The Future of Organometallic Chemistry is Green

By Jonathan L. Kuo

Historically, new reactions were evaluated on the basis of their regio-, chemo-, and stereoselectivity. In the modern era, environmental and socioeconomic factors demand that we also consider green chemistry metrics. Ideally, chemical processes would be reliable and sustainable.

I believe that it is important to develop green alternatives to canonical but inefficient synthetic organic transformations (replacing reactions that generate stoichiometric waste byproducts). The most atom efficient reagents (e.g. H2 as a reductant, O2 as an oxidant) are often kinetically inert to organic substrates, so the reactions will have to be catalyzed. Organometallic chemists can apply their expertise to the development of transition metal based catalysts for carrying out these green transformations.

Reductions with H2 Showcase the Power of Green Chemistry. Reductions promoted by H2 are ubiquitous across all chemical industries. Consider that homogeneous catalysts for hydrogenation were only first reported in the 1960s; this paved the way to innovations like asymmetric hydrogenation (a technology that eventually received the Nobel Prize). In a relatively short timespan, homogeneous hydrogenation catalysts have become a workhorse technology in industrial catalysis. This technology was rapidly adopted because hydrogenation reactions have extremely high atom efficiency, producing no stochiometric waste byproducts. This translates into superior process safety, reductions to production cost, and less damage to the environment. Thus, hydrogenation reactions showcase how green chemistry aligns with various socioeconomic priorities that sometimes conflict with one another.

Progress in Green Oxidation Reactions by O2 has been Comparatively Slow. For oxidation reactions, O2 is the ideal oxidant. Molecular oxygen is inexpensive, abundant, has a low molecular weight, and can generate water as the only byproduct. The flammability of O2 is an important engineering consideration (just as the flammability of H2 is for hydrogenation reactions), but air or pure O2 is used in 14 out of the 16 largest industrial oxidation processes (by metric tonnage); clearly it can be used safely, even on a manufacturing scale.

Despite some successes in aerobic oxidation, there has been comparatively little progress using O2 as an oxidant (relative to using H2 as a reductant). For example, the first homogenous catalyst for the aerobic epoxidation of norbornene was reported by Groves in 1985, achieving 46 turnovers. In 2016, Trewyn, Vedernikov, and Gunnoe reported a catalyst with a max turnover number of 139. These turnover numbers correspond to huge progress in aerobic oxidation reactions, but pale in comparison to the millions of turnovers that can be achieved by modern hydrogenation catalysts. This alludes to a relatively underdeveloped understanding in aerobic oxidation reactions, and a potential area for young organometallic chemists to focus their research efforts.

The relatively slow progress in this area implies that we should expand beyond traditional approaches to organometallic chemistry. For example, an abundance of selective aerobic oxidations are catalyzed by biological systems. Thus, enzymatic systems have much to teach organometallic chemists about how to use O2 in a productive manner. This alludes to the merits of strategies that cut across several disciplines. Organometallic chemists can accelerate progress by embracing diverse and new perspectives, whenever possible.

Oxidation reactions are only one of many potential targets for green catalysis. Regardless of the specific transformation, it would be difficult to argue against developing green chemistry alternatives. This really underscores the inevitability of green chemistry as a central tenet of organometallic chemistry. In other words, the future of organometallic chemistry is green.

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