Not antiaromaticity gain, but increased asynchronicity enhances the Diels–Alder reactivity of tropone

Abstract

Tropone is an unreactive diene in normal electron demand Diels-Alder reactions, but it can be activated via carbonyl umpolung by using hydrazone ion analogs. Recently, the higher reactivity of hydrazone...

The increased Diels–Alder reactivity of umpolung tropone: analysis of individual atoms and bonds using QTAIM and IQA along complete IRC paths

“…One can see that energies of the TSSs, taken relatively to the reactants, are in nice agreement with both the reports from Karas et al 2 and Tiekink et al, 3 and the small differences can be accounted for the different levels of theory used and also the different properties evaluated, as these IRCs feature pure electronic energies (no thermal, enthalpy, or free energy corrections added). This is necessary as we will relate them to the IQA atomic energies for which these further corrections are not available.…”

“…The antiaromaticity would increase tropone's highest occupied molecular orbital (HOMO) energy, and this would lead to a reduction in the gap between the tropone's HOMO and the dienophile's least unoccupied molecular orbital (LUMO), thus facilitating the NDDA. They tested this hypothesis by computing the Diels-Alder (DA) reaction between maleimide (M) and tropone (1) as well as tropone derivatives based on the substitution of the carbonyl's oxygen by =NNH 2 (2prot) and =NNHSO 2 CH 3 (3prot), as well as their anions =NNH À (2) and =NNSO 2 CH À 3 (3), respectively; see Figure 1. Shortly after, Tiekink et al 3 challenged this interpretation, sustaining that although HOMO's energy indeed increases, LUMO's energy also increases, and these two effects effectively cancel each other.…”

“…They tested this hypothesis by computing the Diels-Alder (DA) reaction between maleimide (M) and tropone (1) as well as tropone derivatives based on the substitution of the carbonyl's oxygen by =NNH 2 (2prot) and =NNHSO 2 CH 3 (3prot), as well as their anions =NNH À (2) and =NNSO 2 CH À 3 (3), respectively; see Figure 1. Shortly after, Tiekink et al 3 challenged this interpretation, sustaining that although HOMO's energy indeed increases, LUMO's energy also increases, and these two effects effectively cancel each other. Therefore, although the NDDA is being facilitated, the inverse demand Diels-Alder (IDDA) channel is weakened in about the same extent.…”

“…The blue dashed lines represent the new σ bonds being formed, whereas the red curved arrows represent the standard DA approach found in most introductory textbooks. ANALYSIS Starting from the geometries made available by Tiekink et al, 3 the structures of the reactants (1, 2prot, 2, 3prot, 3, and M), transition state structures (TSSs), and products were fully optimized using Gaussian 16 20 ; vibrational analyses were carried out for all of them, in the same level of theory. All systems were characterized as true energy minima, with no imaginary frequencies for reactants and products, and a single imaginary frequency for the TSSs, whose normal coordinates are in agreement with the DA mechanism.…”

“…Shortly after, Tiekink et al 3 challenged this interpretation, sustaining that although HOMO's energy indeed increases, LUMO's energy also increases, and these two effects effectively cancel each other. Therefore, although the NDDA is being facilitated, the inverse demand Diels–Alder (IDDA) channel is weakened in about the same extent.…”