摘要:

AbstractPhotochemicalE‐Zisomerization of 2‐(methoxyimino)‐1‐indanone (1) and its 7‐methyl derivative (2) was studied under conditions of direct irradiation (λ 350 and 300 nm) and triplet photosensitization. The decay ratiosk5/k6for triplet photosensitization and direct irradiation at λ 350 nm are the same for a given compound, indicating that isomerization of the studied α‐oxo oxime ethers upon direct irradiation at λ 350 nm also proceeds from the (lowest) excited triplet state. Upon direct irradiation at λ 300 nm, the photostationary state (pss) was found to depend on the concentration of the 2‐(methoxyimino)‐1‐indanone: at high concentrations, the (E)‐isomer strongly dominates (>90%), whereas, at low α‐oxo oxime ether concentrations, a substantial amount of the (Z)‐isomer is present (up to 35%). The concentration dependence of the pss is explained in terms of excimer formation of theS2(ππ*) state, the excimer being capable of inducing efficientE‐Zisomerization with a different decay ratio than th

ISSN:0165-0513    

DOI:10.1002/recl.19901091102

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractA detailed mechanistic study of the epoxidation of alkenes by the cytochrome‐P‐450 model sodium hypochlorite/manganese(III) tetraarylporphyrin is presented. From the zero‐order rate dependence on alkene concentration and low‐temperature‐trapping experiments with the hydrogen‐atom donor 1,1‐diphenyl‐2‐picrylhydrazine (DPPH), it is concluded that the formation of the active [(P)MnO]+species from a (P)MnOCl complex is the rate‐determining step of the reaction. The trapping experiments also show that the reverse reaction is possible. Pyridine or imidazole derivatives as axial ligands accelerate the rate‐determining step by electron donation. Imidazoles, however, are destroyed under the reaction conditions. A general base function of pyridine or imidazole derivatives is excluded on the basis of experiments with porphyrin catalysts containing covalently bound pyridine. Large amounts of pyridine or imidazole retard the epoxidation reaction by blocking both sides of the catalyst, and thus preventing the hypochlorite anion to coordinate. In spite of the zero‐order‐in‐alkene concentration, different alkenes are expoxidized at different rates. This is explained by the occurrence of dimerization phenomena,i.e., a reaction of the [(P)MnO]+species with MnIII, which eventually leads to catalyst destruction. Electron‐rich substrates effectively prevent this destruction. In the case of sterically hindered catalysts,i.e., catalysts bearing bulky substituents on theorthopositions of the phenyl rings, no dimerization occurs. With this class of catalysts, however, reaction rates are influenced by steric interactions between catalyst and substrate. Electron‐withdrawing substituents on the 4‐positions of the phenyl rings of the catalyst increase the epoxidation rate. This is a result of increased electrophilicity of the [(P)MnO]+species and decreased electron density at themesopositions making the catalyst less susceptible to destruction. Lowering the pH of the aqueous phase from 13.5 to 12.7 increases the rate of epoxidation, because, at lower pH, a neutral HCl molecule must be expelled from a [(P)MnHOCl]+complex instead of a chloride anion, in order to form the active species. Alcohols increase the reaction rate by providing better solvation of the leaving group (Cl or HCl). Aldehydes and ketones, which are formed as by‐products in epoxidation, accelerate the reaction in the case of sterically not hindered catalysts. These compounds rapidly trap the active species, thus preventing unfavourable dimerization processes occurring. A carbonyl oxide species is produced, which in turn acts as an active epoxidizing agent. The reaction between the [(P)MnO]+species and an alkene is likely to proceed via a non‐concerted process by interaction of the LUMO of the active species with the HOMO of the alkene in an asymmetrical way. With this mechanism, the formation oftrans‐epoxides fromcis‐alkenes as well as the formation of aldehydes and ketones as by‐products are easily explained. Pyridines increase the stereoselectivity of the reaction by pulling the metal centre into the porphyrin plane. This creates increased steric hindrance to

ISSN:0165-0513    

DOI:10.1002/recl.19901091103

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractOptically pure 1‐aryl‐ (phenyl and 2‐chlorophenyl) 1,3‐propanediols,1aand1b, condense smoothly with aldehydes to provide the corresponding cyclic acetals3. Except in cases that the aldehyde is substituted at the α‐carbon atom, a single product is obtained. With the aid of 2D‐NMR the stereochemistry was established to be that with the aldehyde substituent as well as the aryl group equatorial in the favored chair form.The condensation products with some α,β‐unsaturated aldehydes were subjected to cyclopropanation with diiodomethane/diethylzinc. Reactions proceeded in good yields. Deprotection was achieved by ozonolysis. The cyclopropanecarboxylic acids 6 were obtained in 34‐67% yields and in enantiomeric excesses ranging from 21‐78%.A stereochemical rationalization of

ISSN:0165-0513    

DOI:10.1002/recl.19901091104

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractTwo poly(iminomethylenes) with cholesterol‐containing pendant groups were synthesized. Using an aliphatic spacer, a liquid‐crystalline polymer was obtained, with equal amounts of right‐and left‐handed helices. With phenylene as bridging group, only oligomers resulted, probably for steric reasons. Some novel liquid‐crystalline esters of cholesterol are also

ISSN:0165-0513    

DOI:10.1002/recl.19901091105

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

ISSN:0165-0513    

DOI:10.1002/recl.19901091106

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

ISSN:0165-0513    

DOI:10.1002/recl.19901091101

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY