeHC=CHMe)] with the corresponding olefin. Zeise's dimer, [{PtCl2(H2C=CH2)}2] (1), was found to react in chloroform or dichloromethane with the internal alkynes RC≡CtBu (R = Me, tBu), provided that the cleaved‐off ethene was removed, to yield bis(alkyne) complexes of Zeise's dimer type [{PtCl2(RC≡CtBu)}2] (R = Me, 11; tBu, 12). Without removal of the ethene, mononuclear mixed ethene/alkyne complexes cis‐[PtCl2(H2C=CH2)(RC≡CtBu)] (R = Me, 8; tBu, 9) and cis‐[PtCl2(H2C=CH2)2] (10) were formed. Analogous reactions with alkynes RC≡CR′ bearing sterically less demanding substituents (R, R′ = Me, Et, nPr, Ph) led to (cyclobutadiene)platinum(II) complexes [PtCl2(C4R2R′2)] (R/R′ = Me/Me 4; Et/Et 5; Me/nPr 6) and [PtCl2(C4Me2Ph2)] (7). Furthermore, use of acetone (instead of CHCl3/CH2Cl2) as solvent in the reactions between 1 and sterically undemanding substituted alkynes resulted in bridge cleavage of Zeise's dimer (1) and in aldol addition and condensation reactions, thereby confirming why Chatt et al. in his analogous classical experiments (1961) observed only decomposition. Complexes 2–7 and 11/12 were isolated in pure states and fully characterized by elemental analysis, NMR spectroscopy, and X‐ray diffraction measurements (2, 11, 12). For the dinuclear bis(olefin) (1–3) and bis(alkyne) complexes (11, 12) solvent‐dependent equilibria between the transoid and cisoid isomers were observed, and these could even be ascertained crystallographically for complex [{PtCl2(tBuC≡CtBu)}2] (12), which crystallized both as the transoid isomer 12a and as the cisoid isomer 12b·CHCl3. Furthermore, consistent with DFT calculations, NMR measurements provided evidence of fast rotation and of hindered rotation of olefin and alkyne ligands, respectively, resulting in conformational isomers in 8 and 11.

Syntheses, Dynamic Behaviour and Theoretical Studies of [(Piperidinomethyl)silyl]methyl‐Cyclopalladated Dimetallic Complexes

AbstractThe cationic complex [Pd{CH2SiPh2(CH2NC5H10)‐κ2C,N}(NCMe)2]BF4 (CH2NC5H10 = piperidinomethyl) is obtained from the reaction of [Pd{CH2SiPh2(CH2NC5H10)‐κ2C,N}(μ‐Cl)]2 with TlBF4 in the presence of NCMe. It reacts with KO2CCH3 leading to the dimetallic complex [Pd{CH2SiPh2(CH2NC5H10)‐κ2C,N}(μ‐O2CCH3‐κO:κ′O)]2, which exists only as the transoid isomer in solution. A dynamic process due to inversion of the acetato bridges is detected by variable‐temperature 1H NMR spectroscopy. The reaction of the same parent complex with Kdmpz (dmpz = dimethylpyrazolate) leads to the dinuclear complex [Pd{CH2SiPh2(CH2NC5H10)‐κ2C,N}(μ‐dmpz‐κN:κ′N)]2, which in solution exists as a mixture of cisoid and transoid isomers, but no inversion of the bridging pyrazolato groups has been observed. The reaction between equimolar amounts of these two complexes in CH2Cl2 affords low yields of the expected mixed acetato/pyrazolato‐bridged complex [Pd2{CH2SiPh2(CH2NC5H10)‐κ2C,N}2(μ‐O2CCH3‐κO:κ′O)(μ‐dmpz‐κN:κ′N)]. The mixed chlorido‐pyrazolato‐bridged complex [Pd2{CH2SiPh2(CH2NC5H10)‐κ2C,N}2(μ‐Cl)(μ‐dmpz‐κN:κ′N)] is obtained by mixing equimolar amounts of the parent dichlorido‐ and bis(dimethylpyrazolato)‐bridged dinuclear complexes, and exhibits a dynamic process in solution corresponding to the inversion of the bridges. Only the cisoid isomers are detected for both dimetallic complexes with mixed bridging ligands. The experimental findings of the preferred transoid or cisoid structure in these dinuclear Pd complexes were supported by quantum chemical calculations.

