Dedicated to Professor Dieter Seebach on the occasion of his 65th birthday Diaquapalladium(2 ) trifluoromethanesulfonates modified with (4R,4'S)-or (4S,4'S)-2,2'-bis(4-benzyl-4,5-dihydrooxazole) (C s -and C 2 -ligands) produce isotactic poly(1-oxo-2-phenylpropane-1,3-diyl) through copolymerization of styrene with carbon monoxide. However, the same meso-catalyst in the presence of the free ligand leads to prevailingly syndiotactic growth of the copolymer, whereas the optically active catalyst, when used in the presence of the free enantiomeric ligand, gives an atactic copolymer.Asymmetric catalysis leading to the synthesis of stereoregular copolymers has been a matter of intense interest [1] since the discovery of isotactic polypropylene and its rationalization based on the existence of enantiomorphic catalytic sites [2]. The discovery of the relationship between the symmetry properties of the catalytic system and the microstructure of the produced polymers in the case of the propene polymerization by zirconocene catalytic systems is particularly intriguing [3] [4].Despite the development of a variety of palladium-nitrogen-donor ligands catalytic systems to promote the synthesis of stereoregular styrene/carbon monoxide (CO) copolymers 1 [5], the factors involved in the enantioface discrimination during the related copolymerization of olefins with CO are much less understood [6] [7].We showed that [Pd(PP)(H 2 O) 2 ][OTf] 2 complexes (where PP is either (1R,1'S)-or (1RS,1'RS)-1,2-bis[1,1'-(diphenylphosphino)ethyl]benzene, OTf trifluoromethanesulfonate), when used as catalyst precursors for the copolymerization of propene with CO, produce both isotactic poly(1-methyl-2-oxopropane-1,3-diyl) [8]. The meso-ligand shows higher stereospecificity and much higher catalytic activity than the racemic-
Achiral and chiral cationic palladium catalysts, modified with atropisomeric P N ligands with different steric and electronic properties, can efficiently produce poly(styrene-alt-CO) with essentially complete regioregularity and variable tacticity, depending on the ligand geometry; the electronic effect on catalytic activity depends on the geometry of the ligand.The alternating copolymerization of styrene with carbon monoxide to poly[1-oxo-2-phenylpropane-1,3-diyl] has attracted much attention in recent years [1 ± 10]. The catalyst precursors used to produce stereoregular copolymers were[12] complexes (S solvent molecule; X weakly coordinating anion). N N Ligands such as bipyridine or phenanthroline result in syndiotactically specific copolymerization [13], whereas a bis-oxazoline type of ligand [14] [15] or atropisomeric diketo imines [8] result in isotactically specific copolymerization. Cationic palladium complexes of the type [(P N)Pd(S) 2 ](X) 2 (P N 4-substituted 4,5-dihydro-2-phosphinooxazole ligand) also produce highly isotactic copolymers. In contrast to the above-mentioned N N modified compounds, these complexes can be used under high carbon monoxide pressure to achieve higher polymer yields [16]. However, the substituent at C(4) of the dihydrooxazole moiety is necessary for the isotactic microstructure of the copolymer, but also causes rate reduction [17]. In an attempt to combine the good catalytic activity of the bipyridine systems [10] with the stabilizing effect of the diphosphine, without losing the isotactic steric control, we tried to exploit atropisomeric ligands and to compare them with their achiral counterparts. We, therefore, synthesized, where P N are the corresponding ligands 1' ± 4', and used them as catalyst precursors for the copolymerization of styrene with carbon monoxide.The Table lists the results of the copolymerization experiments. They were carried out at high carbon monoxide pressure (320 bar), after identifying the positive order of the carbon monoxide concentration of catalyst precursor 1a, as found for the other P N ligands [16] [19]. The achiral catalyst precursor 1a has the highest catalytic activity of the ligands with diphenylphosphino substituents. It is noteworthy that system 2, with the planar ligand, is not catalytically active. Analogously, for the N N ligand systems under comparable reaction conditions, the phenanthroline catalyst is less active than
Dedicated to the memory of Luigi M. Venanzi in recognition of his significant contribution to transition metal chemistry [(L L')Pd(H 2 O)](OTf) 2 complexes, in which L L' is a chelate ligand containing the chiral 4-benzyl-4,5-dihydrooxazole moiety and either pyridin-2-yl or 2-(diphenylphosphino)phenyl substituents, catalyze the copolymerization of styrene with carbon monoxide with an isotactic or prevailingly syndiotactic microstructure, respectively. The chiroptical properties of the copolymers and model studies for carbon monoxide and olefin insertion on related Pd complexes suggest that the reason for the different stereochemistry of the copolymers is a site-selective coordination of the olefin in the intermediates containing the P N ligand; a lower regioselectivity in the coordination and a different coordination site lead to the different diastereoselectivity for the copolymer formation by the complex containing the N N'-ligand.
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