Dihydroacepentalenediide, the Dianion of Acepentalene

Abstract

The long‐sought dianion of the still unknown acepentalene 2 is formed surprisingly easily upon reaction of triquinacene 1 with n‐butylithium and potassium tert‐amyl alkoxide in hexane at room temperature. 22⊖ reacts with chlorotrimethylsilane to give a mixture of 1,4‐ and 1,10‐bis(trimethylsilyl)dihydroacepentalenes.

Syntheses, Structures, and Reactions of Highly Strained Dihydro- and Tetrahydroacepentalene Derivatives

“…Dipotassium acepentalenediide (10 a) was readily synthesized in a one-pot reaction: triquinacene (8) [20] was treated with the strongly basic mixture of n-butyllithium and potassium-tertbutoxide in n-hexane (Scheme 3). The original protocol required a fairly high temperature (70 8C for 24 h) and a large excess of the poorly soluble so-called Lochmann ± Schlosser base, which could not be completely removed from the crude product.…”

“…[22] Particular efforts have been made to improve the solubility of these superbasic mixtures. In the light of these developments, the disadvantages of the initial dipotassium acepentalenediide synthesis [19,20] could be overcome by addition of TMEDA (N,N,N',N'-tetramethylethylenediamine) [23] to the reaction mixture. Furthermore, the use of ultrasonication decreased the reaction time considerably and led to a much cleaner product (10 a) in a very high yield (93 %), as determined by the yield of the hydrolysis product (see below).…”

“…[20,32] With the optimized preparation of dipotassium acepentalenediide (10 a) (see above) the 4,7-bis(trimethylsilyl)-4,7-dihydroacepentalene (7 c) was isolated in 89 % yield after distillation. The bulky trimethylsilyl substituents provide sufficient steric congestion to protect the highly reactive central double bond and prevent the molecule from undergoing dimerization.…”

“…Its periannelated tricyclic ring system is highly strained and calculations predict a curved molecular surface. Higher members with a curved molecular surface, such as dicyclopenta[cd,gh]pentalene (4) and the completely ball-shaped C 20 -fullerene (5), are still unknown, although attempts to synthesise both 4 [9] and 5 [10] are in progress.…”

Oligounsaturated Five‐Membered Carbocycles – Aromatic and Antiaromatic Compounds in the Same Family

“…However, it took another 30 years to generate acepentalene by the stepwise introduction of double bonds. The breakthrough came with the facile preparation of dipotassium acepentalenediide 3 2À Á2K þ [47] and the purification trick of obtaining the pure dilithium derivative 3 2À Á2Li þ via the bis(trimethylstannyl)dihydroacepentalene 56 (Scheme 9) [48]. The dilithium acepentalenediide 3 2À Á2Li þ permitted the first X-ray crystal structure determination of a bowl-shaped aromatic dianion; it was found to aggregate as a homodimer held together by two dimethoxyethane-ligated lithium counteranions in the solid state (Scheme 9).…”