Lindlar catalyst triple bond reduction

Alkynes can be reduced to yield alkenes and alkanes. Complete reduction of the triple bond over a palladium hydrogenation catalyst yields an alkane partial reduction by catalytic hydrogenation over a Lindlar catalyst yields a cis alkene. Reduction of (he alkyne with lithium in ammonia yields a trans alkene. 

Reduction of enynes to (Z)-atkenes. Lindlar s catalyst is not useful as a hydrogenation catalyst for reduction of trienynes or of dienediynes. The best results can be obtained in CH3OH with zinc activated by successive treatment with Cu(OAc)2 (10%) and AgN03 (10%). This reduction results in conversion of the triple bond to a (Z)-double bond. The system does not reduce simple, nonactivated alkynes, and a-branched enynes are reduced slowly. The reduction is effected at 25° with (Z)-enynes, but temperatures of 45° are necessary for the (E)-isomers. Yields of pure tetraenes are 25-65%. 

Alternative catalysts such as palladium-on-barium sulfate (poisoned by synthetic quinoline) (24), "P-2" nickel boride (with ethylenediamine) (25), and other nickel catalysts (19c) can be used in place of Lindlar catalyst. However, in our hands selective hydrogenation of triple bonds to Z olefins proceeds with the greatest stereoselectivity with Lindlar catalyst. Palladium-on-barium sulfate (in ethanol with quinoline) can give considerable over-reduction and isomerization to the E isomer (22a). Use of "P-2" nickel boride as the catalyst at room temperature usually gives a. 2% of the J5 isomer (e.g. 23). 

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). 

In the second step, the triple bond in 63 is selectively reduced to the cz -alkene using the Lindlar catalyst to form 64. In this case, the Lindlar catalyst is a poisoned heterogeneous palladium catalyst on barium sulfate. The deactivation of the catalyst with quinoline is responsible for the selective hydrogenation to the alkene and not through to the alkane. The reason for the highly stereoselective reduction with syn-addition to the cw-alkene is that one face of the triple bond is shielded by the catalyst surface. 

Unfortunately, the subsequent step using the Lindlar catalyst met with little success, and nonstereoselective partial reduction of the triple bond is observed." It was reported that most catalytic hydrogenations of dialkyl 1-alkynylphosphonates in EtOH using 5% Pd/CaCOj poisoned with quinoline gave mixtures comprising cis- and fra i-l-alkenylphosphonate and starting material, from which the predominant cis isomer was isolated. 

The synthesis of Z-alkenyl derivatives were carried out by partial reduction of triple bond with the Lindlar catalyst (route f). The fully saturated derivatives were obtained starting from the same alkynes by using palladium on charcoal as a catalyst (route g). 

The stereochemistry of many open chain olefins is known. From acetylenic substrates cis or trans olefins can be conveniently prepared by partial hydrogenation over the well-known Lindlar catalyst or by lithium aluminum hydride reduction (in cases when a hydroxyl group is present next to the triple bond). 

Pt or Pd catalysts on CaC03, poisoned by Pb (Lindlar s catalyst). They enable selective reduction of triple bonds to double bonds (Eq. 5-82). 

Addition of 1 mol of hydrogen to the carbon-carbon triple bond can be accomplished stereospecifically. Catalytic reduction leads to the cis isomer. This is most often carried out using Lindlar catalyst, a lead-poisoned palladium-on-calcium carbonate preparation. Palladium on BaS04 is an alternative. Some examples are recorded in Scheme 3.10. Numerous other catalyst systems have been employed to effect the same reduction. Many specific cases are cited in reviews of catalytic hydrogenations. If the trans alkene is desired, the usual method is a dissolving-metal reduction in ammonia. This reaction is believed to involve two successive series of reduction by sodium and protonation ... 

Reduction of an alkyne occurs in two stages first, addition of one mole of H2 to form an al-kene and then addition of the second mole of H2 to the alkene to form the alkane. In most cases, it is not possible to stop the reaction at the alkene stage. However, by careful choice of catalyst, it is possible to stop the reaction at the addition of one mole of hydrogen. The catalyst most commonly used for this purpose consists of finely powdered palladium metal deposited on solid calcium carbonate that has been specially modified with lead salts. This combination is known as the Lindlar catalyst. Reduction (hydrogenation) of alkynes over a Lindlar catalyst is stereoselective syn addition of two hydrogen atoms to the carbon—carbon triple bond gives a cis alkene ... 

Preparation from Methyl Santalbate. Adlof prepared gram quantities of methyl (9Z,ll )-[9,10- H]-octadecadienoate using methyl santalbate [methyl ( )-octadec-ll-en-9-ynoate] obtained from Santalum album seeds (12). Partial reduction of the triple bond of methyl ( )-octadec-ll-enynoate in the presence of Lindlar catalyst, quinoline, and deuterium gas gave the (Z, ) conjugated diene system (Fig. 6.1). 

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. 

The one-pot preparation of (Z, )-2,5-dienol was described by Walsh in 2006 [1]. The strategy is based on the sequential functionalization of bromo-substituted 1,3-enynes such as 4-bromo-l,3-enyne 1 by hydroboration followed by hydride addition to the resulting borane (steps i and ii in Scheme 1). Next, dialkylzinc and carboxaldehyde (steps iii and iv) were sequentially introduced to complete the formal addition of (Z)-dienyl group to an electrophile. The two aldol products 2 and 3 are illustrative of the potential of this methodology. These products contain both (Z)-olefin and thiophen or triple bond appendages and could, with difficulty, be prepared by conventional Lindlar reduction due to risks of catalyst poisoning by the thiophen moiety or over reduction of the triple bond. 

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