Remarkable Solvent Effect Research Articles

Carbon‐13 NMR spectra of chlorophyll a, chlorophyll a′, pyrochlorophyll a and the corresponding pheophytins

AbstractThe 13C NMR spectra of C‐10 epimeric chlorophylls a and a′, pheophytins a and a′, pyrochlorophyll a and pyropheophytin a have been recorded and assigned by chemical shift comparison, by long‐range selective 1H decoupling experiments and by the examination of the fully coupled spectra. Various factors influencing the 13C chemical shifts of the chlorophyll derivatives, e.g. the coordination of magnesium to the chlorin nucleus, the effect of solvent and the steric strain at the periphery of the macrocycle, have been examined. The 13C NMR spectra of chlorophyll a measured in acetone‐d6 and tetrahydrofuran‐d8 (THF) were compared, and remarkable solvent effects on the 13C chemical shifts were observed. These effects were interpreted mostly in terms of specific chlorophyll‐solvent interactions. Different electron donor and steric properties of acetone and THF were considered to cause conformational alterations in the macrocycle, induced by the ligation of the solvent molecule(s) to the axial position(s) of the central magnesium atom of chlorophyll a. These results show that 13C NMR spectroscopy is a method of high information value for investigations of the unique electron donor acceptor (EDA) properties of the chlorophylls. The structural differences between the C‐10 epimeric chlorophylls and pheophytins were examined in terms of the substituent chemical shift (SCS) parameters for the C‐10 methoxycarbonyl group. The analysis showed that the change from the (10R) to the 10(S) configuration induces conformational alterations in the whole macrocycle which are, however, most prominent in rings IV and V. Owing to the increased steric interaction (repulsion) between the bulky substituents at C‐7 and C‐10, the peripheral strain is larger in the (10S) form, and is relieved by more pronounced deviations of rings IV and V from the macrocyclic plane compared with the (10R) form. The examination of the SCS parameters also showed that the peripheral steric strain is dissipated to a larger extent over the entire macrocycle in the Mg‐free derivatives. These results confirm the previous conclusions based on 1H NMR and CD data. The possible function of chlorophyll a′ in photosynthesis is briefly discussed.

Oxidative cleavage of silicon-carbon bonds in organosilicon fluorides to alcohols

Certain functional silyi groups have been shown to be synthetically equivalent to the OH group. All of the C-Si bonds in organosilicon fluorides, K 2(RSiF 5] and R nSiF 4-n, (n = 1,2,3) are cleaved by m-chloroperbenzoic acid (MCPBA) in DMF to give the corresponding alcohols in high yields. Although the reaction with RSiF 5 2- and RSiF 3 proceeds without any additive, cleavage of R 2SiF 2 and R 3SiF requires, respectively, a catalytic or excess amount of KF. A triorganoethoxysilane also undergoes similar oxidative cleavage reactions in the presence of an excess amount of KF. The MCPBA-oxidation of exo- and endo-2-norbornyl-silicate and -trifluorosilane proceeds stereospecifically with retention of configuration at carbon. A remarkable solvent effect has been observed in the oxidative cleavage of RSiF 3. A plausable mechanism involves a hexacoordinate silicon species in which the organic group intramolecularly migrates from Si to O of the coordinated MCPBA.

Photochemical aromatic hydroxylation by aromatic amine N-oxides: remarkable solvent effect on NIH-shift

Photooxygenations of 4- 2H-anisole ( 3 ) and o-xylene ( 5 ) by 3-methylpyridazine 2-oxide ( 1 ) or pyridine 1-oxide ( 2 ) were studied in a variety of solvents at varying irradiation temperatures. Remarkable solvent effect on NIH-shift coupled with their hydroxylation processes was observed.

The asymmetric hydrogenation of α-keto esters catalyzed by rhodium(I) complexes with chiral diphosphine ligands. On the catalytic cycles and the mechanism of asymmetric induction

Asymmetric hydrogenations of n-propyl pyruvate and ketopantoyl lactone catalyzed by rhodium complexes with (—) DIOP and BPPM were carried out under a variety of conditions. It was found that i) the Schrock-Osborn mechanism was not operative in these reactions at all since no acceleration of the reaction rate by the addition of water (1%) was observed, ii) there was a clear difference between cationic and neutral (in situ) rhodium catalysts in their enantioselectivity, and iii) there was a remarkable solvent effect on the extent and direction of asymmetric induction. Possible mechanisms are discussed on the basis of these results.

The Base-catalyzed Tautomerization of Acetylacetone in Organic Solvents. The Correlation of the Catalytic Coefficient with the Partition Coefficient

Abstract The keto-enol tautomerization rate of acetylacetone catalyzed by triethylamine or tributylamine was observed in eight organic solvents, and the catalytic coefficient for each process of the ketonization and enolization was obtained. A remarkable solvent effect on the catalysis was discussed following the linear free-energy relationship between the catalytic coefficient and the liquid-liquid partition coefficients of the keto and enol forms. It was concluded that, in the homogeneous base-catalysis, the solvent affects the free-energy change of the transition state more greatly than those of the keto and enol forms. On the contrary, in the heterogeneous catalysis by the silica surface, the free-energy change of the keto and enol forms was suggested to be the main factor governing the kinetic solvent effect.

