material over the betaine. Stability of the ylide with higher stability similarly leading to greater reversibility. Steric hindrance in the betaine with greater hindrance leading to greater reversibility by disfavoring formation of the intermediate and slowing the rate-limiting rotation of the central bond. Solvation of charges in the betaine by counterions such as lithium with greater solvation allowing more facile rotation in the betaine intermediate, lowering the amount of reversibility. Scope The application of the Johnson–Corey–Chaykovsky reaction in organic synthesis is diverse. The reaction has come to encompass reactions of many types of sulfur ylides with electrophiles well beyond the original publications. It has seen use in a number of high-profile total syntheses, as detailed below, and is generally recognized as a powerful transformative tool in the organic repertoire. Types of ylides Many types of ylides can be prepared with various functional groups both on the anionic carbon center and on the sulfur. The substitution pattern can influence the ease of preparation for the reagents (typically from the sulfonium halide, e.g. trimethylsulfonium iodide) and overall reaction rate in various ways. The general format for the reagent is shown on the right. Use of a sulfoxonium allows more facile preparation of the reagent using weaker bases as compared to sulfonium ylides. (The difference being that a sulfoxonium contains a doubly bonded oxygen whereas the sulfonium does not.) The former react slower due to their increased stability. In addition, the dialkylsulfoxide by-products of sulfoxonium reagents are greatly preferred to the significantly more toxic, volatile, and odorous dialkylsulfide by-products from sulfonium reagents. The vast majority of reagents are monosubstituted at the ylide carbon (either R1 or R2 as hydrogen). Disubstituted reagents are much rarer but have been described: If the ylide carbon is substituted with an electron-withdrawing group (EWG), the reagent is referred to as a stabilized ylide. These, similarly to sulfoxonium reagents, react much slower and are typically easier to prepare. These are limited in their usefulness as the reaction can become prohibitively sluggish: examples involving amides are widespread, with many fewer involving esters and virtually no examples involving other EWG's. For these, the related Darzens reaction is typically more appropriate. If the ylide carbon is substituted with an aryl or allyl group, the reagent is referred to as a semi-stabilized ylide. These have been developed extensively, second only to the classical methylene reagents (R1=R2=H). The substitution pattern on aryl reagents can