Showing papers on "Double bond published in 2002"
TL;DR: The spectroscopic studies together with viscosity experiments and cyclic voltammetry suggest that [CuL(1)](2+) could bind to DNA by partial intercalation via pyridine ring into the base pairs of DNA.
309 citations
TL;DR: In this paper, the authors studied the kinetics of the epoxidation of soybean oil and the extent of side reactions at 40, 60, and 80 °C, and found that the reaction was first-order with respect to the double bond concentration.
Abstract: The kinetics of the epoxidation of soybean oil and the extent of side reactions were studied at 40, 60, and 80 °C. Epoxidation was carried out in toluene with “in situ” formed peroxoacetic and peroxoformic acid and in the presence of an ion exchange resin as the catalyst. The reaction was found to be first-order with respect to the double bond concentration. At higher temperatures and at higher conversions a deviation from the first-order kinetics was observed. The rate constants for the epoxidation with peroxoacetic acid were 0.118 (h−1) at 40 °C, 0.451 (h−1) at 60 °C and 1.278 (h−1) at 80 °C, while those for peroxoformic acid were 0.264, 0.734, and 1.250 (h−1). The activation energy was found to be 54.7 kJ/mol for the epoxidation with peroxoacetic acid and 35.9 kJ/mol for that with peroxoformic acid. Three factors indicated that side reactions did not occur on a large scale: The absence of an OH band in the IR spectra, the formation of less than 2% of higher molecular weight products from gel permeation chromatography and the selectivity values between 0.9 and 1.
285 citations
TL;DR: Epoxidation (C=C) vis-à-vis allylic hydroxylation (C-H) reactions of propene with a model compound I of the enzyme cytochrome P450 were studied using B3LYP density functional theory and found that Cpd I is a chameleon oxidant.
Abstract: Epoxidation (C=C) vis-a-vis allylic hydroxylation (C-H) reactions of propene with a model compound I (Cpd I) of the enzyme cytochrome P450 were studied using B3LYP density functional theory. Potential energy profiles and kinetic isotope effects (KIE) were calculated. The interactions in the protein pocket were mimicked by adding two external NH- - -S hydrogen bonds to the thiolate ligand and by introducing a nonpolar medium (with a dielectric constant, epsilon = 5.7) that can exert a polarization effect on the reacting species. A two-state reactivity (TSR) with high-spin (HS) and low-spin (LS) states were located for both processes (Ogliaro, F.; Harris, N.; Cohen, S.; Filatov, M.; de Visser, S. P.; Shaik, S. J. Am. Chem. Soc. 2000, 122, 8977-8989. de Visser, S. P.; Ogilaro, F.; Harris, N.; Shaik, S. J. Am. Chem. Soc. 2001, 123, 3037-3047). The HS processes were found to be stepwise, whereas the LS processes were characterized as nonsynchronous but effectively concerted pathways. The computed KIE for C-H hydroxylation with and without tunneling corrections are large (>7), and they support the assignment of the corresponding transition states as hydrogen-abstraction species (Groves, J. T.; Han, Y.-Z. In Cytochrome P450: Structures, Mechanism and Biochemistry, 2nd ed.; Ortiz de Montellano, P. R., Ed.; Plenum Press: New York, 1995; Chapter 1; pp 3-48). In the gas phase, epoxidation is energetically favorable by 3.4 kcal mol(-1). Inclusion of zero-point energy reduces this difference but still predicts C=C/C-H > 1. Environmental effects were found to have major impact on the C=C/C-H ratio as well as on the stereoselectivity of the processes. Thus, two NH- - -S hydrogen bonds away from the reaction center reverse the regioselectivity and prefer hydroxylation, namely, C=C/C-H <1. The polarity of the medium further accentuates the trend and leads to a change by 2 orders of magnitude in the regioselectivity, C=C/C-H << 1. Furthermore, since the environmental interactions prefer the LS over the HS reactions, both hydroxylation and epoxidation processes are rendered more stereoselective, again by 2 orders of magnitude. It follows, therefore, that Cpd I is a chameleon oxidant (Ogliaro, F.; Cohen, S.; de Visser, S. P.; Shaik, S. J. Am. Chem. Soc. 2000, 122, 12892-12893; Ogliaro, F.; de Visser, S. P.; Cohen, S.; Kaneti, J.; Shaik, S. Chembiochem. 2001, 2, 848-851; Ogliaro, F.; de Visser, S. P.; Groves, J. T.; Shaik, S. Angew. Chem., Int. Ed. 2001, 40, 2874-2878) that tunes its reactivity and selectivity patterns in response to the protein environment in which it is accommodated. A valence bond (VB) model, akin to "redox mesomerism" (Bernadou, J.; Fabiano, A.-S.; Robert, A.; Meunier, B. J. Am. Chem. Soc. 1994, 116, 9375-9376), is constructed and enables the description of a chameleon transition state. It shows that the good donor ability of the thiolate ligand and the acceptor ability of the iron porphyrin create mixed-valent situations that endow the transition state with a great sensitivity to external perturbations as in the protein pocket. The model is used to discuss the computed results and to relate them to experimental findings.
