Gang Wang,Yiming Xu,Suojiang Zhang,Zengxi Li,Chunshan Li
A one-pot synthesis of α,β-unsaturated esters from unactivated esters and aldehydes using strong bases, such as sodium alkoxide and potassium tert-butoxide, was reported. However, the ionic liquid (IL) catalyzed probase method for producing α,β-unsaturated esters was not reported until now. In this work, a series of ILs with fluoride anions were firstly prepared and used as catalysts in combination with the probase N,O-bis(trimethylsilyl) acetamide (BSA) for the α,β-unsaturated esters synthesis. This process could also be promoted through the introduction of another IL with Lewis acid sites. The yield and selectivity of the product could reach up to 84.2% and 95.0%, respectively, when [Bmim]F was used in combination with [Bmim]Cl/AlCl3 (the molar fraction of AlCl3 is 0.67). The mechanism investigation through GC-MS indicates that BSA would convert into onium amide, which acted as a strong base for α-H abstraction, with the catalysis of [Bmim]F. Meanwhile, [Bmim]Cl/AlCl3 played an important role in the condensation step between enolates and aldehydes. On the basis of mechanism insights, kinetic and thermodynamic studies were also carried out for a better understanding of this new route.
Herein, we demonstrate a hybrid microflow system consisting of a coaxial double-tube microreactor and a capillary microreactor for performing borohydride reduction of α , β -unsaturated esters to allylic alcohols in a highly controlled and safe manner. Using methyl cinnamate as the model substrate and LiBH 4 as reductant, complete conversion was achieved in 75 s residence time at 80 ℃ with up to 85% yield of the target product cinnamyl alcohol under optimized conditions in the hybrid microflow system, enabling selective borohydride reduction of α , β -unsaturated esters to allylic alcohols without the need of using metallic salts as additives or co-reagents. The kinetics of this reduction reaction has been examined and modelled, providing an understanding of the intensified reaction behaviour and the reaction performance of the hybrid microflow process. The results presented can shed new light on the development of continuous microflow processes for the hydridic type of reductions.
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