Organic carbonates. Part XIV. Polar and steric effects of substituents influencing the modes of ring-opening of highly-branched ethylene and trimethylene carbonates by various nucleophiles: A nuclear magnetic resonance study

The appearance of intermediates and the formation of isomeric products resulting from interactions of ten highly branched ethylene (Ib - k) and seven trimethylene carbonates (IIa - g) with nucleophiles such as hydroxide, n-butylamine, ethylenediamine, glycine, and imidazole, were monitored and quantitatively estimated by the n.m.r. technique. The nucleophilic ring-opening of the 1-monosubstituted (Ib, If - k, IIc - g) and 1,1-disubstituted series (Id, IIe - f) gave rise to isomeric hydroxyalkyl carbonates and/or hydroxyalkyl carbamates, the ratio of which varies with substituents and pH. In the 1-monosubstituted carbonates the preferred mode of O-CO bond cleavage is that which leads to the more acidic alcoholic species, whereas in the 1,1-disubstituted series the mode of ring-opening which leads to the more basic alcohol species is the preferred one. The product ratio in the 1-monosubstituted series is linearly dependent on the σ* value of the substituent on the ring, and also yields a linear Brönsted correlation. The two modes of ring-opening in 1-monosubstituted carbonates is governed by the difference in the electronic effects of the leaving oxygens. In the case of 1,1-disubstituted carbonates the polar effect of the leaving group appears to be obliterated by steric factors (conformational and torsional effects) which become mode-determining for the breakdown of the tetrahedral intermediate.