Enhancing triethylborane initiation through mechanistic understanding using a novel radical trapping technique Ivan Ocaña 1 , George Hodges 2 , Neil Griffin 2 , Andrew Rickard 1 , Victor Chechik 1 1 University of York,UK, 2 Syngenta, UK Triethylborane (Et 3 B) in the presence of O 2 is one of the most widely used systems for radical chain initiation in organic synthesis. 1 However, its initiation mechanism is poorly understood, and some reactions fail to initiate by this system. In this work we uncovered a previously unknown secondary mechanism of initiation using a novel radical trapping technique. We developed a new radical trap capable of capturing free radicals generated during initiation and allow their detection by mass spectrometry (MS) and quantification by nuclear magnetic resonance (NMR) (Scheme 1a). 2 This technique was applied to investigate the reactions involved in the initiation mechanism of Et 3 B/O 2 system. We confirmed that the primary initiation mechanism generates ethyl radicals as the initiating radicals. However, as predicted by Davies 3 and in agreement with our computational simulations, the primary mechanism is very inefficient, and it is unlikely to account for the initiation alone. We hypothesized that diethyl(ethylperoxy)borane (Et 2 BOOEt), the product of autoxidation, plays an important role in initiation. We synthesised Et 2 BOOEt separately and reacted it with Et 3 B in the presence of the radical trap. It was found that this reaction generated 1 eq. of ethyl radicals, which were identified by MS and quantified by NMR. Kinetic studies revealed that this secondary initiation was 300 times faster than the primary initiation. 4 Our findings suggest thatEt 2 BOOEt acts as a more efficient initiator than Et 3 B/O 2 as the initiation proceeds (Scheme 1b). When this mechanism was simulated computationally it was found the secondary initiation produces 7 x 10 5 times more initiating radicals than the primary initiation. We exploited this insight to overcome the challenges of initiation by using Et 2 BOOEt in combination with Et 3 B to initiate inefficient chains that could not be accessed using Et 3 B/O 2 alone (Scheme 1c). In the course of our investigations, we discovered that Et 2 BOOEt, in isolation, was also capable of initiating reactions that were previously inaccessible with traditional triethylborane initiation. This work demonstrates the power of our novel radical trapping technique for studying complex radical mechanisms while further expanding our understanding of Et 3 B/O 2 initiation, facilitating its application and opening new avenues for exploration in organic synthesis.
References 1. A. G. Davies, J. Chem. Res., 2008, 7, 361-375. 2. P. J. H. Williams, G. A. Boustead, D. E. Heard, P. W. Seakins, A. R. Rickard and V. Chechik, J. Am. Chem. Soc., 2022, 144, 15969-15976. 3. A. G. Davies, K. U. Ingold, B. P. Roberts and R. Tudor, J. Chem. Soc. B, 1971, 698-712. 4. J. Grotewold, J. Hernandez and E. A. Lissi, J. Chem. Soc. B, 1971, 182.
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