Arenes to alkynes

Table 2 Ortho Photocycloaddition of Arenes to Alkynes and Nitriles...

Activation of the alkyne functionality by In(III) was also implicated in the Friedel-Crafts type addition of arenes to alkynes in the study done by Shirakawa et al. [99]. Several different metal triflates were investigated and efficiencies were comparable. The authors proposed the formation of an intermediate alkenyl carbocation, instead of an In(OTf)3-alkyne complex, to account for the mandatory presence of an aryl group attached to the alkyne and the observation of both cis and trans addition products (Figure 8.49). Song and Lee et al. had found improvements in yields and reaction rates by performing the same reaction in ionic liquids, but with a clear advantage of Hf(OTf)3 over In(OTf)3 in their reaction system [100]. 

Tsuchimoto and Shirakawa et al. again used the In(OTf)3 catalyst to develop the first double addition of heterocyclic arenes to alkynes [101]. The addition occurred geminally and unlike the previous arene addition to alkynes studied by Shirakawa et al., aliphatic alkynes were observed to react. The authors thus proposed that this reaction occurs via an In(OTf)3-alkyne complex, rather than through an alkenyl carbocation intermediate (Figure 8.50). 

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