279 citations
TL;DR: Nine new molecular motors, consisting of a 2,3-dihydro-2-methylnaphtho[2,1-b]thiopyran or 2, 3-diversey-3-methylphenanthrene upper part and a (thio)xanthene, 10,10-dimethylanthracene, or dibenzocycloheptene lower part, connected by a central double bond, were synthesized.
Abstract: Nine new molecular motors, consisting of a 2,3-dihydro-2-methylnaphtho[2,1-b]thiopyran or 2,3-dihydro-3-methylphenanthrene upper part and a (thio)xanthene, 10,10-dimethylanthracene, or dibenzocycloheptene lower part, connected by a central double bond, were synthesized. A single stereogenic center, bearing a methyl substituent, is present in each of the motors. MOPAC93-AM1 calculations, NMR studies, and X-ray analysis revealed that these compounds have stable isomers with pseudoaxial orientation of the methyl substituent and less-stable isomers with pseudoequatorial orientation of the methyl substituent. The photochemical and thermal isomerization processes of the motors were studied by NMR and CD spectroscopy. The new molecular motors all show two cis-trans isomerizations upon irradiation, each followed by a thermal helix inversion, resulting in a 360 degrees rotation around the central double bond of the upper part with respect to the lower part. The direction of rotation is controlled by a single stereogenic center created by the methyl substituent at the upper part. The speed of rotation, governed by the two thermal steps, was adjusted to a great extent by structural modifications, with half-lives for the thermal isomerization steps ranging from t(1/2)(theta) 233-0.67 h. The photochemical conversions of two new motors proceeded with near-perfect photoequilibria of 1:99.
273 citations
TL;DR: This Account describes an experimental system that has been developed to probe, especially by X-ray crystallography, the interactions that occur between Na(+) or K(+) and the neutral arenes of particular biological significance.
Abstract: Cation−π interactions occur between cations and electron-rich species such as double bonds, triple bonds, and arenes. The π-electron system may be neutral or anionic, but the latter are less relevant to biology, at least so far as is currently known. Among the 20 essential amino acids, there are four aromatic residues. These are benzene, phenol, indole, and imidazole, on the side chains of phenylalanine, tyrosine, tryptophan, and histidine, respectively. Of these, imidazole is expected to be a σ-donor, and benzene, phenol, and indole are antipicated to serve as π-donors. Sodium and potassium are the most abundant cations in living systems. This Account describes an experimental system that has been developed to probe, especially by X-ray crystallography, the interactions that occur between Na+ or K+ and the neutral arenes of particular biological significance.
206 citations
TL;DR: In this article, a review summarises some recent results on synthesis, structures, and reactivity of the heavier analogues of carbenes, alkenes, and dienes.
182 citations
TL;DR: The thermal reaction of undecylenic acid with a hydrogen-terminated porous silicon surface takes place at 95°C to yield an organic monolayer covalently attached to the surface through Si-C bonds.
Abstract: The thermal reaction of undecylenic acid with a hydrogen-terminated porous silicon surface takes place at 95°C to yield an organic monolayer covalently attached to the surface through Si-C bonds. The acid terminal group remains intact and is not affected by the chemical process. Under the same conditions, alcohols break the Si-Si back bonds of the PSi matrix. In contrast, the acid function does not react with either the Si-H or the Si-Si bonds of the PSi surface and the reaction takes place at the terminal C-C double bond of the molecule. When the reaction was carried out with decanoic acid, under the same conditions, the reaction was not complete. The functionalized surfaces were characterized using transmission infrared and X-ray photoelectron spectroscopies. The effect of the chemical process on the photoluminescence has been studied, and the stability against corrosion in 100% humidity was verified using chemography. We have demonstrated that the derivatized surface with undecylenic acid can be activated by a simple chemical route using N-hydroxysuccimide in the presence of N-ethyl-N'-(3-dimethylaminopropyl) carbodiimide hydrochloride.
