Bergman, Aza-Bergman, and Protonated Aza-Bergman Cyclizations and Intermediate 2,5-Arynes: Chemistry and Challenges to Computation

Reaction coordinates are computed for the Bergman cyclizations of hex-3-en-1,5-diyne and neutral and protonated 3-azahex-3-en-1,5-diyne at various levels of correlated electronic structure theory, as are singlet−triplet splittings for intermediate arynes. To be effective in low-symmetry situations showing high degrees of biradical character, CCSD(T) calculations benefit from use of Brueckner orbitals. Replacement of a CH fragment by N is predicted to increase the stability of the aryne relative to the iminediyne, and to increase drastically the stability of the isomeric enynenitrile. The barrier for retro-aza-Bergman cyclization of 2,5-pyridyne to pent-3-en-1-ynenitrile is predicted to be only 0.9 kcal/mol, which, combined with a predicted singlet−triplet splitting of −11.6 kcal/mol, suggests that 2,5-pyridynes are poor hydrogen atom abstracting agents. Protonation of nitrogen decreases the singlet−triplet splitting and raises the barrier to retro-aza-Bergman cyclization such that protonated 2,5-pyridynes may be expected to show reactivities similar to all-carbon analogues

Quantum Chemical Characterization of the Bonding of <i>N</i>-Heterocyclic Carbenes to Cp₂MI Compounds [M = Ce(III), U(III)]

The binding of N-heterocyclic carbenes to Ce(III) and U(III) compounds is characterized by quantum chemical methods. Density functional methods are in qualitative agreement with experiment that binding to U(III) is more favorable than to Ce(III); after correcting for basis-set superposition error, quantitative agreement with experiment is achieved with a multireference second-order perturbation theory approach accounting for relativistic effects. The small computed (and observed) preference derives from a combination of several small effects, including differences in electronic binding energies, rovibrational partition functions, and solvation free energies. Prospects for ligand modification to improve the differentiation between lanthanides and actinides are discussed on the basis of computational predictions

Mechanism and Design Principles for Controlling Stereoselectivity in the Copolymerization of CO₂/Cyclohexene Oxide by Indium(III) Phosphasalen Catalysts

Copolymerization of CO2 with cyclohexene oxide (CHO) creates a sustainable polymer that has been a target for homogeneous catalysis. In particular, indium­(III) phosphasalen catalysts generate high proportions of carbonate linkages in isotactic product poly­(cyclohexene carbonate) (iPCHC). We use theory here to characterize the initiation and propagation steps for these indium­(III) catalysts, which involve mononuclear mechanisms for CO2 insertion and CHO ring opening that are distinct from copolymerization mechanisms previously reported for other metal-based catalysts. We find that phosphasalen ligand interactions with CHO and the carbonate-terminated growing chain lead to stereoselection for high levels of isotacticity and suggest further modifications to the ligand that might tune this

Quantum Chemical Characterization of Singlet and Triplet Didehydroindenes

Structural and energetic properties for the lowest energy singlet and triplet states of the 19 possible didehydroindene isomers are predicted using coupled cluster, density functional, and multireference second-order perturbation theories. Singlet−triplet splittings and biradical stabilization energies provide a measure of the degree of interaction between the biradical centers. Comparisons to analogous didehydronaphthalenes are made to understand the influence of the five-membered ring. As in other didehydroarenes, proton hyperfine splittings in antecedent monoradicals are economical predictors of biradical state energy splittings

Quantum Chemical Characterization of Singlet and Triplet Didehydroindenes

Structural and energetic properties for the lowest energy singlet and triplet states of the 19 possible didehydroindene isomers are predicted using coupled cluster, density functional, and multireference second-order perturbation theories. Singlet−triplet splittings and biradical stabilization energies provide a measure of the degree of interaction between the biradical centers. Comparisons to analogous didehydronaphthalenes are made to understand the influence of the five-membered ring. As in other didehydroarenes, proton hyperfine splittings in antecedent monoradicals are economical predictors of biradical state energy splittings

Mechanism and Design Principles for Controlling Stereoselectivity in the Copolymerization of CO₂/Cyclohexene Oxide by Indium(III) Phosphasalen Catalysts

