Evaluation of density functional methods on the geometric and energetic descriptions of species involved in Cu⁺-promoted catalysis

The Reaction Mechanism of the Cu(I) Catalyzed Alkylation of Heterosubstituted Alkynes

Alkynes may be regioselectively alkylated to alkenes by organocopper reagents in a reaction known as “carbocupration”, where an alkylCu(I) binds to the alkyne and transfers its organic moiety to one of the alkyne carbon atoms. Alkynes hetero-substituted with third-row elements yield alkenes with a regiochemistry opposite to that obtained when using alkynes hetero-substituted with second-row elements. Early computational investigations of his reaction mechanism have identified the importance of the organocopper counter-cation (Li+) to the achievement of good reaction rates, but in the subsequent two decades no further progress has been reported regarding the exploration of the mechanism or the explanation of the experimental regiochemistry. In this work, density-functional theory is used to investigate the mechanism used and to describe a model that correctly explains both the reaction rates at sub-zero temperatures and the regiochemistry profiles obtained with each of the heteroalkynes. The rate-determining step is shown to vary depending on the heterosubstituent, and the alkyl transfer is consistently shown to occur, somewhat counter-intuitively, to the alkyne carbon that is complexed by Cu rather than to the “free” alkyne carbon atom, which instead interacts with the counter-cation that stabilizes the developing electronic charge distribution.

Influence of Alkyne and Azide Substituents on the Choice of the Reaction Mechanism of the Cu+-Catalyzed Addition of Azides to Iodoalkynes

The cycloaddition of azides to iodoalkynes is strongly enhanced by some Cu⁺-complexes. We have studied computationally six reaction pathways for the cycloaddition of 24 combinations of azide and iodoalkyne to identify the dominant pathways and the influence of reactant structure on the evolution of the reaction. Two pathways were found to be operating for distinct sets of reactants. In the first pathway, initial complexation of iodoalkyne by Cu⁺ is followed by the binding of the azide to the metal through its substituted nitrogen atom, followed by attack of the nonhalogenated alkyne carbon by the terminal nitrogen atom. This pathway is generally followed by aromatic or electron-deficient azides, unless the iodoalkyne bears an electron-withdrawing group. The second pathway is a single-step mechanism similar (apart from the alkyne bond weakening caused by complexation) to that observed in the absence of catalyst. Electron-deficient iodoalkynes and methyl azides strongly prefer this mechanism, regardless of the identity of the reaction partners. The catalytic gain obtained through the use of Cu⁺ depends only partially on its direct effect on the energy of the transition state (relative to that of the infinitely separated reactants) and may be lost if the iodoalkyne itself strongly interacts with the catalyst through the formation of too strong a π-complex.

Computational exploration of the reaction mechanism of the Cu(+)-catalysed synthesis of indoles from N-aryl enaminones

We have studied the role of Cu(+)-phenantroline as a catalyst in the cyclization of N-aryl-enaminones using density-functional theory computations. The catalyst was found to bind the substrate upon deprotonation of its eneaminone, and to dramatically increase the acidity of the carbon adjacent to the ketone functionality. The deprotonation of this carbon atom yields a carbanion which attacks the aryl moiety, thereby closing the heterocycle in the rate-determining step. This C-C bond forming reaction was found to proceed much more rapidly when preceded by re-protonation of the substrate N-atom (which had lost H(+) in the initial step). Hydride transfer to the catalyst then completes the indole synthesis, in a very fast step. The influence of Li(+) and K(+) on the regio-selectivity of the cyclization of bromo-substituted analogues could not, however, be reproduced by our model. Alternative pathways involving either single-electron transfer from the catalyst to the substrate or ring cyclization without previous carbon α-deprotonation were found to be kinetically or thermodynamically inaccessible.

Performance of Møller-Plesset second-order perturbation theory and density functional theory in predicting the interaction between stannylenes and aromatic molecules

The performances of Møller-Plesset second-order perturbation theory (MP2) and density functional theory (DFT) have been assessed for the purposes of investigating the interaction between stannylenes and aromatic molecules. The complexes between SnX2 (where X = H, F, Cl, Br, and I) and benzene or pyridine are considered. Structural and energetic properties of such complexes are calculated using six MP2-type and 14 DFT methods. The assessment of the above-mentioned methods is based on the comparison of the structures and interaction energies predicted by these methods with reference computational data. A very detailed analysis of the performances of the MP2-type and DFT methods is carried out for two complexes, namely SnH2-benzene and SnH2-pyridine. Of the MP2-type methods, the reference structure of SnH2-benzene is reproduced best by SOS-MP2, whereas SCS-MP2 is capable of mimicking the reference structure of SnH2-pyridine with the greatest accuracy. The latter method performs best in predicting the interaction energy between SnH2 and benzene or pyridine. Among the DFT methods, ωB97X provides the structures and interaction energies of the SnH2-benzene and SnH2-pyridine complexes with good accuracy. However, this density functional is not as effective in reproducing the reference data for the two complexes as the best performing MP2-type methods. Next, the DFT methods are evaluated using the full test set of SnX2-benzene and SnX2-pyridine complexes. It is found that the range-separated hybrid or dispersion-corrected density functionals should be used for describing the interaction in such complexes with reasonable accuracy.

Benchmark study on methanol C-H and O-H bond activation by bare [Fe(IV)O](2+)

We present a high-level computational study on methanol C-H and O-H bond cleavages by bare [Fe(IV)O](2+), as well as benchmarks of various density functional theory (DFT) methods. We considered direct and concerted hydrogen transfer (DHT and CHT) pathways, respectively. The potential energy surfaces were constructed at the CCSD(T)/def2-TZVPP//B3LYP/def2-TZVP level of theory. Mechanistically, (1) the C-H bond cleavage is dominant and the O-H activation only plays minor role on the PESs; (2) the DHT from methyl should be the most practical channel; and (3) electronic structure analysis demonstrates the proton and electron transfer coupling behavior along the reaction coordinates. The solvent effect is evident and plays distinct roles in regulating the two bond activations in different mechanisms during the catalysis. The effect of optimizing the geometries using different density functionals was also studied, showing that it is not meaningful to discuss which DFT method could give the accurate prediction of the geometries, especially for transition structures. Furthermore, the gold-standard CCSD(T) method was used to benchmark 19 different density functionals with different Hartree-Fock exchange fractions. The results revealed that (i) the structural factor plays a minor role in the single point energy (SPE) calculations; (ii) reaction energy prediction is quite challenging for DFT methods; (iii) the mean absolute deviations (MADs) reflect the problematic description of the DFs when dealing with metal oxidation state change, giving a strong correlation on the HF exchange in the DFs. Knowledge from this study should be of great value for computational chemistry, especially for the de novo design of transition metal catalysts.