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âButtressing Effectâ in the HalogenâLithium Exchange in orthoâBromoâN,Nâdimethylanilines and Related Naphthalenes
Non-covalent interactions such as coordination of an organolithium reagent by a directing group and steric repulsion of substituents strongly affect the halogen-lithium exchange process. Here we present the manifestation of the âbuttressing effectâ â an indirect interaction between two substituents issued by the presence of a third group â and its influence on the ease and selectivity of the bromine-lithium exchange and the reactivity of formed aryllithiums. The increase of the size of the âbuttressingâ substituent strongly affects the conformation of a NMe2 group, forcing it to hinder ortho-bromine and thus slowing down the exchange. In naphthalene substrates bearing two bromines, this suppresses regioselectivity of the reaction. The âbuttressing effectâ forces formed aryllithiums to deaggregate, thus boosting their reactivity. This facilitates the decomposition via protolisys by ethereal solvents even at low temperatures and in some cases initiates fast Wurtz-Fittig coupling