This computational exercise is "twinned" with the synthesis experiment 1S in the third year lab. The following computational study and analysis involves the asymmetric epoxidation of an alkene and its characterisation by NMR spectroscopy and chiroptical measurements.
PART 1: Conformational analysis using Molecular Mechanics
Conformations and Hydrogenation of Cyclopentadiene
Cyclodimerisation products of cyclopentadieneHydrogenation scheme of the endo dimer
Upon dimerisation of cyclopentadiene via a Diels Alder [π4s+π2s] cycloaddition two possible products may form: an endo or an exo conformer, as shown in the picture on the right. Endo conformer is formed predominantly rather than exo. The hydrogenation products of endo dimer lead to molecules 3 and 4. This part of the modelling exercise concetrates on establishing whether the cyclodimerisation of cyclopentadiene and its subsequent hydrogenation is under thermodynamic control due to the stability of the products, or under kinetic control due to the lower energy transition state. This is achieved interpreting the results obtained from the molecular mechanics technique.
ENDO and EXO Cyclodimerisation products
In order to assess why the endo product is favoured over exo, both geometries were otpimized using the following settings:
Force Field: MMFF94s
Algorithm: Conjugate gradients
Steps per update: 4
Table 1. Optimized geometries of cyclopentadiene dimer.
ENDO
EXO
Structure
Total Bond Stretching Energy(kcal/mol)
3.46480
3.54444
Total Angle Bending Energy (kcal/mol)
33.22508
30.78197
Total Torsional Energy (kcal/mol)
-2.95330
-2.73235
Total Van der Waals Energy (kcal/mol)
12.33274
12.79366
Electrostatic (kcal/mol)
14.18229
13.01273
Total Energy (kcal/mol)
58.19044
55.37325
Hydrogenation of ENDO dimer
Table 2. Optimized geometries of hydrogenation products.
