Lindlar-type reduction

Redox-type reactions show by far the worst performance in meeting the golden atom economical threshold. Three reductions meet this criterion with (AE)min values of 1 hydrogenation of olefins using the Lindlar catalyst (1952), Noyori stereoselective hydrogenation reaction (1985), and Zincke disulphide cleavage reaction (1911) whereas, oxidations... 

Z, E)-Trienes. Lindlar hydrogenation of substituted diene acetylenic esters of type 1-3 affords the corresponding (Z,E,E)-triene esters in high yield. The site selectivity of this reduction is excellent for all substrates examined except 3 evidently the steric bulk of the isopropyl or methyl substituents of 1 and 2 suppresses the rate of butadiene reduction in these systems. Only cis C2-C3 double bonds are obtained. 

Type 1. Consecutive reactions. The common feature of these examples (Scheme 2.68) is that the product formed in the first step is capable of reacting further under essentially the same reaction conditions. If the requirement for selectivity is to stop the process after the first step, a variety of approaches can be attempted. For example, in case (a) both consecutive steps belong to the same type of chemical process. Therefore to ensure the selective hydrogenation of the alkyne to the alkene, it is necessary to utilize a catalyst that permits the reduction of the triple bond but not the double bond. This requirement is met in Lindlar s catalyst, a palladium metal catalyst adsorbed on a carbonate that is partially deactivated with lead (Pd-CaC03-Pb0). 

Reduction of an alkyne. The best-known reactions of this type are Lindlar reduction, the McMurry reaction and Birch reduction. 

The majority of the examples of this type of reaction are catalytic hydrogenations over Lindlar s catalyst, which usually gives the corresponding alkenes with Z configuration in excellent yield. - For example, hydrogenation of 1 to alkene 2. Similar reductions have been carried out using lithium aluminum hydride, but in this case the... 

Lead(O) is probably most well known for its application as a catalyst poison, especially for transition metals such as palladium, and is most widely used in Lindlar s catalyst, which permits selective reductions of alkynes. In a detailed study of the selective reduction of a triple to a double carbon-carbon bond, it has been shown that two types of surface species, adsorbed lead and bulk lead, are present, and that the preparation of effective catalysts required precise experimental control. Optimisation of the preparation protocol found that the use of surface-oxidised palladium, as opposed to freshly prepared surface-reduced palladium, for the preparation of lead acetate-poisoned catalysts, as well as the removal of excess lead acetate before hydrogenation, is of crucial importance for preparing effective and selective Lindlar catalysts. 

---