Palladium, calcium carbonate, catalyst

Preparation of palladium - calcium carbonate catalyst. Prepare 60 g. of precipitated calcium carbonate by mixing hot solutions of the appropriate quantities of A.R. calcium chloride and A.R. sodium carbonate. Suspend the calcium carbonate in water and add a solution containing 1 g. of palladium chloride. Warm the suspension until all the palladium is precipitated as the hydroxide upon the calcium carbonate, i.e., until the supernatant liquid is colourless. Wash several times with... 

C. [18]Annulene. In a 300-ml. conical flask fitted with a side arm (with a closed stopcock) and a 3.5-cm. Teflon -coated magnetic stirring bar is placed 1 g. of a 10% palladium-calcium carbonate catalyst (Note 20) and 30 ml. of benzene (Note 10). The flask is attached to an atmospheric pressure hydrogenation apparatus, and the air in the system is replaced by hydrogen by evacuating the flask and refilling with hydrogen three times. The catalyst suspension is stirred until no more gas is absorbed. One-third (390 mg., determined spectroscopically)... 

The use of catalytically activated hydrogen as an analytical tool for the dehalogenation of organic compounds was discovered by Busch and Stowe (106), who used a palladium-calcium carbonate catalyst. Later, Kelber (107) reported a similar procedure, using a reduced nickel catalyst. Whitmore and Revukas (108) observed that, in the reductive splitting of substituted azo compounds with hydrogen in the presence of Raney... 

Aldehydes can be obtained homogeneously from ozonides by reductive cleavage, which can be effected either by zinc in glacial acetic acid or by catalytic hydrogenation of the ozonide118,119 in the presence of a 1 19 palladium-calcium carbonate catalyst.120 Adipdialdehyde has thus been obtained from cyclohexene in 60-70% yield 121... 

The most obvious method for the formation of alkenes from alkynes is by partial reduction. This reaction can be effected in high yield with a palladium-calcium carbonate catalyst that has been partially deactivated by addition of lead(II) acetate or quinoline (Lindlar s catalyst). It is aided by the fact that the more electrophilic alkynes are adsorbed on the electron-rich catalyst surface more strongly than the corresponding alkenes. An important feature of these reductions is their high stereoselectivity. In most cases the product consists very largely of the thermodynamically... 

It is very well documented that the carbon-carbon triple bonds (e.g., alkynes) on catalytic hydrogenation gives the completely reduced product, viz. alkanes. Alkynes can also be reduced partially to give z-alkenes by palladium-calcium carbonate catalyst which has been deactivated (partially poisoned) by the addition of lead acetate (Lindlar catalyst) or Pd-BaSO deactivated by quinoline. The lead treatment poisoned the palladium catalyst, rendering it less active and the reaction is more selective. Some examples are given (Scheme 98). 

A benzene solution of the cyclic dimer 61 when refluxed with a saturated solution of t-BuOK in BuOH gave a red solution of the bisdehydro[20]annulene 63 [62]. The cyclic trimer 62 was rearranged similarly to tridehydro[30]annulene 64. Both species show characteristic UV-vis spectra [62]. The bisdehydro[20]annulene was partially hydrogenated in benzene solution over a Lindlar catalyst (quinoline-poisoned palladium calcium carbonate catalyst). This procedure led to the [20]an-nulene 65. Similarly, the reduction of tridehydro[30]annulene 64 with Lindlar catalyst yielded the [30]annulene 66 [62]. 

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