Kinetics and Mechanism of Pyrrolidine Buffer-Catalyzed Fulvene Formation

Abstract

Rapid synthesis of fulvenes is achieved using pyrrolidinium/pyrrolidine buffers in anhydrous acetonitrile. Time-dependent UV−vis absorption and NMR spectroscopy reveal that the rate and yield of fulvene formation depend strongly on both the presence of acid in the medium and the choice of solvent, and they are negatively affected by water. Kinetic data have been collected for various substrates, and the synthetic benefits of the adjusted reaction conditions are showcased. Enhancements of reaction rates are found in comparison to literature procedures. α-Unsaturated fulvenes that were previously difficult to access can now be obtained in good yields.

Crystal structures of 6-cyclopropyl-1,3-diphenylfulvene and 6-(2,3-dimethoxynaphthyl)-1,3-diphenylfulvene

“…Pentafulvenes are a unique class of cross-conjugated organic molecules commonly synthesized using aldehyde and cyclopentadiene starting materials under a variety of conditions (Thiele, 1900;Stone & Little, 1984;Sieverding et al, 2019). Substituted and highly colored pentafulvenes are of particular interest because of their unique optical and thermal properties and for their potential use in electronic applications (Peloquin et al, 2012;Godman et al, 2016;Shurdha et al, 2014).…”

Palladium-Catalyzed Synthesis of 3,6-Diaryl-1-silylfulvenes: A Promising Entry for Preparing 1,3,6-Triarylfulvenes Bearing Three Different Aryl Groups

“…One method involves the introduction of an exomethylene moiety to already existing cyclopentadienes. This is the conventional method, which involves condensation of aldehydes or ketones with cyclopentadienes in the presence of bases. − The other method involves constructing fulvene rings from acyclic starting materials, that is, transition-metal-catalyzed cyclizations for the synthesis of fulvene rings from haloalkenes with alkynes − or alkynyl esters with silyl acetylenes . Most of the above-mentioned reactions gave fulvenes bearing the same substituents, and the fulvenes lack diversity in substitution patterns and types of substituents.…”

FeCl2 Catalyzed Three-Component Reactions of Phospholes, Pyrrolidine, and Ketones (Aldehydes): Chemoselective Synthesis of 1-Phosphafulvenes

“…On the other hand, pentafulvenes are a class of highly reactive compounds with unique electronic properties and have been recognized as the eminent building block of many polycyclic frameworks and natural products . One standard protocol for the preparation of pentafulvene is the condensation of cyclopentadiene with aldehydes or ketones in the presence of secondary amines, particularly pyrrolidine, which was developed by Stone and Little (Scheme c) . Pyrrolidine acts not only as a base but also as a precursor for the iminium ion to render the carbonyl carbon more electrophilic.…”