Cyclopentadienes water promoted

On the basis of Monte Carlo simulations [40] and molecular orbital calculations [26a], hydrogen bonding was proposed as the key factor controlling the variation of the acceleration for Diels-Alder reactions in water. Experimental differences of rate acceleration in water-promoted cycloadditions were recently observed [41]. Cycloadditions of cyclopentadiene with acridizinium bromide, acrylonitrile and methyl vinyl ketone were investigated in water and in ethanol for comparison (Scheme 3). Only a modest rate acceleration of 5.3 was found with acridizinium bromide, which was attributed to the absence of hydrogenbonding groups in the reactants. The acceleration factor reaches about 14 with acrylonitrile and 60 with methyl vinyl ketone, which is the best hydrogen-bond acceptor [41]. 

Lubineau, A. Bouchain, G. (1997) Water-promoted reactivities generation of oxyallyl intermediates and their [4+3] cycloadditions with furan and cyclopentadiene. Facile access to bridged cycloheptenones. Tetrahedron Lett., in press. 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

We were interested in the possibility that Diels-Alder additions could be promoted if the two components bound into the same cavity in a cyclodextrin. We examined three examples 1,3-cyclopentadiene plus butenone, 1,3-cyclopentadiene plus acrylonitrile, and anthracene-9-carbinol plus N-ethylmaleimide (Fig. 1.19). We saw several interesting effects. First of aU, relative to the second-order rate constant in water, the addition of 10 mM -cyclodextrin... 

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