Chemoselective reactions of dimethyl carbonate catalysed by alkali metal exchanged faujasites: the case of indolyl carboxylic acids and indolyl-substituted alkyl carboxylic acids

Abstract

At 160-180 uC, in the presence of alkali metal exchanged faujasites (MX or MY; M = Li, Na, K), the reaction of dimethyl carbonate with indolyl-3-acetic, -3-propionic, and -3-butyric acids proceeds towards the formation of the corresponding methyl esters or carbamate esters which can be isolated in 93-99% yields. The methylation of the indolyl-NH group is never observed. This high chemoselectivity is driven by the nature of the catalyst and the reaction temperature. In particular, among the six different zeolites used, the more basic MX faujasites show better performances in terms of both activity and selectivity than MY solids. A similar trend also holds for the reaction of dimethyl carbonate with indolyl-carboxylic acids, where MX compounds are still efficient catalysts for the formation of methyl esters. In this case, however, the overall reactivity/selectivity also reflects the relative positions of the NH and CO 2 H groups which may account for significant decarboxylation reactions observed for indolyl acids substituted at positions 2 and 3. This process is totally absent for indolyl-6-carboxylic acid.

Manganese Chloride as an Efficient Catalyst for Selective Transformations of Indoles in the Presence of a Keto Carbonyl Group

“…[15] Particularly, when N-(2-hydroxy)ethylindole 9a was used as an alcohol, as the generated product 10a contains both a keto carbonyl group and a reactive C-3 unsubstituted indole fragment, controlling of the reaction selectivity could be a hard task for us. As we expected, various conventional acids, such as H 2 SO 4 3 , SnCl 4 , FeCl 3 , InCl 3 and ZnCl 2 , were found to be ineffective for this transesterification although substrate 2b was almost completely consumed in the most cases (Table 3, entries 1 to 9). The poor efficiencies of these catalysts mainly resulted from a lack of selectivity to 10 a.…”

“…As shown in Table 1, no reaction occurs in the absence of catalyst (entry 1). When some conventional acids, such as p-toluenesulfonic acid (TsOH), ScA C H T U N G T R E N N U N G (OTf) 3 , InCl 3 and FeCl 3 were used as catalysts, 1a was almost completely consumed at the end of the reaction, and a diA C H T U N G T R E N N U N G (indolyl)methane derivative 3a was obtained from a messy mixture in poor yields (entries 2 to 5). In order to control the reaction selectivity, a weak acid, boric acid, was then examined.…”

“…Following treatment of 4a with 2a in the presence of 30 mol% of MnCl 2 tetrahydrate resulted in the quantitative formation of 3i. Two other acids, TsOH and ScA C H T U N G T R E N N U N G (OTf) 3 , were also examined in this procedure in the presence of 1.2 equivalents of water. It was found that 1b was completely consumed in the first step, but, however, 4a was obtained only in a small amount (yield < 20%).…”

“…There are some other by-products that are difficult to isolate in the reaction mixture. Furthermore, TsOH and ScA C H T U N G T R E N N U N G (OTf) 3 were proved to be non-selective for the formation of 3i in the second step because a messy mixture was formed in these cases.…”

“…Although many methodologies for chemoselective control in various reactions have been well established by means of catalysis with enzymes or organic and organometallic compounds, the reported methods often involve practical difficulties due to high costs of catalyst, limited substrate scope or lack of green properties. [3] Therefore, new catalytic systems are being continuously explored in search of improved efficiencies and cost-effectiveness.…”