Coordination chemistry of cyclopropenylidene-stabilized phosphenium cations: synthesis and reactivity of Pd and Pt complexes.
A straightforward synthesis of cyclopropenylidene-stabilized phosphenium cations 1 a-g through the reaction of [(iPr2N)2C3(+)Cl]BF4 with secondary phosphines is described. Their donor ability was evaluated by analysis of the CO stretching frequency in Rh complexes [RhCl(CO)L2](BF4)2 and electrochemical methods. The cyclopropenium ring induces a phosphite-type behavior that can be tuned by the other two substituents attached to the phosphorus atom. Despite of the positive charge that they bear, phosphenium cations 1 a-g still act as two-electron donor ligands, forming adducts with Pd(II) and Pt(II) precursors. Conversely, in the presence of Pd(0) species, an oxidative insertion of the Pd atom into the Ccarbene-phosphorus bond takes place, providing dimeric structures in which each Pd atom is bonded to a cyclopropenyl carbene while two dialkyl/diaryl phosphide ligands serve as bridges between the two Pd centers. The catalytic performance of the resulting library of Pt(II) complexes was tested; all of the cationic phosphines accelerated the prototype 6-endo-dig cyclization of 2-ethynyl-1,1'-biphenyl to afford pentahelicene. The best ligand 1 g was used in the synthesis of two natural products, chrysotoxene and epimedoicarisoside A.
Max-Planck-Institut für Kohlenforschung, Kaiser Wilhelm Platz 1, 45470-Mülheim an der Ruhr (Germany), Fax: (+49) 208-306-2994.
Agnes
A
Kozma
Tobias
T
Deden
Javier
J
Carreras
Christian
C
Wille
Jekaterina
J
Petuškova
Jörg
J
Rust
Manuel
M
Alcarazo
cations; cycloisomerization; homogeneous catalysis; phosphines; platinum