Spin-Forbidden Addition of Molecular Oxygen to Stable Enol Intermediates-Decarboxylation of 2-Methyl-1-tetralone-2-carboxylic Acid

The deprotonation of an organic substrate is a common preactivation step for the enzymatic cofactorless addition of O-2 to this substrate, as it promotes charge-transfer between the two partners, inducing intersystem crossing between the triplet and singlet states involved in the process. Nevertheless, the spin-forbidden addition of O-2 to uncharged ligands has also been observed in the laboratory, and the detailed mechanism of how the system circumvents the spin-forbiddenness of the reaction is still unknown. One of these examples is the cofactorless peroxidation of 2-methyl-3,4-dihydro-1-naphthol, which will be studied computationally using single and multi-reference electronic structure calculations. Our results show that the preferred mechanism is that in which O-2 picks a proton from the substrate in the triplet state, and subsequently hops to the singlet state in which the product is stable. For this reaction, the formation of the radical pair is associated with a higher barrier than that associated with the intersystem crossing, even though the absence of the negative charge leads to relatively small values of the spin-orbit coupling.

Automated summary

The deprotonation of an organic substrate is common in the preactivation step of enzymatic cofactorless addition of O-2. However, the spin-forbidden addition of O-2 to uncharged ligands has been observed in the laboratory and the mechanism for circumventing the spin-forbiddenness is unknown. The computational study of the peroxidation of 2-methyl-3,4-dihydro-1-naphthol reveals that the preferred mechanism involves proton transfer in the triplet state followed by hopping to the singlet state.