180 citations
TL;DR: Comparisons with experiment indicate that steric effects must also be considered for real catalysts containing bulky substituents when rationalizing in terms of electronic effects.
Abstract: Gradient-corrected (BP86) density functional calculations were used to study alternative mechanisms of the metathesis reactions between ethene and model catalysts [(PH(3))(L)Cl(2)Ru[double bond]CH(2)] with L=PH3 (I) and L=C(3)N(2)H(4)=imidazol-2-ylidene (II). On the associative pathway, the initial addition of ethene is calculated to be rate-determining for both catalysts (Delta G(22-25)*[double bond] kcal mol(-1)). The dissociative pathway starts with the dissociation of phosphane, which is rather facile (Delta G(298)* is approximately equal to 5-10 kcal mol(-1)). The resulting active species (L)Cl(2)Ru[double bond]CH(2) can coordinate ethene cis or trans to L. The cis addition is unfavorable and mechanistically irrelevant (Delta G(298)* is approximately equal to 21-25 kcal mol(-1)). The trans coordination is barrierless, and the rate-determining step in the subsequent catalytic cycle is either ring closure of the complex to yield the ruthenacyclobutane (catalyst I, Delta G(298)*=12 kcal mol(-1)), or the reverse reaction (catalyst II, ring opening, Delta G(298)*=10 kcal mol(-1)), that is, II is slightly more active than I. For both catalysts, the dissociative mechanism with trans olefin coordination is favored. The relative energies of the species on this pathway can be tuned by ligand variation, as seen in (PMe(3))(2)Cl(2)Ru[double bond]CH(2) (III), in which phosphane dissociation is impeded and olefin insertion is facilitated relative to I. The differences in calculated relative energies for the model catalysts I-III can be rationalized in terms of electronic effects. Comparisons with experiment indicate that steric effects must also be considered for real catalysts containing bulky substituents.
179 citations
TL;DR: Intramolecular insertion of a C-C double bond into aC-C single bond was achieved by treatment of cyclobutanone bearing an o-styryl group at the 3-position with a catalytic amount of a cationic rhodium(I)-dppp complex.
Abstract: Intramolecular insertion of a C−C double bond into a C−C single bond was achieved by treatment of cyclobutanone bearing an o-styryl group at the 3-position with a catalytic amount of a cationic rhodium(I)-dppp complex. Initially, rhodium is inserted between the carbonyl carbon and the α-carbon of the cyclobutanone. Intramolecular coordination of the vinyl group results in its migratory insertion into the C−Rh linkage. Reductive elimination affords benzobicyclo[3.2.1]octan-3-one. Notably, a ring-opened α,β-unsaturated ketone was obtained when dppe was used instead of dppp. In this reaction, rhodium cleaved the bond between the α sp3 carbon and the β sp3 carbon of the cyclobutanone. The coordinating vinyl group directs this new regioselectivity of cleavage observed with the dppe ligand.
176 citations
TL;DR: In this article, three ammonium salts, hexadecylallyldimethyl ammonium chloride (Allyl16), hexadecylvinylbenzyldimmethyl ammonium (VB16), and hexadecylvinylbenzylbenzine methyl ammonium compound (Bz16), were synthesized and ion exchanged onto montmorillonite and characterized by X-ray diffraction and transmission electron microscopy.
167 citations
TL;DR: In this paper, the crystal and molecular structures of Cs(II) and Rb3(III)·H2O were determined by X-ray diffraction and the latter compound is the first structural characterization of (III) and also contains the nitrocyanamide anion, NCNNO2−.
TL;DR: The formation and unimolecular reactions of primary ozonides and carbonyl oxides arising from the O(3)-initiated reactions of isoprene have been investigated using density functional theory and ab initio molecular orbital calculations, and an OH yield of 0.25 is determined from prompt and thermal decomposition of the carbonies.
