DIELS-ALDER SYNTHESIS OF CYCLOHEX-4-ENE-1,2-CIS-DICARBOXYLIC ANHYDRIDE FROM BUTADIENE SULFONE AND MALEIC ANHYDRIDE

Written by Lauren

ABSTRACT:

New carbon-carbon bonds can be formed from a conjugated diene system heated with a dienophile (alkene or alkyne). It is a concerted cycloaddition reaction between the π-electrons of butadiene sulfone and maleic anhydride to form cyclohex-4-ene-1,2-cis-dicarboxylic anhydride. The product doesn’t need to be recrystallized and its purity is confirmed by a melting point test and ¹H NMR analysis.

INTRODUCTION:

The synthesis of new carbon-carbon bonds between butadiene sulfone and maleic anhydride can be achieved through the Diels-Alder reaction in a pericyclic reaction. The pericyclic reaction of the π-electrons from the diene and dienophile reorganize in a cyclic fashion in the transition state to form the cyclohex-4-ene-1,2-cis-dicarboxylic anhydride¹. The loss of sulfur dioxide from butadiene sulfone provides the conjugated diene system that reacts with the alkene in maleic anhydride. Diels-Alder is stereospecific, reactants maintain their stereochemistry throughout the reaction producing a racemic mixture of the product².

This experiment establishes the Diels-Alder π-electron rearrangement through synthesis of cyclohex-4-ene-1,2-cis-dicarboxylic anhydride.

RESULTS:

Moles butadiene sulfone needed to react with maleic anhydride:

0.0185 moles                        1 mole butadiene sulfone                0.01085 mole maleic anhydride needed

butadiene sulfone                 1 mole maleic anhydride                          (vs. 0.0124 mole used)

therefore, maleic anhydride is the limiting reagent.

Theoretical yield = 0.0124moles                      1mol C₈H₈O₃                                               152.086g           1.883g C₈H₈O₃

maleic anhydride           1 mole maleic anhydride              1 mole

Percentage yield = 1.461/1.883   X   100   =   77.6%

DISCUSSION:

Cyclohex-4-ene-1,2-cis-dicarboxylic anhydride was synthesized in 77.6% yield from the Diels-Alder reaction of butadiene sulfone and maleic anhydride. An isomeric mixture of xylene is used as a solvent for this reaction. Only one product was recovered in a racemic mixture, suggesting that the π-electrons of the diene system and the alkene rearrange in a cyclic manner.

Upon heating butadiene sulfone, sulfur dioxide (SO₂) is lost as a gas to form the butadiene molecule. Each π bond contains two π-electrons, the reaction between butadiene and maleic anhydride is a [4+2] addition with four π-electrons from butadiene and two from the alkene in maleic anhydride. The final product is in cis configuration because the butadiene molecule is cis after sulfur dioxide is lost (1) as shown in scheme 1. It would not be possible for the product to be in trans configuration because the stereochemistry of the reactants are maintained in the products² and also because trans cylcohexene is not known to exist¹.

Scheme 1

                   Cis                              cis-cyclohexene                                   trans                        cyclohexene

The products identity and purity were established by testing the melting point of the product and comparing it to the actual value. The melting point of the recovered product was approximately 104.1°C and the reported value is 105-106°C¹. The data from the ¹H NMR spectrum confirmed the structure of our product with four signals.

In this experiment, maleic anhydride is the limiting reagent; a higher yield may have been achieved if equal amounts in moles of butadiene sulfone and maleic anhydride were used since the reaction is in a 1:1 ratio. It’s also possible the percent yield wasn’t relatively high because some crude product may have been left behind when transferring the solution from the round reaction flask to the beaker to cool or when pouring the reaction mixture into the Buchner funnel for filtration.

The results of this experiment indicate that Diels-Alder reaction is an effective method for forming new carbon-carbon bonds. It can also be used with an alkyne as a starting material rather than an alkene. The reaction will still be a [4+2] reaction in which only two π-electrons from an alkyne would reorganize ultimately forming a diene product².

EXPERIMENTAL SECTION:

General. The ¹H NMRspectrum was recorded at 300 MHz using chloroform-d as the solvent.

Cyclohex-4-ene-1,2-cis-dicarboxylic anhydride. 2.189 g butadiene sulfone, 1.214 g maleic anhydride, and 0.8 mL were added to a 50-mL round bottom flask with a boiling stone. A water-cooled reflux condenser was attached and secured in place with keck clips. The mixture was placed in a sand bath and heated gently. Once the solids dissolved the clear solution was refluxed for one hour at approximately 150°C. After one hour, the sand bath was removed and the solution cooled for ten minutes. the solution was poured into a dry beaker after it was cooled to room temperature. After waiting an additional five minutes, 2 mL hexane were added to dissolve any solid lumps. A Buchner funnel attached to an Erlenmeyer was used to filter out the solid product. It was washed with 2 mL hexane two times and then set in our lab drawer until the next lab to dry out the product. Filtering the product produced 1.461g (78%) of the desired product with a melting point of 104.1°C.

¹H NMR (CCl₄): δ 6.0(triplet, 2H, A), 3.40(multiplet, 2H, B), 2.60(multiplet, 2H, C), 2.30(multiplet, 2H, D)

REFERENCES:

1. Mohrig, J.R.; Hammond, C.N.; Schatz, P.F.; Morrill, T.C. Modern Projects and Experiments in Organic Chemistry: Miniscale and Williamson Microscale, 2^nd ed.; W.H. Freeman and Company: NY, 2003; P. 157 & 158.
2. Carey, F.A.; Giuliano, R.M. Organic Chemistry, 8^th ed.; McGraw-Hill: NY, 2011; Ch. 10