Exploring the photochemistry of lithiated enolates James Mortimer, Prof. Jonathan Clayden, Prof. Andrew Orr-Ewing School of Chemistry, University of Bristol, UK
Low-cost, energy-efficient LED lamps have recently been utilised in our group to promote an enantioselective ring-expansion cascade to form cycloheptatrienes from benzyl benzamides. 1,2 Extending this work, we have found that by irradiating acyclic amide enolates we achieve a nitrogen to carbon shift of a benzyl group (Figure 1a) to give three migration products from reactivity across the trienolate. The versatility of visible-light enolate photochemistry extends to ring-contraction/migration of a six-membered lactam (Figure 1b), where preliminary research has shown a cyclobutyl ring can be formed from simple deprotonation and irradiation of a conjugated lactam. This reaction also provides a ‘ring-swing’ product, forming a cyclohexene from N-C benzylic cleavage and recombination. More recently, an internal chromophore has been used (Figure 1c), where forming a dienolate inside a six- membered ring followed by irradiation leads to an isomeric mixture of ring-opened conjugated trienes. Work is being carried out to investigate the generality of this process, which we believe may offer stereocontrol of the photochemical products. We hope that the unusual chemistry presented could eventually be used to enable facile synthesis of products from conjugated enolates and organolithiums, in particular for the synthesis of novel molecular scaffolds whose formation may otherwise require transition metal-mediated catalysis.
References 1. J. Clayden et. al. , Chem. Sci. , 2022 , 13 , 2079-2085; 2. J. Clayden et. al. , J. Am. Chem. Soc. , 2003 , 125 , 9278-9279.
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