Abstract: The formation and unimolecular reactions of primary ozonides and carbonyl oxides arising from the O(3)-initiated reactions of isoprene have been investigated using density functional theory and ab initio molecular orbital calculations. The activation energies of O(3) cycloaddition to the two double bonds of isoprene are found to be comparable (3.3-3.4 kcal mol(-1)), implying that the initial two O(3) addition pathways are nearly equally accessible. The reaction energies of O(3) addition to isoprene are between -47 and -48 kcal mol(-1). Cleavage of primary ozonides to form carbonyl oxides occurs with a barrier of 11-16 kcal mol(-1) above the ground state of the primary ozonide, and the decomposition energies range from -5 to -13 kcal mol(-1). OH formation is shown to occur primarily via decomposition of the carbonyl oxides with the syn-positioned methyl (alkyl) group, which is more favorable than isomerization to form dioxirane (by 1.1-3.3 kcal mol(-1)). Using the transition-state theory and master equation formalism, we determine an OH yield of 0.25 from prompt and thermal decomposition of the carbonyl oxides.
TL;DR: Ab initio calculations on the haloforms embedded in a homogeneous electric field to model the influence of the ether suggest that the complexation shift of the h Haloform C[bond]H stretching is largely explained by the electric field effect induced by the electron donor in the proton donor.
Abstract: Complexes of haloforms of the type HCCl(n)F(3-)(n) (n = 1-3) with dimethyl ether have been studied in liquid argon and liquid krypton, using infrared spectroscopy. For the haloform C[bond]H stretching mode, the complexation causes blue shifts of 10.6 and 4.8 cm(-1) for HCClF(2) and HCCl(2)F, respectively, while for HCCl(3) a red shift of 8.3 cm(-1) is observed. The ratio of the band areas of the haloform C[bond]H stretching in complex and monomer was determined to be 0.86(4) for HCClF(2), 33(3) for HCCl(2)F, and 56(3) for HCCl(3). These observations, combined with those for the HCF(3) complex with the same ether (J. Am. Chem. Soc. 2001, 123, 12290), have been analyzed using ab initio calculations at the MP2[double bond]FC/6-31G(d) level, and using some recent models for improper hydrogen bonding. Ab initio calculations on the haloforms embedded in a homogeneous electric field to model the influence of the ether suggest that the complexation shift of the haloform C[bond]H stretching is largely explained by the electric field effect induced by the electron donor in the proton donor. The model calculations also show that the electric field effect accounts for the observed intensity changes of the haloform C[bond]H stretches.
TL;DR: In this paper, the authors present a review of O-Radical reactions and their application in organic synthesis, focusing on 5-exo-triggered addition reactions and homolysis of a β-C-C bond.
Abstract: In recent years, powerful new methods for the generation of alkoxyl radicals under mild and neutral conditions have been developed. This progress has led to a thorough investigation of most O-radical elementary reactions.Today, sufficiently reliable thermodynamic and kinetic data are available either from experimental or from theoretical studies in order to predict alkoxyl radical reactivities and selectivities in synthesis. For instance, alkoxyl radicals readily add to carbon-carbon and carbon-nitrogen double bonds. Due to generally low activation barriers and strongly negative reaction enthalpies, inter- and intramolecular addition reactions proceed under kinetic control and are associated with high rate constants. Nevertheless, intramolecular 5-exo-trig additions, i.e. cyclizations, proceed with an astonishing degree of diastereoselectivity and often provide complementary selectivities if compared to commonly used methods such as the bromine cyclization of alkenols. Therefore, several useful applications of O-radical cyclizations in the synthesis of functionalized tetrahydrofurans have been discovered in the last few years. A second major reaction channel of alkoxyl radicals is associated with the homolysis of a β-C-C bond. This fragmentation proceeds under thermodynamic control and affords a carbonyl compound besides an alkyl radical from the starting alkoxyl radical. Regioselectivities for C-C bond homolysis may be predicted by considering strain release (cyclic carbon framework) and the stability of the newly formed carbon radical (cyclic and open chain carbon skeletons). The third major group of alkoxyl radical-based transformations are connected with homolytic substitutions such as intramolecular 1,5-hydrogen shifts which have been applied with considerable success to remote functionalization reactions. In view of the diversity of alkoxyl radical reactions it is the aim of this review to organize basic principles of this type of chemistry and to present its latest useful application in organic synthesis.
