Semi-bullvalenes

Besides simple condensation reactions, valence isomerization reactions, in particular, arc used for the synthesis of unsaturated, eight-membered-ring azaheterocycles. These isomeriz-ations mainly involve rearrangements of nitrogen-containing bicyclo[4.2.0]octatriene or semi-bullvalene systems. 

Efficient syntheses of substituted succinic acids (Table 2, Nos. 5, 6) have been developed in the past a more recent application is the coupling of 16 as part of a semi-bullvalene synthesis [130]. 

Can a fortuitous combination of mechanisms (8.43) and (8.44A) account for the observed hydrogen-label distribution in the formation of semi-bullvalene from barrelene If so, how can you eliminate this possibility ... 

When benzo-2,3-naphthobarrelene is photolyzed, formation of the semi-bullvalene takes place by (50%) j8-naphtho-vinyl and (507 ) benzo-vinyl bridging.<49) However, the quantum efficiency for this reaction (4> = 0.077) is much lower than for the photolysis of 2,3-naphthobarrelene ( = 0.45), where vinyl-vinyl bridging takes place ... 

Earlier dynamic NMR studies had already demonstrated that the substituted semibullvalenes [87], [88] and [89] were indeed rapidly equilibrating Cope systems and not ground-state homoaromatics (Anet and Schenck, 1970 Moriarty et al., 1972 Russell et al., 1973). The bis(trifluoromethyl)semi-bullvalenes [90] and [91] had also been formulated as non-equilibrating Cope systems (Liu, 1968 Liu and Krespan, 1969). 

Following the suggestion that donor-acceptor (Dewar-Hoffmann) semi-bullvalenes [83a] would have a lower activation barrier for the Cope rearrangement, or even a homoaromatic ground state (Hoffmann and Stohrer, 1971 Dewar and Lo, 1971), numerous syntheses and studies of appropriately substituted semibullvalenes have been reported. In fact, this aspect of the search for homoaromatic semibullvalenes has been the most extensively investigated (for a partial summary of this work see Quast et al., 1985 Gompper et al., 1988, and references cited therein). 

Serratose, et al., have succeeded in converting readily available lactone 361 to semi-bullvalene. The scheme, which involves no skeletal rearrangement, is based on diazoketone cyclization chemistry within an oxygenated cyclopentenyl derivative... 

A cycloaddition-cycloelimination sequence transformed 1,2,4,5-tetrazines 50 on reaction with 3,3 -bicyclopropenyl (51) into 3,4-diazanorcaradienes, which, on gentle heating in inert solvents, were transformed into semi-bullvalenes 52 <02EJO791>. 

However, some substrates, generally rigid bicyclic molecules, (e.g., barrelene, p. 152, which is converted to semi-bullvalene) give the di-7i-methane rearrangement only from triplet states. 

Figure 1.2 Extended Htickel treatment of barrelene to semi-bullvalene reaction.

As a proof of principle, the twin states were characterized for the semi-bullvalene rearrangement and found to possess virtually identical geometries. As shown in Figure 19, the twin excited state possesses symmetry as the symmetry of the reaction coordinate of the thermal process. And the transition state mode, bx, which is imaginary for (Ai) was shown to be real for These calculations match the intriguing findings of Quast... 

Novel photoadditions to benzene are reported in Scheme 67.261.412—414 In the low-yield addition of allenes the 1,4-mode of addition is favoured, which contrasts with the addition of simple mono-olefins where the 1,3-mode is favoured. Further study shows that the 1,3-mode of addition is facilitated by strained olefins, i.e. olefins of lower ionization potential. In the case of the olefin adducts the stereochemistry has not been rigorously defined. On treatment with base the allene adducts fail to isomerize to give semi-bullvalenes, but it has not been established whether this is due to kinetic or thermodynamic factors. 

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