The cis—trans isomerization of nitrostilbenes XV: mixed singlet and triplet mechanism for trans → cis photoisomerization of 4-nitro-4′-dialkylaminostilbenes in non-polar solvents

The quantum yields of direct cis trans photoisomerization (φ_c → _t and φ_{t → c}) and of fluorescence of the trans isomers (φ_f) of three 4-nitro-4′-R-stilbenes (R amino (1), dimethylamino (2) and diethylamino (3)) were measured in several saturated hydrocarbons. Formation and decay of the lowest triplet state was observed by nanosecond laser flash photolysis. The triplet yield (φ_T), the triplet lifetime (τ_T), φ_{t → c} and φ_f were measured as a function of temperature and of the concentration of the quenchers ferrocene, azulene (Q) and oxygen. Twisting in the triplet, involving a ³t* ³p* equilibrium, analogous to that in other 4-nitrostilbenes, is suggested on the basis of the effects of temperature and quenchers on φ_T and τ_T. The trans → cis photoisomerization of 1 follows the triplet route almost completely. The existence of a singlet pathway (20% – 30% contribution) for 2 and 3 in non-polar solvents at room temperature is concluded from the non-linear dependence of the φ⁰_{t → c}/φ_{t → c} ratio on the concentration of Q. For these two nitrostilbenes a mixed singlet—triplet mechanism for the trans → cis photoisomerization is suggested.     

Solvent-mediated tautomerization of purine: single to quadruple proton transfer

We present calculations for the structures and the tautomerization reaction of purine and purine – (H₂O)_n (n=1–3) clusters. We find two pathways (via the carbene and the sp³-type intermediate) for the 9 ↔ 7 tautomerization of bare purine. The barrier heights for the 9 → 3 and 9 → 7 tautomerization of bare purine are calculated to be large (60–70 kcal/mol). Hydrogen bonding with the water molecule(s), however, dramatically lowers the 9 → 3 barrier by the concerted multiple proton transfer mechanism, favoring the formation of the conformer 3(H)- relative to the 7(H)-purine in the microsolvated environment, in contrast to the gas phase or the aqueous solution.     

Studies in macrolide synthesis: A highly stereoselective synthesis of (+)-(9S)-dihydroerythronolide a using macrocyclic stereocontrol

(+)-(9S)-Dihydroerythronolide A, 1, is prepared in 8 steps from macrolide 3 by exploiting the conformational preferences of the (5E,11E)-diene intermediate 2. The stereocontrolled introduction of the hydroxyl groups at C₆, C₁₁, and C₁₂ is achieved by osmylation, 2 → 13 and 15 → 1, while that at C₅ is obtained by a Zn(BH₄)₂ reduction, 13 → 14.     

The structure of intermediate products of ‘fragmentation’ of 10-bromodihydrocinchonine

10-Bromodihydrocinchonine 1d, similarly to analogical derivatives of other main cinchona alkaloids, transforms into nicinquine and isonicinquine 2d formally loosing its C2 carbon atom in a form of formaldehyde. This reaction was found to proceed via the so-far unstudied intermediate compounds (5a) 4-S-(Z-propenyl)- and (5 4-S-(E-propenyl)-6-R-7-S-(quinolyl-4)-8-oxa-1-R-azabicyclo[4.3.0]nonane which at the same time are products of a novel rearrangement of the parent cinchonine. The stereostructure of these compounds was determined using, mainly, NMR techniques. The energy minima of conformers 5 and 5a were supported by molecular mechanics calculations. The mechanisms for the 1d → 5 → 2d sequence have been discussed. The alkaloid 5 is sterically preferred to its Z-isomer. The accompanying nucleophilic substitution (1d → 6) and elimination (1d → 7) are also stereospecific.     

