Silica Gel as a Promoter of Sequential Aza-Michael/Michael Reactions of Amines and Propiolic Esters: Solvent- and Metal-Free Synthesis of Polyfunctionalized Conjugated Dienes

We present an efficient, simple, metal- and solvent-free silica-gel-promoted synthesis of functionalized conjugated dienes by sequential aza-Michael/Michael reactions by starting from commercially available primary amines and propiolic esters. The scope and usefulness of the method is demonstrated for 31examples, including a range of propiolic esters, aliphatic amines, and differently substituted aromatic amines. For aliphatic amines, the products were obtained within 0.5 to 4h in 52 to 85% yield, compared with 3.5 to 22h under classical solution-phase synthesis, which proceeds with similar or lower yields. The method was found to be particularly useful for weakly nucleophilic aromatic amines, which provided products in 21 to 73% yield over 2.5 to 9.5h compared with yields of 0 to 49% over 1 to 6d under standard solution-phase conditions, and for more hydrophobic esters that gave products in yields of 47 to 79% over 1 to 3h compared with 0 to 45% over 4 to 114h in solvent.