1,3-dipolar cycloadditions of thionitroso compounds (R-N=S): a density functional theory study

The cycloaddition reactions of 1,3-dipoles, including fulminic acid (HCNO) and the simple azides (XNNN, X = H, CH3, NH2), with thionitroso compounds ( R-N=S, R=H, NH2) as dipolarophiles have been studied by density function pr ocedures using the hybrid B3LYP exchange correlation function and 6-311G(d, p) basis functions. For each system, two (HCNO) or four (azide) distinct t ransition structures have been located and the intrinsic reaction coordinat e path was constructed. It appears in all cases that activation energies fo r the cycloadditions are rather. low, ranging from 0 to 30 kJ mol(-1). The additions are not Stereospecific, the energy difference between transition structures having cis and trans dispositions of the N-R groups being rather small (less than a few kJ mol(-1)). The addition shows a certain regiosele ctivity in which the attack of the O-atom of HCNO and N(R) of the azides to the S-atom of the dipolarophile is preferred. This regiochemistry is expla ined using recently developed DFT-based reactivity descriptors, such as loc al softness and Fukui functions. The hardness and polarizability profiles o f the supersystem along the IRC paths were also discussed.