SYNTHESES, STRUCTURES, AND REACTIONS OF HIGHLY STRAINED DIHYDROACEPENTALENE AND TETRAHYDROACEPENTALENE DERIVATIVES

A versatile approach towards the highly strained acepentalene 3 via th e readily accessible dipotassium acepentalenediide (10a) and the highl y strained tetraenes 7 is reported, An unexpected [4+2] cycloaddition dimer 14 is formed upon protonation of the dipotassium acepentalenedii de (10a) in 93% yield, and the monomeric 4,7-dihydroacepentalene (7a), 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-bridge head alkenes 7c,d can be isolated upon reaction of 10a or 10b with bul kli electrophiles, such as Me-3-SiCl and Me3SnCl, respectively. The X- ray crystal structure analysis of 7d exhibits a highly pyramidalized c entral double bond. The bisstannane 7d is an ideal precursor for acepe ntalene (3) which would be formed by removal of the two trimethylstann yl substituents. It can also be transmetallated to give the pure cryst alline dilithium acepentalenediide (10b) in 78% yield. According to it s H-1, Li-7 and C-13 NMR spectra, the bowl-shaped 12 pi-dianion in 10b is an aromatic species, and it undergoes a rapid bowl-to-bowl inversi on at room temperature.