Solvent effects, nitrile oxide cycloadditions

An unusual solvent effect was observed in cycloadditions of aromatic nitrile N-oxides with alkyl-substituted p-benzoquinones in ethanol-water (60 40) the reaction rates were 14-fold greater than those in chloroform (148). The use of ion pairs to control nitrile oxide cycloadditions was demonstrated. A chiral auxiliary bearing an ionic group and an associated counterion provides enhanced selectivity in the cycloaddition the intramolecular salt effect controls the orientation of the... 

N/L recorded was 16,000 for the reactions using the magnesium aUcoxide of 3-methyl-2-buten-l-ol as a multisubstituted internal allylic alcohol substrate, which is why regiocontrol is still effective in the reactions in a highly coordinating solvent such as THE. Rate enhancement is much lower in the nitrile oxide cycloaddition reactions using homoallylic alcohol substrates. 

Inoue, Y., Araki, K. and Shiraishi, S., Unusual solvent effect on the cycloadditions of aromatic nitrile n-oxide with alkyl-substituted p benzoquinones in ethanol-water systems. Bull Chem. Soc. Jpn., 1991, 64, 3079-3083. 

In all the evaluations of Table 4, the solvent effects on the activation free enthalpies are positive, increase with increasing solvent dielectric constants and tend to be larger for the endo than for the exo adducts. This behaviour, in accord with the experimental trend, is due to the electrostatic contribution the CDS and CDR contributions, in fact, are rather independent of the isomeric reaction considered and, moreover, appear to obtain comparable values in every 1,3-dipolar cycloaddition. For the Tomasi parametrisation in water, for example, the CDR" contribution for the cycloadditions of diazomethane and nitrile oxides to substituted alkenes amounts to -1.85 0.14 kcal mok This finding can be traced back to the view that the CDR term is approximately proportional to the solvent accessible surface area (the cavity area) of solutes and to the feature of TSs of having very alike structures of the new forming pentatomic ring so that the changes of the cavity areas from reactants to TSs are similar. 

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