Flash photolysis/temperature jump study of the vibrational relaxation of N2O(010) by O(P) and NO

This survey begins with the photochemistry at 254 nm and 298 K in the system H₂O₂COO₂RH, the primary objective of which is to determine the rate constants for the reaction OH + RH → H₂O + R relative to the well-known rate constant for the reaction OH + CO → CO₂ + H. Inherent in the scheme is that the reaction HO₂+CO→OH+CO₂ is negligible compared with the OH reaction, and a literature consensus gives k_HO2 < 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹, or some 10⁶ less than k_OH at 298 K. Theoretical calculations establish that the first stage in the HO₂ reaction is the formation of a free radical intermediate HO₂ + CO → HOOCO (perhydroxooxomethyl) which decomposes to yield the products, and that the rate of formation of the intermediate is equal to the rate of formation of the products. The structure of the intermediate and a reaction profile are shown.

High temperature rate data reported subsequent to the data in the consensus and theoretical calculations lead here to a recommendation that, in the range 250–800 K, k_HO2 = 3.45 × 10⁻¹²T^1/2 exp(1.15 × 10⁴/T) cm³ molecule⁻¹ s⁻¹, the hard-sphere-collision Arrhenius modification. This yields k_HO2(298) = 1.0 × 10⁻²⁷ cm³ molecule⁻¹ s⁻¹ or some 10¹⁴ slower than k_OH(298).     

Une synthese totale stereospecifique de c-nor d-homosteroides comportant six carbones asymetriques

An efficient and stereoselective total synthesis of C-nor D-homosteroid compounds is described, following a general pathway of the A→B→C→D type. The A-B ring system was provided by the Wieland-Miescher ketone . the ring C was formed by intramolecular cyclisation of an appropiate γ-diketone such as . Construction of the ring D was achieved by means of a Birch reductive alkylattion, followed by intramolecular cyclisation of the intermediate δ-diketone thus formed. C-nor D-homosteroids and having six asymmetric carbons as well as the required “natural” configuration were thus obtained in eleven steps from the Wieland-Miescher ketone .     

Vibrational relaxation of DF at low temperatures

Temperature dependence of the probability of the deexcitation process DF(ν = 1) + DF(ν = 0) → DF(ν = 0) + DF(ν = 0) has been investigated near and below room temperature based on the model that the vibrational energy is transferred to the hindered rotational (oscillatory) motion as well as the translational motion. The deexcitation probability increases sharply with decreasing temperature; this inverse temperature dependence is attributed to the important contribution of hydrogen-bond attraction.     

The mechanism of photoinsertion of haloalkenes into Fe---C bonds of dieneiron tricarbonyls

1,1-Dichloro-2,2-difluoroethylene (like other fluoroalkenes) and dieneiron tricarbonyls react photochemically to give products in which the haloalkene has inserted into the Fe---C(1) bond of the diene complex. The reaction is regiospecific with respect to both diene and haloalkene. The reaction is shown to occur by (a) photodissociation of a CO ligand from iron; (b) π-complexation of haloalkene; (c) π → σ ligand rearrangements accompanied by CO reattachment. The adducts undergo hydrolysis on silica gel chromatography, to give substituted 2-chlorocyclohexadienoneiron tricarbonyls.     

Thermal stability of osmium mixed oxides I. CaOsO3 decomposition products: A new orthorhombic phase Ca2Os2O7

The thermal decomposition of CaOsO₃ by differential thermal analyses, thermogravimetry and X-ray powder diffraction has been studied. In nitrogen CaOsO₃ decomposes at 880 ± 10°C into CaO, osmium metal and oxygen due to the reaction CaOsO₃ → CaO + Os + O₂. In static air the decomposition occurs in three stages: 2CaOsO₃ + 1/2O₂ → Ca₂Os₂O₇ (in region 775–808°C), Ca₂Os₂O₇ → Ca₂Os₂O_6,5 + 1/4O₂ (at a temperature interval of 850–1000°C) and in the third stage Ca₂Os₂O_6,5 → 2CaO + OsO₄ ÷ 1/4 O₂ (at 1005 ± 5°C). The first intermediate Ca₂Os₂O₇ is isostructural with orthorhombic Ca₂Nb₂O₇ and its cell parameters are: a₀ = 3.745 Å, b₀ = 25.1 Å, c₀ = 5.492 Å, Z = 4, space group Cmcm or Cmc2. Ca₂Os₂O₇ exhibits metallic conductivity and its electrical resistivity is 4.6 × 10⁻² ohm-cm at 296K.     

