Lithiation in Flow: n-Butyl Lithiation and electrophile addition

Abstract

Lithiation reactions are essential in the functionalization of aromatic halogenated compounds. This study investigates the lithiation of an aromatic halogenated compound using n-butyl lithium (nBuLi) as the lithiating agent, followed by the introduction of an acyl chloride derivative as the electrophile (Figure 2).

The reaction was conducted in the SlurryFLO^® reactor, which showcases an exceptionally high heat transfer coefficient, enabling precise thermal control (Figure 3). Reactions were performed at -10°C with varying residence times (1 min, 2 min, 5 min, and 10 min), and 2 minutes was found to give the best results, ensuring high conversion and minimizing side products.

Figure 1: Residence time distribution of SlurryFLO^® Reactor

Figure 2: Lithiation of multihalogenated substrate followed by addition of electrophile

Introduction

Lithiation reactions are invaluable in organic synthesis for functionalizing halogenated aromatic compounds^1,2. However, these reactions are highly sensitive to both temperature and residence time, requiring tight control to avoid side reactions³. Continuous flow technology provides an effective solution, enabling superior control over reaction conditions compared to batch processes.

Figure 3: Heat transfer coefficient vs flow rate plot of SlurryFLO^® reactor

The SlurryFLO^® reactor (Figure 4), which exhibits near-ideal plug flow behavior (with N = 20 in the tanks-in-series model at a flow rate of 300 mL/min and 200 RPM) (Figure 1), offers minimal axial dispersion, allowing for more uniform residence time distribution and better process control. Combined with its excellent heat transfer coefficient, the SlurryFLO^® reactor provides both the thermal management and flow behaviour necessary for optimizing lithiation reactions.

This study aims to optimize the lithiation and subsequent acylation of an aromatic halogenated compound by exploring different residence times under these ideal flow conditions. The reaction was carried out at -10°C to prevent side reactions and decomposition of the organolithium reagent⁴.

Figure 4: SlurryFLO^® Reactor

Figure 5: Schematic of SlurryFLO^® Reactor

The SlurryFLO^® reactor is a versatile continuous chemical reactor designed for performing reactions involving slurries. The key features of the SlurryFLO^® reactor include:

• Homogeneous mixing of the solid particles, liquid (and gas), allowing better contact between reagents and facilitating efficient mass and heat transfer.
• Efficient slurry mixing distributes the heat generated during the reaction, prevents localised heating, and provides uniform temperature throughout the reactor.

Figure 6A: Schematic of reactor setup for lithiation in SlurryFLO^® reactor

Figure 6B: Lithiation of substrate in SlurryFLO^® (insulted) reactor

Reagents

All reagents and solvents were purchased from commercial suppliers and were used without further purification.

Experimental

• Substrate: A multihalogenated aromatic compound (kept general for confidentiality) (1 M)
• Lithiating Agent: n-Butyllithium (n-BuLi), 1.6 M in hexane
• Electrophile: An acyl chloride derivative (1 M)
• Solvent: Tetrahydrofuran (THF)
• Temperature: -10°C, maintained using the SlurryFLO^® reactor’s excellent heat transfer capabilities
• Residence Times Tested: 1 min, 2 min, 5 min, and 10 min for lithiation; 1 min for acylation in all experiments

Results and Discussions

Various residence times were tested to determine the optimal duration for complete lithiation and acylation.

• 1-minute Residence Time: Incomplete conversion due to insufficient reaction time.
• 2-minute Residence Time: Near-complete conversion with minimal side products.
• 5-minute and 10-minute Residence Times: Increased side products due to over-lithiation.

The SlurryFLO^® reactor’s consistent flow conditions and efficient heat management identified 2 minutes as the ideal residence time.

Conclusion

A 2-minute lithiation time was optimal, providing high conversion with minimal side reactions. The consistent 1-minute acylation step ensured uniform downstream conditions.

In summary, the SlurryFLO^® reactor provides a robust platform for precise, scalable lithiation reactions with strong potential for broader flow chemistry applications.

References

1. Power, M.; Alcock, E.; McGlacken, G. P. Organolithium Bases in Flow Chemistry: A Review. Org Process Res Dev 2020, 24 (10), 1814–1838.
2. Natho, P.; Luisi, R. Flow Chemistry as Green Technology for the Genesis and Use of Organometallic Reagents. Tetrahedron Green Chem 2023, 2, 100015.
3. Wong, J. Y. F.; Tobin, J. M.; Vilela, F.; Barker, G. Batch Versus Flow Lithiation–Substitution of 1,3,4-Oxadiazoles. Chem Eur J 2019, 25 (53), 12439–12445.
4. Usutani, H.; Nihei, T.; Papageorgiou, C. D.; Cork, D. G. Flow Chemistry Lithiation–Borylation Route. Org Process Res Dev 2017, 21 (4), 669–673.