Hydrogenation of allenes

The cumulated double bonds of an allenic system are of relatively high energy The heat of hydrogenation of allene is more than twice that of propene... 

Scheme 4.7 Hydrogenation of allene catalyzed by [(j/3-allyl)Pd(PR3)CI] complexes.

The dissociation of a coordinated allene by hydrogen was evidenced for [(// -al-lyl) Pd(PR3)Cl] complexes [11]. The hydrogenation of allene to propene was then invoked to follow the pathway depicted in Scheme 4.7 ... 

The hydrogenations of allenes may proceed through allylic intermediates. The distribution of products formed during the reaction of 1,2-butadiene with D2 depends on the particular nickel catalyst used. Reactions on the more selective catalysts (Ni powder, Ni/Si02) are presumed to proceed mainly via vinyl intermediates reactions on the less selective catalysts (Ni/AhOs) proceed via allylic intermediates (equation 34). A similar dichotomy of mechanisms is assumed to characterize reactions on Pd. 

Problem8.8 (a) Predict the heat of hydrogenation of allene CH2=C==CH2. (b) The actual value is 71 kcal. What can you say about the stability of a cumulated diene ... 

Partial hydrogenation of allenes is possible over catalysts reduced in efficiency. Supported Pd is most selective in achieving partial reduction to cis-olefins. Poisoned finely divided nickel affords a specific partial hydrogenation of the polyene alcohol 4 (vitamin A type) ... 

It is interesting to note that hydrogenation of allenic alcohols can be performed under normal culture conditions using Saccharomyces cerevisiae as the biocatalyst. The reduction leads to ethylenic alcohols75. 

We have just seen that a conjugated diene is more stable than an isolated diene. Now we need to see why a cumulated diene is less stable than an isolated diene. Cumulated dienes are unlike other dienes in that the central carbon is sp hybridized since it has two tt bonds. In contrast, all the double-bonded carbons of isolated dienes and conjugated dienes are sp hybridized. The sp hybridization gives the cumulated dienes unique properties. For example, the —AH° for hydrogenation of allene is similar to the —AH° for the hydrogenation of propyne, a compound with two sp hybridized carbons. 

In a similar study of the hydrogenation of allene in the presence of [14-C]-propylene, over supported Rh and Pd, the amount of propane produced from the further hydrogenation of propylene was found to be very small compared with the total propane yield. By analogy with the acetylene system, it was proposed that the formation of propane occurs by a route not involving the formation of propylene as a gas phase intermediate. 

The heat of hydrogenation of allene is 298 kJ moF, whereas that of propyne is 290 kJ moF. (a) Which compound is more stable (b) Treating allene with a strong base canses it to isomerize to propyne. Explain. 

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