Topic
Substituent
About: Substituent is a research topic. Over the lifetime, 42877 publications have been published within this topic receiving 516716 citations. The topic is also known as: side chain & side group.
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173 citations
TL;DR: A practical sequence involving three consecutive palladium(0)-catalyzed reactions has been developed for synthesizing 3-alkyl-3-aryloxindoles in high enantiopurity and a wide variety of aryl and heteroaryl substituents, including ones of considerable steric bulk, can be introduced at C3 of oxindoles.
Abstract: A practical sequence involving three consecutive palladium(0)-catalyzed reactions has been developed for synthesizing 3-alkyl-3-aryloxindoles in high enantiopurity. The Heck cyclization precursors 10 and 11a-k are generated in one step by chemoselective Stille cross-coupling of 2'-triflato-(Z)-2-stannyl-2-butenanilide 9 with aryl or heteroaryl iodides. The pivotal catalytic asymmetric Heck cyclization step of this sequence takes place in high yield and with high enantioselectivity (71-98% ee) with the Pd-BINAP catalyst derived from Pd(OAc)(2) to construct oxindoles containing a diaryl-substituted all-carbon quaternary carbon center. A wide variety of aryl and heteroaryl substituents, including ones of considerable steric bulk, can be introduced at C3 of oxindoles in this way (Table 4). The only limitations encountered to date are aryl substituents containing ortho nitro or basic amine functionalities and the bulky N-alkyl-7-oxindolyl group. Asymmetric Heck cyclization of butenalide 22 having an o-(N-acetyl-N-benzylamino)phenyl substituent at C2 provided a approximately 1:1 mixture of amide atropisomers 23 and 24 in high yield and high enantioselectivity. These atropisomers are formed directly upon Heck cyclization of 22 at 80 degrees C, as they interconvert thermally to only a small extent at this temperature.
172 citations
TL;DR: The complete synthesis of chlorophyll a starting from Knorr's pyrrole was described in detail in this article, with full experimental detail, in which the four pyrroles which are required for rings A, B, C and D were elaborated largely by known reactions, although with considerable improvements.
172 citations
TL;DR: The substituent electronic effects originated mainly from the perturbation of the core PDI HOMO energy level by the substituents and showed an unusual dual-band absorption that spans from 450 to 750 nm, which was tentatively assigned as the charge-transfer band and the PDI core absorption.
Abstract: Substituent effects on the photophysical and electrochemical properties of 1,7-diaryl-substituted perylene diimides (1,7-Ar2PDIs) have been carefully explored. Progressive red-shifts of the absorption and emission maxima were observed when the electron-donating ability of these substituents was increased. Linear Hammett correlations of 1/λmax versus σ+ were observed in both spectral analyses. The positive slopes of the Hammett plots suggested that the electronic transitions carry certain amounts of photoinduced intramolecular charge-transfer (PICT) character from the aryl substituents to the perylene diimide core which leads to the reduction of the electron density on the substituents. The substituent electronic effects originated mainly from the perturbation of the core PDI HOMO energy level by the substituents. This conclusion was supported by PM3 analyses and confirmed by cyclic voltammetry experiments. More interestingly, the Ph2NC6H4-substituted PDI, 4i, showed an unusual dual-band absorption that sp...
172 citations
TL;DR: In this article, cellulose esters with linear aliphatic acyl substituents ranging in size from C12 (lauric acid) to Eicosanoic acid were prepared in homogeneous solution (DMAc/LiCl) using a novel synthetic method based on the use of a mixed p-toluenesulfonic/carboxylic acid anhydride.
Abstract: Cellulose esters with linear aliphatic acyl substituents ranging in size from C12 (lauric acid) to C20 (eicosanoic acid) were prepared in homogeneous solution (DMAc/LiCl) using a novel synthetic method based on the use of a mixed p-toluenesulfonic/carboxylic acid anhydride. The resulting waxy cellulose esters had a high degree of substitution (DS), between 2.8 and 2.9, and showed little degradation. Thermal analysis of these cellulose derivatives by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) revealed a series of transitions that represented motion by both ester substituents and cellulosic main chain. Broad crystallization and melting transitions attributed to side-chain crystallinity were observed in the range between −19 and +55°C; these side-chain Tm and Tc transition temperatures increased by 10°C per carbon atom of the ester substituent. The Tg of these derivatives increased linearly with increasing substituent size from 94°C for C12 (cellulose laurate) to 134°C for C20 (cellulose eicosanoate). Evidence of “main-chain” crystallization was not observed for these samples, except in the case of peracetylated C12 and C14 esters, which had Tm values of 96°C and 107°C, respectively. © 1996 John Wiley & Sons, Inc.
171 citations