New Methods For Highly Efficient Synthesis Of Allenes From Terminal Alkynes

Allenes are a class of compounds with a1,2-diene functionality, of which two π-orbitals are perpendicular to each other. With the recent development in this area, allenes are playing more and more important roles in organic synthesis. Hence, it is highly desirable to develop simple and efficient methods for the synthesis of all types of allenes.Our group has developed various reactions of allenes. For the future of allene chemistry, new methods for the efficient synthesis of allenes are highly needed. In this dissertation, I have focused on developing simple and efficient methods for the synthesis of allenes from terminal alkynes with paraformaldehyde, aldehydes and ketones.Part I:CuI-mediated synthesis of monosubstituted allenes. Crabbe et al. developed a set of conditions for the synthesis of terminal allenes from terminal alkynes, paraformaldehyde, and diisopropylamine in1979, however, yields are not yet satisfactory for most substrates. We made some modifications over their original conditions. By using CuI instead of CuBr and dicyclohexylamine instead of diisopropylamine, yields of allenes have been improved obviously. Many functional groups such as mesylate, hydroxyl group, ether, amide, etc. may be tolerated during the reaction.Part II:ZnI2-mediated synthesis of1,3-disubstituted allenes. Upon mediation with ZnI2, terminal alkynes can react with aldehydes and morpholine to afford various of1,3-disubstituted allenes. Both aromatic and aliphatic aldehydes may be used.Part III:Catalytic synthesis of1,3-disubstituted allenes. By applying10-20mol%CuI and a dialkylamine as the base, we have developed a catalytic one-step synthesis of functionalized1,3-disubstituted allenes from readily available functionalized terminal alkynes and aliphatic aldehydes.Part IV:CuBr and ZnI2co-promoted synthesis of trisubstituted allenes. Based on Ye's contribution, under the catalysis of CuBr, the reaction of terminal alkynes, ketones, and pyrrolidine can afford corresponding propargylic amine, and then the resulting mixture was filtered to get rid of CuBr. After evaporation, the crude product was transformed into trisubstituted allenes under the mediation of ZnI2. Several types of trisubstituted allenes could be obtained through this protocol.