Allyl Amination of Phosphinoquinoline Allyl Complexes of Palladium. Influence of the Allyl Hapticity on the Reaction Rate and Regiochemistry

The ligands 8-diphenylphosphanylquinoline (DPPQ) and 8-diphenylphosphanyl-2-methylquinoline (DPPQ-Me) react in chlorinated solvents with the allyl dimers [Pd(mu-Cl)(eta(3)-C3H5)](2) and [Pd(mu-Cl)(eta(3)-C3H3Me2)](2), yielding palladium allyl phosphanylquinoline complexes whose structure is strongly influenced by the ancillary ligand and the presence or the absence of chloride ion in solution. Thus, the allyl fragments in the DPPQ derivatives assume eta(3)-hapticity in the absence of chloride and display monohapto coordination when the chloride is not removed from the reaction mixture. In the DPPQ-Me allyl derivatives the allyl fragment is always eta(3)-coordinated, while the ancillary ligand may act as bis- or monochelating in the absence or in the presence of chloride, respectively. The reactivity of the allyl complexes was tested by means of the allyl-amination reaction carried out by either NMR or UV-vis techniques. It was noticed that the eta(3)-derivatives generally display a higher reactivity than their eta(1)-allyl analogues, under the same experimental conditions. It is apparent that chloride in chlorinated solvents is a quite good nucleophile, and therefore it may force the allyl moiety to assume the monohapto coordination or partially displace the hemilabile bis-chelate DPPQ-Me ligand. This experimental finding was also theoretically confirmed by means of an ab initio DFT computation. The crystal structures of the complexes [Pd(eta(3)-C3H5)(DPPQ)]ClO4 and [Pd(eta(1)-C3H5)(DPPQ)Cl] were resolved. The latter represents the seventh structure of a palladium complex with a a-coordinated allyl fragment described in the literature.