Stereoselective Partial Hydrogenation of Allenes by Diimide in situ

G. Nagendrappa, S.N. Moorthy & D. Devaprabhakara
Department of Chemistry, Indian Institute of Technology, Kanpur 208016
4th November 1975

Both cyclic and acyclic allenes have been hydrogenated by diimide formed in situ by oxidation of hydrazine by hydrogen peroxide as well as oxygen gas, to olefins in which the double bounds are found to have cis geometry. Terminal allenes give 2-alkenes. Both observations have been explained by the approach of diimide from the least hindered side of allenic bond. It is also noted that allenic bond is more reactive towards diimide than the olefinic double bond is. The efficiency of reduction increases as the degree of allenic substitution and the ring size of the cyclic allene decrease.

As an unstable species diimide was known long ago. Its recognization as a hydrogenating reagent came about in 1961 (ref. 2-8). Since then there have been sporadic reports9-13 on the reaction and mechanism of hydrogenation by this reagent of such unsaturated functional groups as >C=C<, -C=C-, >C=0, >C=N- and -N=N-. It has been also used as a selective reducing agent in the presence of many other functional groups 14-17. Allenic functional group too could be partially or fully reduced 18. We have studied the reduction of both cyclic and acyclic allenes with various degrees of substitution, by diimide in some detail and the findings are reported here.

Allenes have been reduced, among others by catalytic hydrogenation19 and sodium in liquid ammonia20 quite conveniently. The diimide reduction supplements these methods and offers a choice, more so due to the simplicity of experimental procedure and cheapness of chemicals. The mutually perpendicular double bonds in allenes offer four sides for attack by diimide, and the products study provides lot of information as to the mechanism of this reaction.

Varieties of experimental procedures are available for diimide reduction involving both vapour and solution phases3-8, 21-24. In the present study, diimide was generated by oxidation of hydrazine using hydrogen peroxide or oxygen as an oxidizing agent in presence of Cu2+ ions.

Six acyclic allenes with varying degrees of substitution and five cyclic allenes with ranging ring sizes were chosen in order to study the steric effect on their ease of reduction. All, except 2, 4-dimethyl 2, 3 pentadiene, were synthesized by known methods and characteristics by spectral and gas chromatographic measurements20-25. The results are summarized in Table 1.

Table 1 – Diimide Reduction of Allenes

Allene Product (s) % Conversion(a) Composition % Mole Ratio Of Allene To Hydrazine(e)
  Oxygen H2O2(b)  
1,2-Nonadiene (1) cis-2-Nonene (97) 17 (100) 0.25
4,5-Nonadiene (2) trans-2-Nonenecis-4-Nonenetrans-4-Nonene 30 (3)12 (96)(4)   (c)   0.20
3-Ethyl-1,2-pentadiene (3)Phenylpropadiene(4) 3-Ethyl-2-penteneCis-Propenylbenzenetrans-PropenylbenzeneAllylbenzene (c) (92) 28 (6)(2) 16(100)   (c)    0.20 0.25
3-Phenyl-1,2butadiene (5) 2-Phenyl 1-2-butene(d) 20(100) (c) 0.20
2, 4- Dimethyl-2, 3pentadiene (6) 2, 4Dimethyl-2-pentene 4(100) Zero 0.10
1, 2 Cyclononadiene(7) cis-Cyclononene 81(100) 100 (100) 0.20
1,2,6-Cyclononatriene (8) cis, cis-1, 5-Cyclononandiene 82 (100) 100 (100) 0.20
1,2- Cyclodecadiene(9) cis- Cyclononene 49 (100) 24 (100) 0.20
1,2,6-Cyclodecatriene (10) cis, cis-1, 5-Cyclodecadienecis, cis- 1, 6-Cyclodecadiene  48 (46) (54)   (c)   0.20
1, 2-Cyclotridecadiene (11) cis-Cyclodecadiene 25 (100) 21 (100) 0.20

(a) The product recovery was 95% or more in every case. (b) In the case of H2O2 oxidation only single products were obtained. (c) NO reaction was carried out. (d) It is most likely a mixture of cis- and trans-isomers. Attempt to separate them was not successful. (e) Mole ratios refer to H2O2 oxidation and 0.10 mole ratio was used for all cases of oxidation with O2.

In spite of several-fold excess of diimide generated by H2O2 reagent, it is found that, except the nine-membered cyclic allenes, all allenes undergo poor conversion. This evidently means that the reactivity of the substrate is the deciding factor and not the total amount of diimide alone. If still higher quantities of diimide are used, the olefin that is formed in the first reduction step starts undergoing further reduction to saturated compound. Therefore, for partial reduction of allenes to olefins, it is safer not to use too much excess of diimide.

From an observation of the results it is clear that the reduction is stereoselective giving predominantly cis-olefins. This means that the reduction is not controlled by the thermodynamic stability of the products in which case the trans-isomers should be formed wherever they are more stable. Also in the case of mono and 1, 1-disubstituted allenes (1,3,4 and 5), 1-and 2-alkenes are possible, but only the latter are formed exclusively. Both these observations, i.e. the formation of thermodynamically less stable cis-olefin and the reduction of the less substituted of the allenic double bonds to form more substituted olefins, can be explained by the steric control of the approach of the diimide. The reducing species, diimide, should therefore approach the allenic bond from its least hindered side. Taking into consideration the suggestions that the diimide in its cis form transfers its hydrogens synchronously to the double bond2, 3, 26, 27, in solution as well as gas phase, the reduction of allenic double bond may be visualized to proceed through a six-membered transition state (I).

