Excitation of 5, 7, and 8 M in degassed CH'Cl, at room temperature) by a 20-ps laser pulse at 530 nm was used for transient absorption measurements at 820 nm (triplet state absorption; see Ref. [Ill) with an apparatus already described." 21 Simultaneously, time-resolved emission measurements were performed. In 5, emission at 600 nm is completely quenched whereas emission at 660 nm is enhanced as compared to 7 under the same conditions. The formation rate of the triplet state localized on the non-metalated subunit at room temperature in degassed CH,CI,) is similar to the decay rate of the singlet state (non-metalated ring). Furthermore, the emission quantum yield at 660 nm and the quantum yield of triplet state generation (absorption at 820 nm) for 5 are roughly twice those for 7, corroborating the data of Table 2 obtained by classical emission spectroscopy. No evidence was obtained for light-induced electron transfer. These observations are in full agreement with the postulated reaction of efficient intramolecular singlet energy transfer from the zinc porphyrin part to the non-metalated ring.The present oblique bis-porphyrin may be related to the hypothetical mechanism of the photosynthetic bacterial reaction center. Energy migration from the special pair to the isolated bacteriochlorophyll followed by electron transfer or initial electron transfer from the special pair may be opera- Benzoannelation can increase the stability of cyclically conjugated, nonaromatic hydrocarbons that, owing to unfavorabie electron configuration and/or unfavorable HOMO energies, are unstable and therefore highly reactive. To explore this possibility for acepentalene 1, only the dianion of which, 12e, is so far known, ['] we synthesized tribenzotriquinacene (tetrahydrotribenzoacepentalene) 2 and investigated its deprotonation-dehydrogenation both in solution and in the gas phase.Tribenzotriquinacenes with substituents on the central C atom are accessible by double cyclodehydration of the correspondingly substituted 2-ben~hydryl-l,3-indandiols.'~~ The unsubstituted hydrocarbon 2, however, could be obtained only in low yields via this route. Starting from cinnamic acid, Baker et synthesized the diquinane ketone 3 in three steps; bromination/dehydrobromination then gave the enone 4 in moderate yield. In our attempts to synthesize 2 and higher benzoannelated centrop~lyquinanes,[~~ the ketone 3 and the alcohols 7a, bI3] derived therefrom were identified as 8-exo-phenyl stereoisomers on the basis of their mass spectra;"] thus, the cyclodehydration 7 + 2 attempted by Baker et al.[31 was doomed to failure for steric reasons. The enone 4, together with the isomer 5,[81 is accessible on a preparative scale (overall yield 25%, Scheme 1) by an alternative route involving phenylsulfenylation of 3, oxidation, and thermal elimination of the phenylsulfinyl derivative.'6.Subsequently, 4 and 5 can be rehydrogenated with palladium on charcoal in dioxane to afford the isomeric diquinane ketone 6 with high selectivity. After reduction to the alcohol 8...

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.

Dreifach benzoanellierte Kohlenwasserstoffe sind die vier Titelverbindungen 1 – 4. Während 1 noch nicht synthetisiert werden konnte, ist 2 auf einem neuen Weg gut zugänglich und läßt sich leicht in 3 und 4 überführen. Beide sind stabiler als die analogen Derivate von Acepentalen.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.