LiTMP trans-metal-trapping of fluorinated aromatic molecules: a comparative study of Al and Ga carbanion traps

Fluoro aromatic scaffolds pose a challenge to lithiation due to low stability of lithiated intermediates. Here we apply trans-metal-trapping (TMT) to a series of key fluorinated aromatics. In TMT, LiTMP performs the metallation, while an organometallic trap intercepts the emergent carbanion. This study contrasts the trapping abilities of iBu2AlTMP and Ga(CH2SiMe3)3, structurally mapping their TMT reactions and probing relative stabilities of metallated fluoroaromatic intermediates by NMR studies. Results show the installed Al-C(aryl) bonds are more prone to decomposition by benzyne formation and Li-F liberation, than the Ga-C(aryl) species. The latter are thus better for onward reactivity as demonstrated in cross-coupling reactions with benzoyl chloride that produce ketones.