Question

The X-Ray crystal structure of the α-terpinene-N-phenylmaleimide Diels-Alder adduct was obtained. Several stereodrawings of the structure generated are provided in your lab manual 4) a. Is the adduct obtained the exo- or endo- adduct? EXO or ENDO (circle one) b. The phenyl (CoHs) group bonded to the nitrogen is often written as a cyclohexatriene with alternating double/single bonds. Does the crystal structure support the existence of alternating double and single bonds (yes or no)? (Hint: Compare the bond angles and lengths.) YES or NO (circle one) What structure around the phenyl ring does the structure support? Describe how you came to your conclusion. c. d. What kind of functional group does the phenyl group represent?

Answer 1

The way to differenciate endo versus exo in a diels-alder reaction adduct is to use 1H NMR

The coupling constant for the 3 bond coupling between 1H near to each other has been reported and is given and it follows the Karplus diagram.

Но Hx 3 Hb Hx 3.66 Hz Hx Но HbHn二0.55 Hz Hn Hn

In a diels-alder adduct the endo and exo are defined as

N Ph Endo adduct Exo adduct Ph

So if exo is present the coupling constants should be very small or non exitent whereas if the endo is formed then coupling constant should have a fairly good value.

1) If you look at the 1H NMR recorded for the adduct the coupling constants for the peak at 6 ppm are fairly good so it means it is the endo product and since only one set of coupling are present only the endo is formed.

2) The 1H NMR of the aromatic set in phenylmaleimide shows a signal in the aromatic region so it is not alternate double and single bond but the 3 double bonds are delocalised on all 6 bonds.

I am giving conclusions based on 1H NMR as bond length are not provided.