Efficient Catalyst for Hydroarylation Reaction of Styrene with Phenol to Obtain High DSP Selectivity in Mild Condition

Background: Technical mixture of styrenated phenols including mono-, di-, and tristyrenated phenol, has been commonly applied for industrial materials such as rubber or plastic stabilizer, antioxidant, and nonionic surfactant, etc. Among these styrenated phenols, di-styrenated phenol should be most effective as rubber and plastic stabilizers. Although a number of catalysts for the synthesis of styrenated phenols have been explored, researches on the synthesis of styrenated phenol generally have been focused on selective preparation of mono-styrenated phenol MSP, rather than distyrenated phenol DSP. In this paper, we have investigated the hydroarylation reaction of styrene with phenol to find the optimal catalyst, including single catalysts and mixed catalysts, to get high selectivity to DSP under mild reaction conditions. Method: Hydroarylation reactions of styrene with phenol using various single catalysts, such as inorganic acids, organic acids, Lewis acids, and metal salt catalysts, have been conducted. To optimize the reaction conditions, hydroarylation reactions of styrene with phenol employing InCl3 catalyst were carried out with a variety of styrene amount, catalyst amount, reaction time, and reaction temperature. Halogenpromoted hydroarylation reactions of styrene with phenol were investigated in the presence of NBS or I2 as a halogen source and a variety of metal halides as a Lewis acid catalyst. Br-promoted hydroarylation reactions of styrene with phenol were accomplished using InCl3 along with NBS under a variety of NBS amount and reaction temperature. To explore the scope of Br-promoted hydroarylation, the reactions of various styrene derivatives with phenol were carried out using NBS and InCl3. Results: Hydroarylation reactions of styrene with phenol using various single catalysts, such as inorganic acids, organic acids, Lewis acids, and metal salt catalysts, have been conducted. Among 19 catalysts used, best results in both high conversion of phenol and high DSP selectivity are obtained with InCl3 catalyst. Using InCl3, total yield of styrenated phenols is 98% and product selectivity MSP/DSP/ TSP is 20/65/13. When InCl3 as an optimal catalyst was applied for the hydroarylation reactions of styrene with phenol under various reaction conditions, the optimal reaction conditions for obtaining a high yield, high DSP, and low MSP are as follows: styrene/phenol = 2 molar ratio, catalyst/phenol = 0.1 molar ratio, reaction time 6 hours, reaction temperature 120 °C. In the halogen-promoted hydroarylation reactions of styrene with phenol in the presence of NBS or I2 as a halogen source and various metal halides as a Lewis acid catalyst, best yield (99%) and DSP selectivity (MSP/DSP/ TSP=13/42/41) were obtained using NBS and InCl3. The optimal reaction condition for Br-promoted hydroarylation reaction was found to be phenol 1 eq., styrene 2 eq., InCl3 0.04 eq., NBS 1 eq., 4 hours reaction time, room temperature. For the reactions of various styrene derivatives with phenol using NBS and InCl3, the best DSP selectivity was observed for the CH3-substituted styrene derivative. Conclusion: We have developed hydroarylation reaction of styrene with phenol for obtaining a high yield and a high DSP selectivity even at room temperature. Using NBS as a Br source and InCl3 as a catalyst at room temperature, Br-promoted hydroarylation reaction of styrene with phenol yields good results with respect to both yield and DSP selectivity. Keywords: Styrene, phenol, styrenated phenol, Br-promoted hydroarylation of styrene, InCl3, catalyst.