Mechanism of Propylene Oxide Polymerization Promoted by N-Heterocyclic Olefins

We report a mechanistic DFT investigation of the organopolymerization of propylene oxide (PO) promoted by N-heterocyclic olefins (NHOs) in combination with benzylic alcohol (BnOH). Calculations support the experimentally based hypothesis of two competing pathways, namely, the anionic and zwitterionic pathways. The former is based on an acid-base cooperativity between BnOH and the NHO, promoting ring opening of PO by BnO-. The latter occurs through the formation of a zwitterionic adduct by nucleophilic attack of the exocyclic carbon atom of the NHO on the PO, with the concerted ring opening of PO. The two initiating species cannot interconvert, and chain elongation can proceed from both initiation adducts. Potential energy surfaces were computed for a set of NHOs to clarify the effects of the steric and electronic properties of the NHO on the system reactivity. The results achieved represent useful insight toward the synthesis of PPO with better properties with respect to the polymer obtained with the experimental tested systems because the computationally proposed NHO system is the only one that favors the mechanism leading to higher molecular weight. (Chemical Equation Presented)

Mechanistic Insights into the Organopolymerization of N-Methyl N-Carboxyanhydrides Mediated by N-Heterocyclic Carbenes

We report on a DFT investigation of initiation, propagation; and termination in the organopolymerization of N-methyl N-carboxyanhydrides toward cyclic poly(N-substituted glycine)s, promoted by N-heterocyclic carbenes (NHC). Calculations support the experimentally based hypothesis of two competing initiation pathways. The first leading to formation of a zwitterionic adduct by nucleophilic addition of the NHC to one of the carbonyl groups of monomer. The second via acid base reactivity, starting with the NHC promoted abstraction of a proton from the methylene group of the monomer, leading to an ion-pair-type adduct, followed by nucleophilic attack of the adduct to a new monomer molecule. Chain elongation can proceed from both the initiation adducts via nucleophilic attack of the carbamate chain-end to a new monomer molecule via concerted elimination of CO, from the carbamate chain-end. Energy barriers along all the considered termination pathways are remarkably higher that the energy barrier along the chain elongation pathways, consistent with the quasi-living experimental behavior. Analysis of the competing termination pathways suggests that the cyclic species determined via MALDI-TOF MS experiments consists of a zwitterionic species deriving from nucleophilic attack of the N atom of the carbamate chain end to the C=O group bound to the NHC moiety

Mechanistic Insights into the Organopolymerization of <i>N</i>‑Methyl <i>N</i>‑Carboxyanhydrides Mediated by <i>N</i>‑Heterocyclic Carbenes

We report on a DFT investigation of initiation, propagation, and termination in the organopolymerization of <i>N</i>-methyl <i>N</i>-carboxyanhydrides toward cyclic poly­(<i>N</i>-substituted glycine)­s, promoted by <i>N</i>-heterocyclic carbenes (NHC). Calculations support the experimentally based hypothesis of two competing initiation pathways. The first leading to formation of a zwitterionic adduct by nucleophilic addition of the NHC to one of the carbonyl groups of monomer. The second via acid–base reactivity, starting with the NHC promoted abstraction of a proton from the methylene group of the monomer, leading to an ion-pair-type adduct, followed by nucleophilic attack of the adduct to a new monomer molecule. Chain elongation can proceed from both the initiation adducts via nucleophilic attack of the carbamate chain-end to a new monomer molecule via concerted elimination of CO₂ from the carbamate chain-end. Energy barriers along all the considered termination pathways are remarkably higher that the energy barrier along the chain elongation pathways, consistent with the quasi-living experimental behavior. Analysis of the competing termination pathways suggests that the cyclic species determined via MALDI-TOF MS experiments consists of a zwitterionic species deriving from nucleophilic attack of the N atom of the carbamate chain-end to the CO group bound to the NHC moiety

Mechanism of Intramolecular Rhodium- and Palladium-Catalyzed Alkene Alkoxyfunctionalizations

Density functional theory calculations have been used to investigate the reaction mechanism for the [Rh]-catalyzed intramolecular alkoxyacylation ([Rh] = [Rh-I(dppp)(+)] (dppp, 1,3-bis(diphenylphosphino)propane) and [Pd]/BPh3 dual catalytic system assisted intramolecular alkoxycyanation ([Pd] = Pd-Xantphos) using acylated and cyanated 2-allylphenol derivatives as substrates, respectively. Our results substantially confirm the proposed mechanism for both [Rh]- and [Pd]/ BPh3-mediated alkoxyfunctionalizations, offering a detailed geometrical and energetical understanding of all the elementary steps. Furthermore, for the [Rh]-mediated alkoxyacylation, our observations support the hypothesis that the quinoline group of the substrate is crucial to stabilize the acyl metal complex and prevent further decarbonylation. For [Pcd/BPh3-catalyzed alkoxycyanation, our findings clarify how the Lewis acid BPh3 cocatalyst accelerates the only slow step of the reaction, corresponding to the oxidative addition of the cyanate O-CN bond to the Pd center

Structure–Activity Relationship To Screen Ni–Bisphosphine Complexes for the Oxidative Coupling of CO₂ and Ethylene

Density functional theory calculations have been used to investigate competition between inner- and outer-sphere reaction pathways in the oxidative coupling of CO₂ and ethylene for a set of 12 Ni–bisphosphine complexes, in order to build a QSAR approach correlating catalyst structure to calculated energy barriers for CO₂ activation. The ligands were selected to explore different substituents on the P atoms (cyclohexyl, phenyl, and <i>tert</i>-butyl) and different lengths of the tether connecting the P atoms, −(CH₂)_<i>n</i>– with <i>n</i> = 1–3. As expected, the conclusion is that the inner-sphere reaction pathway is favored with unhindered ligands, while the outer-sphere reaction pathway is favored with hindered ligands. To find a possible correlation with molecular descriptors, we started using the buried volume as a steric descriptor. A reasonable correlation could be found for the energy barrier along the inner-sphere pathway, while scarce correlation was found for the energy barrier along the outer-sphere pathway, indicating that the steric bulkiness of the ligand disfavors approach of CO₂ to the metal center. Much stronger correlation between the ligand structure and the energy barrier along the inner-sphere pathway was achieved when the steric descriptor was augmented by an electronic descriptor, consisting of the partial charge on the Ni atom. The much better correlation suggests that bisphosphine ligands have a non-negligible electronic impact on the catalyst performance