Stille Reactions with Tetraalkylstannanes and Phenyltrialkylstannanes in Low Melting Sugar‐Urea‐Salt Mixtures
Tóm tắt
The transfer of simple alkyl groups in Stille reactions usually requires special solvents (HMPA) or certain organotin reagents (stannatranes, monoorganotin halides) to be efficient. Using low‐melting mixtures of sugar, urea and inorganic salt as solvent, a fast and efficient palladium‐catalyzed alkyl transfer with tetraalkyltin reagents was observed. The high polarity and nucleophilic character of the solvent melt promotes the reaction. Stille biaryl synthesis using electron‐poor and electron‐rich aryl bromides proceeds with quantitative yields in the sugar‐urea‐salt melt. Catalyst loading may be reduced to 0.001 mol % and the catalyst melt mixture remains active in several reaction cycles. Showing the same or improved performance for Stille reactions than organic solvents and allowing a very simple work up, sugar‐urea‐salt melts are a non‐toxic and cheap alternative reaction medium available in bulk quantities for the catalytic process.
Từ khóa
Tài liệu tham khảo
Farina V., 1997, Org. React., 50, 1
2004, Metal‐Catalyzed Cross‐Coupling Reactions
For a recent examples see:
Nicolaou K. C., 1999, Angew. Chem. Int. Ed., 38
Kotha S., 2002, Tetrahedron, 58
Littke A. F., 2002, Angew. Chem. Int. Ed., 41
Based on commercial prices for fine chemicals which do not fully reflect bulk material cost the cost of the sorbitol melt entry 2 in Table 1 is estimated to be approx. € 18.00/kg. Prices for DMSO as a solvent with comparable properties strongly depend on solvent purity and are in the order of € 60.00/kg.
1989, The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals
Imperato G., 2006, Green Chem.
Melts containing dimethylurea are thermally more stable than urea melts which may produce ammonia at elevated temperatures.
No flash points are available for the components of the melt. Only urea and dimethylurea have a significant vapor pressure and sublime in vacuum.