I'm synthesizing a chiral intermediate for a drug, but the enantiomeric excess is only about 70% using asymmetric hydrogenation. What are the key factors affecting selectivity and how can I tweak the reaction to improve it?

Low enantiomeric excess in asymmetric hydrogenation is a common challenge. Here's a comprehensive guide to troubleshoot and optimize your setup:

1. Assess Catalyst and Ligand Quality: Verify the purity and integrity of your chiral ligand and catalyst. Impurities or degradation can significantly reduce selectivity. Consider recrystallizing or repurifying them if necessary.
2. Optimize Reaction Conditions:
   • Solvent: Methanol might not be ideal. Try solvents like dichloromethane, THF, or protic solvents like isopropanol, as polarity can affect hydrogen bonding and stereocontrol.
   • Temperature: Lower temperatures (e.g., 0°C to -20°C) often enhance enantioselectivity by slowing down competing racemization pathways.
   • Pressure: Adjust H2 pressure; sometimes higher pressures (up to 100 psi) improve conversion without compromising selectivity, but test incrementally.
3. Check Substrate and Additives: Ensure your starting material is pure and free from protic impurities. Adding bases like triethylamine or acid scavengers might help stabilize the catalytic cycle.
4. Explore Alternative Systems: If issues persist, consider switching to ruthenium or iridium catalysts with different ligand frameworks, or consult recent literature for optimized protocols specific to your compound class.

Patience and systematic testing are key—document all changes to identify the optimal conditions.