Selective Hydrogenations in the Presence of Other Functional Groups

8 SELECTIVE HYDROGENATIONS IN THE PRESENCE OF OTHER FUNCTIONAL GROUPS 

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Alkyl chlorides are with a few exceptions not reduced by mild catalytic hydrogenation over platinum [502], rhodium [40] and nickel [63], even in the presence of alkali. Metal hydrides and complex hydrides are used more successfully various lithium aluminum hydrides [506, 507], lithium copper hydrides [501], sodium borohydride [504, 505], and especially different tin hydrides (stannanes) [503,508,509,510] are the reagents of choice for selective replacement of halogen in the presence of other functional groups. In some cases the reduction is stereoselective. Both cis- and rrunj-9-chlorodecaIin, on reductions with triphenylstannane or dibutylstannane, gave predominantly trani-decalin [509]. 

Catalytic hydrogenation can be used for the selective reduction of a carbon-carbon double bond in the presence of other functional groups such as a carbonyl group or an aromatic ring. Selective reduction of one double bond in (R)-limonene (6.7), which contains two double bonds, gives (R)-carvomenthene (6.8) by hydrogenation over Ni metal. 

The value of the catalytic transfer hydrogenation route is demonstrated by the selective, high yield and rapid reduction of nitro aliphatic compounds to their corresponding amine derivatives using anhydrous ammonium formate (equation 26). A wide variety of nitro compounds are reduced in the presence of other functional groups including acids, esters and nitriles. Furthermore, the method is stereospecific and proceeds with retention of configuration pure racemic syn-nitro alcohols (39a) and (39b) were converted to the 5yn-amino alcohols (40a) and (40b) and the axial nitrosteroid (41a) afforded the 63-amine (41b). 

Anatase is much more active than rutile in the liquid-phase photooxidation of 2-propanol, and the observed reactivity compared well with that of gaseous alcohol on platinized titania [106a]. Because most early studies employed the alcohol as a sacrificial reagent for the photochemical production of hydrogen, the organic product was often not analyzed and little effort was devoted to the selective activation of alcohols in the presence of other functional groups. In the oxidation of car-... 

Rhodium on alumina is the catalyst of choice, not only for diastereoselective, but also regiose-lective hydrogenations. This catalyst allows selective reduction of the furan nucleus in the presence of other functional groups, such as a keto function. 

To study the selectivity for alcohol oxidation in the presence of other functional groups, allylic alcohols are the substrates of choice since they can undergo a variety of different reactions on each functional group and in addition they can interact with each other undergoing hydrogen rearrangement (Scheme 12.10). 

Pd is an excellent catalyst that is often active and selective. Pd enables selective hydrogenation of double bonds to be carried out in the presence of other functional groups. 

The. selective hydrogenation of a nitro group in the presence of other reactive functionalities is a frequently encountered problem in fine chemicals manufacture. Ciba-Geigy (Novartis). scientists developed, in collaboration with a catalyst manufacturer, a new Pt/Pb on CaCO. catalyst that allows the chemoselective hydrogenation of an aromatic nitro group in the presence of C=C, C=0, C=N as well as Cl or Br substituents in selectivities > 95% (even C C groups react very slowly) (Bader et al., 1996). Eqn. (3) shows an example (Bader eJ a/., 1996). 

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