Cycloadditions possible mechanistic pathways

Using a somewhat similar strategy, Sapi and coworkers [125] have also reported an interesting multicomponent synthesis of carbazoles. While a cycloaddition reaction with a quinodimethane could be postulated as a possible mechanistic pathway for this remarkable reaction, most evidence points in favor of the involvement of a stepwise ionic process. 

SCHEME 20.2 Possible mechanistic pathways for the metal-catalyzed [5+2] cycloaddition of VCP. 

SCHEME 2 Possible mechanistic pathway for the cycloaddition reaction. 

Clearly, in the case of (66) two amide tautomers (72) and (73) are possible, but if both hydroxyl protons tautomerize to the nitrogen atoms one amide bond then becomes formally cross-conjugated and its normal resonance stabilization is not developed (c/. 74). Indeed, part of the driving force for the reactions may come from this feature, since once the cycloaddition (of 72 or 73) has occurred the double bond shift results in an intermediate imidic acid which should rapidly tautomerize. In addition, literature precedent suggests that betaines such as (74) may also be present and clearly this opens avenues for alternative mechanistic pathways. 

These initial mechanistic studies with bis(phosphine) catalysts were able to distinguish between overall associative and dissociative possibilities [86]. As shown in pathway (a) of Fig. 4.30, an associative mechanism involves coordination of an olefin directly to the 16-electron pre-catalyst, to form an 18-electron olefin complex that then undergoes [2 -I- 2] cycloaddition. In contrast, pathways (b) and... 

Other mechanistic possibilities also exist for the [5-1-2] cycloaddition reaction and cannot be mled out at this point In addition to the stepwise pathways depicted in Scheme 13.8, it is also possible that coordination of the 27r-component to 42 could lead directly to 48 through simultaneous insertion and cleavage. Variations on these events, such as direct insertion into the cyclopropane to form a metaUacyclobutane, are also possible. 

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