C–H carboxylation is an attractive transformation for both streamlining synthesis and valorizing CO₂. The high bond strength and very low acidity of most C–H bonds, as well as the low reactivity of CO₂, present fundamental challenges for this chemistry. Conventional methods for carboxylation of electron-rich heteroarenes require very strong organic bases to effect C–H deprotonation. Here we show that alkali carbonates (M₂CO₃) dispersed in mesoporous TiO₂ supports (M₂CO₃/TiO₂) effect CO₃²⁻-promoted C–H carboxylation of thiophene- and indole-based heteroarenes in gas–solid reactions at 200–320 °C. M₂CO₃/TiO₂ materials are strong bases in this temperature regime, which enables deprotonation of very weakly acidic bonds in these substrates to generate reactive carbanions. In addition, we show that M₂CO₃/TiO₂ enables C3 carboxylation of indole substrates via an apparent electrophilic aromatic substitution mechanism. No carboxylations take place when M₂CO₃/TiO₂ is replaced with un-supported M₂CO₃, demonstrating the critical role of carbonate dispersion and disruption of the M₂CO₃ lattice. After carboxylation, treatment of the support-bound carboxylate products with dimethyl carbonate affords isolable esters and the M₂CO₃/TiO₂ material can be regenerated upon heating under vacuum. Our results provide the basis for a closed cycle for the esterification of heteroarenes with CO₂ and dimethyl carbonate.

中文翻译：

---

碳酸盐促进富电子杂芳烃的C–H羧化

C–H羧基化对于简化合成过程和使CO ₂增值都是有吸引力的转化。大多数C–H键的高键强度和极低的酸度，以及CO ₂的低反应性，对该化学构成了根本挑战。富电子杂芳基羧化的常规方法需要非常强的有机碱才能实现C–H去质子化。在这里，我们表明分散在中孔TiO ₂中的碱金属碳酸盐（M ₂ CO ₃）担载了（M ₂ CO ₃ / TiO ₂）效应CO ₃ ^2-促进了200–320°C的气固反应中基于噻吩和吲哚的杂芳烃的C–H羧基化。在此温度范围内，M ₂ CO ₃ / TiO ₂材料是强碱，这使这些基材中的弱酸性键脱质子化，从而生成反应性碳负离子。此外，我们表明，M ₂ CO ₃ / TiO ₂能够通过表观亲电芳族取代机制使吲哚底物进行C3羧化。当用未负载的M ₂ CO ₃代替M ₂ CO ₃ / TiO ₂时，不会发生羧化反应，证明了碳酸盐分散和破坏M ₂ CO ₃晶格的关键作用。羧化后，用碳酸二甲酯处理与载体结合的羧酸盐产物，得到可分离的酯，并且在真空下加热可再生M ₂ CO ₃ / TiO ₂材料。我们的结果为杂芳烃与CO ₂和碳酸二甲酯酯化的封闭循环提供了基础。