Activation of Molecular Hydrogen by a Singlet Carbene through Quantum Mechanical Tunneling

Carbenes are among the few metal‐free molecules that are able to activate molecular hydrogen. Whereas triplet carbenes have been shown to insert into H₂ through a two‐step mechanism that at low temperature is assisted by quantum mechanical tunneling (QMT), singlet carbenes insert in concerted reactions with considerable activation barriers, and are thus unreactive towards H₂ at cryogenic temperatures. Here we show that 1‐azulenylcarbene with a singlet ground state readily inserts into H₂, and that QMT governs the insertion into both H₂ and D₂. This is the first example that shows that QMT can also be important for singlet carbenes inserting into dihydrogen.