Email Discussion: Poster 14 Reduction of Triple Bonds Dr. Rudolf Herrmann

The possibilities of the increasing of Zn-Cu catalyst activity and the influence of pH and solvent composition on the reaction of the reduction of various types of triple bonds have been investigated. The reduction was going in most cases very fast and giving only the corresponding cis-alkene derivative with quantitative yield.

By using of Zn-Cu couple as well as pure Zn the most frequent problem is too long reaction times -- low activity of reagent.² From the other hand the reagent is cheep and the proceeding of synthesis with this is relatively simple.

For the hydrogenation of propargylic alcohols the Zn-Cu catalyst in boiling methanol was used³, but this method was useless for the hydrogenation of acetylenic alcohols with isolated triple bond. Aerssens et. al. have introduced the activation of zinc with 1.2-dibromoethane and finally with CuBr and LiBr.^4,5 Catalyst prepared was relatively active, but not enough for the hydrogenation of long chain alcohols with isolated triple bond.

Results and discussion

Preparation of catalyst. It was investigated in our group the possibilities of the activation of Zn-Cu system for the performing of partial cis-hydrogenation of CC triple bond. The precursor of our Zn-Cu catalyst was prepared by the precipitation of Cu from the solution of CuCl2 2H2O to the surface of Zn dust by 80-90°C in water. The molar ratio of Zn:Cu in catalyst prepared (10:1) was close to optimal. Increasing the ratio decreased the activity of catalyst and decreasing the ratio gave the catalyst with the same activity. Such precursor had relatively low activity and it can be compared with catalyst used by Sondengam³. This material can be used only for the hydrogenation of short chain propargylic alcohols. We have suggested that the low activity of this precipitated Zn-Cu materials caused by several contaminants like ZnO, Zn(OH)2, Zn(OH)Cl etc. To remove these impurities we have activated this precursor material with the water solution of alkalimetal hydroxides by 80-90°C. It was used a 15% KOH solution as usual and the activation continued 30 minutes. Longer activation did not increase the activity of the catalyst. By using higher concentration of the solution of alkali metal hydroxides decreased the required activation time but not the activity of catalyst prepared. Sometimes in very concentrated solutions of alkali (>25%) big granules formed and the catalyst lost fully the activity. After alkaline activation the catalyst was free from Zn compounds as indicated by X-ray diffractografy. In the same time a new type of crystals formed. Corresponding signals of the X-ray diffraction were: 2(deg) = 37.71, 42.05, 57.80, 77.85. It was calculated from the Bragg equation that the diameter of the particles of catalyst diminished 1.5 times. This type of crystals disappeared after hydrogenation and only Zn and ZnO can be detected on X-ray diffraction picture. It was determined that after such activation the rate of hydrogenation of triple bonds in various compounds increased more than 50 times.

Conditions of the hydrogenation. It was determined by kinetic measurements that the rate of hydrogenation of acetylenic compounds is strongly dependent from solvent used. Only mixtures of iPrOH and tBuOH with water were acceptable. Hydrogenation in other solvents (H2O, nPrOH, dioxane, methanol) and their mixtures with water was too slow and stopped sometimes at all. We have find that optimal ratio for iPrOH:H2O mixture is 1:14 (vol.). The pH value of the reaction media has also relatively big influence to the rate of hydrogenation. It was shown that the hydrogenation of 7-dodecyn-1-ol is by pH 7.4 ca five times and by pH 6.4 ca ten times faster than by pH 13. In the same time E2-hepten-4-yn-1-ol was hydrogenated by pH 6.4 as well as by pH 13 with the same velocity, what is ca three times faster than in neutral conditions.⁶

For the better understanding the structure-reactivity relationship electrochemical potential value of the catalyst in pure solvent as well as in the presence of various acetylenic compounds was determined It was used the methodology described earlier by A. D. Sokolski⁷. The anodic drop of the potential in the presence of acetylenic compounds illustrate the adsorption ability of the catalyst. It is well known that such change of potential of the catalyst is bigger when the compound has good adsorption (Table).

