A feasible synthesis route is devised for realizing direct carboxylation of thiophene and CO

2

in a relatively mild solvent-free carboxylate-assisted carbonate (semi) molten medium. The effects of reaction factors on product yield are investigated

and the phase behavior analysis of the reaction medium is detected through the thermal characterization techniques. Product yield varies with the alternative carboxylate co-salts

which is attributed to the difference in deprotonation capacity caused by the base effect within the system. Besides

the detailed mechanism of this carbonate-promoted carboxylation reaction is studied

including two consecutive steps of the formation of carbanion through breaking the C—H bond(s)

via

the carbonate and the nucleophile attacking the weak electrophile CO

2

to form C—C bond(s). The activation energy barrier in C—H activation step is higher than the following CO

2

insertion step whether for the formation of the mono- and/or di-carboxylate

which is in good agreement with that of kinetic isotope effect (KIE) experiments

indicating that the C—H deprotona

tion is slow and the forming presumed carbanion reacts rapidly with CO

2

. Both the activation energy barriers in deprotonation steps are the minimal for the cesium cluster system since there have the weak the cesium Cs-heteroatom S (thiophene) and Cs-the broken proton interactions compared to the K

2

CO

3

system

which is likely to enhance the acidity of C—H bond

lowering the C—H activation barrier. Besides

these mechanistic insights are further assessed by investigating base and C—H substrate effects

via

replacing Cs

2

CO

3

with K

2

CO

3

and furoate (1a) with thiophene monocarboxylate (1b) or benzoate (1c).