Zhang, Yuetao; Schmitt, Meghan L.; Falivene, Laura; Caporaso, Lucia; Cavallo, Luigi; Chen, Eugene You Xian(Journal of the American Chemical Society, American Chemical Society (ACS), 2013-11-27)[Article]
Roesle, Philipp; Caporaso, Lucia; Schnitte, Manuel; Goldbach, Verena; Cavallo, Luigi; Mecking, Stefan(Journal of the American Chemical Society, American Chemical Society (ACS), 2014-12-03)[Article]
Theoretical studies on the overall catalytic cycle of isomerizing alkoxycarbonylation reveal the steric congestion around the diphosphine coordinated Pd-center as decisive for selectivity and productivity. The energy profile of isomerization is flat with diphosphines of variable steric bulk, but the preference for the formation of the linear Pd-alkyl species is more pronounced with sterically demanding diphosphines. CO insertion is feasible and reversible for all Pd-alkyl species studied and only little affected by the diphosphine. The overall rate-limiting step associated with the highest energetic barrier is methanolysis of the Pd-acyl species. Considering methanolysis of the linear Pd-acyl species, whose energetic barrier is lowest within all the Pd-acyl species studied, the barrier is calculated to be lower for more congesting diphosphines. Calculations indicate that energy differences of methanolysis of the linear versus branched Pd-acyls are more pronounced for more bulky diphosphines, due to involvement of different numbers of methanol molecules in the transition state. Experimental studies under pressure reactor conditions showed a faster conversion of shorter chain olefin substrates, but virtually no effect of the double bond position within the substrate. Compared to higher olefins, ethylene carbonylation under identical conditions is much faster, likely due not just to the occurrence of reactive linear acyls exclusively but also to an intrinsically favorable insertion reactivity of the olefin. The alcoholysis reaction is slowed down for higher alcohols, evidenced by pressure reactor and NMR studies. Multiple unsaturated fatty acids were observed to form a terminal Pd-allyl species upon reaction with the catalytically active Pd-hydride species. This process and further carbonylation are slow compared to isomerizing methoxycarbonylation of monounsaturated fatty acids, but selective.
He, Jianghua; Zhang, Yuetao; Falivene, Laura; Caporaso, Lucia; Cavallo, Luigi; Chen, Eugene You Xian(Macromolecules, American Chemical Society (ACS), 2014-11-25)[Article]
A combined experimental and theoretical study on mechanistic aspects of polymerization of conjugated polar alkenes by frustrated Lewis pairs (FLPs) based on N-heterocyclic carbene (NHC) and Al(C6F5)3 pairs is reported. This study consists of three key parts: structural characterization of active propagating intermediates, propagation kinetics, and chain-termination pathways. Zwitterionic intermediates that simulate the active propagating species in such polymerization have been generated or isolated from the FLP activation of monomers such as 2-vinylpyridine and 2-isopropenyl-2-oxazoline-one of which, IMes+-CH2C(Me)=(C3H2NO)Al(C6F5)3 - (2), has been structurally characterized. Kinetics performed on the polymerization of 2-vinylpyridine by ItBu/Al(C6F5)3 revealed that the polymerization follows a zero-order dependence on monomer concentration and a first-order dependence on initiator (ItBu) and activator [Al(C6F5)3] concentrations, indicating a bimolecular, activated monomer propagation mechanism. The Lewis pair polymerization of conjugate polar alkenes such as methacrylates is accompanied by competing chain-termination side reactions; between the two possible chain-termination pathways, the one that proceeds via intramolecular backbiting cyclization involving nucleophilic attack of the activated ester group of the growing polymer chain by the O-ester enolate active chain end to generate a six-membered lactone (δ-valerolactone)-terminated polymer chain is kinetically favored, but thermodynamically disfavored, over the pathway leading to the -ketoester-terminated chain, as revealed by computational studies.
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