Direct Observation of Reactive Intermediates by Time-Resolved Spectroscopy Unravels the Mechanism of a Radical-Induced 1,2-Metallate Rearrangement

Luke J Lewis-Borrell, Mahima Sneha, Ian P. Clark, Valerio Fasano, Adam Noble, Varinder K Aggarwal*, Andrew J Orr-Ewing*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

Abstract

Radical-induced 1,2-metallate rearrangements of boronate complexes are an emerging and promising class of reactions that allow multiple new bonds to be formed in a single, tuneable reaction step. These reactions involve the addition of an alkyl radical, typically generated from an alkyl iodide under photochemical activation, to a boronate complex to produce an α-boryl radical intermediate. From this α-boryl radical, there are two plausible reaction pathways that can trigger the product forming 1,2-metallate rearrangement: iodine atom transfer (IAT) or single electron transfer (SET). Previous steady state techniques have struggled to differentiate these pathways. Here we apply state-of-the-art time-resolved infrared absorption spectroscopy to resolve all the steps in the reaction cycle, by mapping production and consumption of the reactive intermediates over picosecond to millisecond timescales. We apply this technique to a recently reported reaction involving the addition of an electron-deficient alkyl radical to the strained σ‑bond of a bicyclo[1.1.0]butyl boronate complex to form a cyclobutyl boronic ester. We show that the previously proposed SET mechanism does not adequately account for the observed spectral and kinetic data. Instead, we demonstrate that IAT is the preferred pathway for this reaction and is likely to be operative for other reactions of this type.
Original languageEnglish
Pages (from-to)17191 - 17199
Number of pages9
JournalJournal of the American Chemical Society
Volume143
Issue number41
Early online date6 Oct 2021
DOIs
Publication statusPublished - 20 Oct 2021

Bibliographical note

Funding Information:
This research was funded by EPSRC Grant EP/R012695/1 and previous ERC Advanced Grant CAPRI 290966. M.S. gratefully acknowledges award of a Marie Skłodowska-Curie Fellowship (MARCUS 793799). V.F. thanks the University of Bristol for awarding an EPSRC Doctoral Prize Fellowship (EP/R513179/1). The authors are grateful to the Science and Technology Facilities Council for access to the LIFEtime Facility at the Rutherford-Appleton Laboratory. We thank the Bristol Chemical Synthesis Centre for Doctoral Training, funded by EPSRC (EP/L015366/1), and the University of Bristol for a PhD studentship for L.L.B.

Publisher Copyright:
© 2021 American Chemical Society.

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