Talk:Mod:bhheller
Q1. Your calculated energy values are all correct and the explanation for the origin of kinetic control in the reaction is good. For the monohydrogenated products, you correctly state that the difference in energies is due to the relative strain of the two double bond types. The lower energy product is formed in practise – but you should be careful when reading into this fact: The lower energy product could be formed under thermodynamic OR kinetic control depending on the reaction. Diels-Alder reactions are actually a bit of an odd case because usually the lowest energy product is both the thermodynamic product and the kinetic product (i.e. the transition state that leads to it is lowest in energy.
--
Q2. The energy values are again good. The presentation of the variation of dihedral angle vs. energy is an interesting way to depict the question. What were the major structural differences in the conformations you found? A discussion of this and the way you went about finding different structures would have been something else you could have mentioned. The origin of stereoselectivity is well explained (assistance vs. repulsion) as is the reason why you can’t carry out MM2 calculations on magnesium containing intermediates.
--
Q3. Your energy calculations are a little high in this question. For compound 9 a lower energy conformation exists in which the 6-ring is in a twist-boat conformation. Since 6-membered rings have some well-known possible conformations (chair, twist-boat and boat are all possible depending on the substituents) they are often necessary to manipulate in problems such as this. The discussion of energy differences for your isomers is excellent with good rationalisations of the energy contributions. Hyperstable alkenes are correctly defined.
--
Q4. The MOs look good and the IR stretch values are as expected. The discussion about the reactivity of the two alkene groups towards electrophilic attack is good as is the description of the effect of various substituents on the anti-alkene. You mention how the pi-sigma* interaction stabilises the anti-alkene, but it weakens the C-Cl bond, this can be seen in that the C-Cl stretch is greater for the hydrogenated system where such an interaction is no longer possible.
--
MP. This is an interesting choice for a computational project as it is an assessment of whether computational techniques can solve a structural reassignment problem already tackled by synthetic means. As you say experimental differentiation can in this case be carried out by comparison of the NMR shifts of a compound of known configuration (the synthesised material) with the naturally derived compound. You have calculated the 13C NMR shifts for the different isomers for comparison to the spectrum of the original naturally derived material and shown that the technique can be used to rule out the edaxadiene structure because of the significant differences in shift. It would be clearer to see all of your calculated data compared to the experimental spectrum to see which simulated structure gives the best fit. Since you found significant differences between the remaining isomers this would be the next step. This is the typical way in which computational reassignments are conducted in which often many isomers are compared to experimental data. Once the best fitting simulated isomers are decided, it is often the case that these structures are synthesised to confirm (in the way that the paper you found did).