Theoretical ab Initio Study of 1,3-Dipolar Cycloaddition of Fulminic Acid to Acetylene. Support for Firestone's Mechanism

A two-step pathway passing through a highly unsymmetrical transition state and an extended diradical intermediate has been investigated by ab initio computations with the 4-31G basis set and including various levels of electron correlation. Unrestricted Hartree-Fock and restricted Hartree-Fock plus 3X3 configuration interaction calculations are inadequate for the description of the energy surface around the diradical transition structure and for comparison of the concerted and diradical paths. A minimum of 4a’ and 3a^- occupied orbitals must be included in the Cl treatmen. With a multireference iterative perturbation technique using 17 reference determinants and ca. 2.4 million configurations, the extended diradical transition state was found to be 3.6 kcal mol^-1 below the concerted, synchronous structure. While this energy difference is too small to favor one route unambiguously, the present calculations indicate that Firestone's diradical mechanism merits serious consideration, at least for the fulminic acid cycloaddition to acetylene and probably for other propargyl-allenyl type 1,3 dipoles.