Contributed by J. H. Docherty and S. P. Thomas, EaStCHEM School of Chemistry, University of Edinburgh
Chemical catalysis is an engine that powers modern society. The majority of modern chemical transformations, however, rely on the use of precious metals, such as platinum, iridium and rhodium, which are expensive and scarce resources with many high technology applications outside of the chemical sciences.
A sustainable future for catalysis relies on the use of first-row, low cost, low toxicity, Earth-abundant metals. Despite this, the metals that are most abundant have yet to be adopted by the global community.
With this predicament in mind, we questioned:
Why do synthetic chemists not instinctively use iron, manganese or cobalt?
Why do expensive metals such as platinum, palladium and rhodium dominate?
A prevailing answer arose: Precious metals tend to be practical and simple to use. Earth-abundant metals are comparatively difficult, and the non-specialist chemist is simply not equipped to try.
One particular area of interest to our group is reductive transformations (e.g. olefin hydrogenation). In this field, several state-of-the-art Earth-abundant metal catalysts have been reported in recent years for a range of synthetically valuable transformations.1-3 However, these catalysts are often highly air- and- moisture-sensitive, or require the use of a strong reducing agent – such as; lithium aluminium hydride or Grignard reagents – to access the active catalyst,. While these strategies have proven valuable, the sensitive nature of these requisite reagents presents a practical challenge.
The practicality of any synthetic method is key to its modern-day use. Hence, Earth-abundant metal catalysts that will stand the test of time should be those that are easy and simple to use. At the outset, our over-arching aim was to make this catalysis as functional and simple as making a cup of tea.
We began with model reaction conditions, based upon literature established precedents for alkene hydroboration using an isolated low-oxidation-state iron catalyst and in situ organometallic activator. With this as a testing platform, we assessed the viability of easy-to-handle reagents as novel catalyst activators, finding that alkoxide salts were extremely effective. Of the salts tested, sodium tert-butoxide (NaOtBu) proved most robust, giving the best and most consistent results. It is worthy to note that sodium tert-butoxide is a bench-stable solid, with wide commercial availability and minimal associated hazards.
Using this new activator, we assessed the generality with respect to a series of catalysts bearing unique ligand frameworks, finding that this method was successful across a range of previously developed catalysts, reliant on activation using organometallic reagents. Significantly this included expansion to other metals, including: cobalt, manganese and nickel. Now, we had the ability to set-up reactions without the need for a glove box, or strict air-and-moisture-free equipment, all enabled through the use of a simple alkoxide salt.
Throughout our investigations, we used various sources of NaOtBu, with the conclusion that they were all equally active. This included the use of NaOtBu that had been stored on the bench for more than six years.
From a sustainability perspective, these reactions could be conducted in the absence of a solvent (i.e neat) at ambient temperatures, with short reaction times and excellent product selectivities. This is further exemplified by the fact that we targeted only the most atom-economic transformations, namely: hydroboration and hydrosilylation, with expansion to hydrogenation, hydrovinylation, and 2π+2π cycloaddtion – all with 100 percent atom economy.
It is encouraging to hear that our collaborating partner, GlaxoSmithKline, is delighted with the highly practical nature of the methodology, which has been demonstrated in their own laboratories, and that they are actively looking to introduce such transformations to their under-development routes.
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