关键词: 脱羧串联反应, 甲基化, 苯丙酮类, 绿色化学

Decarboxylation reactions have been widely explored in recent ten years, and decarboxylative cascade reaction of cinnamic acids has also attracted much attention. Generally, this class of reaction includes two processes:radical addition and decarboxylation. The result of reaction can introduce kinds of functional groups on the benzene ring, such as halogen atom, nitro-group and trifluoromethyl. Following the continuous studies of our group on oxidative cascade reaction, especially the oxidative reaction in the absence of metal, herein we disclosed a decarboxylative oxidative cascade reaction of cinnamic acids under metal-free conditions. We employed K₂CO₃ as base, tert-butyl hydroperoxide (TBHP) as oxidant and DMSO/H₂O as co-solvent, and cinnamic acids could be converted to propiophenone derivatives in moderate yield. To get an insight into the mechanism of this process, several controlled experiments were conducted. First, DMSO-d₆ was employed under the standard conditions and no d-methyl was detected in the ¹H NMR spectrum of product. It demonstrated that the methyl of product was derived from tert-butyl hydroperoxide. Subsequently, the reaction was carried out under nitrogen and oxygen atmosphere, and it was found that higher reaction efficiency was obtained in N₂. The result indicated that methyl radical was easily quenched by oxygen. On the basis of experiment results, we proposed a plausible mechanism. First, tert-butyl hydroperoxide yielded methyl radical, then the reaction underwent radical addition and decarboxylation to generate the desired product. The reaction featured that tert-butyl hydroperoxide was used as oxidant and methylation reagent in this process, and the reaction proceeded under metal-free and aqueous phase condition. Therefore, it met the requirement of green chemistry. Meanwhile, the operation of reaction was also simple, for example, to a DMSO/H₂O (0.5 mL/0.5 mL) solution of cinnamic acids (0.2 mmol) were successively added K₂CO₃ (0.3 mmol), TBHP (0.8 mmol). The reaction mixture was stirred at 100 ℃. Upon the completion, the desired product was purified by silica gel column chromatography.

Key words: decarboxylative cascade reaction, methylation, propiophenones, green chemistry