The reaction between tetrasulphur tetranitride (S4N4) and electron-deficient alkynes. A molecular orbital study

The regioselectivity of the cycloaddition of tetrasulphur tetranitride (1) to electron-deficient alkynes such as dicyanoacetylene (2) is shown to be largely controlled by interaction between the lowest unoccupied molecular orbital (LUMO) of (2) and either the occupied 3a2 orbital on (1), favouring [2,4]-addition across nitrogen, or the occupied 4b1 orbital on (1), favouring [1,5]-addition across sulphur. The latter regiochemistry in particualr is less readily rationalised by the alternative overlap between the highest occupied orbital (HOMO) of the alkyne and the degenerate LUMO on (1). In an alkyne such as (2), the LUMO is polarised towards the electron-withdrawing groups, increasing the overlap with the 4b1 orbital on (1) and enhancing the selectivity for [1,5]-addition across sulphur. These conclusions are consistent with experimental observations and with the most recent calculations on (1), which predict the lowest cationic state to correspond to ionisation from the approximately degenerate 3a2′ 4b1′ and 8b2 orbitals.