Twisted C=C Double Bonds with Very Low Rotational Barriers in Dioxanediones and Isoxazolones Determined by Low‐Temperature Dynamic NMR Spectroscopy and Computational Chemistry

Abstract

Extremely low rotational barriers for ethylenic C=C double bonds in the range 5–17 kcal/mol have been measured by low temperature 1H and 13C NMR spectroscopy and confirmed by calculations at the M06‐2X level for 5‐methylene‐1,3‐dioxane‐4,6‐diones (Meldrum's acid derivatives) and 4‐methyleneisoxazol‐5(4H)‐ones. The barriers are ascribed to the push‐pull (donor‐acceptor) nature of the alkenes. A correlation between the calculated C=C bond lengths and the ratios of electron occupancies in the ethylenic π and π* orbitals (the π/π* quotient) constitutes a useful measure of the push‐pull character.

Influence of Hydrogen Bonds and π–π Interactions on the Fluorescence of Crystalline (N-Alkylpyridyl)enamino-pyrrolo[2,3-b]quinoxalin-2-one Derivatives

“…Synthesis and UV Data in Solution. The syntheses of push−pull chromophores (E/Z)-2, 43 (E/Z)-3, 44 and (E/Z)-4 45 were published earlier, and the crystal structures of compounds (Z)-2, (E)-2, (E)-4(P2 The compounds occur in the equilibrium of (E)-and (Z)diastereoisomers, 47,48 with the E-form predominating in the solutions. As a result of E/Z-isomerization, the enamines exhibit very weak dual fluorescence (Φ f < 0.1%) in organic solvents.…”

Copper(II)‐Catalyzed Aerobic Oxidative Desulfitative 6π Electrocyclization: Efficient Synthesis of Diverse 4‐Aminoquinolines

“…According to the nature of the reversible isomerization of ketene N,S ‐acetals,, and elimination of EtSH from 9 (Scheme c), a base‐promoted elimination of EtSH from imino ketene N,S ‐acetal 2a would occur along with the generation of ketenimine intermediate B and LCu(SEt) 2 (L=Phen). Ketenimine ( B ) undergoes 6π electrocyclization followed by aromatization to give the thermodynamically favored 4‐anilinoquinolines 3 .…”

“…For the preparation of 2 , catalyzed by BF 3 ⋅Et 2 O (30 mol%), the model reaction (Table ) of the readily available ketene S,S ‐acetal 1a bearing an acetyl and an ethoxycarbonyl group, respectively, at the terminal carbon was reacted with aniline (3 equiv.) in toluene by adding aniline (toluene solution) in 3 portions to the mixture of 1a and BF 3 ⋅Et 2 O within 1 h. As the result, ketene N,S ‐acetal 2a , as an E / Z mixture,, was obtained in 76% yield by running the reaction at 80 °C for 5 h. However, a lower yield of 2d (47%) was afforded from the reaction of 1a with 4‐methoxyaniline under identical reaction conditions due to, probably, the reduced nucleophilicity of 4‐methoxyaniline by the formation of a complex between the 4‐methoxy moiety of 4‐methoxyaniline and BF 3 ⋅Et 2 O. Hopefully, when a super acid catalyst, triflic acid (TfOH), was selected to enhance the electrophilicity of dicarbonyl substrate 1a through protonation with the carbonyl oxygen, both 2a and 2d were obtained in high yields. Thus, catalyzed by TfOH, a series of N ‐arylimino ketene N,S ‐acetals 2a – 2w was prepared from the pseudo‐three‐component reaction of selected arylamines with ketene S,S ‐acetals 1a and 1b , respectively (Table ).…”

“…α‐Oxoketene N,S ‐acetals, which easily isomerize between ( Z )‐ and ( E )‐configurations in both polar and non‐polar media under various conditions via an imine intermediate,, are versatile building blocks in organic synthesis –. This isomerization is important for cyclization reactions due to the “equal contribution”, for example, of the ( Z )‐ and ( E )‐isomers of ketene N,S ‐acetals 2 with dense reactive sites …”

Synthesis and exploration of configurational dynamics in equilibratingE/Z2-aryliminothiazolidin-4-ones using NMR and estimation of thermodynamic parameters

“…Variable temperature dynamic NMR (VT DNMR) is one of the applications of NMR to probe the number and symmetry of conformations in the solution phase where the temperature of the process varies sequentially. This process also provides the rates of internal rotation and corresponding rotational barrier for several chemical systems such as amides, 6,7 [n]cumulenes, 8 metal-coordinated olefins, 2 twisted CQC bonds 9 and phosphoramidates. 10 Analyzing the dynamic equilibrium using VT DNMR helps in distinguishing coalescence temperature and in exploring the rotational energy barrier between two conformations using the Eyring equation.…”