Subtle Balance of Steric and Electronic Effects for the Synthesis of Atactic Polyketones Catalyzed by Pd Complexes with Meta-Substituted Aryl-BIAN Ligands

Abstract

Both symmetric and nonsymmetric bis(aryl)acenaphthenequinonediimine ligands, featured by substituents in meta-positions of the aryl rings, have been applied for the first time as ancillary ligands for the palladiumcatalyzed CO/vinyl arene copolymerization. The nature and the number of substituents affect both the productivity and the molecular weight of the synthesized copolymers. Palladium complexes containing the nonsymmetric ligands are the most efficient catalysts reported so far for the synthesis of atactic polyketones.

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

“…A more interesting comparison can be made between the corresponding k 1 (or k 1 ') and k 3 (or k 3 ') values for the two substrates. 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.…”

“…The NMR characterization of complexes 7a and 7b was in line with data reported in the literature for analogous compounds with symmetrically substituted Ar-BIANs. 12,17,18 In the case of complexes 3a and 3b, bearing the nonsymmetrical ligand 3 on palladium, in analogy with what we reported for complexes 1a, 2a and 1b, 2b, 17 cis and trans isomers were observed in solution differing in the relative position of the Pd-CH 3 fragment with respect to the two halves of the ligand. We define as trans the isomer having the Pd-CH 3 fragment, or more generally the Pd-C bond, trans to the Pd-N bond of the aryl ring substituted with electron- 0/+ Y = Cl, CH 3 CN cis 16 16 withdrawing groups (Scheme 2).…”

“…For Ar-BIANs, (E,E) and (E,Z) isomers are possible regarding to the relative configuration of the aryl rings with respect to the C=N imine bonds. In general, only the E,E isomer is observed for symmetrical Ar-BIANs, 19 but a variable amount of the E,Z isomer is always found in the case of nonsymmetrical Ar,Ar′-BIAN derivatives [16][17][18]20 and for alkyl-BIANs. 22,23 For both ligands 3 and 7, the 1 H NMR spectrum recorded at room temperature in CDCl 3 and CD 2 Cl 2 , respectively, indicated the presence in solution of the E,E isomer only.Though this was expected for the latter, it is surprising for the former.…”

“…17 Both productivity and molecular weight of the synthesized copolymers were affected by the nature of the substituents. Even though some differences in the order of productivity of the catalysts were observed for the two alkenes and the effect of the ligand nature was more pronounced in case of styrene than for 4-methylstyrene, for both alkenes the catalysts with the nonsymmetrical Ar,Ar′-BIANs 1 and 2 were found the most productive within the series.…”

“…We found that moving the substituents on the aryl rings of Ar-BIAN from the ortho to the meta positions resulted in an enhancement of productivity of more than two orders of magnitude, and a productivity value of 3.70 kg CP/g Pd was reached. 17 Polyketones with an atactic microstructure and Mw of 47000 (Mw/Mn = 2.0) were produced. Both productivity and molecular weight were affected by the nature and the number of substituents on the showed a higher productivity and a higher content of the inserted polar monomer with respect to those with the related symmetrical ligands.…”