Benzo-vinyl bridging reactions

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 ... 

The formation of the benzocyclo-octatetraene (26) undoubtedly involves a singlet species which undergoes benzo-vinyl bridging rather than the vinyl-vinyl bridging of the triplet which leads, eventually to compound (27) 51>52>. In accord with this the photo-isomerisation of the adduct (28) formed by the reaction of tetrafluorobenzyne with cyclohexa-1,3-diene, results in the formation of tetrafluorobenzo-dihydrosemibull-valene (29) 53>. 

As mentioned earlier, the solution phase di-jr-methane photorearrangement of dibenzobarrelenes is four-fold degenerate Scheme 10 depicts the four possible initial benzo-vinyl bridging possibilities. As drawn, paths 1 and 2 lead to one enantiomer of dibenzobarrelene photoproduct and paths 3 and 4 give the other. The fact that the reaction of the P2i2] 2i crystals proceeds with 100% enantioselectivity indicates complete discrimination between pathways [1+2] and [3+4] in the solid state. It does not, however, tell us whether [1+2] is favored over [3+4] or vice versa, nor does it indicate the relative importance of 1 versus 2 or 3 versus 4. 

For the di-7t-methane reactions of the (S)-(-)-proline tert-butyl ester salt and the (S,S)-(+)-pseudoephedrine salt in Scheme 36, absolute configuration correlations between reactant and photoproduct could be elucidated by X-ray crystallography. The correct absolute configurations of the reactants and the products are as shown in the scheme. The observed selective benzo-vinyl bridging to one direction is ascribed both to reduced steric repulsion between the vinyl substituents and to better orbital overlap in the transition state [65]. It is concluded for the 11,12-dicarboxydibenzobarrelene derivatives that initial benzo-vinyl bridging is favored at the carboxylate salt-bearing vinyl carbon atom both in the solid state and in solution [66]. 

The parent benzobarrelene upon direct irradiation produces benzocyclooctatetraene exclusively, whereas acetone-sensitized reaction affords the DPM rearrangement product benzosemibullvalene via a triplet-state pathway. Zimmerman s elegant studies on deuterated benzobarrelene (8) revealed that the benzo-vinyl bridging leading to 9a is the predominant route on direct irradiation, and vinyl-vinyl bridging leading to 10a is the predominant route in the sensitized reaction " (Scheme 3). 

In the early seventies, it was reported that irradiation of benzoquinoxalinobarrelene 81a in furan affords SB 85a via the DPM route. Contrarily, it was found upon re-examination of this reaction under similar conditions that ADPM products 82a and 83a arising from the initial quinoxalino-vinyl bridging are the only products. The benzoquinoxalinobarrelenes 81b,c also provide ADPM products 82b,c and 83b,c as the sole products in a similar ratio (Scheme 11). The enhanced quinoxalino-vinyl bridging observed in the reactions of 78a-c and 81 a-c in comparison with 2,3-naphtho-vinyl bridging in benzo-2,3-naphthobarrelene (55) maybe a consequence of the lower triplet-state energy (60.6 kcal mol ) of... 

Transition metal-catalyzed decomposition of a-diazoesters of type 60 result in the formation of a benzo[intramolecular Diels-Alder reaction with a tethered vinyl group followed by spontaneous N-assisted opening of the endoxide bridge to yield 11-azasteroid analogs (Scheme 92) <1999J(P1)59>. 

The azabarrelenes (61)—(63) are all photoreactive on acetone sensitiza-tion." The formation of products is dependent, to some extent, on the substitution pattern. The authors also believe, as a result of calculations, that there is some stereo-electronic control. Thus irradiation of (61) affords a single product (64) involving vinyl-benzo bridging and stabilization of the resultant radicals by overlap with the phenyl group. With the all-methyl system (62) the two products obtained [(65) and (66)] arise from the two modes of vinyl-benzobridging. In the other cases (63) studied, in addition to the formation of the types of product already discussed, an alternative reaction mode affords products (67)—(69) see Scheme (4). 

---