I need help troubleshooting the issue of not obtaining the desired product in a Buchwald-Hartwig reaction.
I plan to carry out the Buchwald-Hartwig coupling reaction using 2,6-diaminopyridine (DAP) and 2,6-dibromopyridine (DBP) in the presence of a palladium catalyst.
My experimental parameters are as follows:
**Reactants and Feed Ratio:**
- 2,6-Diaminopyridine (DAP): 6 mmol
- 2,6-Dibromopyridine (DBP): 3 mmol
- Molar feed ratio: n(DAP) : n(DBP) = 2.0 : 1.0
**Reaction Conditions:**
- Solvent: Toluene, 60 mL
- Base: Potassium tert-butoxide (t-BuOK), 15 mmol
- Catalyst: Pd₂(dba)₃, 0.06 mmol
- Ligand: RuPhos, 0.12 mmol
- Additive: 18-crown-6, 6 mmol
- Reaction Temperature and Time: Stirred under reflux at 90 °C in an oil bath under an inert atmosphere (nitrogen) for 24 hours.
- Workup: After the reaction, the mixture is quenched with saturated ammonium chloride solution, extracted with DCM, washed with saturated sodium chloride solution, dried over anhydrous sodium sulfate with stirring for 30 minutes, evaporated under reduced pressure to remove the solvent, and finally purified by column chromatography.
**Target Product:**
- The target product is N²,N⁶-bis(6-aminopyridin-2-yl)pyridine-2,6-diamine.
- Structural confirmation is planned via ¹H NMR.
**Actual Products:**
- The actual products obtained are N²,N⁶-bis(6-bromopyridin-2-yl)pyridine-2,6-diamine and N²-(6-bromopyridin-2-yl)pyridine-2,6-diamine (mono-substituted byproduct).
These are my reaction conditions, which I followed based on literature references. Why am I not obtaining the desired product? Could any experts provide guidance?
