Equilibrating parent aminomercaptocarbene and CO(2) with 2-amino-2-thioxoacetic acid via heavy-atom quantum tunneling

The search for methods to bind CO(2) and use it synthetically as a C(1)-building block under mild conditions is an ongoing endeavor of great urgency. The formation of heterocyclic carbene–carbon dioxide adducts occurs rapidly when the carbene is generated in solution in the presence of CO(2). Here we demonstrate the reversible formation of a complex of the hitherto unreported aminomercaptocarbene (H(2)N–C̈–SH) with CO(2) isolated in solid argon by photolysis of 2-amino-2-thioxoacetic acid. Remarkably, the complex disappears in the dark as deduced by time-dependent matrix infrared measurements, and equilibrates back to the covalently bound starting material. This kinetically excluded process below ca. 8 K is made possible through heavy-atom quantum mechanical tunneling, as also evident from density functional theory and ab initio computations at the CCSD(T)/cc-pVTZ level of theory. Our results provide insight into CO(2) activation using a carbene and emphasize the role of quantum mechanical tunneling in organic processes, even involving heavy atoms.