Ring Strain

Ring strain is an effect that destabilizes cyclic compounds and in this context may make aziridines and aziridinium ions, which act as versatile intermediates in organic chemistry, susceptible to ring-opening reactions. Ring strain is characterized by the increase in energy due to the ring structure and plays an important role in the thermodynamics of aziridinium ions, as their existence makes the synthetically useful ring-opening reactions of these intermediates favorable.

Despite the important role of ring strain in certain activated aziridinium intermediates, it is rarely assessed quantitatively in experimental or theoretical studies, and the concept has not often been introduced in a formal way. A better understanding of various ring structures will be performed from a detailed theoretical analysis. For that reason, the ring strain of several ammonium species 3–8 (Fig. 1) is investigated.

Ring strain itself is composed of at least four different types of strain: Baeyer or angle strain, stretching strain, Pitzer or conformational strain, and Dunitz-Schomaker or transannular or steric strain (Fig. 2). Baeyer strain arises from bond angle expansion or compression relative to acyclic, unstrained species. This type is most pronounced in small rings. In aziridinium ions, for example, the endocyclic bond angle centered on a tetrahedral center is reduced from 109.5^ο to 60^ο. Baeyer strain is the most obvious type of strain. However, its effects are not always straightforward, as illustrated by the relatively low strain energy of cyclopropane compared to cyclobutane (115.1 and 110.1 kJ/mol, respectively). Indeed, three-membered rings are often stabilized by σ delocalization, which is characterized by an increase of electron density in the center of the ring.

Fig. 1 Cyclopropane 1, aziridine 2, and substituted ammonium ions 3–8

Fig. 2 Types of (ring) strain