Barrelene structure

We reacted 2 first with bromine in chloroform at 10 C. iH NMR studies have revealed that the reaction mixture was very complex and consisted of six products. This mixture was submitted to silica gel column chromatography. Careful repeated chromatography followed by fractional crystallization allowed us to isolate ten products (Scheme 3). IR analysis indicated that a hydroxyl group was incorporated in compounds lfi-19. Therefore, we assume that these products have been formed by partial hydrolysis of compounds lfl-14. Structural determination of compounds lfl-19 revealed that the barrelene skeleton was rearranged completely. 

Unfortunately the structural formula assigned was incorrect. A reinterpretation of the spectral data 44>, showed that the compound was 5,6,7,8-tetrafluoro-1,4-dihydro- 1,4-ethenonaphthalcne (tetrafluorobenzo-barrelene) (24). 

This addition proceeds more readily with the more electron rich 2.6-di-tert-butyl-4-methyl-X -phosphorin 109 b or 2,6-dimethyM-phenyl-X -phosphorin 109 c. The H-NMR signals are in accord with the proposed structure (Table 15). The resonance of the phospha-barrelene 110 a m benzene appears at 6 = + 65 ppm (H3PO4 standard) which corresponds to the resonance of tertiary phosphines. 

Interestingly, the original referees did not like the terms barrelene and semibullvalene with the comment that these were unesthetic and that it was unlikely that any further compounds of these structures would be encountered. 

In fact, barrelene rearranges by the di-7t-methane pathway to semibullvalene only when acetone sensitized direct irradiation produces cyclooctatetraene as the major photoproduct. A number of similar examples exist, each demonstrating the general phenomenon of preferred triplet multiplicity for di-7r-methane rearrangements in rigidly constrained systems, i.e. structures which prohibit free rotation about the Ti-bonds. ... 

From this initial study it was clear that the loss of nitrogen by thermolysis leads to the ground state. Whether this is a concerted process (i.e., a reverse [2 + 2 + 2]) or generates Sq of diradical 1, which then fragments to barrelene, is debatable. In any case, the transition structure for the concerted process is just diradical 1 loosely bonded to a molecular nitrogen molecule. What is clear is that sensitization leads to diradical 1 and preferentially to the semibullvalenes rather than radiationless decay to ground state. 

Clusters with Structures Based on Adamantane or Barrelene Cages and Related Fragments... 

There are few examples amongst II-VI clusters in which the basic structural unit is not related to the adamantane or barrelene cage, granting evidence to the thermodynamic stability of such structures. This is substantiated by the fact that similar structures are accessible via different synthetic routes (see Table 5). Nonadamantoid/barrelenoid structures usually consist of... 

Table 5 Metal chalcogen clusters with structures based on adamantane, barrelene and related structures.

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