Expt #6. Reaction of 1,5-Cyclooctadiene with Excess Dichlorocarbene

Literature References - (1) Fieser, L. F.; Sachs, D. H. J. Org. Chem. 1964 29, 1113-1115. (2) Ault, A.; Wright, B. J. Chem. Ed. 1976 53, 489. (3) Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. "Introduction to Organic Laboratory Techniques A Microscale Approach", 4th ed, Brooks/Cole: Pacific Grove, CA, 2007. pp 217-224.

Overview

Excess dichlorocarbene (Cl₂C:), generated by the reaction of chloroform with concentrated aqueous base (eq 1), will be allowed to react with 1,5-cyclooctadiene to form 5,5,10,10-tetrachlorotricyclo[7.1.0.0^4,6] (structure shown below).

       CHCl₃ +  OH^-(aq) -->  Cl₂C:  +  HOH  + Cl^-         (1)

The product can exist as cis and trans stereoisomers, however one of these can be expected to be strongly favored due to steric hindrance in the attack of the second carbene molecule.

Because chloroform is not soluble in aqueous solution and instead forms a separate organic layer, a phase-transfer catalyst must be used to get it to react with the aqueous hydroxide ions at an appreciable rate. We will use benzyltriethylammonium chloride (PhCH₂N⁺Et₃ Cl^-) for this purpose. This salt is somewhat soluble in both the organic layer (due to the non-polar alkyl groups) and the aqueous layer (due to its ionic nature). It functions as a phase-transfer catalyst by allowing hydroxide ions to pair with the ammonium cation while in the aqueous layer forming an ion pair (PhCH₂N⁺Et₃ OH^-) that then can move into the organic layer, bringing hydroxide ions that can then react with the chloroform molecules (eq 2).

                        PhCH₂N⁺Et₃ Cl^-    +    Na⁺ OH^-   ------->      PhCH₂N⁺Et₃ OH^-        +         Na⁺ Cl^-    (2)
                            in both layers          in aqueous layer               in both layers   

Procedures

Running the reaction.

        To a 5.0-mL conical reaction vial equipped with a magnetic spin vane, add:

1.0 mL 50% NaOH(aq) (take care to not get any of this solution on the ground-glass joint)

1.0 mL CHCl₃

50 mg benzyltriethylammonium chloride

0.25 mL 1,5-cyclooctadiene (this step should be carried out in your own fumehood)

Attach a reflux condenser and reflux the solution with very rapid stirring for 30 min.

Work-up procedures.

Cool the reaction mixture to room temperature and transfer it to a large test tube.

Add 1.5 ml water and extract the mixture three times with 1.5 mL dichloromethane.

Dry the combined DCM extracts over Na₂SO₄.

Remove the DCM solvent on the rotary evaporator.

Determine the crude yield and then recrystallize the crude product from ethyl acetate.

Characterization of Product

Obtain the melting point as well as the ¹H and ¹³C NMR spectra.

One group will be asked to obtain NMR spectra of the crude product as well.

Questions:  

                Use the spectra of the crude product to determine how much, if any, of the minor stereoisomer of the product forms                     and to determine how well the recrystallization procedure worked to remove it from the final product.