ortho-Metalation of Unprotected 3-Bromo and 3-Chlorobenzoic Acids with Hindered Lithium Dialkylamides

Upon treatment of 3-chloro/bromobenzoic acids with hindered lithium dialkylamides (LDA or LTMP) at -50 degrees C, lithium 3-chloro/bromo-2-lithiobenzoates are generated. These dianions can be trapped as such to afford after electrophilic quenching a variety of simple 2-substituted-3-chloro/bromobenzoic acids. The 3-bromo-2-lithiobenzoate is less stable than the corresponding 3-chloro derivative and partly eliminates lithium bromide, thus setting free lithium 2,3- and 3,4-dehydrobenzoates that can be intercepted in situ with the hindered base.