1,3-Dipolar Cycloaddition of Electron-rich Alkynes and Optically Active Allenes in Asymmetric Catalysis
This dissertation includes two parts. The first part focuses on two 1,3-dipolar cycloadditions of electron-rich alkynes. Chapter one describes a copper-promoted cycloaddition reaction of acetylides with diazocarbonyl compounds. This novel cycloaddition offers a direct and efficient approach to the synthesis of pyrazoles. The method is a rare example of an inverse-electron-demand cycloaddition, it represents a conceptually novel approach to this important class of heterocycles. Chapter two investigates a cycloaddition reaction between donor-acceptor cyclopropanes and silyl ynol ethers. Lewis acid promoted ring-opening of donor-acceptor cyclopropanes generates a 1,3-zwitterion; cycloaddition with a silyl ynol ether leads to a general synthesis of cyclopentenones. Substitution is tolerated on the ynol and on all positions of the cyclopropane to give tri-, tetra-, and penta-substituted cyclopentenones in high yield. methyl(methoxyl)aluminumchloride, which is generated from dimethylaluminumchloride by oxidation, appears strong reactivity towards ring-opening of donor-acceptorcyclopropanes and cycloaddition with silyl ynol ethers. This infrequently used species might be extended to other classes of cycloadditions or Lewis acid promoted reactions. The second part describes a new asymmetric catalysis design using optically active allenes as backbone and the application in an asymmetric meso-epoxides opening reaction. Allenes are inherently chiral and can be prepared in optically pure form. They have not been incorporated into ligands or catalysts for asymmetric reactions. Since allenes project functionality differently than either tetrahedral carbon or chiral biaryls, they may create complementary chiral environments. Chapter three demonstrates that optically active, C2-symmetric allene-containing bisphosphine oxides can catalyze the addition of silicon tetrachloride to meso-epoxides with high enantioselectivity. The fact that high asymmetric induction is observed suggests that allenes may represent a new platform for the development other classes of organic catalysts or ligands for asymmetris reactions.