D'Alterio, Massimo Christian (2020) From ring opening catalytic cycles to materials and products: development of an integrated experimental and theoretical approach. [Tesi di dottorato]

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In the last years computational modelling has become a strong ally of catalyst design on rational basis. Calculations based on Density Functional Theory (DFT) is nowadays a well assessed method to model catalyzed chemical reactions and the DFT modeling, giving an atomistic outlook on the reactivity, can return information not easily accessible by means of experiments. One of the main aims of calculations is to explain, drive and sometimes even predict experiments and, on the other hand, experiments are necessary to prove and give reliability to theoretical predictions and is not uncommon that experiments inspire new calculations. In this thesis we demonstrate how powerful this kind of theoretical and experimental synergy can be by focusing on reactions aimed to the production of environmentally benign polymers and products of agrochemical and pharmaceutical interest. More specifically, we focused on ring-opening reactions catalyzed by earth-abundant metals (Al, Zn, Ru) in homogeneous phase. First, we propose a model for stereoselective ROP of rac-LA promoted by chiral aluminum systems based on DFT calculations. The chiral catalyst recognition has been addressed as a promising route to the production of improved PLA microstructures. We demonstrated that the mechanism of enantiomorphic site control dictated by the chiral catalyst shows unusual features, that add complexity and need to be taken into account when addressing the challenge of chiral catalyst recognition. The computational part has been combined also with experimental works made in collaboration with the group of Prof. Claudio Pellecchia at the University of Salerno and we were able to afford "tailor-made" aliphatic polyesters block copolymers by producing stereo-diblock and stereo-multiblock PLA's together with copolymers of LA and ε-caprolactone (CL). These materials attract interest since the obtained copolymer may combine the complementary features of the two homopolymers (the low PCL Tg and the high modulus of PLA). In particular, a novel family of heteroleptic Zn(II) pyridylamido complexes functioning as very active catalysts for the ROP of L-, D- and rac-lactides and ε-caprolactone has been developed. The same family of Zn(II) complexes are also evaluated as catalysts for the alternating copolymerization of carbon dioxide and cyclohexene oxide. The ring-opening copolymerization (ROCOP) of CO2 with energy-rich substrates such as epoxides is a rare example of catalytic process with the potential to deliver large-scale quantities of products from carbon dioxide. Indeed, among all the global concerns, the increase in CO2 concentration in the atmosphere is attracting a lot of attention and it is boosting research efforts toward CO2 utilization as an abundant and renewable carbon source for chemical production. The difference among the activities of the catalysts are justified by DFT calculations. Moving from materials to products, the fundamental understanding of the metal reactivity and the discovery of new chemical reactions are still extremely appealing in view of generating sustainable and environmentally benign processes in academic and industrial laboratories. Focusing on this aspect, it is worth to recall that Ruthenium is attracting increasing interest because it is a promising alternative to precious metals since it displays a large number of oxidation states (from +8 to -2) and coordination geometries comparable to most of the most expensive metals. It has been recently reported a general and high functional group tolerant protodecarbonylation of N-substituted phthalimides into amides using [RuCl2(p-cymene)]2 as pre-catalyst in the presence of K2CO3 and water at high temperatures. Proceeding in the context of the computational modeling of homogeneous catalysis, we show detailed control experiments together with theoretical calculations at the DFT level (carried on during my period abroad in Girona in collaboration with the group of Prof. Albert Poater) to identify the mechanism operating in this ruthenium-catalyzed reaction, in particular, regarding the five-membered ring-opening step.

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