Synthesis of Phenalenyl‐Fused Pyrylium Cations: Divergent C−H Activation/Annulation Reaction Sequence of Naphthalene Aldehydes with Alkynes

Abstract

Described herein is the synthesis of stable oxonium-doped polycyclic aromatic hydrocarbons (PAHs) by the rhodium-catalyzed C-H activation/annulations of naphthalene-type aldehydes with internal alkynes. This protocol provides four divergent reaction types, including two unexpected annulations with an oxygen transposition process, which lead to diverse types of phenalenyl-fused pyrylium cations comprising a four-, five-, or six-ring-fused π-conjugated core. The annulations exhibit an exquisite regioselectivity and a high tolerance of sensitive functional groups. These PAHs feature intriguing photophysical properties such as full-color tunable fluorescence emission, high quantum yield, and positively charged core, and can be reduced easily to the phenalenyl radicals.

Synthesis of Phenalenyl‐Fused Pyrylium Cations: Divergent C−H Activation/Annulation Reaction Sequence of Naphthalene Aldehydes with Alkynes

“…The 19 FNMR spectrum and X-ray single-crystal diffraction indicated that this oxonium-doped PA Hc ontains SbF 6 À as the counteranion. [18] With this consideration in mind, two equivalents of NaSbF 6 were added, thus dramatically improving the yield of 3aa to 38 % (Table S1, entry 3). Further optimization of reaction parameters,such as solvent, oxidant, reaction temperature and time, improved the yield of 3aa to 77 %(Table S1, entry 14).…”

“…As shown, the annulation of naphthalene aldehyde allows awide scope of alkynes,thus producing afamily of four-ring-fused phenalenyl-based pyrylium cations.T he reactions of symmetrical diaryl alkynes with electron-donat-ing groups such as p-methyl, m-methyl, p-methoxy,a nd 2naphthyl smoothly occurred in 55, 69, 67, and 90 %y ields, respectively (3ab-ae). [18] Thed iaryl alkynes bearing an electron-withdrawing group such as fluoro,c hloro,C F 3 , CO 2 Et, and CHO were also be tolerated (3af-aj). 4-Phenylsubstituted a-naphthaldehyde gave 3ea in 72 %yield.…”

“…Forthe unsymmetric alkyl aryl alkyne 2k,t he reaction successfully proceeded to afford 3ak in 68 %y ield with ac omplete regioselectivity. [18] To obtain larger p-conjugated oxonium-doped PA Hs,the aldehyde substrates with alarger degrees of conjugation were examined next. To our delight, phenanthrene-9-carbaldehyde (1f)c ould react with diaryl alkynes,t hus affording the fivering-fused oxonium-doped PA Hs (3faand 3fd;Scheme 3).…”

“…To our delight, phenanthrene-9-carbaldehyde (1f)c ould react with diaryl alkynes,t hus affording the fivering-fused oxonium-doped PA Hs (3faand 3fd;Scheme 3). [18] Thea nnulation of pyrene-1-carbaldehyde (1g)w ith diaryl alkynes worked well and delivered the six-ring-fused oxonium-doped PA Hs (3ga and 3gd). More importantly,t he dialkyl alkyne,o ct-4-yne,c ould react with 1g to give 3gl in 78 %yield.…”

“…X-ray single-crystal diffraction was used to identify the structure of 4ba,i nw hich the oxygen atom of the aldehyde group is expelled. [18] Ther atio of 3ba and 4ba was increased from 1:2.2 to 1:4.6 in as imilar total yield when 10 equivalents of water were added to the reaction system. We assumed that the addition of water may facilitate the cleavage of the CH 3 ÀO + bond and lead to the formation of 4ba.…”