“…Indoles with various carbonyl functional groups can be obtained directly using homo and heterogeneous catalysts. Different types of catalysts can be employed for these reactions, including acid catalysts [3][4][5][6][7][8][9][10][11][12][13], nanocatalysts [14][15][16][17][18][19][20], supported catalysts [21][22][23][24][25][26][27][28][29][30], metal catalysts [31][32][33][34][35][36][37][38], ion exchange resin [39], and enzymes [40]. The acid-catalyzed reactions can be facilitated by HCl [41], P 2 O 5 [42], Amberlyst-15 [43], and solidified acids [44].…”