SYNTHESIS OF 5,6-DIHYDRO-ENDO-DICYCLOPENTADIENE

Dicyclopentadiene is chilled until a major proportion has solidified. The material is filtered and the solid is pressed on a Buchner funnel with a spatula to remove as much liquid as possible. The solid is warmed just to melt and the process is repeated. The resulting solid has m.p. 28-30 °C m.p. 27-28 °C).

STEP 1: In a 2-litre flask, 37.5 g (150 mmol) of nickel(n) acetate tetrahydrate (1) are dissolved in 200 ml of 95 per cent ethanol. The flask is attached to a Brown^e hydrogenator and flushed with hydrogen.

STEP 2: With rapid stirring 150 ml of 1.0 M sodium borohydride solution in ethanol (2) is added to reduce the nickel acetate to P-2 nickel.

STEP 3: The purified endo-dicyclopentadiene (407 g, 3.08 mol) is melted with 200 ml of ethanol and the mixture injected into the hydrogenator.

STEP 4: With vigorous stirring, the reaction proceeds smoothly; hydrogen uptake ceases when 3.08 mol of hydrogen has been absorbed.

STEP 5: Five grams of activated carbon are added to the reaction mixture to aid in catalyst removal, and the warm (40 °C) mixture is filtered through a thin (c. 3 mm) pad of carbon on a Buchner funnel.

STEP 6: The filter pad is washed with 2 x 100 ml of warm acetone.

STEP 7: The combined organic layers are distilled to remove solvent.

STEP 8: The residue is distilled through a short Vigreux column to give 370 g, 2.76 mol (90%) of 5,6-dihydro-endo-dicyclopentadiene, b.p. 178­180 °C, m.p. 48-50 °C.

STEP 9: Recrystallisation from methanol gives m.p. 50 °C (lit.' 48.5-50 °C); p.m.r. (CC1₄, TMS), 6 1.25 (s, 3.8H), 1.45 (s, 2.2H), 2.0-3.2 (m, broad, 6.2H), 5.70 (s, broad, 2.0H). The product was free from starting material by g.l.c. analysis.

Notes to keep in mind:

1. The nickel(n) acetate tetrahydrate was AnalaR grade.

2. A stabilised solution suitable for catalytic reduction was prepared by dissolving 4.0 g of sodium borohydride powder in a mixture of 95 ml of absolute ethanol and 5 ml of 2 M sodium hydroxide and filtering the resulting solution. This solution is best pre­pared freshly the day of use for maximum catalyst reproducibility but may be utilised satisfactorily for up to 5 days if kept refrigerated. Formation of small amounts of sedi­ment under refrigeration is not harmful.

3. Details of a procedure for smaller scale exploratory experiments are given as follows.' Nickel(u) acetate tetrahydrate (1.24 g, 5.00 mmol) is dissolved in (50 — n) ml (n — volume of substrate to be added) of 95 per cent ethanol in a 125 ml Erlenmeyer flask (modified for high stirring). The flask is attached to a borohydride hydrogenator, which is then flushed with nitrogen. With vigorous stirring, 5.0 ml of 1.0 M sodium borohydride solution in ethanol (see above) is injected over 15 seconds. When gas evolution from the mixture ceases, the catalyst is ready for use. The hydrogenator is purged with hydrogen and the reaction initiated by injecting the substrate. Addition of solid substrates is accomplished with the stirrer stopped, just before purging with hydrogen. Samples for analysis are withdrawn from the reactor with a syringe and stainless steel needle. Gas chromatographic columns which are found to be useful have stationary phases of either adiponitrile, triethylene glycol—silver nitrate, tris­(cyanoethoxy)propane, UCON 50 HB 2000, and squalene.