From Organometallics
An easy and straightforward method to obtain a carboxylic acid is to make an organometallic compound react with carbon dioxide.
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This adds one carbon atom to the molecule.
The carbon bonded to the metal, electron rich due to the low electronegativity of the latter, attacks the CO2 carbon, which is electron deficient because of the high electronegativity of the oxygen atoms.
Retrosynthetically speaking, one might plan to have a carboxyl group from a haloderivative, provided the halogen is first replaced by a metal and the resulting organometallic treated with CO2.
Watch out for possible incompatibilities between the functional groups present!
Look how toluene can be the precursor of two very different carboxylic acids.
Bromination is the key step in this scheme. If one uses molecular bromine in the presence of a Lewis acid, bromination takes place at the ring by SEAr. If, instead, N-bromosuccinimide (NBS), a source of bromine radicals, is used, bromination occurs at the benzylic position. Metallation - in one case at the ring and in the other at the benzylic position - leads to different organometallics. The aromatic organometallic with CO2 yields lithium p-methylbenzoate, whereas the benzylic one gives lithium phenylacetate.
Where I put the halogen, I put the carboxylate!