Ligand substitution from the (eta5-DMP)Mn(CO)2(Solv) [DMP = 2,5-dimethylpyrrole, Solv = solvent] complexes: to ring slip or not to ring slip?

The mechanism and energetics of the displacement of solvent from photolytically generated (eta(5)-DMP)Mn(CO)(2)(Solv) complexes has been studied [DMP = 2,5-dimethylpyrrole, Solv = solvent]. Rate enhancement relative to the eta(5)-cyclopentadienyl (Cp) system is not observed in the displacement of weakly bound solvents. The bond dissociation enthalpies obtained from the kinetic analysis are in good agreement with the values obtained by detailed density functional theory (DFT) calculations. The results indicate that for both the Cp and the DMP based systems the displacement of weakly bound solvents proceeds by a dissociative or I(d) mechanism. This is in sharp contrast to CO displacement from (eta(5)-DMP)Mn(CO)(3), which is known to proceed by an associative mechanism by way of an eta(3) ring slip intermediate. The associative substitution pathway only becomes competitive with the dissociative channel when the Mn-Solv bond dissociation enthalpy is more than 33 kcal/mol.

Solid-phase parallel synthesis of phosphite ligands.

Various routes for the synthesis of polymer-bound phosphites and phosphoramidites have been investigated. In the presence of a suitable activator the supported phosphoramidites react cleanly with alcohols to give the corresponding monodentate phosphite ligands in solution. We have applied this novel solid-phase route in the parallel synthesis of several monodentate chiral and achiral phosphite ligands.

Alcoholysis of acylpalladium(II) complexes relevant to the alternating copolymerization of ethene and carbon monoxide and the alkoxycarbonylation of alkenes: the importance of Cis-coordinating phosphines.

The mechanism and kinetics of the solvolysis of complexes of the type [(L-L)Pd(C(O)CH(3))(S)](+)[CF(3)SO(3)](-) (L-L = diphosphine ligand, S = solvent, CO, or donor atom in the ligand backbone) was studied by NMR and UV-vis spectroscopy with the use of the ligands a-j: SPANphos (a), dtbpf (b), Xantphos (c), dippf (d), DPEphos (e), dtbpx (f), dppf (g), dppp (h), calix-6-diphosphite (j). Acetyl palladium complexes containing trans-coordinating ligands that resist cis coordination (SPANphos, dtbpf) showed no methanolysis. Trans complexes that can undergo isomerization to the cis analogue (Xantphos, dippf, DPEphos) showed methanolyis of the acyl group at a moderate rate. The reaction of [trans-(DPEphos)Pd(C(O)CH(3))](+)[CF(3)SO(3)](-) (2e) with methanol shows a large negative entropy of activation. Cis complexes underwent competing decarbonylation and methanolysis with the exception of 2j, [cis-(calix-diphosphite)Pd(C(O)CH(3))(CD(3)OD)](+)[CF(3)SO(3)](-). The calix-6-diphosphite complex showed a large positive entropy of activation. It is concluded that ester elimination from acylpalladium complexes with alcohols requires cis geometry of the acyl group and coordinating alcohol. The reductive elimination of methyl acetate is described as a migratory elimination or a 1,2-shift of the alkoxy group from palladium to the acyl carbon atom. Cis complexes with bulky ligands such as dtbpx undergo an extremely fast methanolysis. An increasing steric bulk of the ligand favors the formation of methyl propanoate relative to the insertion of ethene leading to formation of oligomers or polymers in the catalytic reaction of ethene, carbon monoxide, and methanol.