1,721,242 research outputs found
Theoretical NMR spectroscopy of N-heterocyclic carbenes and their metal complexes
No full textRecent theoretical analysis of the NMR properties of free N-heterocyclic carbenes (NHC) and metal-NHC complexes has complemented experiments, allowing the establishment of structure/property relationships and the rationalization of otherwise surprising experimental results. In this review, the main conclusions from recent literature are discussed, with the aim to offer a vision of the potential of theoretical analyses of NMR properties. (C) 2016 Elsevier B.V. All rights reserved
Organocatalyzed ring-opening polymerization of meso-lactide reveals competing mechanisms of stereocontrol
No full textThe mechanism for the ring-opening polymerization (ROP) of meso-lactide (LA) promoted by the chiral thiourea-amine organocatalyst has been elucidated through density functional theory (DFT) calculations. A comprehensive description of the process was obtained by accounting for the distinct reactivity of the two ring-opening sites of meso-LA with the carbonyl group adjacent to the (R) or (S) stereogenic atoms and the two monomer enantiofaces (re vs si). This mechanistic complexity gives rise to multiple competing pathways with comparable activation barriers, in line with the experimental Pm value of 0.62, which indicates the formation of a weakly syndiotactic polylactic acid (PLA). Compared to the ROP of rac-LA (Pm = 0.90), the reduced selectivity is attributed to a weaker and less cooperative (antagonistic) interplay between enantiomorphic site (ESM) and chain-end (CEM) control models, leading to diminished energetic discrimination among the competing pathways
Mechanistic Insights into the Organopolymerization of N-Methyl N-Carboxyanhydrides Mediated by N-Heterocyclic Carbenes
No full textWe 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
Guidelines To Select the N-Heterocyclic Carbene for the Organopolymerization of Monomers with a Polar Group
No full textWe report on the DFT stability of zwitterion and spirocycle adducts of five polar monomers with nine N-heterocyclic carbenes (NHC), covering the most typical classes of monomers and NHCs used in organopolymerization. Results indicate that the relative stability of the two adducts is dominated by the singlet-triplet energy gap of the free NHC, with low energy gaps favoring the spirocycle adduct, while high energy gaps favor the zwitterionic adduct. This basic structure/property relationship can be tuned by the hindrance of the NHC and the nature of the monomer. In addition to rationalize existing systems, the 45 NHC/monomer combinations we examined can be used as a guideline to predict the behavior of a new NHC/monomer combination
Nitrite to nitric oxide interconversion by heme Fe-II complex assisted by [Cu-I(tmpa)](+)
No full textThe present computational study complements the recent experimental efforts by Karlin and coworkers to describe the interconversion of nitrite to nitric oxide by means of an iron porphyrin complex together with a Cu chemical system, i.e., the iron(II) complex (F8TPP)Fe-II [F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate(2-)] and a preformed copper(II)-nitrito complex [(tmpa)Cu-II(NO2)][B(C6F5)(4)] [tmpa = tris(2-pyridylmethyl)amine], being the latter an oxidized species of [(tmpa)Cu-I(MeCN)](+). By DFT calculations, we unravel how the reduction of nitrite to nitric oxide takes place through a mu-oxo heme-Fe-III-O-Cu-II complex, following a mimetic path as in the cytochrome c oxidase. Mayer bond order (MBO) and energy decomposition analyses are used to analyze the bonding strength of such nitro derivatives to either copper or iron
N-Heterocyclic olefins as initiators for the polymerization of (meth)acrylic monomers: A combined experimental and theoretical approach
No full textThe zwitterionic organopolymerization of four different acrylic monomers (N,N-dimethylacrylamide, methyl acrylate, methyl methacrylate and tert-butyl methacrylate) based on neutral initiators, so-called N-heterocyclic olefins (NHOs), is presented. The scope and underlying (deactivation-)mechanisms were studied in a combined experimental and computational effort. From a range of differently structured NHOs it was shown that imidazole-derivatives, in contrast to imidazoline- and benzimidazole-derivatives, readily polymerize the selected monomers. While the additive-free reactions proceed with a relatively low degree of control to yield a largely atactic material, for the acrylamide the addition of LiCl as a Î1⁄4-type ligand has been shown to result in a rapid and quantitative monomer consumption. The thus generated poly(N,N-dimethyl acrylamide) was found to be highly isotactic (>90% isotactic dyads) with high molecular weight (Mn= 250000-650000 g mol-1, DM= 1.3-1.6). Complementing DFT calculations considered the zwitterionic chain growth with respect to competing side reactions, namely spirocycles and enamine formation. It was found that NHOs with an unsaturated backbone better support the zwitterionic chain growth, with the spirocycles acting as dormant species that slow down but do not quench the polymerization process. Contrasting this, enamine formation irreversibly terminates the polymerization and is found to be energetically favored. This pathway can be blocked by the introduction of substituents on the exocyclic carbon of the NHO, resulting in structures like 2-isopropylidene-1,3,4,5-tetramethylimidazoline (4) which consequently deliver the most controlled polymerizations. Finally, a good correlation of the initiation energy barrier with the buried volume (%VBur) and the Parr electrophilicity index is described, allowing for a quick and reliable screening of potential monomers based on these two readily accessible parameters
Mechanism of Propylene Oxide Polymerization Promoted by N-Heterocyclic Olefins
No full textWe 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)
Unusual NHC-Iridium(I) Complexes and Their Use in the Intramolecular Hydroamination of Unactivated Aminoalkenes
No full textN-heterocyclic carbene (NHC) ligands with naphthyl side chains were employed for the synthesis of unsaturated, yet isolable [(NHC)Ir(cod)](+) (cod=1,5-cyclooctadiene) complexes. These compounds are stabilised by an interaction of the aromatic wingtip that leads to a sideways tilt of the NHC-Ir bond. Detailed studies show how the tilting of such N-heterocyclic carbenes affects the electronic shielding properties of the carbene carbon atom and how this is reflected by significant upfield shifts in the (CNMR)-C-13 signals. When employed in the intramolecular hydroamination, these [(NHC)Ir(cod)](+) species show very high catalytic activity under mild reaction conditions. An enantiopure version of the catalyst system produces pyrrolidines with excellent enantioselectivities
Inverting the Diastereoselectivity of the Mukaiyama−Michael Addition with Graphite-Based Catalysts
No full textHere, we show that graphite-based catalysts,
mainly graphite oxide (GO) and exfoliated GO, are effective
recyclable catalysts for a relevant stereoselective Mukaiyama−
Michael addition, outperforming currently available catalysts.
Moreover, the graphite-based catalysts described here invert
the diastereoselectivity relative to that observed with known
catalysts, with the unprecedented large prevalence of the anti
diastereoisomer. This inverted diastereoselectivity is increased
when the catalyst concentration is reduced and after catalyst
recycling. Density functional theory calculations suggest that
the selectivity is determined by two types of supramolecular
interactions operating between the catalyst and the substrates
at the diastereoselectivity-determining transition state, specifically, the π-stacking of β-nitrostyrene with graphite and the van der
Waals interaction between the SiMe3 group of the silyl ether and the graphite
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