The application of aromaticity and antiaromaticity to reaction mechanisms

Aromaticity, in general, can promote a given reaction by stabilizing a transition state or a product via a mobility of π electrons in a cyclic structure. Similarly, such a promotion could be also achieved by destabilizing an antiaromatic reactant. However, both aromaticity and transition states cannot be directly measured in experiment. Thus, computational chemistry has been becoming a key tool to understand the aromaticity-driven reaction mechanisms. In this review, we will analyze the relationship between aromaticity and reaction mechanism to highlight the importance of density functional theory calculations and present it according to an approach via either aromatizing a transition state/product or destabilizing a reactant by antiaromaticity. Specifically, we will start with a particularly challenging example of dinitrogen activation followed by other small-molecule activation, C-F bond activation, rearrangement, as well as metathesis reactions. In addition, antiaromaticity-promoted dihydrogen activation, CO2 capture, and oxygen reduction reactions will be also briefly discussed. Finally, caution must be cast as the magnitude of the aromaticity in the transition states is not particularly high in most cases. Thus, a proof of an adequate electron delocalization rather than a complete ring current is recommended to support the relatively weak aromaticity in these transition states.

Computational Study of Iron-Catalyzed Intramolecular [2 + 2] Cycloaddition and Cycloisomerization of Enyne Acetates: Mechanism and Selectivity

The mechanism of iron-catalyzed intramolecular [2 + 2] cycloaddition and cycloisomerization of enyne acetates has been investigated with DFT computations. Both mechanisms start the catalytic cycle from the stepwise 1,2-acyloxy migration to afford the iron carbene. The [2 + 2] cycloaddition mechanism involves subsequent key steps of [2 + 2] cycloaddition, 1,2-acyloxy migration, and reductive elimination to generate the azabicyclo [3.2.0] heptane product, with the reductive elimination being the rate-determining step. The cycloisomerization mechanism involves subsequent key steps of [2 + 2] cycloaddition, stepwise 1,4-acyloxy migration to produce the allenylpyrrolidine product, with the 1,4-acyloxy migration being the rate-determining step. Reaction potential energy surfaces for two model substrates that have or do not have alkene-terminal substituents have been investigated and the origins of the selectivities have been disclosed. Moreover, energy profiles with three possible spin states (S_Fe = 0, 1, 2) have been considered. The reaction is suggested to occur mainly on the singlet potential energy surface with a few spin crossovers between singlet and triplet states involved, which indicates that this reaction should have two-state reactivity (TSR).

Systematic Design of a Frustrated Lewis Pair Containing Methyleneborane and Carbene for Dinitrogen Activation

Activation of atmospherically abundant dinitrogen (N₂) by metal-free species under mild reaction conditions has been one of the most challenging areas in chemistry for decades. Very recent but limited progress in N₂ activation by boron species, including two-coordinated borylene and methyleneborane and three-coordinated borole and borane, has been made toward metal-free N₂ activation. Here, we systematically probe an experimentally viable frustrated Lewis pair (FLP) containing two moieties (methyleneborane and carbene) for N₂ activation via density functional theory (DFT) calculations, which has proven to be an efficient approach for N₂ activation in a thermodynamically and kinetically favorable manner. Aromaticity is found to play a crucial role in stabilization of the product. This study could be a valuable alternative for the development of metal-free N₂ activation chemistry, highlighting great potential of FLP for N₂ activation and functionalization.

Understanding the sigmatropic shifts of cyclopenta-2,4-dien-1-yltrimethylsilane in its Diels-Alder addition

To understand its dynamic processes in the Diels-Alder addition, temperature-dependent dynamic processes of cyclopenta-2,4-dien-1-yltrimethylsilane [1-(C5H5)Si(Me)3] are investigated by DFT computations. The degenerate sigmatropic migration of Si(Me)3 is found to be the dominant factor with a Gibbs free energy of activation of 13.2 kcal mol-1. The contribution from suprafacial hydrogen shifts is negligible, and the antarafacial sigmatropic hydrogen shifts are excluded due to their relatively high Gibbs barriers. Thermodynamically controlled Diels-Alder addition is obtained, which involves the critical interconversion between the 2,4-dienyl isomer 1 and 1,4-dienyl isomer 3.

Tuning the hyperconjugative aromaticity in Au(III)-substituted indoliums

As a fundamental concept in chemistry, aromaticity has been extended from traditional organics to organometallics. Similarly, hyperconjugative aromaticity (HCA) has also been developed from main group to transition metal systems through the hyperconjugation of the substituents. However, it remains unclear that how the oxidation state of transition metal in the substituents affects the HCA. Herein, we demonstrate via density functional theory calculations that HCA could disappear in indoliums when the Au(i) substituents are changed to the Au(iii) ones. By tuning the ligand or cis-trans isomerization, HCA could be regained or enhanced in indoliums containing Au(iii) substitutents.

Reaction Mechanisms on [3 + 2] Cycloaddition of Azides with Metal Carbyne Complexes: Significant Effects of Aromaticity, Substituent, and Metal Center

Density functional theory calculations were used to investigate the reaction mechanisms on [3 + 2] cycloaddition reactions of azides with metal carbyne complexes. Our results reveal that the formation of a 1,4-metallatriazole regioisomer is a kinetically favorable process in comparison with the formation of 1,5-metallatriazole. Aromaticity plays an important role in stabilizing the products in these reactions. Further analyses show that the electron-donating ligand on metal centers or the electron-withdrawing group on the azide could accelerate the [3 + 2] cycloaddition reaction. All of these findings could be useful for experimental chemists to develop "click reactions" in organometallic chemistry.

Reaction Mechanisms on Unusual 1,2-Migrations of N-Heterocyclic Carbene-Ligated Transition Metal Complexes

Unusual 1,2-migration reactions of N-heterocyclic carbene (NHC) on transition metals were investigated using density functional theory calculations. Our results reveal that the electronic properties, ring strain of the four-membered ring, and aromaticity of NHC play crucial roles in the thermodynamics of such a 1,2-migration. Further studies show that changing the methylene on the metal center in the reactant with a more electronegative group (NH or O) will lead to the formation of products with nitrogen coordinating to the metal center, whereas other groups (BH, CF₂ , and SiH₂ ) will make such a 1,2-migration reverse. In addition, the reversed rearrangement of 1,2-boron, silyl migration could be thermodynamically and kinetically favorable.