Hydrogenation, catalytic, alkene functional group

Until the second half of the twentieth century, the structure of a substance—a newly discovered natural product, for example—was determined using information obtained from chemical reactions. This information included the identification of functional groups by chemical tests, along with the results of experiments in which the substance was broken down into smaller, more readily identifiable fragments. Typical of this approach is the demonstration of the presence of a double bond in an alkene by catalytic hydrogenation and subsequent determination of its location by ozonolysis. After-considering all the available chemical evidence, the chemist proposed a candidate structure (or structures) consistent with the observations. Proof of structure was provided either by converting the substance to some already known compound or by an independent synthesis. 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

Catalytic hydrogenation transfers the elements of molecular hydrogen through a series of complexes and intermediates. Diimide, HN=NH, an unstable hydrogen donor that can be generated in situ, finds specialized application in the reduction of carbon-carbon double bonds. Simple alkenes are reduced efficiently by diimide, but other easily reduced functional groups, such as nitro and cyano are unaffected. The mechanism of the reaction is pictured as a concerted transfer of hydrogen via a nonpolar cyclic TS. 

The catalytic system employing (2 - Fur)3P as ligand was applied to the coupling of methyl vinyl ketone and ethyl vinyl ketone to aromatic, aliphatic, acetylenic, and olefinic aldehydes (Scheme 23) [37]. Despite the hydrogenation conditions, alkyne and alkene moieties, as well as benzylic ether and nitro functional groups all remained intact. Furthermore, extremely high lev-... 

You can also catalytically reduce aldehydes and ketones to produce 1° and 2° alcohols. Reduction conditions are very similar to those used to reduce alkene double bonds. If a molecule possesses both a double bond and an aldehyde or ketone functional group, reduction of the aldehyde or ketone group is best carried out using sodium borohydride. The reduction of cyclohexanone by hydrogen gas with a platinum catalyst produces cyclohexanol in good yield. 

In the synthesis we should not wish to make 21 as it would cyclise and, in any case, we d rather reduce nitrile, nitro and alkene all in the same step by catalytic hydrogenation. The very simple method used for the conjugate addition is possible only because of the slow aldol reaction of the hindered aldehyde 24. The aldol 25, also called a Henry reaction, needs a separate dehydration step but the three functional groups in 26 are reduced in one step in good yield.7... 

All the forward reactions are important steps in commercial homogeneous catalytic processes. Reaction 2.2 is a step in methanol carbonylation (see Chapter 4), while reaction 2.3 is a step in the hydrogenation of an alkene with an acetamido functional group. This reaction, as we will see in Chapter 9, is... 

The substitution of functional groups at the electrophilic double bond or catalytic replacement of the hydrogen atom of alkene fragment by pyrroles or indoles has found wide use in the synthesis of U-vinylpyrroles or -indoles. Thus, indole was reacted with ethyl 2-nitro-3-ethoxyacrylate 662 to give ethyl 2-nitro-3-(3-indolyl)acrylate 663 as a 1 1 mixture of the (E)- and (Z)-isomers (Equation 159) <1996TL3309>. 

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