Acepentalene dianions

A more versatile approach to 4,7-disubstituted dihydroacepentalenes 65 is via the stable acepentalene dianion 64 as an easily accessible intermediate. Dipotassium acepentalenediide 64 can be obtained in virtually quantitative yield by treatment of triquinacene 10 with the superbasic mixture of potassium f-pent-oxide and butyllithium [or even better potassium f-butoxide, butyllithium and tetramethylethylenediamine (TMEDA)] in hexane, the so-called Lochmann-Schlosser base (Scheme 15) [62, 63]. Mechanistically this transformation has... 

Fig. 6. Structures and calculated structural parameters of acepentalene 3, acepentalene dianion 32 (experimental values for dilithium acepentalendiide 71 in parentheses [63]) and acepentalene dication 32+...

While some complexes of dihydroacepentalene derivatives have been reported some time ago [178-180], metal complexes of the unstable acepentalene (137), although they were the subject of DFT calculations [181], are still elusive. The acepentalene dianion (138) was obtained as its lithium salt in pure form by treatment of triquinacene (147) with BuLi/KOtBu/TMEDA giving 138 as its impure potassium salt, which was trapped with MegSnCl. The l,7-bis(trimethylstaimyl)-4,7-dihydroacepentalene (148) obtained was then treated with methyllithium, affording pure dilithium acepentalenediide (138-2Li+) (Scheme 10.52) [152]. 

Acepentalene (55) and its dianion (552-) have attracted much interest due to then-unique tricyclic structure, curved molecular surface and interesting electronic properties86. Because of the large strain in 55 it has not been isolated at ambient temperatures. However, de Meijere and coworkers were able to prepare 552 86,87, which is less strained and has a closed-shell system, and therefore electronically more favorable than the neutral system. 

The fully unsaturated dicyclopenta[cd,g/z]pentalene 4 (Fig. 7) remains elusive. Several attempts at generating 4 have been made, but so far all available knowledge about the system rests on computational studies which predict the structure and properties of this highly strained 7r-system [35]. In contrast to computational results for pentalene 2 and acepentalene 3, calculations for 4 predict an aromatic singlet ground state with a delocalized 7r-system. While the dianion of 4 is calculated to also be aromatic, the dication 42+ should be antiaromatic [35]. 

The dianion of dihydroacepentalene is of considerable interest as a potential precursor for the elusive hydrocarbon acepentalene, CioHe, which has been predicted to have a triplet diradical ground state. The corresponding dianion, CioHe ", is significantly more stable and can be generated in solution by treating triquinacene with a superbasic mixture of n-butyllithium and potassium t-pentoxide in the presence of tmeda [27]. More recently a crystalline sample of the dimethoxyethane solvate [ Li(DME) 2CioH6]2, 33, has been successfully prepared according to Eq.(l) by means of a transmetallation reaction between 4,7-bis(trimethylstannyl)-dihydroacepentalene and methyllithium in DME solution at -60 °C [28]. 

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