Nonradioactive assay for detecting isoprenyl diphosphate synthase activity in crude plant extracts using liquid chromatography coupled with tandem mass spectrometry

Terpenoids form the largest class of plant metabolites involved in primary and secondary metabolism. Isoprenyl diphosphate synthases (IDSs) catalyze the condensation of the C5 terpenoid building blocks, isopentenyl diphosphate and dimethylallyl diphosphate, to form geranyl diphosphate (C10), farnesyl diphosphate (C15), and geranylgeranyl diphosphate (C20). These branch point reactions control the flow of metabolites that act as precursors to each of the major terpene classes—monoterpenes, sequiterpenes, and diterpenes, respectively. Thus accurate and easily performed assays of IDS enzyme activity are critical to increase our knowledge about the regulation of terpene biosynthesis. Here we describe a new and sensitive nonradioactive method for carrying out IDS assays using liquid chromatography coupled with tandem mass spectrometry (LC–MS/MS) to detect the short-chain prenyl diphosphate products directly without dephosphorylation. Furthermore, we were able to separate cisoid and transoid isomers of both C10 enzyme products (geranyl diphosphate and neryl diphosphate) and three C15 products [(E,E)-, (Z,E)-, and (Z,Z)-farnesyl diphosphate]. By applying the method to crude protein extracts from various organs of Arabidopsis thaliana, Nicotiana attenuata, Populus trichocarpa, and Picea abies, we could determine their IDS activity in a reproducible fashion.

Photochromic properties of phenanthroline-annulated spirooxazine in the solid state

Photochromic transformations of phenanthroline-annulated spirooxazine were observed in polycrystalline powders and solid-phase films obtained from solutions by solvent evaporation. The spectral and kinetic properties of the solid-phase films were studied by stationary absorption spectroscopy and laser flash photolysis. The mechanism of photoinitiated reactions of solid-phase samples of phenanthroline-annulated spirooxazine includes mutual transformations of different transoid isomers of the merocyanine form.

Combined NMR, SERRS, and DFT study of photochemical and thermal reactions of acetylene‐ and thienylacetylene‐substituted chromenes

AbstractNuclear Magnetic Resonance (NMR), Surface‐Enhanced Resonance Raman Scattering (SERRS), and Density Functional Theory (DFT) were used to study 8‐substituted acetylene‐ and thienylacetylene‐diphenylnaphthopyran photochromic compounds. The purpose of this work is to determine the effect of the 8‐acetylene substituent on the photochromic equilibrium. NMR spectroscopy shows that photocoloration by UV irradiation at low temperature leads to a mixture of transoid isomers (TC and TT) and a small amount of an allenyl‐naphthol structure. The mixture is further enriched in this latter isomer by subsequent visible irradiation. Some photoproducts are identified by analysis of the changes in the SERRS spectra of an irradiated solution with time. To assist spectral assignment, the vibrational frequencies of compounds (1) and (2) and their photoproducts were computed at optimized geometries and compared with the observed Raman bands. Copyright © 2007 John Wiley & Sons, Ltd.

New Dinuclear Face‐ and Edge‐Sharing Bioctahedral Ruthenium Compounds Containing 1,2‐Bis(diphenylphosphanyloxy)ethane

