Regioselective synthesis of 1,2- and 1,3-diaminothiacalix[4]arenes via nucleophilic aromatic substitution and their X-ray structures

Thiacalix[4]arenes bearing two amino groups instead of two hydroxy groups of p-tert-butylthiacalix[4]arene (2) at the distal 1,3- (5b) and proximal 1,2-positions (5c) are regioselectively synthesized via the nucleophilic aromatic substitution (S_NAr) reaction of lithium benzylamide with tetra-O-methylsulfinylcalix[4]arenes 4(rctt) and 4(rcct), respectively. The S_NAr reaction selectively proceeds at the benzene rings between two neighboring sulfinyl groups of a trans conformation. This is rationalized by the cooperative coordination of a methoxy group with a neighboring sulfinyl oxygen atom toward the metal ion to form a six-membered chelate, through which the amide ion attacks the aromatic nucleus. The N,O-hybrid thiacalixarenes 5b and 5c adopt cone conformations in their acetonitrile inclusion crystals. The distances between adjacent N and/or O atoms of their narrow-rim substituents indicate that the four substituents form a circular hydrogen-bond network but it is weaker than that of original thiacalixarene 2. Compound 5c also adopts a cone conformation in the guest-free crystal. This, combined with the fact that tetraaminated p-tert-butylthiacalix[4]arene 5a adopts a 1,3-alternate conformation in its guest-free crystal, indicates that the intramolecular O⋯N hydrogen bonds, not to mention the O⋯O hydrogen bond, in compound 5c play a crucial role in maintaining a cone conformation in the crystal.