The Diels Alder Adduct

Discussion
The reaction of (-)-α-phellandrene, 1, and maleic anhydride, 2, gave a Diels-Alder adduct, 4, 7- ethanoisobenzofuran- 1, 3- dione, 3a, 4, 7, 7a- tetrahydro- 5- methyl- 8- (1- methylethyl), 3, this reaction gave white crystals in a yield of 2.64 g (37.56%). Both hydrogen and carbon NMR as well as NOESY, COSY and HSQC spectrum were used to prove that 3 had formed. These spectroscopic techniques also aided in the identification of whether the process was attack via the top of bottom face, as well as if this reaction was via the endo or exo process. These possible attacks give rise to four possible products, however, in reality due to steric interactions and electronics only one product is formed.

Scheme 1. Formation of the Diels-Alder adduct with maleic anhydride and (-)-α-phellandrene.

The IR spectrum that was obtained of the white crystals showed several functional groups present in the molecule. The spectrum shows weak sharp peak at 2865 to 2964 cm-1, which is often associated with C-H, sp3 hybridised, stretching in the molecule, peaks in this region often represent a methyl group or CH2 groups. There are also peaks at 1369 cm-1, which is associated with CH3 stretching. There is also C=O stretching at 1767 cm-1, which is a strong peak due to the large dipole created via the large difference in electronegativity of the carbon and the oxygen atom. An anhydride C-O resonates between 1000 and 1300 cm-1 it is a at least two bands. The peak is present in the 13C NMR at 1269 and 1299 cm-1 it is of medium intensity.
Figure 1. Atoms labelled in 3 used in the NMR assignment.

The 1H NMR spectrum shows that there are 18 protons in 11 different proton environments. This fits with the Diels-Alder reaction taking place a...

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...teraction of the HOMO of the diene and the LUMO of the dienophile. This reaction was done at relatively low temperatures as the dry ether has a boiling point of 34.6 °C. At low temperature the endo preference predominates unless there is extreme steric hindrance, which in this case there is not. The endo product forms almost exclusively because of the activation barrier for endo being much lower than for exo. This means that the endo form is formed faster. When reactions proceed via the endo for the reaction is under kinetic control. Under kinetic control the adduct is more sterically congested, thus thermodynamically less stable. The endo form has a lower activation energy, however, the EXO form has a more stable product. As this is a symmetrical Diels-Alder reaction there is not two possible isomers of the product.
Figure 5. Mechanism of the formation of 3.