Hydrogen borrowing with heterogeneous catalysis.

Abstract

Hydrogen borrowing (HB) is a transition metal-catalyzed methodology that enables the formation of C-N, C-C, and C-S bonds using alcohols (or sometimes amines) as alkylating agents. This sustainable approach only produces water as a byproduct and eliminates the need for stoichiometric oxidants and reductants. HB is particularly relevant for the (future) valorization of oxygen-rich renewable feedstocks, which typically contain OH groups, allowing upgrading to higher-value products, thereby contributing to the CO2 footprint reduction of the chemical industry. While homogeneous catalysts have demonstrated high selectivity and efficiency, their industrial applicability remains limited due to cost and difficulty in recovery and reuse. On the other hand, heterogeneous HB catalysts offer the advantage of improved recyclability and scalability (continuous production). This review is structured around the three key HB reactions, namely C-N, C-C, and C-S bond formation. The first section focuses on C-N bond formation, highlighting the alkylation of amines, ammonia, and nitrogens in conjugation with XO (X = C, S, SO, or P). Also, aromatic alcohol amination (i.e. phenols) and the synthesis of N-heterocycles are covered. The second section discusses C-C bond formation covering homo- (i.e. Guerbet reaction) and heterocoupling of alcohols, α-alkylation of ketones, amides and nitrile derivatives, and alkylation of fluorenes and electron-rich aromatics (i.e. phenols and indoles). Finally, the third section summarizes the less-studied C-S bond formation, which presents opportunities for greener thiolation strategies using H2S, thiols, and dithiocarbamates. Through these three sections, this review is further subdivided per support type (based on carbon, inorganic, coordination polymers and MOFs, non-supported, and supported catalysts) and provides an overview of recent advancements in heterogeneous HB catalysis and its potential for sustainable chemical synthesis. A specific section on the most important renewable alcohols indicating their current technology readiness levels is included, giving an idea on the state of the upstream processes to access them.

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