Origin of the Synchronicity on the Transition Structures of Polar Diels−Alder Reactions. Are These Reactions [4 + 2] Processes?

Abstract

The transition structures (TSs) for a series of related Diels-Alder reactions between cyclopentadiene and mono-, di-, tri-, and tetracyanoethylene derivatives have been studied with use of DFT methods at the B3LYP/6-31G computational level. The increase of the electron-withdrawing substitution on ethylene increases the rate of these polar cycloadditions. However, the symmetric arrangement of cis and trans 1,2-di- and tetracyanoethylenes decreases the effectiveness of the substitution, which can be related to the symmetry found at the corresponding TSs. A DFT analysis of the global and local electrophilicity power of these series of cyano ethylenes provides a sound explanation about the nature of these synchronous processes. The present theoretical study is in agreement with the experimental outcomes.

Origin of the synchronicity in bond formation in polar Diels–Alder reactions: an ELF analysis of the reaction between cyclopentadiene and tetracyanoethylene

“…7,8 In addition, the degree of asynchronicity follows a direct relationship with the polar character of the reaction, 9 which can be anticipated with the analysis of the electrophilicity index ω, 10 defined within the conceptual DFT. 11 In 2003, we performed a DFT study 12 of the P-DA reactions between cyclopentadiene (Cp) 4 and the series of cyanoethylenes 5-10, which was studied experimentally by Sauer et al (see Scheme 2). 13 In this series of P-DA reactions, a clear relationship between the experimentally observed acceleration rate 13 and the polar character of the DA reaction was established (see Table 1 and Fig.…”

Revisiting the stability of endo/exo Diels-Alder adducts between cyclopentadiene and 1,4-benzoquinone

“…[12][13][14][15] An evidence of the importance of this reaction to the development and application of some computational calculations is the large number of published papers involving considerations about the concerted and nonconcerted, synchronous and asynchronous mechanism nature of some Diels-Alder reactions. [15][16][17][18] Most of these studies attempt to explain some experimental results non-consistent with empirical rules or expected results concerning the selectivity. While experimental measurements can provide accurate rate constants for the different reaction pathways, high-level quantum chemical calculations are often necessary to explain the observed phenomena at an electronic level, to predict the substituent effects and also to evaluate steric interactions between the participating species.…”

“…1, 2010 Besides this, the estimation of several chemical properties and reactivities indexes of dienes and dienophiles can be performed with good experimental agreement. [12][13][14][15] An evidence of the importance of this reaction to the development and application of some computational calculations is the large number of published papers involving considerations about the concerted and nonconcerted, synchronous and asynchronous mechanism nature of some Diels-Alder reactions. [15][16][17][18] Most of these studies attempt to explain some experimental results non-consistent with empirical rules or expected results concerning the selectivity.…”

“…30 According to Alder's rule the major stereoisomer in Diels-Alder reactions is the one that is formed by maximum accumulation of double bonds in the transition state (through-space). [26][27][28][29][31][32][33][34] On the other hand, the endo product is usually less stable than exo.While the stereoelectronic interactions, involved in the transition state stability, are widely studied in the literature, 14,15,35 the same attention has not been dedicated to understand the stability of the products. One of the most used examples of the Diels-Alder reaction, which is listed in several Organic Chemistry textbooks, [36][37][38] is that between cyclopentadiene (1) and maleic anhydride which, at room temperature, gives only the endo adduct that is then converted at 200 o C to the thermodynamically more stable exo adduct through a retro Diels-Alder reaction.…”

“…While the stereoelectronic interactions, involved in the transition state stability, are widely studied in the literature, 14,15,35 the same attention has not been dedicated to understand the stability of the products. One of the most used examples of the Diels-Alder reaction, which is listed in several Organic Chemistry textbooks, [36][37][38] is that between cyclopentadiene (1) and maleic anhydride which, at room temperature, gives only the endo adduct that is then converted at 200 o C to the thermodynamically more stable exo adduct through a retro Diels-Alder reaction.…”