Crystallography as a tool in elucidating activation modes of boron Lewis acid catalysts with nitrogen-containing substrates Yara van Ingen, Rebecca Melen Cardiff University, UK The presence of an empty p-orbital on boron renders it Lewis acidic and thus able to activate polar, Lewis basic substrates. As a result, it has seen a plethora of catalytic applications paving the way for metal-free catalysis. Functionalised nitrogen-containing scaffolds are important building blocks towards pharmaceuticals and agrochemicals. In this poster I will show recent efforts from our group that show mild and efficient reaction protocols for the decarbonylation of isocyanates to form urea scaffolds, 1 as well as the amidation of indoles with isocyanates 2 – both utilising boron Lewis acid catalysts. Detailed mechanistic studies will be described to better understand the activation modes of the borane catalysts, utilising computational methods and crystallographic insights as the main tools. In this poster I will show why the sterically encumbered B(C 6 F 5 ) 3 was found to be the optimum catalyst for the C‒H amidation of indoles with isocyanates, whilst the more Lewis acidic BCl 3 was efficient for the N ‒carboxamidation of indoles as well as the decarbonylation of isocyanates. In the former case, DFT studies showed that the reactivity between indoles and isocyanates was found to proceed through B(C 6 F 5 ) 3 activation of the isocyanate and subsequent nucleophilic attack of the C3 position of 1-methylindole. BCl 3 catalysed C‒H amidation of indoles on the other hand was found to proceed via a key, isolable, indole→BCl 3 adduct. Crystallography was a key tool in determining the activation modes of the effective boron Lewis acid catalysts in these transformations and these reported methodologies further the understanding of metal-free catalysed reactions of nitrogen-containing substrates (Scheme 1).
Scheme 1: Key intermediary crystal structures in boron-catalysed reactions of isocyanates and indoles. References 1. A. Dasgupta, Y. van Ingen, M. G. Guerzoni, K. Farshadfar, J. M. Rawson, E. Richards, A. Ariafard and R. L. Melen, Chemistry – A European Journal , , DOI:10.1002/chem.202201422. 2. A. Dasgupta, M. G. Guerzoni, N. Aloitaibi, Y. van Ingen, K. Farshadfar, E. Richards, A.Ariafard, R. L. Melen, Manuscript submitted.
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