### Reactivity of Indoles through the Eyes of a Charge-Transfer Partitioning Analysis

OROZCO-VALENCIA Ulises1,GÁZQUEZ José L.2,VELA Alberto1,*()

1. 1 Departamento de Química, Centro de Investigación y de Estudios Avanzados, Av. Instituto Politécnico Nacional 2508, Colonia San Pedro Zacatenco, Ciudad de México, 07360, México
2 Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco 186, Ciudad de México, 09340, México
• 收稿日期:2017-11-24 发布日期:2018-03-20
• 通讯作者: VELA Alberto E-mail:avela@cinvestav.mx

### Reactivity of Indoles through the Eyes of a Charge-Transfer Partitioning Analysis

Ulises OROZCO-VALENCIA1,L. GÁZQUEZ José2,Alberto VELA1,*()

1. 1 Departamento de Química, Centro de Investigación y de Estudios Avanzados, Av. Instituto Politécnico Nacional 2508, Colonia San Pedro Zacatenco, Ciudad de México, 07360, México
2 Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco 186, Ciudad de México, 09340, México
• Received:2017-11-24 Published:2018-03-20
• Contact: Alberto VELA E-mail:avela@cinvestav.mx

A global and local charge transfer partitioning model, based on the cornerstone theory developed by Robert G. Parr and Robert G. Pearson, which introduces two charge transfer channels (one for accepting electrons (electrophilic) and another for donating (nucleophilic)), is applied to the reaction of a set of indoles with 4, 6-dinitrobenzofuroxan. The global analysis indicates that the prevalent electron transfer mechanism in the reaction is a nucleophilic one on the indoles, i.e., the indoles under consideration transfer electrons to 4, 6-dinitrobenzofuroxan. Evaluating the reactivity descriptors with exchange-correlation functionals including exact exchange (global hybrids) yields slightly better correlations than those obtained with generalized gradient-approximated functionals; however, the trends are preserved. Comparing the trend obtained with the number of electrons donated by the indoles, and predicted by the partitioning model, with that observed experimentally based on the measured rate constants, we propose that the number of electrons transferred through this channel can be used as a nucleophilicity scale to order the reactivity of indoles towards 4, 6-dinitrobenzofuroxan. This approach to obtain reactivity scales has the advantage of depending on the intrinsic properties of the two reacting species; therefore, it opens the possibility that the same group of molecules may show different reactivity trends depending on the species with which they are reacting. The local model allows systematic incorporation of the reactive atoms based on the their decreasing condensed Fukui functions, and the correlations obtained by increasing the number of reactive atoms participating in the local analysis of the transferred nucleophilic charge improve, reaching an optimal correlation, which in the present case indicates keeping three atoms from the indoles and two from 4, 6-dinitrobenzofuroxan. The atoms selected by this procedure provide valuable information about the local reactivity of the indoles. We further show that this information about the most reactive atoms on each reactant, combined with the spatial distribution of the nucleophilic and electrophilic Fukui functions of both reactants, allows one to propose non-trivial candidates of starting geometries for the search of the transition state structures present in these reactions.

Abstract:

A global and local charge transfer partitioning model, based on the cornerstone theory developed by Robert G. Parr and Robert G. Pearson, which introduces two charge transfer channels (one for accepting electrons (electrophilic) and another for donating (nucleophilic)), is applied to the reaction of a set of indoles with 4, 6-dinitrobenzofuroxan. The global analysis indicates that the prevalent electron transfer mechanism in the reaction is a nucleophilic one on the indoles, i.e., the indoles under consideration transfer electrons to 4, 6-dinitrobenzofuroxan. Evaluating the reactivity descriptors with exchange-correlation functionals including exact exchange (global hybrids) yields slightly better correlations than those obtained with generalized gradient-approximated functionals; however, the trends are preserved. Comparing the trend obtained with the number of electrons donated by the indoles, and predicted by the partitioning model, with that observed experimentally based on the measured rate constants, we propose that the number of electrons transferred through this channel can be used as a nucleophilicity scale to order the reactivity of indoles towards 4, 6-dinitrobenzofuroxan. This approach to obtain reactivity scales has the advantage of depending on the intrinsic properties of the two reacting species; therefore, it opens the possibility that the same group of molecules may show different reactivity trends depending on the species with which they are reacting. The local model allows systematic incorporation of the reactive atoms based on the their decreasing condensed Fukui functions, and the correlations obtained by increasing the number of reactive atoms participating in the local analysis of the transferred nucleophilic charge improve, reaching an optimal correlation, which in the present case indicates keeping three atoms from the indoles and two from 4, 6-dinitrobenzofuroxan. The atoms selected by this procedure provide valuable information about the local reactivity of the indoles. We further show that this information about the most reactive atoms on each reactant, combined with the spatial distribution of the nucleophilic and electrophilic Fukui functions of both reactants, allows one to propose non-trivial candidates of starting geometries for the search of the transition state structures present in these reactions.