AbstractThe bidentate diphosphinite ligand 1,2‐bis(diphenylphosphanyloxy)ethane (L), Ph2P–O–(CH2)2–O–PPh2, can be sequentially incorporated into a series of dinuclear chloro‐bridged ruthenium compounds using [RuCl2(DMSO)4] as a precursor to give the new bioctahedral compounds [Ru2(μ‐Cl)3(DMSO)3Cl(η2‐L)] (1), [Ru2(μ‐Cl)3(DMSO)Cl(η2‐L)2] (2) and [Ru2Cl2(η2‐L)2{η1‐L(O)}2(μ‐Cl)2] (3). Compound 3 contains two partially oxidised diphosphinite ligands that act in a monodentate manner. These compounds can also be obtained by direct reaction of [RuCl2(DMSO)4] with the appropriate stoichiometric amount of L but in the case of 3 (in which prolonged heating is necessary) the reaction affords two mononuclear compounds [RuCl2(η2‐L)(Ph2POPPh2)] (7) and [RuCl(CO)(η2‐L)(Ph2POCH2)] (8), with ligand fragments coordinated to the metal probably originating in metal‐mediated disruptions of L. Reaction of 2 with the monodentate ligands PPh3 – n(OEt)n (n = 1–3) (L′) affords the dinuclear doubly chloro‐bridged compound [Ru2(μ‐Cl)2Cl2(η2‐L)2(L′)2] (4), the dinuclear triply chloro‐bridged compound [{Ru(η2‐L)(L′)}2(μ‐Cl)3]Cl (5) and the mononuclear compound [RuCl2(η2‐L)(L′)2] (6). The solid‐state structures of 2 (transoid isomer), 5 (transoid isomer), 6, 7 and 8 are reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

A B3LYP study of the effects of phenyl substituents on 1,5-hydrogen shifts in 3-(Z)-1,3-pentadiene provides evidence against a chameleonic transition structure.

The effects of one or two phenyl substituents on the activation enthalpy for a 1,5-hydrogen shift in 3-(Z)-1,3-pentadiene (1) and on the geometry of the transition structure (TS) have been investigated by B3LYP/6-31G calculations. The phenyl-substituent effects on the experimentally measured activation enthalpies are predicted to be sizable, spanning a range of nearly 10 kcal/mol. However, if differences between steric effects in the transoid isomers of the reactants are factored out by comparing the activation enthalpies in the cisoid conformers, the electronic components of the phenyl-substituent effects on both the barrier heights and the TS geometries are found to be quite modest in size. Unlike the TS in the Cope rearrangement, the TS for a 1,5-hydrogen shift in 1 is not highly variable in nature, and the reason the 1,5-hydrogen shift TS is not chameleonic is discussed.

Origin of the Color of π-Conjugated Polymers: Poly(N-n-octyl-3-carbazoyl)acetylene Prepared with a [Rh(norbornadiene)Cl]2 Catalyst

Stereoregular polymerization of (N-n-octyl-3-carbazoyl)acetylene was successfully performed with a [Rh(NBD)Cl]2 (NBD = norbornadiene) catalyst to afford poly(N-n-octyl-3-carbazoyl)acetylene (PNOCzA) as a new type of possible electron-transporting polymer in fairly high yields when triethylamine, chloroform, or dimethylformamide was used as the polymerization solvent. The resulting polymers were composed of amorphous cis−transoid isomers bearing a yellow color or crystalline cis−transoid isomers having an orange color, called a columnar, as a π-conjugated molecular assembly at the solid state. The cis-to-trans isomorization of the yellow and orange polymers was induced by compression at 200 kg/cm2 under vacuum at room temperature for 10 min. The polymers obtained before and after compression were characterized in detail using 1H NMR, wide-angle X-ray diffraction, laser Raman, electron spin resonance, and diffuse reflective UV−vis (DRUV−vis) methods. The data showed that compression of the orange polymers r...

A Monovacant Heteropolytungstate Thioderivative: Synthesis and Characterization of [(PW11O39)2 (H4Mo4S4O6)]10‐ and Related Isomers.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A monovacant heteropolytungstate thioderivative: Synthesis and characterization of [(PW11O39)2(H4Mo4S4O6)](10-) and related isomers.

