Concerted addition of aldehydes to the singlet biradical [P(μ-NTer)] 2 .

The reaction of the singlet biradical [P(μ-NTer)] 2 with various aldehydes selectively yielded the corresponding [2.1.1]-bicyclic addition products in a very fast reaction. All products were fully characterized, including by NMR and vibrational spectroscopy as well as single-crystal X-ray diffraction. The mechanism of the addition was investigated theoretically using high-level ab initio methods (CCSD(T) with triple- and quadruple-zeta basis sets) and corresponds to a concerted cycloaddition reaction with a very low activation barrier. For comparison, the mechanisms of the literature-known cycloadditions of H 2 , alkenes, and alkynes were also studied, indicating a similar reaction profile for all unsaturated reactants.