Syndiotactic CO/Styrene Copolymerization Catalyzed by α-Diimine Pd(II) Complexes:  Regio- and Stereochemical Control

Abstract

Using the catalytic system [(Pr i DAB)Pd(Me)(NCMe)] + [{3,5-(CF 3 ) 2 C 6 H 3 } 4 B] -, where Pr i DAB ) 1,4-diisopropyl-1,4-diaza-1,3-butadiene, it has been possible to obtain copolymers of styrene or p-methylstyrene with carbon monoxide, which have alternating, head-to-tail, syndiotactic structure and a stereochemical purity of 92%. To investigate the reaction mechanism, the first steps of the copolymerization were studied by isolating and characterizing the reaction intermediates. The reactivity of the catalyst toward carbon monoxide was studied, and the corresponding methyl carbonyl Pd initiator complex was isolated. The regiospecific 2,1-insertion of p-methylstyrene in the Pd-acyl bond was proved by NOE measurements performed on the first intermediate, a five-membered palladacycle, in which the acetylic oxygen is coordinated to the palladium. Moreover, the intermediate resulting after the second sequence of CO and styrene insertion unexpectedly showed a doubling of the signals, both in the proton and carbon-13 NMR spectra. This finding, attributed to the presence of two diastereoisomeric complexes, showed that the chain-end stereochemical control is not fully effective during the initial steps of the copolymerization. Last, the fluxional character of some examined complexes was studied and the relative activation energies were determined.

Catalyst activity or stability: the dilemma in Pd-catalyzed polyketone synthesis

“…In a previous paper in which some of the same ligands employed here had been tested, it had been noted that a higher productivity was obtained with unsubstituted styrene, 17 contrary to what was generally reported in the literature. 39,[40][41][42][43] The new, more extensive, series of data supports this observation, but also shows that the higher activity is also associated with a faster deactivation, since not only k 1 , but also k 3 values are consistently higher in the case of styrene. To the best of our knowledge, an active role of the olefin in the deactivation process has not been proposed before.…”

New Pyridine−Imidazoline Ligands for Palladium-Catalyzed Copolymerization of Carbon Monoxide and Styrene

“…[1Ϫ6] Catalysts containing planar, achiral ligands give syndiotactic copolymers with high stereocontrol. [3,7,8] This stereoregularity has been attributed to a chain-end control due to the interaction of the growing chain with the incoming styrene unit, which inserts exclusively in a 2,1-fashion. [3] Enantiomerically pure C 2 -symmetrical N-N ligands yield highly isotactic copolymers due to the strict enantiomorphic site control provided by the chiral ligand.…”

Mono- and di-nuclear 2,3-diazabutadiene and 2-azabutadiene complexes of Rhenium(I): Syntheses, luminescence spectra and X-ray structures

“…1 The asymmetric unit of monoclinic lattice contains one di-nuclear complex molecule and one CH 2 Cl 2 solvent molecule. The molecular structure of 2a is quite similar to a series of complexes of the type [(OC) 3 Re(l-E-E)(l-Br) 2 ). The structure consists of two pseudo-octahedral Re(I) centers joined by a common BrÁ Á ÁBr edge and a N-N azine bridge.…”

“…With the objective to synthesize some rhenium complexes ligated by these 2-azabutadiene ligands for luminescence studies, we reacted [Re(CO) 3 (THF)(l-Br)] 2 [14a,14b] with two equivalent of 1a. Unfortunately, the outcome of the reaction was not straightforward and gave a mixture of several products.…”