Since the allenic moiety contains two mutually perpendicular double bonds, the formation of cis and/or trans alkene(s) is expected depending on the approach of the diimide towards the allene molecule, as depicted in Charts 1 and 2.

Sterically the most favourable approach of the diimide in the case of monosubstituted allene is the one shown in Chart1. Approaches of diimide from other directions would be hindered by the alkyl group. This explains at once the formation of 2-alkene with cis geometry. In the case of 1, 3- disubstituted acyclic and cyclic allenes, the -orbital in the same plane with hydrogen on the third carbon atom is less hindered and therefore more easily approachable. This again gives only the cis-olefin. In fact the diimide appears to be so sensitive to steric hindrance that the percentage of acyclic allene converted into olefin falls dramatically when the degree of substitution is increased from 1 to 6, being almost zero with tetrasubstitution. In the case of cyclic allenes ring strain also appears to play an important role. In the nine-membered ring the strain is apparently the dominant factor. Otherwise, when we go from nine-membered ring through ten-membered to the thirteen-membered ring there should not be such a remarkable drop in the percentage conversion considering the fact that all of them have allenic bond with same degree of substitution. Indeed the nine-membered cyclic allene has been observed to exhibit some physical and chemical properties28, that are different from the other higher homologues, which have been ascribed to the ring strain.

One more point that has to be noted in this study is that the allenic bond is slightly mor reactive than the ordinary carbon-carbon double bond. This is evident from the fact that no detectable amount of standard compounds are formed under the reaction conditions that are employed in the present investigation, till the concentration of the olefin becomes substantially more than that of the allene. This is especially true in cyclic allenes. However, when the concentration of olefin and the allene for diimide becomes important. But this depends on the relative reactivity of the individual allene and its first reduction product. For example, in the case of 1,2-cyclodecadiene (7) practically no second step reduction was observed, whereas in the case of 1,2-nonadiene (1) formation of very small amount of ?-nonane could be detected. This assumes significance when one considers the comparatively high conversion rate of 1,2-cyclodecadiene (7).

Experimental Procedure

The boiling points (b.p.) are uncorrected. All the IR spectra were recorded on a Perkin-Elmer model 700 and model 137 spectrophotometers as neat liquids between sodium chloride plates. GLC analyses were made on a Varian Aerograph model 90-P instrument, on columns filled with 15% silicone rubber SE-30, 20% Carbowax 20M and 15% Carbowax 20M 5% silver nitrate as liquid phases supported on 60/80 mesh chromosorb P. The products were identified by comparison of their IR spectra with those of authentic samples. The composition of the product mixture was calculated by measuring the areas of the peaks of standard runs. 2,4- Dimethyl-2,3-pentadiene was purchased from Aldrich Chemical Co. All other allenes were prepared by the known procedures and their purity was checked by IR and physical constants 20, 25. Hydrazine hydrate (98%) and hydrogen peroxide (30%) were BDH commercial samples and were used without estimation. The oxygen gas was obtained from Indian Oxygen Co. and was used without further purification.

The general procedure for hydrogenation: (A) By hydrogen peroxide oxidation of hydrazine-In a three-necked round-bottomed flask fitted with a gas outlet, a mercury sealed mechanical stirrer and a dropping funnel with a fine nozzle, was placed solution of allene in ethanol distilled over quicklime. To this solution were added a 4-10-fold excess of 98% hydrazine hydrate and 1 ml of 1% copper sulphate solution. The flask with its contents kept stirred, was cooled by ice-salt mixture. The gas outlet was led into a water filled measuring cylinder to measure the volume of nitrogen evolved. After allowing the system to attain steadiness, required quantity of 30% hydrogen peroxide was added from the dropping funnel at the rate of 5-6 drops per minute. After no more nitrogen evolved, water was added into the reaction mixture, the product was extracted three times with pentane or petroleum ether (b.p. 40-60°), and the combined extracts, after washing with water, were dried over magnesium sulphate. The solvent was then removed, and the product analysed by GLC on a suitable column.

(B) By oxidation of hydrazine by oxygen gas- The reaction was carried out in a 100 ml two-necked round-bottomed flask fitted with and inlet and an outlet for oxygen gas. The inlet carried a stopcock and sintered glass tip dipping into the reaction mixture. The end of the outlet was connected to a mercury bubbler. In each case 0.01 mole of allene, 0-10 mole of 98% hydrazine hydrate, 50 ml of absolute ethanol and 10 mg of copper sulphate were placed in the flask. The reaction mixture maintained at 40-45° was kept stirred magnetically. The oxygen gas was passed into the solution at the rate of 75 ml per min. Under these conditions no perceptible amount of allene escaped from the solution. Aliquots of the reaction mixture were removed at intervals of 60 min, quenched with water and extracted with pentane. After drying the extract on magnesium sulphate the solvent was removed carefully and the residue analysed by GLC. It was observed that, under these reaction conditions, a 180 min reaction time was best for partial reduction. Further passing of oxygen was found to start hydrogenating the olefin. The yields of the products were calculated by GLC, based on the allene consumed in the reaction. The total recovery was in the range of 95-100%. The proportion of allene reduced depended on the individual allene.

Acknowledgment

One of the authors (S.N.M) thanks the CSIR, New Delhi, for the award of a senior research fellowship.

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