Understanding the Mechanism of Diels–Alder Reactions with Anionic Dienophiles: A Systematic Comparison of [ECX]⁻ (E = P, As; X = O, S, Se) Anions

Abstract

While Diels–Alder (DA) reactions involving neutral or cationic dienophiles are well-known, the characteristics of the analogous reactions with anionic dienophiles are practically unexplored. Herein we present the first comparative computational investigations on the characteristics of DA cycloadditions with anionic dienophiles on the basis of the reactions of [ECX] − anions (E = P, As; X = O, S, Se) with 2 H -pyran-2-one. All of these reactions were found to be both kinetically and thermodynamically feasible, enabling synthetic access toward 2-phosphaphenolate and arsaphenolate derivatives in the future. This study also reveals that the [ECO] − anions show clear regioselectivity, while for [ECS] − and [ECSe] − anions, the two possible reaction channels have very similar energetics. Additionally, the activation barriers for the [ECO] − anions are lower than those of the heavier analogues. The observed differences can be traced back to the starkly differing nucleophilic character of the pnictogen center in the anions, leading to a barrier-lowering effect in the case of the [ECO] − anions. Furthermore, analysis of the geometries and electron distributions of the corresponding transition states revealed structure–property relationships, and thus a direct comparison of the cycloaddition reactivity of these anions was achieved. Along one of the two pathways, a good correlation was found between the activation barriers and suitable nucleophilicity descriptors (nucleophilic Parr function and global nucleophilicity). Additionally, the tendency of the reaction energies can be explained by the changing aromaticity of the products.

The case of a μ2-P aromatic phosphinine as a 4-electron donor forming σ- and π-three–center-two-electron bonds

“…The triphosphininyl phosphite ( 1 , Scheme 1) was prepared in 70% yield. 1 h ,4 a ,16 The 31 P{H} NMR spectrum of 1 shows a doublet at δ ( 31 P) = 151.8 ppm and a quartet at δ ( 31 P) = 127.9 ppm ( 3 J PP = 68.0 Hz) for the phosphinine and phosphite moieties, respectively. We then envisioned treating 1 with half equivalents of the chloro(1,5-cyclooctadiene)rhodium( i ) dimer [Rh(COD)Cl] 2 affording the red complex [ 2 ]Cl in 92% yield.…”

Deciphering the Differences in Ambident Reactivity between the Cyanate, Thiocyanate Ions, and their P‐ and As‐Containing Analogues

“…In the case of the ambident [ECX] − anions, however, the IE cannot differentiate between the E and X centres, as all of the HOMOs represent delocalised π‐systems involving the p orbitals of all the E, C, and X atoms, and thus the IE would be the same for both attacks. To solve this problem, here we recommend the use of local charge transfer (Δq, local CT) as a more applicable alternative for quantifying the covalent character of the interaction in the transition state [97] . The Δq value accounts for the transferred electron density from the nucleophile toward the electrophile and is obtained as the difference between the NPA (natural population analysis) partial charges of a given attacking centre in the TS and that in the free anion: Δq=q(TS)−q(anion).…”

“…To solve this problem, here we recommend the use of local charge transfer (Δq, local CT) as a more applicable alternative for quantifying the covalent character of the interaction in the transition state. [97] The Δq value accounts for the transferred electron density from the nucleophile toward the electrophile and is obtained as the difference between the NPA (natural population analysis) partial charges of a given attacking centre in the TS and that in the free anion: Δq = q(TS)À q(anion). Note that the larger Δq value indicates a higher covalent character for the interaction in the transition state (TS).…”