Talk:Mod:sp31

Q1: Your energy values are spot on – but you have the wrong structure for compound 3 (it should be derived from the endo-product as shown in your Figure 1). The major difference between 1 and 2 is indeed torsional strain (deviation from ideal dihedral angles); for 3 and 4 it is bending strain due to differences in deviation from ideal sp2 bond angles – you still found that to be the case with the wrong structure. Another point to take care on is the naming of these compounds: it would be fine to label them all just compound or molecule 1, 2 etc...None of these structures can be described as “cyclopentadiene” however. The discussion about kinetic vs thermodynamic control is fine. The endo-dimer is shown to be a kinetic product because it is formed in spite of it not being the most stable product. It is not true to that the kinetic product is by definition the one that is higher in energy (i.e. it is not some kind of opposite to the thermodynamic product). The kinetic product is simply the one that comes from the lowest energy transition state and in fact the kinetic product and thermodynamic product is often the same. I wasn’t entirely sure if this had been understood (the relevant statement could be interpreted either way) so this is here to clear this up in case. As for the hydrogenation it is fine to say the lower energy compound is expected to form if you include the qualifier “under thermodynamic control”.

Q2. Your structures have the correct configuration but there is a lower energy conformation available for intermediate 1. It would have been worth discussing how you came to these minimised structures – e.g. show some other higher energy conformations. A key feature that is a good starting point here is the 6-ring; there are well-known conformations available for such structures and it is worth analysing some of them (chair vs twist-boat). In your structure for int 1 the ring is in the twist-boat when it should be chair. You mention why the structures have different energies which is good, but a fuller answer could have compared the strain contributions as in Q1. Part of the course is training in the techniques – so if you see an opportunity to apply something that was used in an earlier question go ahead because you’ll get extra marks for it. The definition of hyperstable alkenes is good.

Q3. Your MOs are good and the rationalisation for the regioselectivity is correct. The IR stretches you have listed are a little off in some cases – the C-Cl bond is expected to be strengthened when the double bond anti to it is removed, this is because there is a pi-sigma* interaction between it and the C-Cl antibonding orbital (LUMO+1). This is also the reason for the difference in the stretching frequencies of the two alkenes in the initial compound. Without seeing the actual stretches it is hard to say whether this is because the wrong peaks are listed – it is worth including even some images that illustrate the vibrations so that they could be checked.

Q4. R=Me is the right choice. Your PM6 energies are all pretty close and the structures look good. For some of them (B for example) a lower energy conformation is accessible if the carbonyl oxygen is positioned directly above the oxonium. This may not be the case in the MM2 calculations but the difference with MOPAC is the neighbouring group participation can be taken into account directly. With the oxygen and the oxonium close together it will recognise the possibility for bonding and the structure for A will be the same as C (likewise B=D). As you say the major deciding factor for the selectivity is the C/C’ or D/D’ ratio which is heavily biased towards the cis-fused structures.

MP. You have taken a good approach here in comparing both sets of calculated data to the experimental data. As you have shown it doesn’t appear possible to distinguish these isomers using computational chemistry. It can be difficult to calculate the NMR spectrum for cyclopropyl compounds because they are highly strained (as you said) and the associated MOs are unusual. For the phosphorus NMR, the inaccuracy could be due to conformational flexibility of the phosphorus substituents. 31P NMR can sometimes be hard to predict even experimentally, so this problem could require application of different computational methods. Other things to look at would have been IR and optical rotation which in principle could be different for the different isomers and the difference in energy and its significance to the reaction mechanism.