Palladium-catalyzed cross-couplings — Suzuki, Heck, Sonogashira, and Buchwald–Hartwig among them — build the biaryl and C–N bonds at the heart of modern API manufacture. DMSO is the preferred high-temperature solvent when unreactive aryl chlorides or sluggish couplings demand heat that decomposes amide solvents such as DMF.
Ar–X + Ar′–B(OH)2Pd(0) cat., K2CO3, DMSO, 120–150 °C→Ar–Ar′ + B(OH)3 + HX
Reaction 1: Suzuki–Miyaura cross-coupling in DMSO (representative Pd coupling)
Where Pd-Catalyzed Coupling in DMSO Is Used
The largest use is in building biaryl drug molecules. Suzuki couplings in DMSO join aryl boronic acids to aryl chlorides under Buchwald biaryl phosphines such as XPhos and SPhos, and the same medium supports Heck, Sonogashira, and C–N Buchwald–Hartwig aminations used across oncology, cardiovascular, and anti-infective API families.
DMSO dissolves the inorganic base (K₂CO₃, Cs₂CO₃, or K₃PO₄) and the boronic acid together, which lets a single-phase reaction reach the 120–150 °C needed to activate aryl chlorides — the cheap, abundant halide that most couplings cannot turn over in colder solvents. Buchwald biaryl phosphines such as XPhos and SPhos are used where electron-rich, bulky ligands are needed to activate these aryl chlorides. The base is selected to balance solubility with the requirement to avoid competitive reduction of the aryl halide. In Suzuki couplings DMSO dissolves both the boronic acid and the inorganic base, and in Heck reactions its boiling point allows couplings of less reactive substrates that would reflux too cold in dioxane.
Coupling Classes in Production
Each coupling class is chosen for a different bond in the target molecule, and DMSO supports all of them at the temperatures where catalyst turnover becomes limiting. The table below maps the coupling to the API family it serves.
| Coupling | API / industry class | DMSO role |
|---|---|---|
| Suzuki | Biaryl oncology / anti-inflammatory APIs | Aryl chloride + boronic acid |
| Buchwald–Hartwig | Aniline / amine-containing drugs | C–N bond formation |
| Heck / Sonogashira | Alkene / alkyne-linked APIs | High-temperature C–C coupling |
Table 1: Representative Pd-catalyzed couplings run in DMSO
Why DMSO Rather Than DMF or Dioxane
DMSO boils at 189 °C and stays thermally stable above 140 °C, whereas DMF begins to decompose near 120 °C, releasing dimethylamine that coordinates palladium, competes with the designed phosphine, and suppresses turnover. Couplings that stall in hot DMF typically run to completion in DMSO at the same temperature because the catalyst keeps its ligand sphere.
Dioxane cannot reach the temperatures required for stubborn aryl chlorides, and toluene is a poor solvent for the inorganic base. NMP, the other common amide, carries REACH reproductive-toxin restrictions. The trade-off is that DMSO is not removed by simple evaporation, so isolation by aqueous dilution and extraction is accepted in exchange for reliable, production-compatible conversion.
References
- Miyaura, N.; Suzuki, A. Chem. Rev., 1995, 95(7), 2457–2483.
- Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed., 1998, 37(24), 3387–3388.