The syntheses of a mixture of the racemic (a) and the meso form (b) of several derivatives of 2,2‘-bi(tricyclo[3.3.0.03,7]octylidene) are described: The 6,6‘-diisopropylidene- (4), 6,6‘-dimethylidene- (5), 6,6‘-dione- (6), syn-4,4‘-dione- (7), anti-4,4‘-dione- (8), syn-4-methylidene-4‘-one- (9), anti-4-methylidene-4‘-one- (10), syn-4,4‘-dimethylidene- (11), and anti-4,4‘-dimethylidene- (12) derivatives. Starting materials for all compounds were tricyclo[3.3.0.03,7]octane-2,4-diol (19) and its 2,6-isomer 20. Key reaction was the McMurry coupling of 6-isopropylidenetricyclo[3.3.0.03,7]octan-2-one (23) or 6-methylidenetricyclo[3.3.0.03,7]octan-2-one (24) to yield 4 or 5, respectively. The McMurry coupling of the ketone 26 yielded 7 and 8. From the latter, species 9−12 could be generated via the Wittig reaction. In most cases only the racemates could be isolated. X-ray investigations on 4a revealed a separation of the isopropylidene groups by 10.3 Å. The central C−C bridging bonds of the stellane units in 4a, 7a, 8a, 12a, and 30 C(1)−C(5), C(3)−C(7), C(1)−C(5‘), C(3‘)−C(7‘) were found to be between 1.596(2) and 1.616(3) Å. In 4a we found a twisting between the terminal isopropylidene groups by a torsion ((CH3)2CC···CC(CH3)2) of 74° and 83°, respectively, in both independent molecules. The twist angle between the CO groups in 7a amounts to 53°, and in 8a to 118°. The same twist angle was found between the exo-methylidene groups in 12a.
A number of condensations could be carried out using tricyclo[3.3.0.03,7]octane‐2‐one (stellanone, 4) and tricyclo[3.3.0.03,7]octane‐2,6‐dione (2,6‐stelladione, 5) as starting materials. The components for condensations were 2‐trimethylsilyl‐1,3‐dithiane (6), 1,1‐bis(trimethylsilyl)‐1H‐cyclopropa[b]naphthalene (7), its 3,6‐dimethoxy‐substituted analogue 8, fluorene (12), xanthene (13), diethyl malonate (14), and malononitrile (15). The condensation reactions with 5 yielded mono‐ and disubstituted products, among them were the donor–acceptor‐substituted 2,6‐stellanes 33–35. The structures of 18 (prepared from stellanone and fluorene), 19, 24, 27, 31 and 32 (synthesized by condensation of 2,6‐stelladione and 2‐trimethylsilyl‐1,3‐dithiane and malononitrile, respectively) were determined by X‐ray crystallography.
The photoelectron spectra of rac-6,6′-diisopropylidene-2, 2′-bis(tricyclo[3.3.0.0 3,7 ]octylidene) (12), syn-and anti-2, 2′-bis(tricyclo[3.3.0.0 3,7 ]octane)-4,4′-dione (13 and 14), syn-and anti-4-methylidene-2,2′-bis-(tricyclo[3.3.0.0 3,7 ]octylidene)-4′-one (15 and 16), and anti-4, 4′-dimethylidene-2,2′-bis(tricyclo[3.3.0.0 3,7 ]octylidene) ( 18) have been recorded. The energy difference between the 2p-type lone pairs of the angular oriented oxygen atoms of 13 and 14 amounts to 0.6 eV, while the corresponding energy difference in 4, where the oxygen atoms are linearly orientated, is 0.2 eV. For 3 and 12, in which the terminal π-bonds are arranged linearly, the difference between the first two peaks in their spectra is 0.4-0.5 eV, while the angular arrangement of the terminal π-bonds in 18 gives a splitting of only 0.2 eV. Hartree-Fock SCF calculations, using a 6-31G* basis set, have been used to interpret the photoelectron spectra of 12-16, 18, and 19. The interactions between the σ frame and the n and π orbitals in these molecules have been investigated and analyzed in terms of the through-space through-bond concept. It is concluded that a maximum interaction between π bonds is attained, if they are arranged in a linear fashion, while the interaction between π bonds and keto groups reaches its maximum in an angular orientation.
In several instances the last name of one of the authors (U. E. H.) is misprinted; it should read as above. In Tables 4 and 5 the different k factors are misprinted (104 instead of 104 etc.).
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