Investigations of prenol reactions catalyzed by zinc and molybdenum complexes

Abstract

We demonstrate the formation of symmetrical and unsymmetrical allylic ethers with molybdenum catalysts for the first time. While conducting reactivity studies of biobased oxygenates as fuel blending components in the presence of molybdenum or zinc metal complexes commonly used as lubricant additives, we discovered that prenol (3-methyl-2-buten-1-ol), a promising oxygenate with good fuel properties, undergoes unexpected chemical transformations. Interestingly, its isomer, isoprenol (3-methylbut-3-en-1-ol) was unreactive towards the same metal complexes. Moreover, the results suggest that the transformations were catalytic in nature as only 1% (w/w) of lubricant molybdenum complex (Molyvan L – molybdenum di (2-ethylhexyl) phosphorodithioate) led to the almost complete consumption of prenol under certain conditions. The zinc complex (zinc dialkyldithiophosphate) also led to a rich product profile, perhaps to a lower extent than the molybdenum analog. Intrigued by these results, we fine-tuned reaction conditions (e.g., temperature, solvents, closed reactor, or reflux set-up) to maximize the conversion of prenol and generation of byproducts, enable scale-up, and subsequently separate and identify the main components found in the mixture. We determined that the most efficient reaction was neat (prenol with 1% (w/w) molybdenum catalyst) under reflux conditions and at mild temperatures (<120 °C) with 95% conversion of prenol and formation of four major products. The product profile was identified spectroscopically (nuclear magnetic resonance spectroscopy, gas chromatography via flame ionization detection, and gas chromatography-mass spectroscopy) and included isoprene, 2-methylbut-3-en-2-ol (prenol isomer alcohol), 3-methyl-3-((3-methylbut-2-en-1-y)oxy)but-1-ene (mixed alcohol ether), and 1,1′-oxybis(3-methyl-2-butene) (prenol ether). The products are consistent with the catalytic isomerization of prenol to generate a tertiary unsaturated alcohol, as well as inter and intramolecular dehydration chemistries of the two alcohols that formed ethers and isoprene, respectively. This finding is significant as it demonstrates potentially undesirable chemistries that take place when lubricants and fuels come in contact, such as the sump pump or in-cylinder. Moreover, the fuel chemistry observed in real-life scenarios may be due in part to formed by-products and not the starting oxygenate. This work highlights potential catalytic chemistries that can occur with other metal complexes commonly found in lubricants. To the best of our knowledge, this is the first demonstration of the formation of symmetrical and unsymmetrical allylic ethers with molybdenum catalysts. We propose likely mechanisms for the observed chemistries.