Cycloalkenes strain energies

Even some cis cycloalkenes have very large strain energies when compared to analogous cycloalkanes. 

Strain energies (kcal mol 1) of cyclic ketones and cycloalkenes relative to cycloalkanes from force-field calculations... 

Heats of formation, strain energies, and olefinic strain for cycloalkenes. 

We have once earlier used the term cycloethene for acetylene in the context of defining the strain energy of acetylene in terms of ethylene, i.e. AHf(g, C2H2) vs 2AH g, C,H4) AH g, C2H6)], in order to compare that quantity with the strain energies of cyclopropene and other cycloalkenes cf Reference 4, p. 94. 

Cyclodecene is the smallest cycloalkene, which can accommodate a trans double bond without significant deformation of bond angles and/or dihedral angles. The strain energies and structures of smaller fraws-cycloalkenes have been the subjects of considerable research over the years. 

The equilibrium constants for a series of cycloalkenes decrease in the order norbomene > c -cyclooc-tene > cyclopentene > cycloheptene > cyclohexene, which correlates with the calculated strain energies as well as the kinetically determined relative adsorption constants on Pt (Table 2). Tolman states that electron donation from a filled metal rf-orbital to an empty alkene Tr -orbital is extremely important in determining the stability of these complexes. Steric effects of substituents are relatively unimportant compared to electronic effects, and resonance is more important than inductive interactions. The ability of the metal to back bond is lowered progressively in the series Ni° > Pt° > Rh > Pt" > Ag which reduces the importance of resonance and decreases the selectivity of the metal for different substituted alkenes. 

TABLE 6. Heats of formation and strain energies of cycloalkanes and cycloalkenes (kcal mol )... 

Kinetic and computational studies by Shea and Kim on MCPBA epoxidations of a series of cyclic alkenes including bridgehead alkenes and tra/w-cycloalkenes have shown that the reactivity depends primarily on the strain energy relief in the transition state <92JA3044>. 

Experimental Heats of Formation AH°f(g) (kcal/mol), Strain Energies (SE), and Olefmic Strain Energies (OS) of Cycloalkenes (218)... 

In another analysis of structure-reactivity relationships in free radical addition reactions of cycloalkenes with ring size 5-8, it was concluded that strain effects as well as the electronegativity of the attacking radical affect the reactivity (259). The rate constants for the addition reactions of the p-chlorophenylthiyl radical to cycloalkenes of ring size 5-8 and 10 have been discussed in terms of released strain energy and the polar nature of the transition state (260). 

For two different (but numerically and conceptually comparable) sets of strain energies of the simple cycloalkenes, see A. Greenberg and J. F. Liebman, Strained Organic Molecules, Academic Press, New York, 1978, p. 94. 

Strain energies in some alicyclic compounds Relative stabilities of cw- and cycloalkenes... 

Highly constrained (E)-cycloalkenes, generated readily through the photoisomerization of (Z)-isomers, are known to possess unique physical and (chir)optical properties. These properties are of particular interest and importance from both the experimental and theoretical points of view. For example, the experimental barriers of thermal E-to-Z isomerization of cycloalkenes are compared with the theoretical values obtained by semiempirical calculations. Among the calculations reported, the PM3 method appears to give good fits to the experimental results e.g., the experimental strain energy of (Z)-cyclooctene (20Z) is 11.37 kcal/mol, which is close to the value (10.55 kcal/mol) obtained by PM3. The relevant experimental (chir)optical properties, such as specific rotations and absorption maxima and thermodynamic parameters for isomerization, are summarized in Table 16.2. 

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