Imidazolium Salt-Catalyzed Friedel-Crafts-Type Conjugate Addition of Indoles: Analysis of Indole/Imidazolium Complex by High Level ab Initio Calculations

Recent advances in the organocatalytic applications of cyclopropene- and cyclopropenium-based small molecules

The development of novel small molecule-based catalysts for organic transformations has increased noticeably in the last two decades. A very recent addition to this particular research area is cyclopropene- and cyclopropenium-based catalysts. At one point in time, particularly in the mid-20^th century, much attention was focused on the structural aspects and physical properties of cyclopropene-based compounds. However, a paradigm shift was observed in the late 20^th century, and the focus shifted to the synthetic utility of these compounds. In fact, a wide range of cyclopropene derivatives have been found serving as valuable synthons for the construction of carbocycles, heterocycles and other useful organic compounds. In the last few years, the catalytic applications of cyclopropene/cyclopropenium-based compounds have been uncovered and many synthetic protocols have been developed using cyclopropene-based compounds as organocatalysts. Therefore, the main objective of this review is to highlight recent developments in the catalytic applications of cyclopropene-based small molecules in different areas of organocatalysis such as phase-transfer catalysis (PTC), Brønsted base catalysis, hydrogen-bond donor catalysis, nucleophilic carbene catalysis, and electrophotocatalysis developed within the past two decades.

Bis(amino)cyclopropenium Ion as a Hydrogen-Bond Donor Catalyst for 1,6-Conjugate Addition Reactions

The catalytic application of the bis(amino)cyclopropenium ion has been investigated in conjugate addition reactions. The hydrogen atom, which is attached to the cyclopropene ring of bis(amino)cyclopropenium salts, is moderately acidic and can potentially serve as a hydrogen-bond donor catalyst in some organic transformations. This hypothesis has been successfully realized in the 1,6-conjugate addition reactions of p-quinone methides with various nucleophiles such as indole, 2-naphthol, thiols, phenols, and so forth. The spectroscopic studies (NMR and UV-vis) as well as the deuterium isotope labeling studies clearly revealed that the hydrogen atom (C-H) that is present in the cyclopropene ring of the catalyst is indeed solely responsible for catalyzing these transformations. In addition, these studies also strongly indicate that the C-H hydrogen of the cyclopropene ring activates the carbonyl group of the p-quinone methide through hydrogen bonding.

Synthesis of 1-(β-coumarinyl)-1-(β-indolyl)trifluoroethanols through regioselective Friedel-Crafts alkylation of indoles with β-(trifluoroacetyl)coumarins catalyzed by Sc(OTf)3

A highly efficient Friedel-Crafts alkylation of indole derivatives with β-(trifluoroacetyl)coumarins using Sc(OTf)3 as a catalyst has been developed, which gives regioselective 1,2-adducts to afford 1-(β-coumarinyl)-1-(β-indolyl)trifluoroethanols. A series of tertiary trifluoroethanols containing different indole and coumarin groups were synthesized in moderate to excellent yields (up to 95%) in the presence of 5 mol% catalyst in a short time (only 2 minutes at least). A mechanism of the reaction, in which the trace amount of water plays the role of proton transfer in catalyzing circulation was proposed and confirmed.

Development of an imidazole salt catalytic system for the preparation of bis(indolyl)methanes and bis(naphthyl)methane

Imidazolium salts are shown to catalyze the rapid room temperature reaction of indoles or naphthol with aldehydes to provide bis(indolyl)methanes or bis(naphthol)methane in excellent yields and the reaction proceeds optimally in dichloromethane with no base additives. The reaction exhibits a broad substrate tolerance and occurs through nucleophilic activation of the indoles and naphthols through a cation-π interaction.

Onium Ion-assisted Organic Reactions Through Cation–π Interactions

The cation–π interaction is an attractive noncovalent interaction between a cation and a π-face. Owing to the stronger interaction energy than those of the other π interactions, such as π–π and CH–π interactions, the cation–π interaction has recently been recognized as a new tool for controlling the regio- and stereoselectivities in various types of organic reactions. This chapter attempts to cover a variety of organic reactions assisted by interactions between unreactive onium ions and π-faces, which will provide comprehensive knowledge on the role of cation–π interactions in organic synthesis.