What factors cause the complete substrate-controlled selectivity in Rh2(Piv)4-catalyzed cycloadditions of 1,2,3-triazoles with isocyanates or isothiocyanates

• Cycloaddition mechanisms of 1,2,3-triazoles with isocyanates/isothiocyanates were studied.
• The nucleophilic trend (CS > CN > CO) is the main selective origin of these reactions.
• The presence of Rh2(Piv)4 is another selective origin via stabilizing azavinyl carbene.

DFT studies on Rh2(Piv)4-catalyzed cycloadditions of 1,2,3-triazoles with isocyanates or isothiocyanates show that 1, 2, 3-triazoles undergo ring opening → coordination of Rh2(Piv)4 → dissociation of N2 to give Rh2(Piv)4-azavinyl carbene, where Rh2(Piv)4 exerts a stabilizing effect on azavinyl carbene. The formed Rh2(Piv)4-azavinyl carbene prefers to nucleophilically cyclize with CN bond of isocyanatobenzene to give the imidazolone, rather than CO bond of isocyanatobenzene to give the oxazolimine. In comparison, Rh2(Piv)4-azavinyl carbene preferentially cyclize with CS bond of isothiocyanatobenzene to give the thiazole, instead of CN bond of isothiocyanatobenzene to give the thioimidazolone. The nucleophilic capacity follows CS > CN > CO trend, which is the key selective origin of these reactions. The presence of Rh2(Piv)4 would help to form the carbene and to stabilize it via donating its d-electron into the azavinyl carbene, which is another selective origin of these reactions.

Rh2(Piv)4-azavinyl carbene nucleophilically cyclizes with isocyanatobenzene to give imidazolone, while with isothiocyanatobenzene to give thiazole. The nucleophilic capacity follows CS > CN > CO trend, which is main selective origin of these reactions. The presence of Rh2(Piv)4 help to form the carbene and to stabilize it, which is another selective origin.Figure optionsDownload as PowerPoint slide