Synthesis of a Tetraphenyl-Substituted Dihydropentalene and Its Alkali Metal Hydropentalenide and Pentalenide Complexes

Abstract

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.

Connect Four: Tetraarylated Dihydropentalenes and Triarylated Monocyclic Pentafulvenes from Cyclopentadienes and Enones

“…Pyrrolidine has been shown to be superior to other bases in facilitating PnH 2 formation from Ph 2 CpH and chalcone. , As discussed in more detail by Hayashi et al, , this effect is likely due to a double activation of both substrates, with pyrrolidine increasing the nucleophilicity of the cyclopentadiene by deprotonation while increasing the electrophilicity of the enone by in situ iminium formation. Applying our previously optimized conditions for the synthesis of 1,3,4,6- Ph 4 PnH 2 (1 equiv R 2 CpH, 1.1 equiv enone, 1.1 equiv pyrrolidine, toluene/MeOH at 70 °C for 40 h) to the reaction of p -Tol 2 CpH with 1,3-di- p -tolylprop-2-en-1-one initially showed no noticeable reaction progress after 40 h. Applying slightly more forcing conditions of 75 °C with 5 equiv of pyrrolidine did produce a characteristic color change within 2 h and showed complete conversion after 46 h (Scheme ).…”

“… , Note that pathway 1 could have a contribution from transient iminium ion formation from the activation of the enone by pyrrolidine . However, as dihydropentalene formation may also be induced by, for example, alkoxide bases, ,, the anionic Cp – route is likely the main reaction pathway.…”

“…The same trend prevailed in the trisubstituted pentafulvenes which had absorption maxima at slightly higher energy than their corresponding dihydropentalenes ( Ph 3 Fv = 266 nm, 8b = 274 nm, 9b = 268 nm). None of the pentafulvenes investigated showed any of the broad absorptions in the visible range at 400–550 nm reported for the very strongly absorbing hydropentalenide Na­[Ph 4 PnH] …”

“…As observed with Ph 4 PnH 2 , 16 no nucleophilic attack from the C5 position of the cyclopentadiene on the enone seemed to take place, presumably due to steric hindrance by the substituents in the flanking 1,4 positions. Purification of the crude reaction mixture via column chromatography on silica using 5:1 cyclohexane/toluene as the eluent mixture yielded two products.…”

“…The latter method typically gives lower product yields but at higher dihydropentalene selectivity with lower amounts of pentafulvene formation. All dihydropentalenes were isolated as air-stable solids with lower melting points and higher solubility compared to the symmetrically substituted tetraphenyl-dihydropentalene . The products consisted exclusively of the fulvenic 1,2 isomers with no sign of 1,4 or 1,5 double bond isomer formation.…”