A total synthesis of (±)-isocomene and (±) β-isocomene by an intramolecular ene reaction

The racemic sesquiterpenes isocomene 1 and β-isocomene 22 have been synthesized starting from 1,7-octadien-3-one 10 in a stereoselective manner. In the key step 12 → 13 (Scheme 5) the C-7, C-8-bond was formed by an intramolecular thermal ene reaction. Further transformations of 13 (Scheme 6) involved successively ring contraction 18 → 19, elimination 21 → 22 and olefin isomerization 22 → 1.     

Novel triterpenoid saponins from Mimusops elengi

Two novel triterpenoid saponins, mimusopin ( 3-O-β-D-glucopyranosyl-2β, 3β, 6β, 23-tetrahydroxyolean-12-en-28-oic acid 28-O--L-rhamnopyranosyl-(1→3)-β-D-xylopyranosyl-(1→4)[a-L-rhamnopyranosyl-(1→ 3)]--L-rhamnopyranosyl-(1→2)--L-arabinopyranoside)(1) and mimusopsin 3-O-[β-D-glucopyranosyl-(1→3)β-D-gluco-pyranosyl]-2β, 3β, 6β, 23-tetrahydroxyolean-12-en-28-oic acid 28-O--L-rhamnopyranosyl-(1→3)-β-D-xylopyranosyl-(1→4)--L-rhamnopyranosyl-(1→2)--L-arabinopyranoside (2) were isolated from the seeds of Mimusops elengi. Their structures were elucidated by a combination of 2D-NMR (COSY, HOHAHA, HETCOR, HMBC and NOESY), FAB-MS/MS and strategic chemical degradation. In addition, molecular mechanics and dynamics studies showed that the lack of a ¹³C glycosylation shift at the C-4 of the inner rhamnose in 1 could be correlated with distortion in the corresponding torsion angles.     

Analysis of the vibration correlation diagram of base · HCl(DCl) H-bonded complexes in Ar matrices: Type II and intermediate type I → II systems

The Fourier transform infrared spectra of the H-bonded complexes between HCl and 4-aminopyridine, 4-aminopyrimidine, 4-hydroxypyridine, 2-hydroxypyridine, benzimidazole and purine were investigated in Ar matrices. From the analysis of these spectra, the H-bonds N HCl appear to be of the pseudosymmetric type II for 4-aminopyridine, 4-aminopyrimidine and 4-hydroxypyridine, while benzimidazole forms a slightly weaker complex. H-bonding of HCl with the bases 2-hydroxypyridine and purine is of the intermediate type I → II. In the case of 4-aminopyrimidine, additional bonding of the Cl atom of HCl to an amino N---H bond yields a closed complex which explains the type II behaviour. In all other cases, bonding of additional HCl molecules to the 1:1 complexes results in proton transfer towards N---H⁺…Cl⁻(HCl)_π species, but n is much lower for type II than for the intermediate type I → II complexes. The results allow us to investigate the vibration correlation diagram and the isotopic ratio ν(HCl)/ν(DCl) for B - HCl complexes in Ar matrices into more detail.     

Semiempirical SCF/CI interpretation of the origin of the catalytic activity of transition metal-macrocycle complexes

Semiempirical self-consistent field (SCF) and configuration interaction (CI) calculations of the intermediate neglect of differential overlap (INDO) type are applied to the analysis of the electronic transitions of the hexaazacyclophane base and its Ni and Cu complexes. The ground states (¹A_g for the ligand and Ni complex, ²B_1g for the Cu complex) are planar structures of D₂h symmetry. The low-energy region of the UV-visible spectra, whose analysis may help to recognize the catalytic active sites of the complexes is associated with d → d transitions in the Ni complex, and M → L charge transfer in the Cu complex.     

