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A general treatment of macrocyclization reactions occurring under thermodynamic control is presented. The fundamental quantities on which the treatment is based are the effective molarities of the cyclic oligomers and the equilibrium constant for the intermolecular model reaction between monofunctional reactants (Kinter). Four typical cases have been considered, namely, addition and condensation of a monomer of the type A-B, addition of A-A, and addition of A-A + B-B. A critical comparison with the classical theory of Jacobson and Stockmayer is presented.It is shown that the phenomenon characterized by the critical monomer concentration (cut-off point) is a limiting phenomenon which would occur only for infinitely large values of Kin,,,. The treatment has been successfully applied to the DOS/DTC-induced cyclooligomerization of /3-propiolactone in CDC13 solution that yields well-behaved ringchain equilibrates closely adhering to the theoretical model. Best fit of the experimental product distributions to the general equations gave the equilibrium constant (Kinter) of the intermolecular model reaction, as well as the effective molarities (EMi) for the cyclic oligomers from trimer to octamer. The EMi values decrease in proportion of the -2.5 power of the oligomerization degree, thus providing a strong indication that the oligomeric polylactones are essentially strainless. The extremely low value of Kin,,, (2.5) is responsible for the absence of a cut-off point, which is usually present in ring-chain polymeric equilibrates.

The role of ring strain on the ease of ring formation over a wide range of ring sizes is discussed on the basis of a comparison of transition state versus product‐ring strain energies. A general procedure is illustrated for the assessment of transition‐state strain energies, based on experimental effective molarities (EMs) and an extra‐thermodynamic treatment of entropy data for ring closure. It is found that the ring product is a good model of the transition state of cyclisation for all but the shortest chains. Our earlier interpretation of this remarkable lack of correspondence between transition state and product‐ring strain energies in intramolecular nucleophilic substitutions of the very short chains, is compared with the results of recent theoretical calculations.

Amine-imine exchange reactions of sterically unhindered reactants were found to be surprisingly fast at room temperature in a variety of nonaqueous solvents in the absence of proton and metal catalysts. The reaction mechanism suggested by ab initio calculations in the gas phase involves nucleophilic addition to the CN bond in concert with proton transfer from the amine NH bond to the imine nitrogen via a highly imbalanced transition state. These very fast transimination reactions were utilized in the catalysis of imine metathesis. Imine metathesis, usually carried out in organic solvents at high temperature in the presence of metal catalysts, occurs smoothly at room temperature in the presence of primary amines under nonacidic conditions as a result of coupled transimination processes. Kinetic data fully consistent with the proposed reaction mechanism were obtained. © 2013 The Royal Society of Chemistry