Strategies for the synthesis of chiral, bidentate bis(perfluoroorganyl)phosphane ligands

Abstract

Chiral, bidentate phosphane ligands, so-called PP ligands, are most frequently synthesized by reacting chiral ditosylates with diarylphosphanide ions. To use the P(CF 3) 2 − ion in nucleophilic substitution reactions, it is necessary to reduce the negative hyperconjugation, which is associated with a C–F activation. For this reason we synthesized different bis(trifluoromethyl)phosphanido complexes of mercury, silver and tungsten and investigated their use in nucleophilic substitution reactions. The most reactive compound, resulting from this study so far, is the pentacarbonylbis(trifluoromethyl)phosphanidotungstenate, [W{P(CF 3) 2}(CO) 5] −, which exhibits nearly the same bonding situation for the P(CF 3) 2 unit as in the free P(CF 3) 2 − ion. For use in synthesis of bis(trifluoromethyl)phosphane derivatives, Lewis acids are desirable, which stabilize the P(CF 3) 2 − ion by an intermediary formation of a donor acceptor adduct and can be split off after the synthesis of bis(trifluoromethyl)phosphane derivatives, as could be achieved using the extremely weak Lewis acids, CS 2 and acetone. These results could be established in the synthesis of the first example of an chiral, bidentate bis(trifluoromethyl)phosphane derivative. To synthesize a chiral, bidentate bis(pentafluorophenyl)phosphane derivative, a different synthetic strategy is necessary that does not involve the P(C 6F 5) 2 − ion, which decomposes even at low temperature. The implementation of functional P(CN) 2 − ions leads to the synthesis of functional, chiral bidentate dicyanophosphane derivatives which finally can be transformed into the corresponding bis(pentafluorophenyl)phosphane derivatives.