# Understanding the Role of Base in Catalytic Transfer Hydrogenation: A Comparative Review

## TL;DR

This review compares how bases influence catalytic transfer hydrogenation reactions using different metal catalysts and highlights sustainable alternatives to traditional base usage.

## Contribution

The paper systematically compares base-assisted, base-free, and base-as-co-hydrogen-donor CTH methods across diverse systems.

## Key findings

- Bases like triethylamine and K2CO3 activate catalysts and modulate hydride formation in CTH.
- Base-free CTH systems use ligand frameworks and metal-ligand cooperativity to maintain activity without additives.
- Formic-acid-driven systems show dual base roles and synergistic effects with mixed-base additives.

## Abstract

Catalytic transfer hydrogenation (CTH) provides a practical and sustainable approach for reducing unsaturated compounds, serving as an alternative to high-pressure H2 in laboratory and fine chemical contexts. This broad reaction class includes asymmetric transfer hydrogenation (ATH), a key strategy in enantioselective synthesis due to its operational simplicity, high stereocontrol, and compatibility with sensitive functional groups. A central variable governing CTH efficiency is the role of bases, which may function as essential activators, co-hydrogen donors, or be entirely absent depending on the catalytic system. This review provides a comparison of base-assisted, base-free, and base-as-co-hydrogen-donor CTH methodologies across diverse metal catalysts and substrates. We highlight how bases such as triethylamine, K2CO3, and NaOH facilitate catalyst activation, modulate hydride formation, and tune reactivity and selectivity. The dual function of bases in formic-acid-driven systems is examined alongside synergistic effects observed with mixed-base additives. In contrast, base-free CTH platforms demonstrate how tailored ligand frameworks, metal-ligand cooperativity, and engineered surface basicity can eliminate the need for external additives while maintaining high activity. Through mechanistic analysis and cross-system comparison, this review identifies the key structural, electronic, and environmental factors that differentiate base-assisted from base-free pathways. Emerging trends—including greener hydrogen donors, advanced catalyst architectures, and additive-minimized protocols—are discussed to guide future development of sustainable CTH processes.