Volume parameters of some Diels-Alder reactions involving C=C, C=S, and N=N bonds

Abstract

The effect of a hydrostatic pressure of up to 1000 kg cm -2 on the rate constants of the Diels-Alder reactions of maleic anhydride with 1,2,3,4 tetraphenylcyclopentadiene and with 6,13 dichloropentacene, of 4 phenyl 1,2,4 triazoline 3,5 dione with hexachlorocyclo pentadiene, and of thiobenzophenone with isoprene was studied at 25 °С. The volume param eters and ratios of the activation to reaction volumes make it possible to exclude electrostriction of the solvent during transition state solvation in all the reactions studied, which corresponds to the nonpolar nature of the transition state.Elevated pressure facilitates substantially chemical pro cesses if the volume of the system decreases during the reaction. The origin of a volume change is not the main problem for practical purpose. However, the use of vol ume parameters of the reaction to refine the reaction mechanism requires information on the contributions that form these volume parameters. The factors that affect the activation volume (∆V ≠ ) and Diels-Alder reaction vol ume (∆V ) have been considered earlier. 1-4 These volume parameters can be determined from the dependence of the rate (k) or equilibrium (K ) constant on the pres sure (p).Real and imaginary changes in the volume parameters should be distinguished. Real values of the volume pa rameters are caused by the general volume changes in the system (addends, transition state, and solvents) and in clude changes of the intrinsic (van der Waals, ∆V W ) vol umes due to the bond redistribution and changes in inter molecular volumes (∆V solv ) in the solvate shell of the cor responding states (reactants, transition state, products). The reaction volume values can be obtained both by Eq.(2) and directly from the difference between partial molar volumes (PMVs) of the products and reactants. The activation volumes can be calculated only using the dependence (1). If an elevated pressure can induce a change in the properties of the system, which, in turn, affects the free activation or reaction energy, then the volume parameter values including the imaginary contri butions can be obtained from Eqs (1) and (2). With a pressure increase, the rate (k) or equilibrium (K ) con stants can change due to the energy р•∆V ≠ or р•∆V and pressure induced changes: in the dielectric constant of the medium, 3 concentration of active species in the cata lytic process, 5 and viscosity of the medium to a level of the diffusionally controlled rate of the process. 4,6Two new C-C bonds are consistently formed in a nonpolar Diels-Alder reaction. However, the values of the activation and reaction volumes can strongly differ (from -15 to -40 cm 3 mol -1 ), depending on the size and structure of the reactants. 7 The formation of the products from the reactants with elevated packing coefficients (η = V W /V ) was found 8 to be usually accompanied by a weak increase in this parameter, while an increase in the packing coefficients on going to the adducts is much higher in the reactions involving "loose" reactants. The observed reaction...

Thia-Diels–Alder reactions of hetaryl thioketones with nonactivated 1,3-dienes leading to 3,6-dihydro-2H-pyrans: evidence for a diradical mechanism

“…15 In a recent study, the volume parameters of the reaction of thiobenzophenone with isoprene were determined, and on this basis the authors concluded that a concerted reaction mechanism is followed. 16 In recent reports, asymmetric thia-Diels-Alder reactions of activated dithioesters 17 as well as of aryl-and hetaryl thioketones 18 with an in-situ-generated chiral trienamine were presented. In the second case, a stepwise reaction mechanism via diradical intermediates was proposed.…”

A New Way to Prepare Ferrate Solutions by Microwave Assisted Hypochlorite-Oxidation Method

“…It has been well documented [1] that sodium ferrate, Na 2 FeO 4 , is a strong and environmental friendly oxidant. Under the acidic conditions, the redox potential of ferrate (VI) ions is as high as 2.2V.…”

“…Additionally, the product of ferrate oxidation reaction is considered to be ferric hydroxide, which is useful coagulant and will enhance the water remediation performance. Up to now, there are three methods have been reported to synthesis of ferrate: (1) the electrochemical method by which anodic oxidation uses iron or alloy as the anode and NaOH/KOH as the electrolyte. The production yield is revealed to be strongly dependent on the electrolyte temperature and current density; (2) the dry method by which various iron oxide-containing minerals are heated or melted under strong alkaline conditions and with oxygen flow.…”