Cyclopropenium ion catalysed Beckmann rearrangement

Abstract

1-Chloro-2,3-diphenylcyclopropenium ion was found to be a very efficient organocatalyst (3 mol% loading) for liquid phase Beckmann rearrangement of various ketoximes to the corresponding amides/lactams within 2 h in acetonitrile at reflux temperature. This is the first example of the application of the cyclopropenium ion as a catalyst, which opens up a new aspect of the synthetic utility of aromatic cation based catalysis.

Asymmetric Synthesis of Nonracemic Primary Amines via Spiroborate-Catalyzed Reduction of Pure (E)- and (Z)-O-Benzyloximes: Applications toward the Synthesis of Calcimimetic Agents

“…Therefore, different approaches to the ( Z )-isomers were studied. 10 Following Sharghi and coworkers’ procedure, 10a acetophenone was treated with hydroxylamine hydrochloride using K 2 CO 3 under solvent-free conditions. However, the ( E )-acetophenone oxime was the major product.…”

“…Likewise, the ( E )-isomer was the only product obtained by the method of Coustard and coworkers using naphthalene and nitroethane in the presence of H 2 SO 4 to obtain the 1-(naphthalen-1-yl)ethanone oxime. 10b However, the desired ( Z )-isomers were conveniently prepared according to the method first developed by Smith et al . 10c,d Representative ( E )-α-bromo acetophenone oximes and 1-(naphthalen-1-yl)ethanone oxime were synthesized from their corresponding α-bromo ketones by treatment with 3 equivalent of hydroxylamine in methanol/water solvent (Scheme 2).…”

“…10b However, the desired ( Z )-isomers were conveniently prepared according to the method first developed by Smith et al . 10c,d Representative ( E )-α-bromo acetophenone oximes and 1-(naphthalen-1-yl)ethanone oxime were synthesized from their corresponding α-bromo ketones by treatment with 3 equivalent of hydroxylamine in methanol/water solvent (Scheme 2). The corresponding ( Z )-acetophenone oximes were then successfully prepared in excellent yield by the nucleophilic reaction of α-bromo acetophenone oximes with 1 equivalent of NaBH 4 .…”

“…81–83 °C, Lit. 10d 81–83°C; 1 H NMR (400 MHz, CDCl 3 ) δ 2.20 (s, 3H), 7.35–7.39 (m, 3H), 7.42–7.43 (m, 2H), 7.44 (br s, 1H); GC/MS m/z 135.1 (M + ).…”

Cyclopropenium Ion

“…Thus, they easily combine with an anion or a Lewis basic heteroatom leading to the reversible generation of the corresponding neutral carbocyclic species (Scheme 1). Owing to their unique reactivity, cyclopropenium ions, especially 2,3-diarylcyclopropenium ions, have been strategically exploited for the activation of the hydroxyl group of alcohols, carboxylic acids, oximes, and diols in various synthetically useful dehydrative processes to afford alkyl chlorides, 2,3 acid chlorides, 4 amides/lactams, 5,6 and cyclic ethers, respectively. 7…”