Reaction of nitrosonium cation with resorc[4]arenes activated by supramolecular control: Covalent bond formation

Resorc[4]arenes 1 and 2, which previously proved to entrap NO+ cation within their cavities under conditions of host-to-guest excess, were treated with a 10-fold excess of NOBF4 salt in chloroform. Kinetic and spectral UV-visible analyses revealed the formation of isomeric 1:2 complexes as a direct evolution of the previously observed event. Accordingly, three-body 1-(NO+)2 and 2-(NO+)2 adducts were built by MM and fully optimized by DFT calculations at the B3LYP/ 6-31G(d) level of theory. Notably, covalent nitration products 4, 5 and 6, 7 were obtained by reaction of NOBF4 salt with host 1 and 2, respectively, involving macrocycle ring-opening and insertion of a nitro group in one of the four aromatic rings. In particular, compounds 4 and 6, both containing a trans-double bond in the place of the methine bridge, were oxidized to aldehydes 5 and 7, respectively, after addition of water to the reaction mixture. Calculation of the charge and frontier orbitals of the aromatic donor (HOMO) and the NO+ acceptor (LUMO) clearly suggests an ipso electrophilic attack by a first NO+ unit on the resorcinol ring, mediated by the second NO+ unit.