Intensification of Renewable 4,4′-Dimethylbiphenyl Synthesis for Recyclable Diesters

Abstract

Reducing the global dependence on petroleum-derived chemical products requires renewable alternatives to replace established materials. Recent investigations demonstrated a biobased pathway to prepare the platform chemical 4,4′-dimethylbiphenyl (4,4′-DMBP). The synthesis of 4,4′-DMBP follows a two-step process: (1) 2-methylfuran (2-MF) oxidative coupling to 5,5′-dimethyl-2,2′-bifuran (5,5′-DMBF) and (2) 5,5′-DMBF tandem Diels–Alder-dehydration with ethylene to afford 4,4′-DMBP. Here, we report the intensification of reaction conditions in step (1), improving 5,5′-DMBF space-time yield up to 1.10 mol L^–1h^–1, an 86% increase from the baseline. Scale-up of step (1) was hindered by oxygen-deprivation-induced palladium black formation and reaction exotherms decreasing yields at larger scales. Oxygen sparging, mechanical mixing, and internal cooling implemented simultaneously enabled a 108× increase in 5,5′-DMBF production to an average of 13 g/batch. In step (2), the use of a homogeneous La(OTf)₃ catalyst in the Diels–Alder-dehydration reaction─instead of heterogeneous γ-Al₂O₃─led to a 54% increase in 4,4′-DMBP yield with a 70 °C temperature reduction to 180 °C. Scale-up of the Diels–Alder-dehydration to 3 g/batch maintained para-selectivity for 4,4′-DMBP with full conversion to the product within 20 h. Renewable 4,4′-DMBP is achieved from the improved pathway and isolated in 96.7% purity for further utilization downstream.