An extension of the study of selectivity of the heavy carbons versus light carbons is the study of regioselectivity in unsymmetrical reactions. No significant
regioselectivity is normally expected in additions to internal disubstituted alkenes, for example additions to cis-2-undecene (17). The explanation for this routine experience is subtly built on transition state theory. The size of the groups makes no difference in the
barriers for reactions, and when two reactions face an equal free-energy barrier for passage through their competitive transition states, the two occur at the same rate.
There are many similar situations, for example the Diels-Alder cycloaddition of methacrolein and analogous enones. For these reactions little selectivity might be expected among the products 22 and 23 from the reaction of 20 and 21, no matter the size of the chain in 20. But in actuality the selectivity of the reaction would not be controlled by transition state energies.71 This reaction instead involves a bifurcating
energy surface and would be controlled by dynamics. The addition of dichloroketene to unsymmetrical cis-2-alkenes like 17 would also involve a bifurcating energy surface along with significant amounts of recrossing (as seen earlier in this chapter). Because dynamics controls these reactions instead of TST the standard rules of selectivity no longer apply.
Similar experiments could be carried out in the additions of dichloroketene to unsymmetrical cis-2-alkenes as done with cis-2-butene to test whether group size on the alkene has the same effect on recrossing that is seen from an isotopic substitution in
inertial isotope effects. The alkene studied for this experiment was cis-2-heptene and the resulting product ratios were measured by looking at a 1H NMR of a completed reaction and looking at the protons beta to the carbonyl for each product (Figure 2.14).
Figure 2.14. Protons that were used to determine product ratios in the 1H NMR. Proton for 24 was at a frequency of 3.0 ppm and proton for 25 was at a frequency of 2.9 ppm.
The full spectra can be seen in the experimental chapter and the protons were assigned based literature values for 2,2-dichlorocyclobutanones which arise in the 3.0 ppm region. The assignment for the proton of 24 versus 25 was done based on coupling constants. Because the proton of 24 is coupled by 4 other protons while the proton from 25 is coupled by only 3 other protons, the proton for 24 should have a larger total coupling constant associated with its spectral peak. Integrations of these peaks give an relative ratio of 46.5 % for 24 and 53.5 % for 25. While the product ratio difference is not large there is still a slight preference for the dichloroketene to attack the side with the longer chain over the side with the methyl group. This correlates well with the isotopic observation of cis-2-butene where there was a preference for the dichloroketene
to attack the heavier atom. As explained earlier this could be due to the preference to form the second bond to be associated with a lighter atom or in this case a smaller chain.
In order to further study these experimental results we turn to theoretical calculations. The transition state and starting materials were optimized with a
MPW1K/6-31+G** level of theory for the cycloaddition of dichloroketene and cis-2- undecene (the optimized transition state can be seen in Figure 2.15). Cis-2-undecene was chosen to study theoretically instead of cis-2-heptene for hopes to enhance the recrossing effect that we predict as the cause of the product ratio difference observed
experimentally.
Figure 2.15. Transition state for the cycloaddition of dichloroketene and cis-2-undecene with a MPW1K/6-31+G** level of theory.
The associated bond distance for the ketene olefin C-C distance is 2.171 Å and 2.128 Å for the C2 and C3 carbon respectively. Showing a slight preference for the attack on the C3 carbon. Instead of two separate transition state structures to afford the two products there is one transition state on a bifurcating energy surface leading to the
Cl
O
2.128
2.171
two products. In order to examine the product ratios from a theoretical perspective dynamic trajectories were employed in much the same way as with the cis-2-butene and cyclohexene studies. The trajectory studies are summarized in Table 2.9.
Table 2.9. Trajectory results for the cycloaddition of dichloroketene and cis-2-undecene from a MPW1K/6-31G* energy surface.
Total runs Total recrossing Product 15 Product 16
231 158 47 26
As can be seen in the table the trajectory study predicts the opposite effect that is seen experimentally. While the amount of recrossing reaches an astounding 68 % of the trajectories, the dynamic runs show a 1.8 : 1 preference for formation of product 24. While we would predict based on the previous results of cis-2-butene and the experimental evidence that the trajectories would favor the formation of 25, the theoretical results tell a different story. Clearly the theory used in this case does not encompass the true nature of this reaction. As to the reasoning why the theory fails in this case there are many possible explanation and the true nature of the erroneous
prediction will never truly be understood. However, it is possible that a different method or basis set could give a more accurate description of the energy surface and ultimately correct this discrepancy. Further investigation will need to be done to in order to obtain a better understanding of this system.
2.5 Experimental Procedures