Homologations via Carbene‐Mediated Rearrangement Reactions

Abstract

This book chapter will provide a detailed discussion about homologation reaction by [1,2]-sigmatropic rearrangement of onium ylides via carbene transfer reactions. This chapter will be divided into four subsections. (1) 1,2-Sigmatropic Rearrangement of Onium Ylides Sigmatropic rearrangement reactions of ylides are an important reaction of carbenes, and this reaction is today one of the most important transformations. In this chapter we will discuss the advances that have been made using metal-catalyzed, bio engineered, and photochemical carbene transfer reaction for the 1,2-sigmatropic rearrangement reaction of onium ylides (oxonium, sulfonium, selenium, ammonium, iodonium, etc.). (2) Homologation Reaction by X–X and X–Y Insertion into Diazoalkane Carbene insertion reaction is an important fundamental interest in organic synthesis for the formation of CC and CX/Y bond. Widely used carbene precursor such as diazoalkanes, N -sulfonyl-1,2,3-triazoles, and tosylhydrazones can be easily inserted into the CH; XH, YY, and YZ bond under metal-catalyzed, thermal, and photochemical reaction conditions where X is N, Si, O, S, Se, P, or halogen and Y and Z are B, S, Se, etc. The key step for these insertion reactions is [1,2]-sigmatropic rearrangement or [1,2]-proton shift. In this chapter we will discuss the homologation reaction by carbene insertion reactions via [1,2]-sigmatropic rearrangement or [1,2]-proton shift. (3) Ring Expansion Reaction The application of [1,2]-sigmatropic rearrangement on the ring expansion reactions will be discussed in detail in this chapter. The detailed DFT calculations for the mechanism study of the ring expansion reactions of oxygen and sulfur heterocycles have been studied by Koenigs and Xu group, and they have uncovered the reaction pathway proceeding via a diradical pathway under photochemical carbene transfer reaction conditions, which is really important for understanding free ylide mechanism, and this will be discussed in this chapter.