| Description |
An improved method for the [2+2+2] cycloaddition of diynes with CO2 to synthesize pyrones was developed using a Ni/NHC catalyst system. The new catalyst system boasts improved yields, lower reaction temperatures, and lower reaction pressures compared to the previously known methods. Furthermore, certain unsymmetrical diynes could be employed in the reaction to generate a single pyrone regioisomer. Extensive studies suggest that the regioselectivity arises from steric interactions between the NHC ligand and the substituents on the alkyne termini. In an effort to synthesize pyrans, attempts at cyclizing aldehydes and ketones with diynes and enynes were made. When aldehydes and diynes were employed, the expected pyran cycloadduct was not obtained. Instead, the [3,3] electrocyclic ring opened tautomer was obtained. The use of enynes with aldehydes prevented this tautomerization, but the synthesis of the desired pyran was highly substrate dependent due to competitive side product formation. The use of ketones and enynes in the cycloaddition allowed for the chemoselective formation of dihydropyrans in excellent yields. However, it was necessary to carefully tune the reaction conditions to each particular substrate in order to achieve high yields. Nonetheless, this represents the first [2+2+2] cycloaddition of ketones and enynes. Due to the air sensitivity of the Ni(0) precursor and the carbene ligand, a method was devised for generating the active catalyst in situ from air stable precursors. While this method worked very well for the cyclization of diynes and nitriles, the coupling of diynes with CO2, aldehydes, and isocyanates was less successful. The Ni/NHC cycloisomerization of enynes to 1,3-dienes was also discovered. While the yields of this reaction were excellent, the substrate scope was limited. Attempts at improving the system were initiated though an in-depth study reaction mechanism. Deuterium labeling experiments suggest that the active catalyst is formed by an unprecedented C-H activation of the carbene ligand by the Ni(0). |