TL;DR: Ab initio methods are used to characterize the ground and first excited state of the chromophore in the rhodopsin family of proteins: retinal protonated Schiff base and show that energetic considerations cannot explain the observed bond selectivity.
Abstract: Ab initio methods are used to characterize the ground and first excited state of the chromophore in the rhodopsin family of proteins: retinal protonated Schiff base. Retinal protonated Schiff base has five double bonds capable of undergoing isomerization. Upon absorption of light, the chromophore isomerizes and the character of the photoproducts (e.g., 13-cis and 11-cis) depends on the environment, protein vs. solution. Our ab initio calculations show that, in the absence of any specific interactions with the environment (e.g., discrete ordered charges in a protein), energetic considerations cannot explain the observed bond selectivity. We instead attribute the origin of bond selectivity to the shape (topography) of the potential energy surfaces in the vicinity of points of true degeneracy (conical intersections) between the ground and first excited electronic states. This provides a molecular example where a competition between two distinct but nearly isoenergetic photochemical reaction pathways is resolved by a topographical difference between two conical intersections.
TL;DR: In this paper, a planar SiClSi=SiClSi framework of disilene was analyzed and the formation mechanisms of which were discussed − the formation mechanism of which are discussed − is discussed.
Abstract: The reaction of R*2MeSi−SiX2X′ (X/X′ = Cl/Br, Br/Cl) with NaR* (R* = supersilyl SitBu3) leads, probably via the silylenes R*2MeSi−SiX, to the trans-configurated disilene (R*2MeSi)ClSi=SiCl(SiMeR*2), which could be isolated as orange-red crystals. According to an X-ray structure analysis, the central SiClSi=SiClSi framework of the disilene − the formation mechanisms of which are discussed − is planar. The Si=Si double bond is as short as 2.163(4) A and shows a band in the Raman spectrum at 589 cm−1. Due to its insolubility in organic solvents, no NMR spectra of the disilene in solution were obtained. The spatially overcrowded disilene is stable towards H2O, MeOH, HF, and NaR*. It melts at 228 °C with decomposition. Reduction of the disilene occurs with LiC10H8 in THF with formation of a reaction mixture containing a chlorine-free product that gives a low field 29Si NMR signal at δ = 91.5 (the region for unsaturated Si atoms). It transforms in solution with traces of oxygen into a substance which, according to mass spectrometry (chemical ionization), shows a mass for the disilyne R*2MeSi−Si≡Si−SiMeR*2 plus two oxygen atoms. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)
TL;DR: In this article, the rate constants for the gas phase reactions of atomic chlorine with a series of alkenes, relative to n-heptane as a reference, have been investigated at (298 ± 3) K and 1 atmosphere.
Abstract: Chlorine atoms are important oxidants at dawn in the marine boundary layer where a variety of organics are also present, including alkenes. Using the relative rate technique, the kinetics of the gas phase reactions of atomic chlorine with a series of alkenes, relative to n-heptane as a reference, have been investigated at (298 ± 3) K and 1 atmosphere in either synthetic air or nitrogen. The rate constant for n-heptane, relative to n-butane whose rate constant was taken to be 2.18 × 10−10 cm3 molecule−1 s−1, was also measured and found to be (3.97 ± 0.27) × 10−10 cm3 molecule−1 s−1
(2s). Based on this value for the n-heptane reaction, the following absolute values for the rate constants, k
(in units of 10−10 cm3 molecule−1 s−1) for the chlorine atom reactions were determined: propene, 2.64 ± 0.21; isobutene, 3.40 ± 0.28; 1-butene, 3.38 ± 0.48; cis-2-butene, 3.76 ± 0.84; trans-2-butene, 3.31 ± 0.47; 2-methyl-1-butene, 3.58 ± 0.40; 2-methyl-2-butene, 3.95 ± 0.32; 3-methyl-1-butene, 3.29 ± 0.36; 2-ethyl-1-butene, 3.89 ± 0.41; 1-pentene, 3.97 ± 0.36; 3-methyl-1-pentene, 3.85 ± 0.35; and cis-4-methyl-2-pentene, 4.11 ± 0.55 (±2s). The errors reflect those in our relative rate measurements but do not include the 10% error in the absolute value of the n-butane rate constant upon which these rate constants are ultimately based. A structure–reactivity scheme is presented that assumes that rate constants for addition of chorine atoms to the double bond, as well as that for abstraction of an allylic hydrogen atom, depend upon the degree of alkyl substitution at the double bond and allylic carbons. The surprising result is that the allylic hydrogen atoms react less rapidly with chlorine atoms than the analogous alkyl hydrogens in alkanes. The atmospheric implications for loss of alkenes in the marine boundary layer are discussed.
