Hydrogenation of conjugated dienes

A selective hydrogenation of conjugated dienes was carried out with a Pd complex which was preactivated with oxygen. Besides the conversion of dienes with good selectivity (98%), diene esters, ketones and nitro compounds were also hydrogenated with fairly good selectivities (equation 9)29. 

Much experience concerning the hydrogenation of conjugated dienes was obtained with butadiene hydrogenation. On Pt single crystals the reaction was found to be structure sensitive the activity sequence of different planes (marked with Miller s index) is... 

The most disputed question about the hydrogenation of conjugated dienes is whether 1,2- or 1,4-addition takes place as in the following reaction (equation 49)128. 

Certain monohydride catalysts exhibit high selectivity toward hydrogenation of conjugated dienes and polyenes to give monoenes. Potassium pentacyanocobaltate prepared from CoCl2 and KCN is one of the most active catalysts, but reduces only conjugated double bonds. The reduction of dienes with nonequivalent double bonds follows the usual substitution trend noted for monoenes. 1,3-Pentadiene, for example, is converted selectively to trans-2-pentene via 1,2 addition 134... 

Certain chromium complexes also exhibit similar selectivities in the hydrogenation of conjugated dienes.139 140 Coordination of the diene in the cisoid conformation and 1,4 addition can account for the selectivity. 

Selective partial hydrogenation of conjugated diene via alkylallyl anion intermediates. 

In DMF as solvent, PdCl2 or [PdCl2(DMF)2] will catalyze the hydrogenation of conjugated dienes and alkynes to alkenes.194 The above systems quite generally give selective reduction of dienes to monoenes. 

The cobalt(II) cyanide complex Co(CN)5 " in aqueous solution acts as a homogeneous catalyst for the selective hydrogenation of conjugated dienes to monoenes (81). The initial step in this catalysis is the reaction... 

The hydrogenation of conjugated dienes to monoalkenes poses questions of both regio- and stereoselectivity and is a function of the metal and the conditions.The distribution of products depends not only on the inherent selectivity of the initial addition process but also on the competition between the diene and the first-formed alkene(s) for the reactive sites. The latter may add hydrogen or be isomerized. If the structure of the diene allows the change, the diene itself may be isomerized. As for alkynes, palladium and nickel catalysts tend to be the most selective to the monoene. 

Partial hydrogenation of conjugated dienes or polyenes is a nontrivial synthetic transformation. This reaction commonly yields a mixture of products arising from 1,2- and 1,4-addition of hydrogen to the butadiene moiety, as well as complete hydrogenation of both double bonds. Unless there is a substantial difference between the two double bonds in terms of substitution pattern and electronic density, selectiv-... 

Fig. 20. Proposed intermediates in the hydrogenation of conjugated dienes to monoenes.

Then hydrogenation of conjugated dienes can also have stereochemical features however, only few synthetically [35] or mechanistically [36-38] valuable studies have been reported. 

Conjugated Dienes. General Observations, The catalytic hydrogenation of conjugated dienes by pentacyanocobaltate(II) is completely selective, yielding only monoolefin, which cannot be reduced further (19, 20, 21), Lack of reduction of 2,5-dimethyl-2,4-hexadiene indicates that dienes in which the 5-cis conformation is sterically hindered might not be catalytically hydrogenated (21). However, reduction of 3-methylene cyclohexene, a diene with a fixed s-trans conformation, disproves this and indicates that other steric factors are probably involved (24). 

As wilt be described in the next section, the selective hydrogenation of conjugated dienes is accomplished more readily than the partial hydrogenation of... 

The partial hydrogenation of conjugated dienes generally takes place more easily than the saturation of a monoalkene, which improves the prospect for selective hydrogenation of the conjugated species. This reaction is an example of Type III selectivity (Eqn. 15.40) (Chapter 5). Active catalysts can enhance organic substrate diffusion limitations and decrease the selectivity of monoalkene formation. Low hydrogen availability will minimize oversaturation. 

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