Similar to Cu(I) chemistry mentioned above, Rh(I) complexes can also transmetallate with arylboronic esters to get aryl rhodium intermediates, to which CO is inserted giving carboxylic acids. Later, Iwasawa et al. described C-H carboxylation strategy. Rh(I) undergoes oxidative addition to aryl C-H bond followed by transmetallation with alkyl aluminum species. Ar-Rh(I) regenerates by reductive elimination releasing methane. Ar-Rh(I) attacks CO then transmetallates with aryl boronic acid to release the boronic acid of product, giving final carboxylic acid by hydrolysis. Directed and non-directed versions are both achieved.
Iwasawa and co-workers developed Rh(I) catalyzed carbonation reaction initiated by Rh-H insertion to vinylarenes. In order to regenerate reactive Rh-H after nucleophilic addition to CO, photocatalytic proton-coupled electron transfer approach was adopted. In this system, excess amount of diethylpropylethylamine works as sacrificial electron donor (Scheme 5).