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

Abstract

A series of Pd-complexes containing nonsymmetrical bis(aryl-imino)acenaphthene (Ar-BIAN) ligands, featured by substituents on the meta positions of the aryl rings, have been synthesized, characterized and applied in CO/vinyl arene copolymerization reactions. Crystal structures of two neutral Pd-complexes have been solved allowing comparison of the bonding properties of the ligand. Kinetic and mechanistic investigations on these complexes have been performed. The kinetic investigations indicate that in general ligands with electron-withdrawing substituents give more active, but less stable, catalytic systems, although steric effects also play a role. The good performance observed with nonsymmetrical ligands is at least in part due to a compromise between catalyst activity and lifetime, leading to a higher overall productivity with respect to catalysts based on their symmetrical counterparts. Additionally, careful analysis of the reaction profiles provided information on the catalyst deactivation pathway. The latter begins with the reduction of a Pd(II) Ar-BIAN complex to the corresponding Pd(0) species, a reaction that can be reverted by the action

Unprecedented Comonomer Dependence of the Stereochemistry Control in Pd‐Catalyzed CO/Vinyl Arene Polyketone Synthesis

“…[16,51] The opposite trend, analogous to that observedh erefor 1b,was,however,reported forpalladium catalysts containing bis(aryl-imino)acenaphthenes,f or which productivities in the CO/MSc opolymerization were similaro rl ower compared to those obtained in the CO/S counterpart. [22,49] The literature catalytic systemsw ith iminopyridines showedr emarkably lower productivities than those reported here, that is, from 0.097 to 0.541 kg CP g Pd…”

“…This phenomenon appearst ob eam ore general behavior, whichi sa lso valid for analogousp alladium complexes with other nonsymmetric nitrogenl igands, such as pyridiylimidazolines, [46] and the nonsymmetric bis(aryl-imino)acenaphthenes. [22,44,49] Notably,i nt he NMR spectra of the neutrald erivatives and of both isomers of the cationic complexes, the singlet of the PdMe fragment moves at low frequencyg oing from complexes with ligand L1 to those with ligand L2,c onfirming that the presence of the electron-releasing group on the imino carbon atom is reflected in ah igher electron density on palladium.…”

“…The catalyst with ligand L2 was more than one order of magnitude more productivet han catalyst with ligand L1.T his result is in agreement with the data previously reportedf or the ethylene homopolymerization catalyzed by iron complexes with bis(imino)pyridine ligands, in which ketimine catalysts were found to be more productive than their aldimine analogues. [50] In the copolymerization under investigation, the ligand effect on catalyst productivity appeared to be related to catalyst stability:w ith precatalyst 1b,t he formation of inactive palladium black was already observed after 3h of reaction, whereas for precatalyst 2b,n op alladium metal was observed fora tl east 72 h. Because the deactivation pathways imply the dissociation of the N-N' ligand from the palladiumc oordination sphere, [49] the ligand effect on catalysts tability is in agreement with the NMR data about the ligand coordination capability.…”

Bis‐Alkoxycarbonylation of Acrylic Esters and Amides for the Synthesis of 2‐Alkoxycarbonyl or 2‐Carbamoyl Succinates

“…[16] These results clearly indicate that not only the electronic character and the steric hindrance of the ortho-substituents of the aromatic rings, but also the rigidity of the resulting catalysts, influenced by the backbone, [17] greatly affected the reactivity. [16] These results clearly indicate that not only the electronic character and the steric hindrance of the ortho-substituents of the aromatic rings, but also the rigidity of the resulting catalysts, influenced by the backbone, [17] greatly affected the reactivity.…”

Easy Entry into Reduced Ar‐BIANH2 Compounds: A New Class of Quinone/Hydroquinone‐Type Redox‐Active Couples with an Easily Tunable Potential

“…Compounds of the family bis(aryl)acenaphthenequinonediimine (Ar‐BIAN) ( 1 ; see Scheme ) have been known for some time,1–3 but have been brought to the general attention only in the 90s by Elsevier and his group 4. Since then, they have found widespread use mostly as ligands for transition metals and the corresponding complexes have been employed as catalysts for a wide variety of reactions, such as alkene hydrogenation,5 polymerization,6–11 copolymerization,12–18 aziridination,19 cyclopropanation,19 and epoxidation20 alkene‐CO copolymerization,21–24 alkyne coupling in the presence of halogens or organic halides and tin compounds25–27 or just tin compounds,28 selective semihydrogenation of alkynes29 and allenes,30 allylic aminations of olefins by nitroarenes in the presence of CO,31–33 the synthesis of pyrroles and oxazines from dienes, nitroarenes and CO,34 the reduction of nitroarenes to anilines by CO/H 2 O,35–37 the hydroamination of alkynes,38 the homo‐coupling of organic halides39 and their cross‐coupling reaction with organomagnesium, ‐zinc, and ‐tin reagents,40–44 the Suzuki–Miyaura cross coupling,45 the synthesis of 4‐quinolones and 2,3‐dihydroquinolones from 2′‐nitrochalcones and CO,46, 47 the synthesis of indoles from o ‐nitrostyrenes and CO,47, 48 the Heck arylation of olefins,49, 50 and oxidative Heck reaction,51, 52 the synthesis of dihydrofurans and dihydrobenzoxepines53 and N ‐acyl vinilogous carbamic acids,54 the cyclotrimerization of alkynes,55 the click reaction,56 the dimerization of vinyl ethers to acetals,57 and the diboration of α,β‐unsaturated carbonyl compounds 58. For some of these syntheses, the use of the Ar‐BIAN ligands was instrumental in achieving a high performance of the catalytic system.…”