Strain in medium rings

Rings larger than six-membered are always puckered unless they contain a large number of sp atoms (see the section on strain in medium rings, p. 185). 

Release of strain in medium rings becomes apparent in the mild reaction conditions used in the synthesis of large rings, e.g., 9940. 

Unrestricted LCAO-SCF-MO calculations using the INDO approximation have been carried out on various deformed geometries of allene the results show that simultaneous twisting and bending from the ideal geometry is the most efficient way of relieving strain in medium-ring allenes. ... 

The naphthodiazocine monocation 185 was found to undergo 100% equilibrium N-protonation. This was in contrast to the 9- and 10-memb-ered ring homologues, which underwent 100% C-protonation on the naphthalene nucleus to give an sp2-hybridized imminium ion C=N +. This phenomenon is believed to be favored in medium rings. However, models of C-protonated 185 did seem strained (82JCS(P2)477). 

In 1972, Mock considered double-bond reactivity and its relationship to torsional strain, by which he understood the strain imposed on a double bond in medium-ring fra 5-cycloalkenes or by steric compression of large cis substituents [28]. He argued that the loss of 7t overlap due to a torsion about the double bond can be partially compensated by rehybridization in these two situations, leading, respectively, to syn and anti pyramidalization of the double bond consequently, such bonds will favor different modes of addition (cis and trans). The proposition was supported by examples of X-ray structures of strained olefins, STO-3G energy calculations for the twisted and pyramidalized ethylene geometries, and by analysis of the out-of-plane vibrational frequencies of ethylene. Mock concluded that small ground-state distortions may produce sizable effects in the transition states. 

As mentioned in Chapter 2, these molecules all suffer a high level of strain in the rings. Medium sized rings experience steric strain and the introduction of trans double bonds increases this significantly. This stored energy has dramatic effects on the chemistry of the caryophyllenes and humulenes as we shall see. 

PROBLEM 5.6 Use models to look for the sources of strain in medium-sized rings. 

Figure 4.3 Cycloalkane strain energies, calculated by taking the difference between cycloalkane heat of combustion per CH2 and acyclic alkane heat of combustion per CH2, and multiplying by the number of CH2 units in a ring. Small and medium rings are strained, but cyclohexane rings are strain-free.

The relation between matter and ether was rendered clearer by Lord Kelvin s vortex-atom theory, which assumed that material atoms are vortex rings in the ether. The properties of electrical and magnetic systems have been included by regarding the atom as a structure of electrons, and an electron as a nucleus of permanent strain in the ether— a place at which the continuity of the medium has been broken and cemented together again without fitting the parts, so that there is a residual strain all round the place (Larmor). 

However, it is also possible to avoid an intramolecular reaction as the first step, for example if the cycle being formed in this transformation would be somehow strained, as observed for the formation of medium rings. In such a case, an inter-molecular first takes place, followed by an intramolecular reaction. 

This efficient cyclobutanol forming reaction has been utilized in the synthesis of terpenes404) (4.4), medium ring compounds40s) (4.5) and strained molecules as in (4,6)406) and (4.7) 407). In the last example the tricycloP. O.O jhexane skeleton is formed. 

Medium-ring bicyclic diamines have now been prepared and their proton transfer behaviour has been investigated (Alder, 1983). The diamine 1,6-djflza-bicyclo[4.4.4]tetradecane [73] is reluctant to undergo outside-prptonation, presumably because outward pyramidalisation of the nitrogen atoms intro duces strain the diamine exists in the in-in conformation. The strain involved... 

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