Syntheses, structures, and reactions of highly strained dihydro- and tetrahydroacepentalene derivatives.
A versatile approach towards the highly strained acepentalene I via the highly strained tetraenes II and the readily accessible dipotassium salt of acepentalenediide III is reported. An unexpected [4+2] cycloaddn. dimer is formed upon protonation of the dipotassium acepentalenediide in 93% yield, and the monomeric 4,7-dihydroacepentalene (II, R = H), as the reactive intermediate, can be trapped with anthracene to form the corresponding Diels-Alder adduct. In contrast, the highly strained, but sterically protected monomeric bridgehead-bridgehead alkenes II (R = SiMe3, SnMe3) can be isolated upon reaction of the dipotassium or dilithium salts of III with bulky electrophiles, such as Me3SiCl and Me3SnCl, resp. The X-ray crystal structure anal. of II (R = SnMe3) exhibits a highly pyramidalized central double bond. According to its 1H, 7Li and 13C NMR spectra, the bowl-shaped 12p-dianion III is an arom. species, and it undergoes a rapid bowl-to-bowl inversion at room temp.
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