Synthesis and properties of TRANSDIP, a rigid chelator built upon a cyclodextrin cavity: Is TRANSDIP an authentic trans-spanning ligand?

The C(2)-symmetrical diphosphane TRANSDIP was obtained in high yield by treating 6(A),6(B),6(D),6(E)-tetra-mesylated, permethylated alpha-cyclodextrin with PPhLi(2) in excess. The double cascade cyclisation thus produced is re-gioselective as phosphinidene capping involves only adjacent glucose units. It is also stereospecific, as both lone pairs on the phosphorus atoms are orientated towards the cyclodextrin axis. The restricted flexibility of the phosphorus atoms, which are part of nine-membered heterocyclic rings, is responsible for J(P.C) spin-spin couplings with the eight-bond distant CH(2)OMe carbon atoms of glucose units C and E The treatment of TRANSDIP with Group 10 metal dihalides quantitatively gave square-planar chelate complexes, in which a M-X bond points towards the centre of the cavitand. The favourable P...P separation and the directional control of the lone pairs on the phosphorus atoms rule out the possibility of forming binuclear complexes or higher oligomers. Further, in all the complexes, the phosphorus atoms are in a trans arrangement. TRANSDIP may therefore be regarded as an authentic trans-spanning diphosphane. In the complex [NiBr(2).TRANSDIP], the cavity provides effective protection of the encapsulated M-X bond towards nucleophilic attack by MeLi. The same complex, upon activation with methyl-aluminoxane, efficiently dimerises ethene and propene.