Studies on Mixed Chelates. V. Mixed Nickel(II) Chelates with N,N,N′,N′-Tetraethylethylenediamine and β-Diketones

Abstract Fourteen mixed nickel(II) chelates with the general formula, Ni teen(β-dik)X, were newly obtained. Here teen is N,N,N′,N′-tetraethylethylenediamine, i.e., a sterically hindered ligand with four bulky ethyl groups on the donor N atoms, β-dik is a β-diketonate ion, and X is an anion such as B(C6H5)4−, ClO4−, NO3− or β-dik. According to the coordination ability of the anion X, the structure of the mixed chelate changes from square planar to octahedral, remarkable solvent effects being observed on the absorption spectra of these chelates. The steric influence of the ligand “teen” was compared with that of the ligand “tmen”, i.e., its tetramethyl analog.

The Apparent Solvent Effect on the Fluorescence Efficiency of 9-Methyl-10-(chloromethyl)anthracene

Abstract 9-Methyl-10-(chloromethyl)anthracene (I) is immediately transformed into 9-methyl-10-(ethoxymethyl)-anthracene (II) in ethanol, and a very strong fluorescence of II is observed. While I is practically nonfluorescent, II fluoresces strongly in all solvents. These facts explain well a phenomenon misinterpreted by Melhuish as a remarkable solvent effect on the fluorescence efficiency of I.

Solvent effects on radical polymerization of cyclododecyl acrylate, 1. Homopolymerization

AbstractThe radical homopolymerization of cyclododecyl acrylate at 60°C in benzene, tetrahydrofuran, or dioxane was studied and compared with that of cyclohexyl acrylate. The difference in the polymerization rates of the acrylates could not be interpreted in terms of the polar and steric substituent constants of ester groups in Taft's equation. Remarkable solvent effects on the polymerization rates for both acrylates were also found in the same manner and ascribed to the polar nature of the used solvents.

The ESR Spectra of Bis(t-butyl and ethyl acetoacetato)copper(II) Complexes in Various Hydrocarbon Solvents

Abstract The X-band ESR spectra of bis(t-butyl acetoacetato)copper(II) and bis(ethyl acetoacetato)copper(II) in several non-coordinating solvents of hydrocarbon were measured at 77 K. Remarkable solvent effects on dimer formation were revealed for the complexes.

Remarkable solvent effects on the lifetime of 1.DELTA.g oxygen

Remarkable solvent effects on the lifetime of 1.DELTA.g oxygen

Oxidation by Transition Metal Complexes. II. Oxidation of Styrenes Catalyzed by Rhodium Compounds

Abstract Catalytic activities of chlorotris(triphenylphosphine)rhodium(I) and rhodium(III) chloride for the oxidation of olefins have been studied. It has been found that the complex and RhCl3 can cause catalytically the oxidation of aliphatic olefins such as styrene, α-methylstyrene or β-methylstyrene to produce carbonyl compounds under oxygen atmosphere. A remarkable solvent effect was observed in the reaction.

Reactions between oximes and organo-derivatives of group III elements. Part I. Acetoxime

Acetoxime reacts at 100° with trimethylborane to form methane and (Me2C:NOBMe2)n, which is monomeric (n= 1) in the gas phase, partially associated (n= 1·3–1·4) in benzene solution, and probably dimeric in the crystal phase. A six-membered “BONBON” ring structure seems likely for this compound, although a four-membered “BOBO” ring cannot be ruled out on present evidence. Trimethyl-aluminium, -gallium, and -indium react with acetoxime at –78° to form related compounds (Me2C:NOMMe2)2(M = Al, Ga, or In), which are thought to be dimeric in the gas, solution, and crystal phases and to contain similar rings. Trimethylthallium at room temperature gives the analogous derivative Me2C:NOTlMe2. Methyl-lithium and acetoxime give the polymeric Me2C:NOLi. The mass i.r., and 1H n.m.r. spectra of these compounds are discussed, the latter showing a remarkable solvent effect. Reaction between diborane and acetoxime leads to general decomposition.

Fluorescence Quenching due to the Interactions between π-Electron Systems in the Excited State and Acid-Base Relation: Nitrogen Heterocycles, Naphthylamine and Naphthol in Aqueous and Alcoholic Solutions

Abstract The acid-base relation in the fluorescence quenching in aqueous and alcoholic solutions, due to the interaction between π-electron systems in the excited state, has been pointed out. Roughly speaking, this is parallel with the fluorescence quenching due to hydrogen bonding between the π-electronic fluorescer and quencher in non-polar solvent. Remarkable solvent effects on the quenching efficiency have been found and some important roles of hydrogen bonding in this phenomenon have been suggested.