Rhodium(I)-catalyzed synthesis of indoles: amino-Claisen rearrangement of N-propargylanilines

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Synthesis of 4-amino-5-allenylisoxazoles via gold(I)-catalysed propargyl aza-Claisen rearrangement

Propargyl aza-Claisen rearrangement of 4-propargylaminoisoxazoles 1 proceeded in the presence of cationic gold(i) catalysts to give 4-amino-5-allenylisoxazoles 2 in good to high yields. The silyl group at the terminal alkyne and a cationic gold(i) catalyst bearing a sterically bulky ligand are essential for the generation of isolable allene intermediates. The N-protection of the generated 4-amino-5-allenylisoxazoles 2 allowed the isolation of 5-allenylisoxazoles 4 that have never been synthesized. N-Propargyl aniline 5 was successfully converted to the corresponding ortho-allenyl aniline 6 under the current reaction conditions.

One-pot, three-component Fischer indolisation-N-alkylation for rapid synthesis of 1,2,3-trisubstituted indoles

A one-pot, three-component protocol for the synthesis of 1,2,3-trisubstituted indoles has been developed, based upon a Fischer indolisation-indole N-alkylation sequence. This procedure is very rapid (total reaction time under 30 minutes), operationally straightforward, generally high yielding and draws upon readily available building blocks (aryl hydrazines, ketones, alkyl halides) to generate densely substituted indole products. We have demonstrated the utility of this process in the synthesis of 23 indoles, benzoindoles and tetrahydrocarbazoles bearing varied and useful functionality.

Recent advances in the synthesis of indoles from alkynes and nitrogen sources

This review highlights ten years of success in the synthesis of indoles using alkynes and nitrogen sources as substrates.

Homogeneous rhodium(i)-catalysis in de novo heterocycle syntheses

Rh(i)-catalysed reactions often employ mild reaction conditions and offer excellent functional group tolerance, making them ideal transformations for the preparation of complex molecules. This review surveys examples of these synthetically useful transformations as applied to the synthesis of various heterocycles.

Synthesis of quinoxalines or quinolin-8-amines from N-propargyl aniline derivatives employing tin and indium chlorides

The utility of stannous chloride dihydrate as well as indium/InCl₃ in combination with “ortho-nitrogen”-N-propargylanilines for the construction of either quinoxaline- or quinolin-8-amine-scaffolds is demonstrated. Depending on the nature of the alkyne, different bicyclic nitrogen heterocycles are formed under aerobic conditions.

Toward a symphony of reactivity: cascades involving catalysis and sigmatropic rearrangements

Catalysis and synthesis are intimately linked in modern organic chemistry. The synthesis of complex molecules is an ever evolving area of science. In many regards, the inherent beauty associated with a synthetic sequence can be linked to a certain combination of the creativity with which a sequence is designed and the overall efficiency with which the ultimate process is performed. In synthesis, as in other endeavors, beauty is very much in the eyes of the beholder. It is with this in mind that we will attempt to review an area of synthesis that has fascinated us and that we find extraordinarily beautiful, namely the combination of catalysis and sigmatropic rearrangements in consecutive and cascade sequences.

Gold versus silver catalyzed intramolecular hydroarylation reactions of [(3-arylprop-2-ynyl)oxy]benzene derivatives

The scope and the generality of gold versus silver catalyzed intramolecular hydroarylation reactions of 3-[(3-arylprop-2-ynyl)oxy]benzene derivatives in terms of rings substitution were investigated. Only products deriving from 6-endo cyclization were exclusively formed. The features of substituents had a considerable effect on the reaction outcome in the presence of silver catalysis, whereas gold catalysis revealed a unique blend of reactivity and selectivity and represented the only choice for the intramolecular hydroarylation reaction of the starting substrates bearing electron deficient arenes.

One-pot synthesis of 1,2-dihydropyridines: expanding the diverse reactivity of propargyl vinyl ethers

The catalyzed synthesis of 1,2-dihydropyridines starting from easily accessible propargyl vinyl ethers was realized. The reaction sequence involving a transition metal-catalyzed propargyl-Claisen rearrangement, a condensation step, and a Brønsted acid-catalyzed heterocyclization furnishes the highly substituted heterocycles in moderate to excellent yields. Additionally, a practical one-pot protocol toward 1,2-dihydropyridines and 2H-pyrans starting from propargylic alcohols was developed.

Cationic rhodium(I)-dppf complex-catalyzed olefin isomerization/propargyl Claisen rearrangement/carbonyl migration cascade

We have established that a cationic rhodium(I)-dppf complex catalyzes isomerizations of allyl propargyl ethers to allenic aldehydes in good yields from room temperature to 40 degrees C. On the other hand, carbonyl migration reactions from allenic aldehydes further proceed to give dienals in good yields at 80 degrees C.