Schleyer hyperconjugative aromaticity and Diels–Alder reactivity of 5‐substituted cyclopentadienes

Abstract

Schleyer's discovery of hyperconjugative aromaticity and antiaromaticity in 5-substituted cyclopentadienes further expanded our understanding of the pervasive influence of aromaticity. Acceptors induce antiaromatic character by Schleyer's negative hyperconjugative aromaticity, and donors have the opposite effect. We computationally explored the Diels-Alder reactivity of 5-substituted cyclopentadienes with ethylene and maleic anhydride. The predicted billionfold difference in the computed gas phase rate constants at room temperature for the Diels-Alder reactions of 5-substituted cyclopentadienes with ethylene or maleic anhydride results from differences in the transition state distortion energies, which are directly related to the hyperconjugative aromaticity of these molecules.

Predicting an Antiaromatic Benzene Ring in the Ground State Caused by Hyperconjugation

“…Hyperconjugation, first proposed by Mülliken [30] in 1939 and later developedb yS chleyer,O ' Ferrall, Houk et al, [31][32][33]48] can make the cyclopentadiene ring aromatic or antiaromatic, depending on the substituents. Specifically,e lectropositive substituents (e.g.,S iH 3 ,G eH 3 ,S nH 3 )a tt he sp 3 -hybridized carbon lead to aromaticity in the five-membered ring (5MR) whereas electron-withdrawing groups (e.g.,F )p roduce an antiaromatic ring.…”

Effect of an α‐Methyl Substituent on the Dienophile on Diels‐Alder endo:exo Selectivity

“…Within this localized orbital‐based method, we split orbital interactions as: (a) primary orbital interactions, directly related to the formation of new C−C σ‐bonds (not included in Figure ); (b) secondary orbital interactions (classical SOI) as defined by Hoffmann and Woodward, that is interaction between p orbitals not directly involved in the formation of new C−C σ‐bonds (named as π C−X− >π* C−X′ in Figures –); and (c) hyperconjugative interactions: interactions between σ‐ and π‐orbitals. These later interactions have been proven to be relevant in the observed selectivity of some chemical reactions ,. Note that E(2) is somehow related to Δ E oi ( ζ ) but considering NBO localized orbitals instead of canonical Kohn‐Sham ones.…”

“…These later interactions have been proven to be relevant in the observed selectivity of some chemical reactions. [18,24] Note that E(2) is somehow related to ΔE oi (ζ) but considering NBO localized orbitals instead of canonical Kohn-Sham ones.…”

“…Remarkably, Houk et al . computationally analyzed reactions of 5‐substituted of cyclopentadienes with maleic anhydride ( 2 ).…”

“…[16] The most recent studies are focused on the reaction of substituted cyclopropenes with butadiene, showing the relevance electrostatic interactions [5c] as well as SOI and charge transfer interactions. [17] Remarkably, Houk et al [18] computationally analyzed reactions of 5-substituted of cyclopentadienes with maleic anhydride (2). The authors described a correlation between the hyperconjugative aromaticity of the substituted cyclopentadienes and the activation barriers, showing that stabilization through hyperconjugative interactions within the 5-member ring reduces the reactivity by increasing the energy required to deform this fragment, but no evaluation of the endo:exo selectivities were addressed.…”