关键词: 离子液体, 生物质基N-杂环, 脱氢环化, 可切换选择性, 催化剂控制

α-Hydroxyl esters have been explored as tunable carbon synthon for divergent synthesis of a range of N-heterocycles through modulating the reactivity of their devised functionalities enabled by ionic liquids (ILs). Specifically, the ILs with sulfonic acid functionality catalyzed dehydrogenative and decarboxylative annulation of α-hydroxyl ester with aromatic amines effectively. In contrast, with carboxylic acid substituted ILs, a number of cyclic amines bearing quaternary carbon center was accessed through dehydrogenative annulation, wherein the decarboxylation was inhibited. Notably, the dehydrogenation of α-hydroxyl ester was achieved with the aid of ILs, which is distinct from the traditional metal promoted dehydrogenation. The protocol featured versatile transformations of α-hydroxyl esters, renewable starting materiel, green and recycle catalytic system, high selectivity, broad substrate scope, and easily scaled-up. Especially, carboxylic acid functionalized catalyst (e.g. [HO₂CMemim]OTf) typically gave rise to dihydroquinoxalines bearing quaternary carbon through dehydrogenative annulation, while quinazolinone, benzimidazole, benzothiazole, and quinoxalines can be smoothly provided by the catalysts bearing sulfonic acid functionalities (e.g. Cat-8). Interestingly, the Hammett acidity (H₀) determination of several catalysts has revealed the correlation between the catalytic activity and acidity. Mechanistic studies demonstrated the combination of hydrogen bonding effect, synergistic interaction of anions and cations, and acidic functionality play multiple role in promoting dehydrogenation, annulation, and decarboxylation. Moreover, the acidic difference of two catalysts was responsible for the product diversity. A representative procedure is described as follows: A pressure tube was charged with IL (30~50 mol%) as catalyst, 2-(1H-indol-1-yl)aniline or arylamine (0.1 mmol), and ethyl lactate or its analogues (0.3 mL). The reaction mixture was then stirred at 140 ℃ for 6~12 h under air condition. After cooling to room temperature, the system was washed with saturated NaHCO₃ aqueous solution, extracted by ethyl acetate, and dried over anhydrous Na₂SO₄. After removal of the solvent under reduced pressure, the crude product was purified by flash column chromatography on silica gel with the mixture of ethyl acetate and petroleum ether to provide the corresponding dihydroquinoxaline or quinoxaline product.

Key words: ionic liquids, bio-based N-heterocycles, dehydrogenative annulation, switchable selectivity, catalyst control