Copolymerization of CO2 with cyclohexene oxide (CHO) creates a sustainable polymer that has been a target for homogeneous catalysis. In particular, indium­(III) phosphasalen catalysts generate high proportions of carbonate linkages in isotactic product poly­(cyclohexene carbonate) (iPCHC). We use theory here to characterize the initiation and propagation steps for these indium­(III) catalysts, which involve mononuclear mechanisms for CO2 insertion and CHO ring opening that are distinct from copolymerization mechanisms previously reported for other metal-based catalysts. We find that phosphasalen ligand interactions with CHO and the carbonate-terminated growing chain lead to stereoselection for high levels of isotacticity and suggest further modifications to the ligand that might tune this

B−N Distance Potential of CH₃CN−BF₃ Revisited: Resolving the Experiment−Theory Structure Discrepancy and Modeling the Effects of Low-Dielectric Environments

We have re-examined the B−N distance potential of CH3CN−BF3 using MP2, DFT, and high-accuracy multicoefficient methods (MCG3 and MC-QCISD). In addition, we have solved a 1-D Schrödinger equation for nuclear motion along the B−N stretching coordinate, thereby obtaining vibrational energy levels, wave functions, and vibrationally averaged B−N distances. For the gas-phase, MCG3//MP2/aug-cc-pVTZ potential, we find an average B−N distance of 1.95 Å, which is 0.13 Å longer than the corresponding equilibrium value. In turn, this provides solid evidence that the long-standing discrepancy between the experimental (R(B−N) = 2.01 Å) and theoretical (R(B−N) = 1.8 Å or R(B−N) = 2.2−2.3 Å) distances may be genuine, stemming from large amplitude vibrational motion in the B−N stretching coordinate. Furthermore, we have examined the effects of low-dielectric media (ε = 1.1−5.0) on the structure of CH3CN−BF3 by calculating solvation free energies (PCM/B97-2/aug-cc-pVTZ) and adding them to the gas-phase, MCG3 potential. These calculations demonstrate that the inner region of the potential is stabilized to a greater extent by these media, and correspondingly, the equilibrium and average B−N distances decrease with increasing dielectric constant. We find that the crystallographic structural result (R(B−N) = 1.63 Å) is nearly reproduced with a dielectric constant of only 5.0, and also predict significant structural changes for ε values of 1.1−1.5, consistent with results from matrix-isolation−IR experiments

Modeling the Peroxide/Superoxide Continuum in 1:1 Side-on Adducts of O₂ with Cu

The character of singlet (C3N2H5)CuO2 ranges smoothly between copper(III) peroxide and copper(II) superoxide with variation of the electronic character of the supporting β-diketiminate ligand. Over the range of the variation, multireference second-order perturbation theory predicts the 1A1 singlet state always to be lower in energy than the lowest triplet state (3B1). The multideterminantal character of the biradical-like superoxide mesomer causes density functional theory sometimes to fail badly in predicting the relative energies of these same states, although its predictions of other properties, such as geometry, are of good quality

Quantum Chemical Characterization of Singlet and Triplet Didehydroindenes

Structural and energetic properties for the lowest energy singlet and triplet states of the 19 possible didehydroindene isomers are predicted using coupled cluster, density functional, and multireference second-order perturbation theories. Singlet−triplet splittings and biradical stabilization energies provide a measure of the degree of interaction between the biradical centers. Comparisons to analogous didehydronaphthalenes are made to understand the influence of the five-membered ring. As in other didehydroarenes, proton hyperfine splittings in antecedent monoradicals are economical predictors of biradical state energy splittings

Quantum Chemical Characterization of Factors Affecting the Neutral and Radical-Cation Newman–Kwart Reactions

Details of the electronic and geometric structures of stationary points along the reaction coordinate of the Newman Kwart rearrangement, which transforms an O-arylthiocarbamate into an S-arylcarbamothioate, are examined using quantum-chemical methods for a large number of compounds considering both the thermal reactions of uncharged substrates as well as the corresponding reactions of radical-cation substrates generated under photoredox conditions. The uncharged mechanism, which has intrinsically high 298 K free energies of activation (in excess of 30 kcal mol–1), has the character of nucleophilic aromatic substitution and is thus accelerated by electron-withdrawing substituents on the aromatic ring. The radical-cationic mechanism, by contrast, has 298 K free energies of activation that are typically below 20 kcal mol–1 and is accelerated by electron donating substituents on the aromatic ring, which stabilize the hole character that is transferred to this fragment from the thiocarbamate fragment as the reaction proceeds. Opportunities to further accelerate the radical-cation reaction are revealed by computational assessment of alternative amino groups for the thiocarbamate functionality