Evaluation of Strain Effects on the Reactivity of Small Rings

Abstract

Abstract. The qualitative effect of ring strain on the reactivity of small ring systems is briefly reviewed. It is pointed out that nucleofugalities of groups cannot be measured for nucleophilic substitution but that such measurements are possible for activated eliminations. Application of the techniques for determination of nucleofugality to eliminative fission of small strained rings shows that nucleofugality is very greatly enhanced. The extent of enhancement cannot be determined for epoxides, but has been determined for cyclopropanes, the results showing that about 60% of the ring strain energy is released in the transition state of ring fission. Catalysis of ring fission requires very specific placement of proton donor groups; gem-dimethyl substitution raises the enthalpy of the transition state for eliminative fission of cyclopropanes.This account considers the question: How much does ring strain energy contribute to reactivity in heterolytic ring cleavage? A striking feature of small ring chemistry is that in spite of the fact that ring strain is a major factor in controlling the reactivity of small rings, almost no quantitative data relating to this question is available. This account describes quantitative answers to this question for the first time and, within the compass of the account, the results have a bias towards recent work in these laboratories.strain energy that appears in reducing the energy of activation of the ring c1earage reaction?There are two serious difficulties in answering such a question. First, unstrained acyclic analogues, with which to compare the reactivity of strained cyclic systems, are not readily available particularly as the leaving groups must be at least two-coordinate. Second, the relationship between reactivity in nucleophilic substitution and 'leaving group ability' in a quantitative sense is unexplored. RING STRAIN AND SUBSTITUfIVE CLEAVAGE OF SMALL RINGSRing strain energies are summarised in Table 1. The very large strain energies of three-and four-membered rings are interpretable as arising from inefficient bonding by overlap of orbitals of high p-character.' These large strain energies are clearly responsible for reactions shown particularly by three-and four-membered ring systems, but not by their acyclic (unstrained) analogues or by larger ring systems of lesser strain. Typical of such reactions are nucleophilic substitutions on cyclopropanes and epoxides, Eq. (I), (Fig. 1). Such reactions, involving SN2 displacements of uncharged oxygen and carbon leaving groups, do not have analogues in acyclic or larger ring systems (Eq. (2». Dimethyl ether, for example, is not susceptible to nucleophilic substitution with displacement of methoxide ion. In alkaline hydrolysis of an epoxide, the strain of the ring reduces the energy of activation for the reaction by transforming the 'poor' alkoxide leaving group into a 'good' leaving group. The intriguing question is, therefore, how much does the (known) strain of the oxiran ring promote displacement of an alkoxide leaving group...

A theoretical investigation of the substituent effect on the chemical reactivity of NHC(R2 )-B and NHC-B(R'2 ) Boriranes

“…Historically, Stirling studied the qualitative effect of ring strain on the reactivity of small ring systems. It pointed out that nucleofugalities of groups cannot be measured for nucleophilic substitution but that such measurements are potential for activated eliminations [44]. Due to severe deformation from the ideal 180, the triple bond in cycloalkynes having less than nine ring atoms displays high reactivity with dienes, 1,3-dipoles, and other systems [17].…”