Uncharacteristic thione behavior in a Huisgen cycloaddition reaction: a kinetic and theoretical study

In the reaction of phthalazinium-2-dicyanomethanide with adamantanethione the rare ring system [1,3]thiazolo[2,3-a]phthalazine was obtained. An X-ray crystal structure of the product shows regioselectivity with the thione carbon bonded to the dicyanomethanide terminus of the 1,3-dipole. UV kinetic measurements and DFT calculations showed that the rate of cycloaddition of adamantanethione was significantly slower than that of DMAD and no super-dipolarophile character was exhibited. This arose from exceptional lowering of the HOMO energy of the 1,3-dipole by the cyano substituents.