Substituent effects strain energy

TABLE 3. Effect of substituents on the strain energies (kcalmol 1) at the HF/6-31G level... 

A recent theoretical study on the effect of substituents on the strain energies of small ring compounds has provided some valuable insight into the differences between 1,2-dioxetanes and 1,3-dioxetanes <2002JOC2588>. The C-H bonds within 1,2-dioxetane have been calculated to be stronger than those within 1,3-dioxetane by some 8 kcal mol-1 at the G2 level of theory. Calculations at the same level of theory indicate that 1,2-dioxetane is more strained than 1,3-dioxetane by some 6 kcal mol-1. Somewhat surprising is that this study has also shown that 1,2-dioxetanes are more strained than dioxiranes by some 7-12 kcal mol-1, which is in stark contrast to the case for the parent hydrocarbons and our expectations. The vibrational frequencies and the moments of inertia have also been calculated for the parent 1,2- and 1,3-dioxetanes <1997PGA2471>. 

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. 

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