Interaction of sodium cholate with dioctadecyldimethylammonium chloride vesicles in aqueous dispersion

Mixtures of dioctadecyldimethylammonium chloride (DODAC) cationic vesicle dispersions with aqueous micelle solutions of the anionic sodium cholate (NaC) were investigated by differential scanning calorimetry, DSC, turbidity and light scattering. Within the concentration range investigated (constant 1.0 mM DODAC and varying NaC concentration up to 4 mM), vesicle → micelle → aggregate transitions were observed. The turbidity of DODAC/NaC/water depends on time and NaC/DODAB molar concentration ratio R. At equilibrium, turbidity initially decreases smoothly with R to a low value (owing to the vesicle–micelle transition) when R = 0.5–0.8 and then increases steeply to a high value (owing to the micelle–aggregate transition) when R = 0.9–1.0. DSC thermograms exhibit a single and sharp endothermic peak at T_m ≈ 49 °C, characteristic of the melting temperature of neat DODAC vesicles in water. Upon addition of NaC, T_m initially decreases to vanish around R = 0.5, and the main transition peak broadens as R increases. For R > 1.0 two new (endo- and exothermic) peaks appear at lower temperatures indicating the formation of large aggregates since the dispersion is turbid. All samples are non-birefringent. Dynamic light scattering (DLS) data indicate that both DODAC and DODAC/NaC dispersions are highly polydisperse, and that the mean size of the aggregates tends to decrease as R increases.     

Computational studies on glyceraldehyde and glycine Maillard reaction-II

Density-functional theory (BLYP/6-31G*) and semiempirical theory was used to study compounds related to a strained cyclic hexaalkyne that was postulated by Staab et al. as a trimeric intermediate in the copper-mediated oxidative coupling of 2,2′-diethynylbiphenyl. Among the products of this reaction is an ortho-arene cyclyne. It was found that the hexaalkyne→ortho-arene cyclyne rearrangement is quite exothermic (ca. 130 kcal/mol). Of the semiempirical methods tested, the AM1 results were closer than PM3 or MNDO to BLYP/6-31G*. For the intermediate, two conformations were found: the expected D₃-symmetric conformation and an unexpected C₂-symmetric conformation. Structures and energies of hexamethyl- and dodecamethyl-substituted analogues of the intermediate and product were also calculated at the AM1 level, in order to test whether steric effects might stabilize the intermediate, relative to the product. It was found that even very sterically crowded products were much more stable than the relevant intermediates. This suggested that the large exothermicity of the hexaalkyne→ortho-arene cyclyne rearrangement might be used to drive formation of interesting strained products. An example used to illustrate this is the rearrangement of a binaphthyl analogue of Staab's biphenyl intermediate. BLYP and AM1 calculations suggest that the rearrangement of this intermediate would be exothermic by ca. 105–110 kcal/mol, despite strain induced by formation of three [5]helicene moieties in the product.     

In situ observation of the pressure-induced mesophase for 4'-n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acid

In situ observation of the optical texture, and X-ray patterns of the pressure-induced mesophase seen for 4'-n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC-16) was performed under hydrostatic pressures up to 100MPa using a polarizing optical microscope equipped with a high pressure hot stage and a wide angle X-ray diffractometer equipped with a high pressure vessel respectively. It was found that the pressure-induced mesophase (hereafter refered to as 'X') appeared at pressures above 60 MPa, and exhibits a birefringent broken-fan or a sand-like texture that remain unaltered in the SmC phase. The POM-transmitted light intensity curve measured on heating clearly showed the Cr₄ →Cr₁ →SmC →'X' →SmA →I transition sequence at 80 MPa. The optical texture and the POM-transmitted light intensity measured during a pressure cycle at 185°C showed a reversible change between the cubic and 'X' phases. The WAXD pattern of the 'X' phase showed a spot-like pattern, suggesting no layered structure for this phase, and also revealed a substantial decrease in the d-spacing of the low angle reflection at 80 and 100 MPa, compared with the d-spacings of the (0 0 1) reflection of the SmC phase and also the (2 1 1) reflection of the cubic phase. It is concluded from these data that the 'X' phase is a birefringent hexagonal columnar phase.     

