Annulation Cascade of Aryl Alkynes Induced by Alumina‐Mediated CF Bond Activation

Alumina‐mediated CF activation (AmCFA) is an established method to generate incipient phenyl cation (IPC) through polarization of C(aryl)F bonds. Herein, it is reported that AmCFA can be used to induce intramolecular arylation of aryl alkynes containing one, two, and three triple bonds. During the reaction, a single formally eliminated hydrogen fluoride (HF) leads to the formation of two, three, and four CC bonds, respectively. Thus, the transformation gives rapid access to π‐extension and serves as an interesting example of solid‐state rational domino annulation enabling the bottom‐up construction of nanographenes.


Results and Discussion
To demonstrate the scope of the developed reaction, we have synthesized precursor alkynes 1-8 (the synthesis procedures can be found in the Supporting Information).
We have initiated our study with precursor 1, annulation of which under our initial assumption could proceed via 5-exo or 6-endo routes. The exposure of 1 to γ-Al 2 O 3 at 190 ºC has, indeed, resulted in the formation of the 1a-1b (3:1) mixture. What is noteworthy is that unlike previously reported AmCFA syntheses, in this setup HF elimination induces the formation of two C─C bonds. Interestingly, in the case of 2 the ratio of the two chemical routes was drastically shifted toward 6-endo because 2a was obtained as the only major product. Synthesizing the precursor 3, we have attempted to push the substrate to extreme energy barriers associated with the construction of the corannulene core. Despite this severe obstacle, we have been able to isolate 3a. Although the yield of the transformation is fairly poor, such synthesis serves as a rare example of the nonpyrolytic approach to coranullene's derivatives. [21] From a structural standpoint, these three transformations resemble an annulative π-extension (APEX) reaction recently developed by Itami and coworkers [22] The transformation enables π-extension of PAHs bearing bay-regions. In contrast, our approach is also suitable for the extension of PAHs with zigzag (i.e., L-region) periphery. Thus, the transformations of 4 and 5 lead to the formation of the anticipated products 4a and 5a, respectively. In the first case, we have also isolated the product of 5-exo cyclization 4b. As the formation of the four-membered ring is energetically unfavorable, the cationic intermediate undergoes a 1,3-hydrogen shift, leading to the redislocation of the cation and subsequent formation of the hexagon. The mechanism of this cascade is confirmed by the transformation of 6 into 6a. While the former does not have alternative possibilities for intramolecular chemistry, a corresponding product is not observed in the case of perylene's derivative apparently due to a higher reactivity in S E Ar of the carbon attached to H a .
We assumed that the incorporation of additional triple bonds could enable a stepwise transfer of the cationic intermediates that could extend the domino cascade to more than two C─C bonds. To check this assumption, we designed the precursors 7 and 8 containing two and three C≡C bonds, respectively. The transformation of these molecules into 2a and 8a, respectively, represents an extrapolation of the previous examples. Thus, instead of terminating the cascade, the first cationic intermediates yields the second cationic species. The whole process reminds us of the chain transfer occurring in a polymerization reaction. Considering the number of formed C─C   bonds, the yields of these folding processes can be estimated as 53% and 47% per C─C bond for 7 and 8, respectively (Figure 3).

Conclusion
In summary, we have designed and implemented the cascade annulation of aryl alkynes initiated via AmCFA. The reaction can be formally considered as a single HF elimination that depending on the substrate induces two, three, or four C─C bonds. The reaction occurs on the surface of the activated γ-alumina and represents an important transformation that can be potentially transferred to the nonconducting surfaces for the bottom-up construction of sp 2 -carbon-based materials directly within a device.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.