[(PW(11)O(39))(2)(Mo(4)S(4)O(4)(OH(2))(2))](10-) anions were obtained through the stereospecific addition of the [Mo(2)S(2)O(2)](2+) oxothiocation to the monovacant alpha-[PW(11)O(39)](7-) anion. K(10)[(PW(11)O(39))(2)(Mo(4)S(4)O(4)(OH(2))(2))].25H(2)O has been isolated as crystals and characterized by X-ray diffraction. The structure revealed a "sandwich-like" dimer of two alpha-[PW(11)O(39)](7-) subunits assembled by the noteworthy central cluster [H(4)Mo(4)S(4)O(6)]. The crystallization of the crude product produces an isomerically pure compound, which was characterized by (31)P and (183)W NMR. IR data were also supplied. In solution, the compound isomerizes, giving a second diastereoisomer. A kinetic experiment, carried out by (31)P NMR, allowed the conditions of the thermodynamic equilibrium to be determined. A structural relationship between the two isomers is proposed, fully consistent with NMR data. Cisoid and transoid isomers result in the relative disposition of each [PW(11)O(39)](7-) subunit, either staggered or eclipsed. An investigation of the formation of the [Mo(2)O(2)S(2)](2+) unit from the polycondensed cyclic precursor [Mo(10)S(10)O(10)(OH)(10)(H(2)O)(5)] and the aggregation process resulting in the oxothio [(PW(11)O(39))(2)(Mo(4)S(4)O(4)(OH(2))(2))](10-) compound has been undertaken. The studies were monitored by (31)P NMR and UV-vis spectroscopies. The reaction is quantitative in nearly stoichiometric conditions.

AB Initio MO Study of P--Si π Systems: Structures and Energies

The origin of the relative stability of isomers of 1,2-diphosphinodisilene and related compounds is discussed. The geometries of 18 isomers of (H 2 X)HE=EH(XH 2 ), (H 2 X)HE=EH(XH 2 ) 2+ , and HX=EH--HE=XH (E = C, Si; X = P, N) have been optimized at the MP2/6-311++G(d,p) level of theory. Since nitrogen and phosphorus have lone-pair electrons, six electrons can delocalize in four orbitals of (H 2 X)HE=EH(XH 2 ) (6e/4o), while four electrons can delocalize in four orbitals of (H 2 X)HE=EH(XH 2 ) 2+ and HX=EH--HE=XH (4e/4o). To examine the interaction that governs the structure and relative stability of their cis and trans (or cisoid and transoid) isomers, the ~ - * delocalization energies and isodesmic reaction energies of the compounds were calculated. It is concluded that the ~ - * delocalization affects the bending structure of disilene derivatives, and phosphorus substituents prefer 2e/4o and 4e/4o systems.

Photoinduced Cis-to-Trans Isomerization of Poly(2-ethynylthiophene) Prepared with a [Rh(norbornadiene)Cl]2 Catalyst. 1H NMR, UV, and ESR Studies

Poly(2-ethynylthiophene) was stereoregularly prepared with a Rh complex catalyst, [Rh(norbornadiene)Cl]2, to selectively produce the cis−transoid isomer in high yields when triethylamine was used as the polymerization solvent at 20 °C. Photoinduced cis-to-trans isomerization was newly found to take place when the film of a pristine cis−transoid polymer was photoilluminated using a Xe lamp with light of wavelength, 320−500 nm under vacuum for 5 h. The cis and trans polymers obtained before and after the illumination were characterized in detail using 1H NMR, UV−vis of solution and film, and electron spin resonance methods.

Polymerization of o-Trifluoromethyl(phenylacetylene) Initiated by [Rh(norbornadiene)Cl]2 and MoOCl4−n-Bu4Sn−EtOH Catalysts. Formation of Order and Disorder Trans Sequences

o-Trifluoromethyl(phenylacetylene) (oTFMPA) was successfully polymerized by a Rh complex, [Rh(norbornadiene)Cl]2 catalyst, in the presence of triethylamine as the polymerization solvent under mild conditions. Structural differences between the Rh polymer and the Mo polymer prepared with a metathesis initiator, MoOCl4−n-Bu4Sn−EtOH, using the same monomer were compared using laser Raman, 1H and 19F NMR, solution and diffuse reflective UV, and electron spin resonance (ESR) methods. The Rh complex produces the aromatic polyacetylene bearing regular head-to-tail, ordered trans sequences where more mobile unpaired electrons generated through the rotational scission of the cis CC bonds are stabilized, although the Rh catalyst is known to selectively generate the cis−transoid isomer. The Mo-based initiators produce also complete trans−transoid polyacetylene involving irregular head-to-head and/or tail-to-tail units of ca. 32% disordered trans sequences, where less mobile unpaired electrons are stabilized in the i...

