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Novel approach in organic chemistry enables boron to be added to molecules with high precision

A simple chloride catalyst enables selective C–H borylation under mild photochemical conditions

28 October 2020

A major goal in organic synthesis is to develop efficient reactions to convert feedstock chemicals (otherwise known as raw or natural materials) into valuable reagents that can be used to create pharmaceuticals and agrochemicals.

A powerful approach to this core scientific challenge is toconvert carbon–hydrogen bonds into carbon–heteroatom bonds. However, organic molecules are composed of a “sea”of carbon–hydrogen bonds, making it very difficult to achieve high selectivity for reaction at a specific bond. 

To add another layer of difficulty, carbon–hydrogen bonds are generally unreactive, which means that harsh reaction conditions are required, often using high temperatures and expensive precious metal catalysts.

Now, scientists at the University of Bristol have discovered a mechanistically unique approach to facilitate this process in a way that is more efficient and less costly than traditional methods.  

Their findings, as reported in the journal Nature today [28 October] open up new possibilities for converting feedstock chemicals into valuable boron-containing compounds, which play a major role in the manufacture of numerous products, from medicines to TV screens.

This latest study, led by Professor Varinder Aggarwal FRS and Dr Adam Noble from the School of Chemistry, describes how a process known as C–H borylation can be used to convert carbon–hydrogen bonds oforganic moleculesinto carbon–boron bonds, which are some of the most versatile in chemical synthesis.

“An important feature of this new C–H borylation is that, unlike all previously reported methods, these reactions proceed at ambient temperature and do not require a metal catalyst, which can significantly reduce their cost,” says Professor Aggarwal. 

The study involved usinga simple chloride catalyst in place of preciousmetals.Key to the success of the reaction was the use of violet-light irradiation, which provides the chloride catalyst with enough energy to break unreactive carbon–hydrogen bonds.

"Crucially, themild reaction conditions of this photoinduced C–H borylationenablea laser-like precision as to which of the myriad of C–H bonds is transformed. In addition, since the mechanism is distinct from those that use metal catalysts, this new method allowsthe synthesis of boron-containing organic molecules that cannot be accessed using existing approaches, thus generating new opportunities for the synthesis of these valuable compounds from feedstock chemicals.”

Further information

Paper: ‘Metal-free photoinduced C(sp3)–H borylation of alkanes', by Chao Shu, Adam Noble and Varinder K. Aggarwal, DOI 10.1038/s41586-020-2831-6, https://www.nature.com/articles/s41586-020-2831-6

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