The Effect of Pressure on Retro Diels–Alder Reactions

Abstract

The activation and reaction volumes (∆V /∆V) were showing negative volumes of activation (∆V Ͻ 0) whereas the others are slightly retarded (∆V Ͼ 0). From the analysis determined for the retro Diels-Alder reactions of the parent dihydrobarrelene 1a, its 2-cyano derivative 1b, the exo and of the volume data including the van der Waals volumes (V w ) one can conclude that the packing coefficients of the endo Diels-Alder adducts of maleic anhydride to naphthalene exo-, endo-4, and the exo and endo Diels-Alder pericyclic transition states are equal to or even larger than those of the corresponding Diels-Alder adducts. This finding adducts of N-phenylmaleic imide to 6,6-dimethylfulvene exo-, endo-8 from the pressure dependence of the rate may be explained with the restriction of the degrees of freedom in the transition states leading to a contraction of constants and the partial molar volumes (V) of reactants and products at various temperatures. The cleavage of exo-4, the expansion volume. endo-4, and exo-8 are slightly accelerated by pressure Many DielsϪAlder reactions show a powerful pressure-The intrinsic molar volumes, the so-called van der Waals volumes, V W , calculated for the pericyclic transition strucinduced acceleration which is often turned to good synthetic purposes.[1][2] The activation volumes ∆V resulting tures of DielsϪAlder reactions by the use of the structural parameters obtained from quantum-mechanical calcufrom the pressure dependence of the rate constants are usually highly negative (∆V ഠ Ϫ25 to Ϫ45 cm 3 mol Ϫ1 ), some-lations [6] and the van der Waals radii of the different types of atoms derived from X-ray data are generally larger than times even more negative than the corresponding reaction volume ∆V so that the ∆V /∆V ratio is close to or even those calculated analogously for the corresponding cycloadducts. [3g,7] Grieger and Eckert [8] considered two explalarger than unity (Θ ϭ ∆V /∆V Ն 1). [3] Within the scope of the transition-state theory, activation nations of the ratio Θ > 1 in the DielsϪAlder reaction of isoprene as diene with maleic anhydride as dienophile: a volumes can be considered to be a measure of the relative partial molar volume of the transition states [∆V ϭ larger dipole moment of the transition state or secondary orbital interactions [9] from which the secondary orbital in-V (TS) Ϫ ΣV(reactants)]. Accordingly, the molar volumes of transition states of many DielsϪAlder reactions are ap-teractions can only occur in endo DielsϪAlder reactions.The finding that the difference between the activation volproximately equal to or even smaller than those of the corresponding cycloadducts. These surprising results could be umes of many endo and exo DielsϪAlder reactions is small (∆∆V > 1Ϫ2 cm 3 mol Ϫ1 ), [1e] seems to rule out that secondconfirmed by two independent studies. In the DielsϪAlder reactions of furan with acrylonitrile [4] and in that of N-ary orbital interactions are important and induce a larger contraction of the volume of the endo transition state. benzoylpyrro...

The Effect of Pressure on Organic Reactions

“…In a more recent investigation F.-G. Klärner and V. Breitkopf observed that the retro Diels-Alder reactions of dihydrobarrelene and its 2-cyanoderivative and of the endo-DielsAlder adduct between dimethylfulvene and N-phenylmaleic imide are slightly decelerated by pressure showing positive volumes of activation whereas the retro Diels-Alder reactions of the endo and exo [4+2] cycloadducts between naphthalene and maleic anhydride and that of the exo adduct between dimethylfulvene and N-phenylmaleic imide are accelerated by pressure showing a negative volume of activation (Scheme 4) [25]. Grieger and Eckert [26] considered two explanations of the ratio Φ > 1 in the Diels-Alder reaction of isoprene with maleic anhydride: a larger dipole moment of the transition state or secondary orbital interactions which can only occur in endo Diels-Alder reactions.…”

Diels-Alder reaction between naphthalene and N-phenylmaleimide under ambient and high pressure conditions

“…12 The rate of the Diels-Alder reaction between substituted anthracenes and activated dienophile (7) NMR spectra differ significantly for the endo-and exo-adducts. 7 However, the NMR spectra of the adduct obtained in the catalyzed reaction 2 + 7 (Scheme 2) indicate the presence of the only one isomer (8 or 9). Exo-isomer 8 with m.p.…”

“…More drastic temperature conditions are unfavorable for the yield of the target adduct, because the process is reversible. Only elevated temperature (100 °C) and pressure (10-12 kbar) make it possible to achieve 90% yield of the product (adduct of naphthalene with maleic anhydride) 6,7 The purpose of this work was to estimate the reactivity of simple arenes as dienes in the Diels-Alder reaction, and to use different ways of activation of reactants in this reaction.…”

Microwave Assisted Synthesis of Biologically Active 4-Hydroxy-7v-(Phenylcarbonyl)-2h-1,2-Benzothiazine-3-Carbohydrazide 1,1-Dioxide Derivatives

“…To achieve this, innovative and unconventional synthetic procedures using water as solvent 1 under normal 2 and super heated conditions 3 , in supercritical fluids 4 , ionic liquids 5 and micro-emulsions 6 are being used now a days. Besides these, ultrasound 7 and microwaves 8 assisted reactions both in solvent as well as in solventless conditions are becoming popular due to their shorter reaction times and greater yields.…”