QUÍMICA DOS COMPOSTOS HIPERVALENTES: ESTRUTURA E REATIVIDADE DO REAGENTE DE TOGNI I

Abstract

The chemical bond, given by the sharing of an electron pair, and the octet rule proposed by Lewis are able to explain the electronic structure of most organic compounds formed by H, C, N, O, F. However, such concepts find difficulties in explaining the PCl5 molecule, in which the sharing of 5 electron pairs results in a violation of the octet rule for phosphorus. To remedy this difficulty, Linus Pauling proposed that the phosphorus would be able to use the 3d orbital to extend its octet in a model known as sp3d hybridization. Computational studies, however, have shown that the energy and extent of the d orbitals are incompatible with those of the 3s and 3p orbitals. Such an observation is not an exception, but a general rule for molecules formed by elements of the third period and subsequent periods of the p-block in the well-known hypervalent molecules. The hypervalent bonds of these molecules give them properties similar to transition metals, with the advantage that some elements such as iodine are found in large quantities and have low toxicity. In this perspective, attention is drawn to hypervalent iodine reagents, such as those from Togni I and II, which are economically and environmentally viable options for transition metals, useful for trifluoromethylation in organic syntheses. These reagents are activated in the presence of a Lewis/Brönsted acid, capable of increasing its electrophilic character. In the present work, a review of the models used to describe the hypervalent bonds was carried out and the stability and reactivity of the Togni I reagent was evaluated through the study of its isomerization and heterolytic bond dissociation reactions using a computational method. The literature review revealed that both the research on hypervalent bonds and those on the hypervalent iodine reagents, only intensified in 2009. A timeline of publications on hypervalent bonds was then delimited and a selection of articles considered relevant here was discussed to understand the applications of Togni's reagents. The theoretical calculations were performed using the level of theory M06-2X/def2-TZVPD in acetonitrile solution. Structural parameters were obtained for the Togni I molecule consistent with the crystallographic data available in the literature, suggesting that the theoretical method applied is accurate. The reaction path that leads to the formation of the acyclic isomer of Togni I was investigated, having obtained the energy barriers involved in each stage of the process. It was observed that the acyclic isomer is more stable, but it is less reactive because the C-O bond is stronger than the hypervalent bond involving C-I present in Togni I. The study of the heterolytic cleavage showed that the energy needed to break the C-I bond in Togni I is close to the largest isomerization energy barrier and is relatively high. This corroborates the idea that there must be an acid activation of the Togni I reagent to improve its action as a trifluomethylator. Thus, the results of the present study point to a strong relationship between the structural parameters of Togni molecules and their stability and reactivity.