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Reactions of an Aluminium(I) Reagent with 1,2-, 1,3- and 1,5-Dienes: Dearomatisation, Reversibility, and a Pericyclic Mechanism

preprint
submitted on 18.12.2019 and posted on 23.12.2019 by Clare Bakewell, Martí Garçon, Richard Y Kong, Louisa O'Hare, Andrew J. P. White, Mark Crimmin
The reactions of an aluminium(I) reagent with a series of 1,2-, 1,3- and 1,5-dienes are reported. In the case of 1,3-dienes the reaction occurs by a pericyclic reaction mechanism, specifically a cheletropic cycloaddition, to form aluminocyclopentene containing products. This mechanism has been interrogated by stereochemical experiments and DFT calculations. The stereochemical experiments show that the (4+1) cycloaddition follows a suprafacial topology, while calculations support a concerted albeit asynchronous pathway in which the transition state demonstrates aromatic character. Remarkably, the substrate scope of the (4+1) cycloaddition includes dienes that are either in part, or entirely, contained within aromatic rings. In these cases, reactions occur with dearomatisation of the substrate and can be reversible. In the case of 1,2- or 1,5-dienes complementary reactivity is observed; the orthogonal nature of the C=C π-bonds (1,2-diene) and the homoconjugated system (1,5-diene) both disfavour a (4+1) cycloaddition. Rather, reaction pathways are determined by an initial (2+1) cycloaddition to form an aluminocyclopropane intermediate which can in turn undergo insertion of a further C=C π-bond leading to complex organometallic products that incorporate fused hydrocarbon rings.

Funding

We are grateful to the European Research Council (FluoroFix:677367) and the Royal Society (UF090149). Pete Haycock is gratefully acknowledged for assistance with NMR spectroscopy. M.G. thanks “la Caixa” Foundation (ID 100010434) for a postgraduate scholarship (LCF/BQ/EU19/11710077). RK thanks the President’s Scholarship scheme of Imperial College London.

History

Email Address of Submitting Author

m.crimmin@imperial.ac.uk

Institution

Imperial College London

Country

United Kingdom

ORCID For Submitting Author

0000-0002-9339-9182

Declaration of Conflict of Interest

The authors declare no conflict of interest

Version Notes

version 1.0

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