Carbene transfer from thianthrenium ylides for cyclopropanation

Carbenes are reactive divalent carbon intermediates that serve as one-carbon synthons to access cyclopropanes. The synthetic utility of these three-membered carbocycles has led chemists to accept the dangers associated with the explosive starting materials¹ or intermediates² required for both conventional and modern cyclopropanation reactions. Sulfonium salts, although safer, have not been competitive for cyclopropanation because they typically do not function as efficient carbene donors³. In all cyclopropanation reactions reported, diversity can be obtained through either the olefin or carbene partner, but not both; for example, several modern reactions are limited to activated olefins, such as styrenes^4-7. Here we report how alkylthianthrenium salts differ conceptually from all other carbene precursors and allow for carbene-transfer chemistry, including the synthesis of cyclopropanes, in which both the olefin and the carbene partner can be diverse. The steric bulk and low Lewis basicity of thianthrene can rationalize the superior reactivity of the thianthrenium salts for cyclopropanation because undesired energetically low-lying local minima on the potential energy surface, as present for other sulfonium salts, are avoided. Despite high reactivity, thianthrenium salts exhibit a desirable safety profile that allows scale up, also in the solid state through ball-milling, which is dangerous with many other cyclopropanation reactions. The combination of reactivity and safety highlights thianthrenium ylides as a general compound class for metal-carbene reactivity that extends beyond cyclopropanation to diverse carbene-transfer reactions, including σ-bond insertion and sigmatropic rearrangements.