Unconjugated dienes, hydrogenation

FIGURE 12.22 Hydrogenation of 1-butene is exothermic by 30.3 kcal/mol. Hydrogenation of unconjugated dienes is exothermic by almost exactly twice the heat of hydrogenation of 1-butene. 

The same authors also used this approach for an enantioselective synthesis of the natural product (-i-)-royleanone (4-54), a member of the abietane diterpenoid family [17]. The enantiopure sulfoxide 4-50 was oxidized using DDQ to give crude 1,4-ben-zoquinone 4-51, which by reaction with the diene 4-52 in CH2C12 under high pressure led to the tricyclic compound 4-53 with 97 % ee and 60% yield based on 4-50 (Scheme 4.11). Hydrogenation of the unconjugated double bond in 4-53 afforded 35% of the desired compound 4-54 after crystallization to separate it from the unwanted cis-isomer. 

For hydrogenation in water with an inexpensive catalyst, solutions containing cobalt salts and excess cyanide are useful10,11. The catalysts are selective for conjugated C=C bonds and are relatively unreactive with unconjugated dienes such as 1,5-cyclooctadiene. 

Concerning chemical selectivities in multiple unsaturated acetylenic compounds, conjugated enynes are reduced with modest selectivity. The most difficult case involves the selective reduction of an enyne with a terminal double bond and an internal triple bond, because the difference in the rates of their hydrogenation is minimal. The hydrogenation of nonterminal, unconjugated diynes to (Z,Z)-dienes can be achieved in good yield. 

Photochemically, a [1,3] suprafacial shift of hydrogen is allowed, and has been observed with several 1,3-dienes rearranging to the unconjugated 1,4-... 

The use of copper chromite at 40°C and atmospheric pressure was not very effective for selective carbonyl group hydrogenation. Unsaturated alcohols were produced from unsaturated aldehydes in low yields at low conversions and not at all from methyl vinyl ketone. 28 With unconjugated, unsaturated aldehydes, copper chromite is effective as a selective hydrogenation catalyst. Hydrogenation of 46 at 140°-160°C and 200 atmospheres gave better than 70% of the diene diol, 47. Increasing the temperature to 240°C resulted in the complete saturation of 46 (Eqn. 18.28). 29... 

Total hydrogenation of an unconjugated diene is achieved under pressure over Pt02, Pd—C, finely divided Ni, and under homogeneous conditions over Rh complexes. Aqueous Rh reduces limonene to the corresponding alkane under mild conditions. The catalyst is not air sensitive and can be stored and immediately reused. 

The Stability of a conjugated radical accounts for the greater reactivity of vinylarenes and conjugated dienes versus that of unconjugated monoenes. A well-documented example of hydrogen atom addition to olefins by this radical pathway is the hydrogenation of a-methyl styrene by HMn(CO)j Halpem used CIDNP (cherrucally induced dynamic nuclear polarization) to establish this radical reaction path. 

The simplest conjugated diene is 1,3-butadiene, but because this molecule has only four carbon atoms, it has no unconjugated constitutional isomer. However, we can estimate the effect of conjugation of two double bonds in this molecule in the following way. The heat of hydrogenation of 1-butene is -127 kJ (—30.3 kcal)/mol. 

Both of our sample dienes in Figure 12.22 were unconjugated. Now let s look at the heat of hydrogenation of the conjugated diene, 1,3-butadiene A// is only -57.1 kcal/mol (Fig. 12.23). 

So now we have two measures of the energetic value of conjugation in 1,3-butadiene, and they are remarkably close to each other. The first is the height of the barrier to rotation about the C(2)—C(3) bond (4-5 kcal/mol), and the second is the lowering of the heat of hydrogenation of a conjugated diene relative to that of an unconjugated diene ( 3.6 kcal/mol). 

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