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.