Catalytic Methods to Convert Allylic Substrates through Hydride and Proton Shifts : Transition Metal-Catalyzed and Organocatalyzed Approaches

Abstract: The present thesis describes the development of new catalytic protocols to transform allylic substrates into a wide variety of versatile carbonyl and vinyl organic compounds. All procedures that are described in this work have in common the existence of one or more hydrogen shifts as key steps in the mechanism of the reactions. The thesis is divided into two mayor sections depending on the strategy employed, metal catalysis or organocatalysis. The introductory chapter (Chapter 1) starts with an overview of the different types of catalysis and the importance of allylic substrates in organic chemistry. The chapter continues with an extensive description of the isomerization of allylic alcohols and finishes with a short introduction about hypervalent iodine chemistry. The goals of the thesis are also depicted at the end of this chapter.Chapters 2, 3 and 4 embody the use of iridium catalysis as an effective tool to synthesize α-functionalized carbonyl compounds selectively as single constitutional isomers from allylic alcohols. The first two chapters of this section describe the employment of several electrophiles to trap enolate derivatives formed from the corresponding allylic alcohols. Chapter 2 shows the development of two new protocols for the preparation of challenging α-iodinated carbonyl compounds. In chapter 3, the synthesis of α-aminooxy and α-hydroxyketones is investigated by employing an N-oxoammonium salt as electrophilic agent. Chapter 4 describes the development of an umpolung strategy that allows the synthesis of α-functionalized carbonyls through the reaction of two formal nucleophiles: enolate derivatives and alcohols. Mechanistic investigations performed in this section point to the presence of an iridium-catalyzed hydride shift operating in the reaction pathways.The last three chapters (5, 6 and 7) describe the development of metal-free methods for the conversion of allylic substrates into valuable products by means of base catalysis. Chapter 5 and 6 depict the stereospecific isomerization of a large scope of allylic alcohols, ethers and halides. A simple guanidine-type base, TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene), is an effective catalyst to isomerize allylic substrates with excellent levels of transfer of chirality. The mechanism of this transformation is studied in detail experimentally and computationally and it is suggested to involve a [1,3]-proton shift through the formation of a tight ion-pair. Chapter 7 shows that base-catalysis allows the isomerization of conjugated polyenyl alcohols and ethers which has been proved to be challenging with metal–catalysis. Experimental and computational investigations in this last chapter suggests that the mechanism may proceed through a series of iterative [1,3]-proton shifts or “base-walk”.