New temperature effect on tunneling reaction in hydrogen, alkane, and ethanol in the solid phase

Rate constants for the tunneling reaction (HD + D → h + D₂) in solid HD increase steeply with increasing temperature above 5 K, while they are almost constant below 4.2 K. The apparent activation energy for the tunneling reaction above 5 K is 95 K, which is consistent with the energy (91–112 K) for vacancy formation in solid hydrogen. The results above 5 K were explained by the model that the tunneling reaction was accelerated by a local motion of hydrogen molecules and hydrogen atoms. The model of the tunneling reaction assisted by the local motion of the reactans and products was applied to the temperature dependence of the proton-transfer tunneling reaction (C₆H₆⁻ + C₂H₅OH → C₆H₇ + C₂H₅O⁻) in solid ethanol, the tunneling elimination of H₂ molecule of H₂ molecule ((CH₃)₂ CHCH(CH₃)₂⁺ → (CH₃)₂ C = C(CH₃)₂⁺ + H₂) in solid 2,3-dimethylbutane, and the selective tunneling reaction of H atoms in solid neo-C₅H₁₂-alkane mixtures.     

Action of manganese dioxide on carbohydrates—I Some disaccharides

Manganese dioxide degrades 1 → 3, 1 → 4 and 1 → 6 linked hexose disaccharides to, inter alia, 1 → 2, 1 → 3 and 1 → 5 linked hexosyl-pentoses, respectively. The yields (18–31 per cent) compare favourably with those recorded for other degradations. The methods which have been worked out for the isolation of these products are experimentally straightforward, and should be generally applicable to mixtures obtained by oxidation of 3-, 4- and 6-substituted hexoses with manganese dioxide.     

Interfacial criteria for stabilization of liquid foams by solid particles

The stability criteria of liquid foams, stabilized by solid particles have been derived, based on the interfacial separating pressure, acting between two neighboring bubbles (foam cells). Different structures of solid particles in the cell walls have been considered, all being able to stabilize liquid foams with an increasing probability, according to the following row: structure LP1 (loosely packed single layer of particles) → structure CP1 (closely packed single layer of particles) → structure LP2C (loosely packed double layer of clustered particles) → structure LP2+C (loosely packed ‘double+’ layer of clustered particles) → structure CP2 (closely packed double layer of particles) → structure CP2+ (closely packed ‘double+’ layer of particles). It has been shown that the contact angle should be higher than a certain value Θ_o, in order to ensure stability of bubble–particles agglomerates. On the other hand, different structures of particles can stabilize the foam, if the contact angle is below the certain value (90° for the CP1 and LP1 structures, 129° for the CP2, LP2C and LP2+C structures and 180° for the CP2+ structure). The optimum value of the contact angle, being able to stabilize the foam is a difficult function of different parameters, but has been found in the interval between 50 and 90°. It has been shown that the possibility to stabilize liquid foams is connected with the value of the dimensionless quantity PR_s/σ (P: the pressure, destabilizing the foam; R_s: the radius of the stabilizing particles; σ: the surface tension of the liquid). When PR_s/σ>40, foam stabilization is absolutely impossible. When PR_s/σ<40, foam stabilization becomes possible, but it has high probability only at PR_s/σ<4. From this condition the maximum size of the particles, being able to stabilize liquid foams can be found. Trial calculations showed that particles smaller than 3 and 30 μm in diameter are requested for stabilizing water based, and liquid aluminum based foams, respectively.