TL;DR: The cyclic vinyl sulfides that have agonist activity were both shown to possess low-energy conformers compatible with the previously proposed 3D model for the bioactive conformation of Ang II.
Abstract: Vinyl sulfide cyclized analogues of the octapeptide angiotensin II that are structurally related to the cyclic disulfide agonist c[Hcy(3,5)]Ang II have been prepared. The synthesis relies on the reaction of the mercapto group of a cysteine residue in position 3 with the formyl group of allysine incorporated in position 5 of angiotensin II. A mixture of the cis and the trans isomers was formed, and these were separated and isolated by RP-HPLC. Thus, the three-atom CH(2)[bond]S[bond]S element of the AT(1) receptor agonist c[Hcy(3,5)]Ang II has been displaced by a bioisosteric three-atom S[bond]CH[double bond]CH element. A comparative conformational analysis of the 13-membered ring systems of c[Hcy(3,5)]Ang II and the 13-membered cyclic vinyl sulfides with cis and trans configuration, respectively, suggested that all three systems adopted very similar low-energy conformations. This similarity was also reflected in the bioactivity. Both of the compounds that contained the ring systems encompassing the cis or trans vinyl sulfide elements between positions 3 and 5 exhibited K(i) values less than 2 nM and exerted full agonism at the AT(1) receptor. In contrast, vinyl sulfide cyclization involving the amino acid residues 5 and 7 rendered inactive compounds. The cyclic vinyl sulfides that have agonist activity were both shown to possess low-energy conformers compatible with the previously proposed 3D model for the bioactive conformation of Ang II.
TL;DR: The structures of the meromycolic acids, including those with three functional groups extracted from Mycobacterium tuberculosis H37Ra, MycOBacterium bovis BCG and Myc Cobacterium microti, were established and were structurally closely related to those with one cis cyclopropane ring.
Abstract: Mycobacterial α-, methoxy- and keto-mycolic acid methyl esters were separated by argentation chromatography into mycolates with no double bond, with one trans double bond or with one cis double bond. Meromycolic acids were prepared from each methyl mycolate fraction by pyrolysis, followed by silver oxide oxidation, and analysed by high-energy collision-induced dissociation/fast atom bombardment MS to reveal the exact locations of the functional groups within the meromycolate chain. The locations of cis and trans double bonds, cis and trans cyclopropane rings, methoxy and keto groups, and methyl branches within the meromycolate chain were determined from their characteristic fragment ion profiles, and the structures of the meromycolic acids, including those with three functional groups extracted from Mycobacterium tuberculosis H37Ra, Mycobacterium bovis BCG and Mycobacterium microti, were established. Meromycolic acids with one cis double bond were structurally closely related to those with one cis cyclopropane ring, whereas the meromycolic acids with one trans cyclopropane ring were closely related to the corresponding meromycolic acids with one cis cyclopropane ring. A close relationship between methoxy- and keto-meromycolic acids was also implied. The relationship between the meromycolic acids with a trans double bond and the other meromycolic acids was not clearly revealed, and they did not appear to be immediate substrates for trans cyclopropanation.
TL;DR: The results indicate that self-assembling properties strongly depend on the number of cis double bonds.
Abstract: Four long-chain phenyl glucoside amphiphiles possessing a saturated or unsaturated long alkyl chain group as the self-assembling unit of a highly organized molecular architecture were synthesized. Their self-assembling properties were investigated by EF-TEM, SEM, CD, FT-IR, and XRD. Compound 2 possessing one double bond in the lipophilic portion showed twisted helical fibers, which formed a bilayered structure with a 3.59 nm period, while compound 3 showed the helical ribbons and left-handed nanotubular structures with 150-200 nm inner diameters and ca. 20 nm of wall. Very interestingly, compound 4 possessing three double bonds showed a nanotubular structure with ca. 70 nm of inner diameter through a helical ribbon, which formed a loose bilayered structure with 4.62 nm. These results indicate that self-assembling properties strongly depend on the number of cis double bonds.