It was found that the optimal substrate-catalyst ratio depends on the chain length of compound hydrogenated and also on the position of the triple bond in chain. We suppose that the long aliphatic chain is shielding the active surface of the catalyst and the reaction rate is decreasing. From the other hand the active type of crystallites in catalyst decompose with relatively constant velocity and a bigger amount of catalyst for slow reaction is needed. This is the case by weak adsorbing substrates like alkynes. So for the hydrogenation of substituted propargylic alcohols and 2-penten-4-yn-1-ols up to 7-8 carbons 10-12 mmol substrate can be used per standard amount of catalyst indicated in experimental part. The hydrogenation finished after 20-30 min by 80-90deg.C and the reaction proceeded according to zero kinetic order.The calculated activation energy for E2-hepten-4-yn -1-ol was 68.3 +/- 8.4 kJ/mol. The reaction velocity increased in alcaline solutions. All these characteristics are similar to the characteristics of the reaction going according to the radical-ionic mechanism⁷. By longer molecules (up to 14 carbon atoms) 5mmol of substrate per standard amount of catalyst is recommended.

For the hydrogenation of alkynols with chain length up to 12-14 carbon atoms and with isolated triple bond 3-5 mmol of substrate per standard amount of catalyst was recommended. The reaction was complete after 60-90 min and proceed according to the first order kinetic. Velocity of the reduction of these compounds increased by pH 6.4 and decreased by pH 13. Alkyns are reacting according to the same scheme but substantially slower like alkynols. The velocity of the reaction was increased by pH 6.4 up to 50 times. So 10 mmol of 1-decyne was reduced in a period of 20 minutes and 1 mmol of 3-dodecyne in 90 minutes.

Hydrogenation of the following compounds was performed:

Experimental

X-ray analyses were done with diffractometer Dron-1 (Russia) by using Cu anode, U=23kV and I=16 mA. Electrochemical measurements were performrd in thermostated flask equipped with a platin wire as measuring electrode and calomel electrode (connected through saturated KCl bridge) as reference.For the determination of potentials of catalyst a digital millivoltmeter was used. All potential values were in normal H-scale.

Activated Zn-Cu catalyst. To a suspension of 4g Zn powder in 5 ml distilled water 1g of CuCl2 2H2O in 5 ml of distilled water was added by shaking or vigorous stirring by temperature of 80-85deg.C. The black precipitate prepared was washed twice with 10 ml of distilled water and activated then with 5.6 g KOH in 30 ml distilled water by vigorous stirring at temperature of 80-85deg.C in a period of 0.5 h. Then the KOH solution was separated by decantation and the remaining voluminous black material was washed with distilled water to neutral .

Hydrogenation. To the catalyst prepared 15 ml of iPrOH - H2O mixture 1:14 as solvent and 1g of corresponding actylenic alcohol was added. When the hydrogenation was complete as detected by GLC the catalyst was filtered off and washed with 5 ml of iPrOH and twice with 5 ml of diethyl ether. Organic layer was separated and the water layer was extracted with diethyl ether (3x10 ml), combined extracts were dried over MgSO4 and the solvent was evaporated with rotavapor. Products so prepared were pure ( >97%,) for most type of synthetic purposes. For the separation of residues of solvents the destillation or evaporation under high vacuum can be used.

References

2. A.J. Clarke, L. Crombie, Chemistry and Industry, 1957, 143; S. G. Morris, S.F. Herb, P. Magidman, F.E. Luddy, J. Am. Oil Chem. Soc., 1972, 49, 92.

3. B.L. Sondengam, G. Charles, T.M. Akam, Tetrahedron Lett., 1980, 21, 1069.

4. M.H.P.J. Aerssens, L. Brandsma, J. Chem. Soc., Chem. Commun., 1984, 735.

5. M.H.P.J. Aerssens, R. van der Heiden, M. Heus, L. Brandsma, Synt.Commun., 1990, 20, 3421.

6. For the preparing of solutions with required pH value 3N HCl or KOH was used.

7. D.V. Sokolski, Hydrogenation in Solution, 1979 (in russian).