Acepentalene

The unknown tricyclic compounds 127 and the cation 128 along with acepentalene 129 (Figure 14) have been evaluated computationally using DFT B3LYP/6-31 G <1998JOC3417>. 

CH4/O2 and CF4 as the reactant gases and observed the formation of [M — 4 H] ions in the Cl plasma (Scheme 13)166. Thus, repeated deprotonation and electron transfer processes appear to offer an efficient access to more highly unsaturated and/or ring condensed trimethylenemethane radical anions. The [M — 4 H]- ion is considered identical to the molecular radical anion (42) of acepentalene (43), which was generated as a short-lived species from the former by neutralization-reionization mass spectrometry167. Efforts to apply Squires methodology to triquinacene 41 and the tribenzotriquinacenes 44 have been made168. 

The bis(stannane) 56 also proved to be a perfect precursor for the generation of the neutral acepentalene 3 [28]. By deavage of the two labile tin-carbon bonds in 56 a diradical is... 

In view of the high purity and thermal stability of dilithium acepentalenediide 32 -2Li+ [48], it is the most feasible precursor of other stable acepentalene metal derivatives and... 

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. 

Keywords Polyquinanes (unsaturated), fulvene, pentalene, acepentalene, C20-fullerene. 

Acepentalene (C10H6) and Related Unsaturated Triquinane Derivatives... 

Triquinacene 10 which is now reasonably well accessible by a six to seven step synthesis [1,2], has long been envisaged as the logical precursor to acepentalene 3 [43] (see Scheme lb). Woodward et al. developed the first synthesis for triquinacene 10 [43] and suggested that it should be a potential precursor to both the fully unsaturated tricycle acepentalene 3 and dodecahedrane 88 [ 1,44]. Although many attempts have been made to cyclodimerize triquinacene 10,... 

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... 

Dilithium Acepentalenediide and Other Metal Complexes of Acepentalene... 

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+...

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 fully unsaturated linear and angular tetraquinanes 99-103, the C13-fenes-tranes 104,105 and the bisangular tetraquinanes 107,108 with two quaternary carbon atoms each (Fig. 8), resemble hydrocarbon skeletons consisting of four five-membered rings. While saturated congeners of 99 (see Note added in proof, p. 165) and 100 are known as skeletons of natural products [1, 2], none of the fully unsaturated compounds has even been tackled. While 99 and 100 consist of a pentalene and an added fulvene unit each, and 102 is a cyclopentene-annela-ted acepentalene, they have no chance to be stable at ambient temperature. Compounds 101,107 and 108, however, should be less strained. 

The fully unsaturated linear hexaquinanes 124-126, 128 and the angular derivative 129 as well as other isomers thereof have no chance ever to be prepared in view of the instability of pentalene 2 and acepentalene 3. The fully unsaturated hexaquinane 132, formally arising by bridging the two ends of the bent linear hexaquinane system 128, is a particular case in that it is a corannulene (compare Fig. 6a) with an inner 6n and an outer 1271 system. 

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