- Clay-catalyzed solventless addition reactions of furan with α,β- unsaturated carbonyl compounds
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The reaction of furan with α,β-unsaturated carbonyl dienophiles catalyzed by K10 montmorillonite in the absence of organic solvents produces the corresponding Diels-Alder adducts and, in the case of methyl vinyl ketone, Michael-type products, under much milder conditions than the conventional protocols. The results are consistent with acid catalysis on the clay surface. Acrylates gave lower yields and/or decomposition products. The reaction can be extended to alkynic substrates such as DMAD to afford cycloadducts in good yields.
- Avalos, Martin,Babiano, Reyes,Bravo, Jose L.,Cintas, Pedro,Jimenez, Jose L.,Palacios, Juan C.
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p. 9301 - 9304
(2007/10/03)
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- CONJUGATE ADDITION OF FURANS TO ENONES OR THEIR SYNTHETIC EQUIVALENTS
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Conjugate addition of furans 5 to enones 3 in the presence of an alcohol and boron trifluoride etherate is described.A same reaction product is obtained with hemiacetal vinylogs 1, synthetic equivalents of enones 3.Acidic treatment of adducts 6 and 7 leads to tri- and tetraketones 15 and 16.
- Poirier, Jean-Marie,Dujardin, Gilles
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p. 399 - 407
(2007/10/02)
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- Influence de la pression sur les reactions de substitution electrophile du furanne et de methyl-2 furanne
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We previously investigated the (4+2) cycloaddition reactions of 2-methylfuran, which are known to be very difficult, even impossible, under the usual conditions for Diels-Alder reactions.We found that the adducts are generated only at low temperatures (20-40 deg C) and high pressures (ca. 10 kbar)(Bull.Soc.Chim., 1978, p.461).However, by increasing the temperature, we have observed the alternative reaction consisting in the α-substitution of furan when furan compounds are allowed to react with α,β-unsaturated carbonyl compounds at temperatures above 100 deg C.We found that this reaction, which proceeds slowly (or not at all) at atmospheric pressure, is noticeably faster at high pressures.Having in mind a simple method for facile synthesis of mono- and disubstituted furans, we thought that pressure could be an interesting parameter in producing such compounds in high yield.Additionally, since the reaction was found some time ago to be very sensitive to acidic media (electrophilic substitution of the Friedel-Crafts type), we imagined that a possible pressure induced creation of charges in the transition state would give rise to an electrostriction contribution to the reaction rate.We first investigated the thermal synthesis without catalyst and found that only unsaturated carbonyl compounds show some reactivity, while unsaturated nitriles or esters do not react.However, the substitution reaction is considerably perturbed by the polymerisation reaction, which is also positively influenced by pressure and thus reduces drastically the yield of substituted products, except in some rare cases.The use of a suitable catalyst (AcOH-H2O) combined with pressure (3 000 bar) allows satisfactory yields of substituted furans with little formation of polymer.The pressure kinetics of two selected reactions reveals a strong pressure acceleration with activation volumes in the range (-45 to -50 cm3/mol) independent of the solvent.We suggest that starting from a neutral initial state, during the activation process there is a development of electrostriction with the possible appearance of strongly polarized species, the polarization being enhanced by pressure.However, in this case we would normally anticipate a solvent dependence of the activation volume ΔV(excit.); in fact this effect is not observed.Nevertheless, the hypothesis of electrostriction is quite plausible by the fact that, when the reaction is carried out in the presence of the catalyst (AcOH-H2O), the value ΔV(excit.) is reduced to half the former value (-24.5 cm3/mol), because the initial state is no longer neutral and charges are present along the activation process.
- Jenner, Gerard,Rimmelin, Jean,Antoni, Francois,Libs, Suzanne,Schleiffer, Elisabeth
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