We report a high-yielding solution phase synthesis of 1,3,4,6-tetraphenyl-dihydropentalene based on a simple annulation reaction of cyclopentadiene with a chalcone. Deprotonative metalation of 1,3,4,6-tetraphenyl-dihydropentalene with alkali metal bases (Li, Na, K) of moderate basicity (pK a > 15) cleanly yields the corresponding hydropentalenide complexes, which exist as solvent-separated ion pairs in coordinating solvents such as THF, pyridine, and DMSO. A second deprotonative metalation with stronger alkali metal bases (pK a > 25) yields the double-anionic, 10 π aromatic tetraphenyl-pentalenide complexes, which are of low solubility in the case of Li 2 , Na 2 , and K 2 . In the solid state the two metals are bound η 5 to each anionic ring in anti configuration as in the unsubstituted pentalenide Li 2 [C 8 H 6 ]. Mixed-metal tetraphenyl-pentalenide complexes of Li/Na/K display remarkably enhanced solubility, allowing for solution phase characterization via NMR and UV−vis spectroscopy and application in transmetalation reactions.

Challenges and opportunities in the synthesis of pentalenide complexes are reviewed from a practical point of view.

In search of novel pentalenide ligands for use in organometallic chemistry and homogeneous catalysis, we report the scope of a straightforward base-promoted Michael annulation of cyclopentadienes with α,β-unsaturated ketones that allows the introduction of symmetrical as well as unsymmetrical aryl and alkyl substitution patterns including electron-donating as well as electron-withdrawing substituents. More than 16 examples of various isomers of 1,3,4,6-tetraarylated dihydropentalenes have been synthesized in isolated yields of up to 78%, representing a substantial expansion of the range of dihydropentalene scaffolds known to date. Double bond isomerization between the two pentacyclic rings in 1,2-dihydropentalenes with electronically different substituents occurred depending on the polarization of the molecule. The melting points of the air-stable dihydropentalenes decrease, and their solubilities in organic solvents improve with increasing substitution and decreasing symmetry of the molecule. A competitive pseudo-retro-aldol pathway produces 1,3,6-triarylated monocyclic pentafulvenes as side products in yields of 9−68%, which can be cleanly isolated (8 new examples) and used for other synthetic purposes, including separate cyclization to other dihydropentalenes.