| Summary: | The studies in the present dissertation were aimed at developing stereoselective and regioselective organic transformations mediated by squaramide catalysis and palladium catalysis.
Squaramides have shown great potential as an emerging class of hydrogen bonding catalysts owing to their unique structural features. In Chapter I, the recent development in squaramide-catalyzed enantioselective reactions was reviewed. The examples presented were categorized based on the types of electrophiles, and demonstrated the capability of squaramides as effective hydrogen bonding catalysts for electrophilic activation.
The phospha-Michael addition of diphenyl phosphite to nitroalkenes provides facile access to β-nitro phosphonates, precursors to biologically active β-amino phosphonic acids. In Chapter II, squaramide-catalyzed enantioselective Michael addition of diphenyl phosphite to nitroalkenes was developed. A series of cyclohexane-1,2-diamine-derived squaramide catalysts were synthesized. A readily prepared piperidinyl-substituted chiral squaramide was identified as an effective catalyst for the phospha-Michael addition. High yields and excellent enantioselectivities were obtained for both aryl- and alkyl-substituted nitroalkenes, including those bearing acidic protons or sterically demanding substituents.
In the past few decades, advances in palladium-catalyzed reactions of indole have provided useful methods for the functionalization of this privileged heterocycle. In Chapter III, a general method for palladium-catalyzed regioselective C3-benzylation of indoles was developed. The studies showed that the choices of DPEphos as a supporting ligand and of triethylborane as an additive were important. This transformation provides facile access to a wide range of 3-benzylindolenines from various 1H-indoles and benzyl methylcarbonates under mild reaction conditions.
In Chapter IV, palladium-catalyzed enantioselective C3-allylation of indoles was developed. Two modes of enantioinduction were identified. In the first case, a bidentate chiral phosphine ligand (Fu-MeO-Biphep) was utilized. Interestingly, a high palladium to ligand ratio was required to achieve optimal reaction rate and enantioselectivity. In the second case, the enantioselectivity was induced using a combination of an achiral monodentate phosphine ligand and a chiral phosphate ligand. A variety of 3-allylindolenines were prepared in modest to good enantioselectivities under the first set of conditions.
In Chapter V, the preparation and characterization of new compounds were provided. Copies of NMR spectra were included as an appendix.
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