Reduction of Alkenes and Cycloalkenes

Isolated double bonds in alkenes and cycloalkenes are best reduced by catalytic hydrogenation. Addition of hydrogen is very easy and takes place at room temperature and atmospheric pressure over almost any noble metal catalyst, over Raney nickel, and over nickel catalysts prepared in special ways such as P-1 nickel [13] or complex catalysts, referred to as Nic [49]. 

Isomerizations may take place over some catalysts more than over others. Regio- as well as stereoisomers may be formed [13, 49. Such side reactions are undesirable in the reduction of pro-chiral alkenes, and especially in the hydrogenation of some cycloalkenes. 

Hydrogenation over Raney nickel was found to be even less stereoselective. 2-, 3- and 4-methylmethylenecyclohexenes gave different mixtures of cis and irons dimethylcyclohexanes depending not only on the structure of the starting alkene but also on the method of preparation and on the freshness of the catalysts. The composition of the stereoisomers ranged from 27-72% cis to 28-73% irons [340], 

The stereoselectivity of different catalysts in catalytic hydrogenations is discussed in the chapter on catalysts (pp. 4, 50). In addition to catalytic hydrogenation, a few other methods of reduction can be used for saturation of carbon-carbon double bonds. However, their practical applications are no match for catalytic hydrogenation. 

The indirecl reduction by borones consists of two steps addition of a borane across the double bond, and decomposition of the alkylborane by an organic acid, usually propionic acid [341  

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