Projection Experiment: (4+2)-Cycloadditions (Diels-Alder)

Demonstration of Microscale Projection Experiments - Chemistry en miniature

(4+2)-Cycloadditions (Diels-Alder)

Objective: Diene and Dienophile Components in Diels-Alder Products

Peter Keusch

German version

Chemicals:

cyclopentadiene (freshly distilled)

1 mmol anthracene / toluene (0.178 g / 20 mL)

1 mmol cyclopentadiene / toluene (82.6 ml / 20 mL)

1 mmol p-benzoquinone / toluene (0.108 g / 20 mL)

0.2 mmol 4-phenyl-1,2,4-triazolin-3,5-dione (N-PTD) / toluene (0.035 g / 20 mL)

Note: The freshly distilled cyclopentadiene must be kept on ice or it will react with itself within an hour to form dicyclopentadiene (dimerization).

Hazards and safety precautions:

Dicyclopentadiene is harmful harmful by ingestion, inhalation and through skin contact.

Cyclopentadiene is toxic by ingestion, inhalation or through skin contact.

p-Benzoquinone is toxic. May be fatal if swallowed, inhaled or absorbed through skin. Severe irritant.

Toluene is toxic by inhalation, ingestion or by absorption through skin. Serious irritant. Experimental teratogen.

Safety glasses, gloves. The experiment should be carried out in a portable fume hood giving all-round visibility!

Experimental procedure:

1 mL of a solution of N-PTD is pipetted into each of two test tubes (T1, T2). A drop of anthracene solution is added to the solution in T1 and a drop of the solution of cyclopentadiene is given to the solution in T2.

1 mL of benzoquinone solution is added to two further test tubes (T3 and T4). The solution in T4 is mixed with 5 drops of freshly distilled cyclopentadiene.

Results:

Test tube 1	The red N-PTD solution is decolorized - this takes about 5 minutes.
Test tube 2	The red N-PTD solution fades immediately to colorless.(Fig. 1).
Test tube 3	yellow solution of p-benzoquinone (reference solution)
Test tube 4	The yellow color fades gradually and the solution becomes colorless after 2 minutes. (Fig. 2).

Photo 1

Photo 2

Discussion:

· In a Diels-Alder cycloaddition a system with 2 p-electrons (dienophile) is added in 1,4-position to a conjugated p-system with 4 p-electrons (diene). The reaction is classified as a [4+2] process. This means that 4 p-electrons of the diene and 2 p-electrons of the dienophile are involved simultaneously in the reaction. A rearranging of the 6 electrons occurs. Two new s-bonds (colored blue in the equations below) and one p-bond are formed as three p-bonds (colored red in the equations below) are broken. The driving force of the reaction is the formation of the new s-bonds, which are energetically more stable than the p-bonds. The reaction product is a six-membered ring. A typical example is the reaction of 1.3 butadiene with ethene resulting in the formation of cyclohexene (1).

· The Diels-Alder reaction is stereospecific with respect to both the diene and the dienophile. Addition is syn on both components. Thus, the reactivity of open-chain dienes depends on the fraction of s-cis conformers available in the conformation equilibrium. Cyclic dienes which are locked by the ring in s-cis conformation e.g. cyclopentadiene and 1,3-cyclohexadiene are therefore especially reactive, while cyclic dienes that are permanently in the s-trans conformation and cannot adopt the s-cis conformation will not undergo the Diels-Alder reaction at all.

· The dimerization of cyclopentadiene already proceeding at room temperature is a Diels-Alder reaction. One molecule of cyclopentadiene acts as a 4 p-electron diene and the other as a 2 p-electron dienophile. This dimeric material can be 'cracked' back to cyclopentadiene by heating at 150°C for an hour and then distilling off the diene monomer.

· In general, electron-donating substituents on the diene (methyl, methoxy) promote cycloadditions as do electron-withdrawing groups (formyl, nitro) on the dienophile.

· Cycloadditions belong to the pericyclic reactions. Breaking and forming of bonds takes place in a concerted manner (simultaneously) as the result of a cyclic arrangement of electrons in the transition state.

· N-PTD and p-benzoquinone are dienophiles, cyclopentadiene and anthracene are dienes. The decolorization of the red solution of N-PTD or of the yellow solution of p-benzoquinone is due to the disruption of the conjugated double bond system in N-PTD and p-benzoquinone, respectively ((2) and (3)).

· The different reaction rates allow an evaluation of the dienophile and diene character: N-NPT is a better Diels-Alder dienophile than p-benzoquinone. Cyclopentadiene is a better Diels-Alder diene than anthracene.

· Anthracene undergoes cycloaddition on the central ring (resonance stabilized cycloadduct).

References:
Pericyclic Reaction Chemistry

General experimental instructions and index of experiments