Oxidative addition of palladium(0) complexes generated from [Pd(dba)(2)] and P-N ligands: A kinetic investigation

The major complex formed in solution from ([Pd-0(dba)(2)]+1P-N} mixtures is [Pd-0(dba)(P-N)] (dba = trans,trans-dibenzylideneacetone; P - N = PhPN, 1- dimethylamino-2-diphenylphosphinobenzene: FcPN, N,N-dimethyl-1-[2-(diphenyl phosphino)ferrocenyl]- methylamine; OxaPN, 4,4'-dimethyl-2-(2-diphenylphosp hinophenyl)-1,3-oxa-zoline). Each complex consists of a mixture of isomers involved in equilibria: two 16-electron rotamer complexes [Pd-0(eta(2)-dba) (eta(2)-P-N)] and one 14-electron complex [Pd-0(eta(2)-dba)(eta(1)-P-N)] ob served for FcPN and OxaPN. [Pd-0(dba)(PhPN)] and [SPd0(PhPN)] (S = solvent) react with PhI in an oxidative addition; [SPd0(PhPN)] is intrinsically mor e reactive than [Pd-0(dba)(PhPN)]. This behavior is similar to that of the bidentate bis-phosphane ligands. When the PhPN ligand is present in excess, it behaves as a monodentate phosphane ligand, since [Pd-0(eta(2)- dba)(eta (1)-PhPN)(2)] is formed first by preferential cleavage of the Pd-N bond ins tead of the rd-olefin bond. [Pd-0(eta(1)- PhPN),I is also eventually formed . [Pd-0(dba)(FcPN)] and [Pd-0(dba)(OxaPN)] are formed whatever the excess o f ligand used. [SPd0(FcPN)] and [SPd0(OxaPN)] are not involved in the oxida tive addition. The 16-electron complexes [Pd-0(eta(2)-dba)(eta(2)-FcPN)] an d [Pd-0(eta(2)-dba)(eta(2)-OxaPN)] are found to react with Phl via a 14-ele ctron complex as has been established for [Pd-0(eta(2)-dba)(eta(1)-OxaPN)]. Once again, the cleavage of the Pd-N bond is favored over that of Pd-olefi n bond, This work demonstrates the higher affinity for [Pd-0(P-N)] of dba c ompared with the P-N ligand, and emphasizes once more the important role of dba, which either controls the concentration of the most reactive complex, [SPd0(PhPN)], or is present in the reactive complexes, [Pd-0(dba)(FcPN)] o r [Pd-0(dba)-(OxaPN)], and thus contributes to their intrinsic reactivity.