Gilbert 2 Isomerization Reactions

The orientation on addition of asymmetric electron-rich olefins has also been studied by Padwa et al. (83JOC2330). Only one regioisomer could be isolated from such reactions. These reactions were assumed to be products resulting from the attachment of the olefinic electron-rich moiety to the exocyclic nitrogen. This assignment was based on H-NMR, which revealed the triazine H-4 in 96 at 88.5. This, however, does not rule out isomeric 97, as Ege and Gilbert (81JHC675) reported a shift of 8.7 for H-3 in a system believed to be a 3-unsubstituted-4-arylpyrazolo[5,l-c]-l,2,4-triazine (Scheme 13). 

The photolysis of perfluoro(l,3,5-trimethyl)benzene yields perfluorotrimethyl-prismane 19 The latter was considered to be formed via a Dewar benzene. The prismane was isomerized to another Dewar compound which is an intermediate of perfluoro(l,2,4-trimethylbenzene) (see Eq. 16). There are many reactions where prismanes are postulated as intermediates. Some of them are described in the review of Bryce-Smith and Gilbert 29). 

The formation of benzvalene is formally an x[2 + 2] cyclo-addition. The S, (Bju) reaction path from benzene toward prefulvene starts at an excited-state minimum with symmetry and proceeds over a transition state to the geometry of prefulvene, where it enters a funnel in S, due to an S,-So conical intersection and continues on the Sg surface, mostly back to benzene, but in part on to benzvalene (Palmer et al., 1993 Sobolewski et al., 1993). At prefulvene geometries, Sg has a flat biradicaloid region of high energy with very shallow minima whose exact location depends on calcula-tional details (Kato, 1988 Palmer, et al., 1993, Sobolewski et al., 1993). Fulvene has been proposed to be formed directly from prefulvene or via secondary isomerization of benzvalene (Bryce-Smith and Gilbert, 1976). Calculations support the former pathway with a carbene intermediate (Dreyerand Klessinger, 1995). 

---