Pentafulvenes are a unique class of compounds that originally attracted attention due to their propensity to display nonbenzenoid aromaticity. Subsequently, they were recognized as valuable synthons for the construction of a wide range of compounds by virtue of their ability to display multiple cycloaddition profiles. Naturally, this area of organic chemistry has experienced rapid growth over the last five decades, fueled by elegant work showcasing the unique reactivity of pentafulvenes in a plethora of cycloaddition reactions. In this Review, we have attempted to provide a systematic account of the methods for the generation of pentafulvenes, their rich and varied cycloaddition chemistry, organometallic reactions, and theoretical studies that support their versatility. Further, we have highlighted their applications in the synthesis of a variety of complex structural frameworks. It is our conviction that this Review will be useful to a wide range of chemists, and will spur further research in this promising area.

This work describes an experimental kinetic study and the development of a kinetic model for the dimerization of cyclopentadiene (CPD) to dicyclopentadiene (DCPD) in multicomponent reaction mixtures at temperatures ranging from 80 to 160 °C. The reaction mixtures consist of CPD and other reactive C 5 compounds, including isoprene, trans-1,3-pentadiene, 1-pentene, cis-2-pentene, and trans-2-pentene, which are normally present in the CPD stream derived from steam cracking of naphtha. Reaction rate constants for (a) dimerization of CPD to form exo-and endo-DCPD, (b) the codimerization of CPD with the other reactive components, and the homodimerization of each of the reactive components are presented along with the corresponding activation energies and pre-exponential factors. By including the retro Diels−Alder reaction of endo-DCPD and the formation of CPD trimers, our kinetic model accurately predicts concentrations of CPD, endo-DCPD, exo-DCPD, and codimers with respect to time. The model identifies CPD self-dimerization as the dominant reaction occurring in the multicomponent mixture, followed by the codimerization between cyclopentadiene and isoprene. At 120 °C, codimerization reactivity with CPD was found in the following order: 1-pentene > cis-2-pentene > 2-methyl-1-butene > cyclopentene > 2methyl-2-butene > trans-2-pentene, with reaction rate constants 1 to 2 orders of magnitude lower than that of exo-DCPD formation. Overall, the model established herein allows an accurate prediction of the DCPD purity that can be obtained from a petroleum pyrolysis stream no matter what the impurity concentrations may be.

A simple and efficient method for the stereoselective ring opening of bicyclic hydrazines with various aryl halides under palladium catalysis has been elaborated. The reactions afforded trans-3,4-disubstituted cyclopentenes or alkylidene cyclopentenes in good to excellent yields. By taking advantage of multiple points of functionalization in the synthesized trans-3-phenyl-4hydrazino-cyclopentene, we have synthesized the antidepressant Cypenamine (trans-2-phenylcyclopentylamine).

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A diverse approach toward the catalytic regioselective nucleophilic addition of nitrogen heterocycles to Lewis acid activated pentafulvenes has been established. The developed protocol introduces pentafulvenes as nonsymmetrical alkenes for the hydroheteroarylation reaction, providing alkylidenecyclopentenylation at the C-3 position of indoles and the C-2 position of pyrrole.