Olefin metathesis is among the most powerful tools for forming new C-C bonds and is comprised of ring-closing metathesis (RCM), ring-opening metathesis (ROM), and cross metathesis (CM). These techniques can be used in industrial-scale reactions for producing polypropylene, linear α-olefins, and pharmaceuticals. The most common metathesis catalysts include Ru, Mo, and W, which are non-abundant transition metals, elevating production costs. Vanadium, a more abundant metal, can be used as a greener, more cost-effective alternative. Our group has previously synthesized a bis-phosphine vanadium alkylidene catalyst, which yielded a turnover number (TON) of 6.4 for RCM of our model substrate (N,N-diallyltosylamide). In response to this, we began testing a variety of imido and neutral ligands on the bis-phosphine vanadium alkylidene and synthesized a mono-phosphine mono-iMes NHC catalyst, which yielded a TON of up to 170 for RCM of the model substrate. Because of the presence of the trimethyl-phosphine ligand in our catalyst, we hypothesize free phosphine can exist in solution and bind to the catalyst in place of the iMes-NHC to form the bis-phosphine catalyst, which we know from previous research allows for β-hydride elimination to occur, leading to premature decomposition. We anticipate that by using a bidentate ligand, we can erase the possibility of β-hydride elimination by synthesizing a class of phosphine-free catalysts. The process of synthesizing these phosphine-free catalysts involves adding alkoxide, styrene, and NHC over the course of 24 hours. O-methoxy phenol potassium salt, O-methoxymethyl phenol potassium salt, and methylated binol potassium salt show promising bidentate alkoxide activity at NMR scale reactions and are currently being explored in a variety of solvents. Additionally, we are testing adding the three phenol complexes at the trialkyl step, premature of forming alkylidene, to release a tetramethylsilane. By adding iMes NHC and styrene after this, we anticipate trapping the alkylidene formed following α-hydride abstraction, which occurs when ß-hydride elimination is blocked. By never adding phosphine, we eliminate the possibility of forming the bis-phosphine complex, allowing for a higher TON and more catalytic activity overall.

Vanadium-based alkylidene catalysts for olefin metathesis

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