TL;DR: The time-dependent isomerizations and thiol additions of several Z- and E-monounsaturated fatty acid methyl esters catalyzed by alkanethiyl radicals during gamma-radiolysis of tert-butyl alcohol solutions are analyzed on the basis of the radiation chemical yield of radicals and established rate data to provide room-temperature rate constants for the reversible thiyl addition.
Abstract: The time-dependent isomerizations and thiol additions of several Z- and E-monounsaturated fatty acid methyl esters catalyzed by alkanethiyl radicals during γ-radiolysis of tert-butyl alcohol solutions are analyzed on the basis of the radiation chemical yield of radicals and established rate data. This provides room-temperature rate constants for the reversible thiyl addition. Within experimental errors, they do not depend on the double bond position in the alkyl chains. Particularly noteworthy is the very fast β-elimination of thiyl radicals from alkyl radicals which carry a second β-substituent. It is supported by additional evidence obtained with a radical clock methodology, and the large preference of fragmentation to the E-isomers is attributed to different barriers for the formation of the E- and Z-transition states from the equilibrium radical structure.
TL;DR: The Wittig reaction and related methods for synthesis of C=C double bonds belong to the standard repertoire of the synthetic chemist and have been extensively studied in recent years and applications of such processes to complex molecule synthesis have begun to emerge.
Abstract: The Wittig reaction and related methods for synthesis of C=C double bonds belong to the standard repertoire of the synthetic chemist. Studies of asymmetric versions of these reactions have been increasing in recent years and applications of such processes to complex molecule synthesis have begun to emerge. In this review, we will emphasise the recent advances in developing methods and synthetic applications of these reactions, but earlier results will be covered as well to place the recent results in context.
TL;DR: The indium-mediated radical addition to an electron-deficient C=C bond also proceeded effectively to provide the new carbon-carbon bond-forming method in aqueous media.
TL;DR: The interactions of these Diels-Alderase ribozymes with their substrates and products have now been elucidated by chemical substitution analysis by using 44 different, systematically varied analogues.
Abstract: Ribozymes have recently been shown to catalyze the stereoselective formation of carbon−carbon bonds between small organic molecules. The interactions of these Diels−Alderase ribozymes with their substrates and products have now been elucidated by chemical substitution analysis by using 44 different, systematically varied analogues. RNA−diene interaction is governed by stacking interactions, while hydrogen bonding and metal ion coordination appear to be less important. The diene has to be an anthracene derivative, and substituents at defined positions are permitted, thereby shedding light on the geometry of the binding site. The dienophile must be a five-membered maleimidyl ring with an unsubstituted reactive double bond, and a hydrophobic side chain makes a major contribution to RNA binding. The ribozyme distinguishes between different enantiomers of chiral substrates and accelerates cycloadditions with both enantio- and diastereoselectivity. The stereochemistry of the reaction is controlled by RNA−diene ...
Patent•
18 Jul 2002TL;DR: In this paper, a photoinitiator compound is defined as an oligomer or polymer containing at least one carboxylic acid group in the molecule and having a molecular weight of 200′ooo or less.
Abstract: A photosensitive composition comprising, (A) an oligomer or polymer containing at least one carboxylic acid group in the molecule and having a molecular weight of 200′ooo or less; (B) at least one photoinitiator compound of formula I, R 1 , is lenear or banched C 1 -C 12 alkyl; R 2 is linear or branched C 1 -C 4 alkyl; R 3 and R 4 independently of one another are linear or branched C 1 -C 8 alkyl; and (C) a monomeric, oligoneric or polymeric compound having at least one olefinic double bond, is especially suitable for preparting photoresists, in particular color filters.
TL;DR: A significant ground-state destabilization of carboxylic acid esters, and Carbonyl compounds in general, due to the decreased resonance stabilization, is proposed as a novel concept to explain both the increase in their reactivity and the changes in the chemical shifts and carbonyl frequencies induced by electron-withdrawing substituents.
