Rhenium-catalyzed Deoxydehydration Of Vicinal Diols To Alkenes

The deoxydehydration(DODH) of polyols to alkenes is one of the important methods for the conversion of biomass-based platform molecules to useful chemicals and liquid fuels. In comparison to the current widely used procedures of high-temperature pyrolysis, acid-catalyzed dehydration and hydrogenolysis reactions, metal-catalyzed DODH reactions have simpler reaction condition, higher resource utilization, less environmental pollution. Although the study of the reaction in recent years has made great progress, it is still in the early stages.Currently, only a handful of the precious metal catalyst can catalyze the DODH reactions of vicinal diols, and there are such problems as large catalyst loading, expensive, difficult to separate and recycle, etc. Therefore, the development of new catalytic system for glycol DODH reaction is of great significance.In this paper, we explored new catalytic system for glycol DODH, mainly studied the rhenium-catalyzed DODH reactions of vicinal diol,and developed a kind of catalytic system with a newly synthesized imidazolinium perrhenate as catalyst.(1) We synthesized a series of MoO2(acac’)2 type catalyst. through detailed optimization of reaction conditions, in the use of MoO2(acac)2 as catalyst, we achieved a vicinal diol to olefin product transformation in 30% yield;(2) We synthesized nine new imidazolinium and imidazolium perrhenate catalysts. The deoxydehydration of 1,2-tetradecanediol was chosen as the model reaction for studying the influence of catalysts. we found that perrhenate with different cations exhibit different catalytic activity, suggesting that the structure of the cation play a significant regulatory role of the catalytic activity.With 1,3-(2,6-diisopropylphenyl) imidazolinium perrhenate as catalyst, the reation gave the highest yield, using 2.5 mol % catalyst delivered product in 85% yield.(3) For the 1,3-(2,6-diisopropylphenyl) imidazolinium perrhenate catalyzed DODH reaction, the reaction conditions, including reductants and solvents were investigated, the substrates were expanded. The results obtained demonstrate that with 3-octanol(3 equiv) as the reductant in chlorobenzene at 180 ℃ under argon atmosphere, several diol and epoxide substrates with different carbon chain lengths and different substituents were subjected to the DODH reaction and afforded their corresponding olefin products in moderate to good yields(41~85%) after 24~60 h. In addition, the catalyst could be reused after recoverd.(4) The reaction using the recovered catalyst gave the olefin in 55% yield. We characterized its structure by NMR and X-ray single crystal diffraction, suggesting that structural change has not occurred before and after the reaction. However, the recovered catalyst containing crystal water, may be one cause of lower yield.