Abstract

A versatile approach towards the highly strained acepentalene 3 via the readily accessible dipotassium acepentalenediide (10 a) and the highly strained tetraenes 7 is reported. An unexpected [42] cycloaddition dimer 14 is formed upon protonation of the dipotassium acepentalenediide (10 a) in 93 % yield, and the monomeric 4,7-dihydroacepentalene (7 a), as the reactive intermediate, can be trapped with anthracene to form the corresponding Diels ± Alder adduct 16 in 15 % yield. In contrast, the highly strained, but sterically protected monomeric bridgehead ± bridgehead alkenes 7 c, d can be isolated upon reaction of 10 a or 10 b with bulky electrophiles, such as Me 3 -SiCl and Me 3 SnCl, respectively. The Xray crystal structure analysis of 7 d exhibits a highly pyramidalized central double bond. The bisstannane 7 d is an ideal precursor for acepentalene (3), which would be formed by removal of the two trimethylstannyl substituents. It can also be transmetallated to give the pure crystalline dilithium acepentalenediide (10 b) in 78 % yield. According to its 1 H, 7Li and 13 C NMR spectra, the bowl-shaped 12 p-dianion in 10 b is an aromatic species, and it undergoes a rapid bowl-to-bowl inversion at room temperature.

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