Abstract
In molecular similarity there is a premise “similar molecules tend to behave similarly”; however in the actual quantum similarity field there is no clear methodology to describe the similarity in chemical reactivity, and with this end an analysis of charge-transfer (CT) processes in a series of Diels-Alder (DA) reactions between cyclopentadiene (Cp) and cyano substitutions on ethylene has been studied. The CT analysis is performed in the reagent assuming a grand canonical ensemble and the considerations for an electrophilic system using B3LYP/6-31G(d)and M06-2X/6-311 +G(d,p)methods. An analysis for CT was performed in agreement with the experimental results with a good statistical correlation(R2=0.9118)relating the polar character to the bond force constants in DA reactions. The quantum distortion analysis on the transition states (TS) was performed using molecular quantum similarity indexes of overlap and coulomb showing good correlation(R2=0.8330)between the rate constants and quantum similarity indexes. In this sense, an electronic reorganization based on molecular polarization in terms of CT is proposed; therefore, new interpretations on the electronic systematization of the DA reactions are presented, taking into account that today such electronic systematization is an open problem in organic physical chemistry. Additionally, one way to quantify the similarity in chemical reactivity was shown, taking into account the dependence of the molecular alignment on properties when their position changes; in this sense a possible way to quantify the similarity of the CT in systematic form on these DA cycloadditions was shown.
Highlights
Since its discovery, the Diels-Alder (DA) reactions have become one of the most relevant reactions in synthetic organic chemistry due to their ability to create cyclic unsaturated compounds with a predictable stereochemistry and regioselectivity [1,2,3,4,5,6,7,8]
We present a combined approach of molecular quantum similarity (MQS) and chemical reactivity descriptors supported in density functional theory (DFT) [79,80,81,82,83,84] to study the electronic reorganization in terms of the formation of the zwitterionic character and their CT analysis along the series of reactions shown in Figure 1 and Table 1
The B3LYP in many cases has replaced the traditional ab initio methods and for this reason was used in this study; this calculation level can be adequate for the purpose of this study, in order to characterize the dispersion energy for the (HOMO) and (LUMO) frontier orbitals which are important on the chemical reactivity in the reactions studied [109,110,111]
Summary
The Diels-Alder (DA) reactions have become one of the most relevant reactions in synthetic organic chemistry due to their ability to create cyclic unsaturated compounds with a predictable stereochemistry and regioselectivity [1,2,3,4,5,6,7,8]. One of the fields in quantum chemistry for feasibility study through the TSs in the DA reactions is the molecular quantum similarity (MQS) presenting possible ways of electronic systematization [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78] In this sense, we present a combined approach of MQS and chemical reactivity descriptors supported in DFT [79,80,81,82,83,84] to study the electronic reorganization in terms of the formation of the zwitterionic character and their CT analysis along the series of reactions shown in Figure 1 and Table 1.
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