Abstract: (13)C NMR chemical shifts and nu(C[double bond]O) frequencies have been measured for several series of phenyl- or acyl-substituted phenyl acetates and for acyl-substituted methyl acetates to investigate the substituent-induced changes in the electrophilic character of the carbonyl carbon. Charge density, bond order, and energy calculations have also been performed. The spectroscopic and charge density results indicate that opposite to the conventional thinking, electron-withdrawing substituents do not increase the electrophilicity of the carbonyl carbon but instead decrease it. On the other hand, reaction energies of the isodesmic reactions designed show that electron-withdrawing substituents destabilize the carbonyl derivatives investigated. So, a significant ground-state destabilization of carboxylic acid esters, and carbonyl compounds in general, due to the decreased resonance stabilization, is proposed as a novel concept to explain both the increase in their reactivity and the changes in the chemical shifts and carbonyl frequencies induced by electron-withdrawing substituents.
TL;DR: High-level ab initio calculations show that the singlet photochemical cis–trans isomerization of organic molecules under isolated conditions can occur according to two distinct mechanisms, characterized by the different structures of the conical intersection funnels controlling photoproduct formation.
Abstract: High-level ab initio calculations show that the singlet photochemical cis–trans isomerization of organic molecules under isolated conditions can occur according to two distinct mechanisms. These mechanisms are characterized by the different structures of the conical intersection funnels controlling photoproduct formation. In nonpolar (e.g. hydrocarbon) polyenes the lowest-lying funnel corresponds to a (CH)3 kink with both double and adjacent single bonds twisted, which may initiate hula-twist (HT) isomerization. On the other hand, in polar conjugated systems such as protonated Schiff bases (PSB) the funnel shows a structure with just one twisted double bond. The ground-state relaxation paths departing from the funnels indicate that the HT motion may take place in nonpolar conjugated systems but also that the single-bond twist may be turned back, whereas in free conjugated polar molecules such as PSB a one-bond flip mechanism dominates from the beginning. The available experimental evidence either...
TL;DR: In this paper, a theoretical study of the ring-opening polymerization (ROP) mechanism of 1,5-dioxepan-2-one (DXO) and glycolide with Sn(II) and Al(III) alkoxide initiators (SnMe3MeO, SnMe2(MeO)2, and AlMe2MeO).
Abstract: A theoretical study is presented of the ring-opening polymerization (ROP) mechanism of 1,5-dioxepan-2-one (DXO) and glycolide with Sn(II) and Al(III) alkoxide initiators (SnMe3MeO, SnMe2(MeO)2, and AlMe2MeO). The B3LYP density functional method has been used to perform the quantum chemical calculations. A coordination−insertion mechanism is presented with two principal reaction steps. First, the alkoxide of the initiator performs a nucleophilic attack on the carbonyl carbon, and the carbonyl bond is broken. An intermediate is formed at this point, where the former carbonyl oxygen of the monomer is coordinated to tin via an alkoxide bond, while the carbonyl carbon assumes a sp3 bonding geometry. The second step involves the acyl−oxygen cleavage of the monomer. For all three initiators it was found that the transition state involving the breaking of the carbonyl double bond (TS1) represented the highest point on the potential energy surface for DXO. For glycolide, however, the transition state of the acyl−o...
TL;DR: The present strategy opens new opportunities to manipulate reactive carbon species using redox processes in organic synthesis by examining electrochemical reduction of "cation pools" generated by low-temperature electrolysis of carbamates.
Abstract: Carbocations, carbon radicals, and carbanions are important reactive carbon intermediates in organic chemistry, and their interconversions can be carried out by redox processes. Although, such relationships have been well recognized, experimental work has been limited to analytical studies on highly stabilized intermediates. In this study such interconversions were examined using electrochemical reduction of “cation pools”. Acyliminium cations, which were generated by low-temperature electrolysis of carbamates, were reduced electrochemically in the absence of radical acceptors. The homo coupling products formed effectively, suggesting that the one-electron reduction of the acyliminium cation produced the corresponding carbon-centered radical. Next, the electrochemical reduction of the acyliminium cations in the presence of electron-deficient olefins was examined. The cross coupling products were obtained in good-to-moderate yields. A mechanism involving radical addition to the double bond followed by the ...
TL;DR: The results suggest that the light metal atom ion is responsible for low intersystem crossing probability to the triplet excited state and the structural rigidity of the hexa-coordinated complexes for reducing the probability of internal conversion.