Iron-Catalyzed Regioselective Alkoxycarbonylation of Imidazoheterocycles with Carbazates

Abstract

A regioselective alkoxycarbonylation of imidazoheterocycles using carbazates as ester group sources in DMSO was developed, in which an inexpensive FeCl2·4H2O was used as the catalyst and (NH4)2S2O8 was the oxidant. The reaction proceeded smoothly under an air atmosphere to give the 3-alkoxycarbonylated products in moderate to good yields.

Metal‐free Deoxygenative 2‐Amidation of Quinoline N‐oxides with Nitriles via a Radical Activation Pathway

“…First, the oxidation of the methyl carbazate by Selectfluor occurred via a single electron transfer to generate a carbon-centered ester radical with the release of molecular nitrogen, HF and tetrafluoroborate. [17] Subsequently, the ester radical coupled with quinoline N-oxide 1 to produce a radical…”

“…Based on the above experimental results and related literature, a possible reaction mechanism is depicted in Scheme . First, the oxidation of the methyl carbazate by Selectfluor occurred via a single electron transfer to generate a carbon‐centered ester radical with the release of molecular nitrogen, HF and tetrafluoroborate . Subsequently, the ester radical coupled with quinoline N ‐oxide 1 to produce a radical cation intermediate A which could be further oxidized to a quinolinium intermediate B .…”

Recent Advances in Radical C−H Bond Functionalization of Imidazoheterocycles

“…In 2016, Sun and co-workers developed alkoxycarbonylation of imidazo[1,2-a]pyridines 1 with carbazates 49 using FeCl 2 • 4H 2 O as a catalyst and (NH 4 )S 2 O 8 as an oxidant in DMSO at 55°C (Scheme 33). [53] A series of imidazo [ Based on a set of the mechanistic study, it is proposed that the radical B was produced from DMA via SET oxidation in presence of Cu(II) catalyst. Then, radical addition of B on 1 followed by cascade events such as SET oxidation, aromatization and hydrogen radical abstraction afforded the intermediate C. The radical C quickly trapped by O 2 followed by hydrogen to produce peroxide intermediate D, which upon loss of water yielded the desired product 54.…”

“…In 2016, Sun and co‐workers developed alkoxycarbonylation of imidazo[1,2‐ a ]pyridines 1 with carbazates 49 using FeCl 2 ⋅ 4H 2 O as a catalyst and (NH 4 )S 2 O 8 as an oxidant in DMSO at 55 °C (Scheme ) . A series of imidazo[1,2‐ a ]pyridines 1 , benzo[ d ]imidazo[2,1‐ b ]thiazoles 2 and imidazo[2,1‐ b ]thiazole 3 bearing various functional groups reacted well with 49 and produced the desired products 50 in 32–92% yields.…”

Mn(OAc)3-Mediated Regioselective Radical Alkoxycarbonylation of Indoles, Pyrimidinones, and Pyridinones

“…Organic free radicals have been widely used for direct functionalization of Csp 2 -H and other unsaturated bonds. For example, alkoxycarbonyl radicals generated from the decomposition of carbazates could add to alkenes, 2 isocyanides, 3 and heteroarenes 4 (Scheme 1, A-D). We have recently reported a series of Mn(OAc) 3 -promoted radical phosphorylations of pyrimidinones, pyridinones, and indoles.…”