Picosecond Time-Resolved Resonance Raman Spectroscopy and Vibrational Analysis in Spirooxazine Photochromism

Picosecond time-resolved resonance Raman (TR3) spectroscopy was used to study the dynamics and structure of transient species involved in the photochromic process in solution of 1,3,3-trimethylspiro[indoline-naphthoxazine]. TR3 spectra recorded in n-hexane, at 10 ps and 50 ps time-delays were compared with those obtained on the ns-ms timescale and to the resonance Raman spectrum of a permanent opened form. These data and the results of a detailed ab initio vibrational analysis, have allowed us to determine the structure of the tran-soid isomer already present at 10 ps and to precise the photochromic mechanism.

Synthesis and molecular structure of cisoid and transoid isomers of [Re2(CO)6(1,1′-bis(indenylidene)] derived from the reaction of [Re2(CO)8(MeCN)2] and diazoindene

Abstract The compound [Re 2 (CO) 8 (MeCN) 2 ] reacts with diazoindene (C 9 H 6 N 2 ) while refluxing in THF to afford three dirhenium products in which C 9 H 6 N 2 is cleaved with loss of N 2 and with incorporation of the residual indenylidene group into the products. Two indenylidene groups are coupled in two diastereomers of [Re 2 (CO) 6 (μ,η 5 :η 5 -1,1′-C 18 H 12 )] where C 18 H 12 =bis(indenylidene). X-ray structures show that these isomers are related as RR / SS and RS isomers. These have the two Re(CO) 3 groups coordinated transoid and cisoid , respectively to a trans bis(indenylidene) bridge. The third product is the μ-indenylidene complex [Re 2 (CO) 8 (μ,η 1 :η 5 -C 9 H 6 )], which was also structurally characterised by X-ray diffraction.

Picosecond time-resolved spectroscopy of the photocolouration reaction of photochromic naphthoxazine-spiro-indolines

The photochemical formation of the open merocyanine forms of several naphthoxazine-spiro-indolines in different solvents have been studied using both picosecond transient absorption (PTA) and picosecond time-resolved resonance Raman (PTR3) methods. The PTA studies have established the presence of several metastable species in the photochemical formation of the coloured merocyanine form of these photochromic compounds. The primary photochemical step occurs on the sub-ps timescale and is followed by the formation of a cisoid intermediate over the next 6–30 ps. This cisoid species then isomerises to the equilibrated distribution of transoid isomers of the merocyanine form with a lifetime that is dependent upon both solvent viscosity and polarity as well as the nature of the substituents on the naphthalene part of the molecule. However, the rate of this cis → trans isomerisation is unaffected on changing the N-alkyl group on the indoline part of the molecule from a methyl to an isobutyl group. The PTR3 studies have demonstrated that, in butan-1-ol, there are at least three different transient species with characteristic vibrational spectra which evolve with different lifetimes to give the final isomeric distribution over the first few ns of the reaction. In cyclohexane it is likely that the formation of a single species is being probed, which is fully developed after the first 200 ps of the reaction. This single species has a spectrum which is the same as the equilibrated steady-state resonance Raman merocyanine spectrum. It is likely that the evolution in the PTR3 spectra obtained here, in butan-1-ol, results from an equilibration of initially formed transoid merocyanine isomers to give a more stable distribution in this polar hydrogen-bonded solvent. Such an equilibration appears to be unnecessary in a non-polar solvent such as cyclohexane and it is suggested that this is because the transoid isomer, initially formed, is already in its most stable form.