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Author Topic: Partial reduction of alkynes to cis-alkenes by using Lindlar catalyst  (Read 6294 times)

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affa

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I am working on the partial reduction of alkyne to cis-alkene using Lindlar's catalyst. However, I am not really sure the amount of Lindlar catalyst to be added. Commerical source is 5% Pd on CaCO3, poisoned with lead. How much catalyst should be used relative to the substrates?

My second question is how much quinoline should be added (relative to the Lindar catalyst). In practice, trace amount of trans-alkene would also be produced under Lindlar's partial reduction conditions. When the amount of quinoline (relative to Lindlar catalyst) is insufficient, more undesired trans-alkene would be produced (difficult to separate trans- and cis-alkenes). However, the reaction rate becomes sluggish when too much quinoline is added.
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kiwi

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Re: Partial reduction of alkynes to cis-alkenes by using Lindlar catalyst
« Reply #1 on: October 24, 2007, 12:50:59 AM »

i've reduced internal alkyne -> alkene with lindlars before, and it worked well. i forgot the loading, it was a while ago, but i'll try to find it. quinoline is generally needed for terminal alkynes as far as i remember
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affa

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Re: Partial reduction of alkynes to cis-alkenes by using Lindlar catalyst
« Reply #2 on: October 24, 2007, 08:42:10 AM »

My experience told that addition of quinoline is always required to suppress the formation of trans-alkenes. I agree that  reduction of terminal alkynes may form some over-reduced alkanes especially when the amount of quinoline is insufficient. That's why I hope to find out the standard loading of quinoline to Lindlar catalyst.

This week, I worked on the partial reduction of 3-phenyl-2-propyn-1-ol to form cis-cinnamyl alcohol. In the first trial, the loading of quinoline was found insufficient as a mixture of cis- and trans-alcohol in 3.5 : 1 ratio was afforded. The reaction conversion goes to completion within 24 hrs. In the second trial, similar amount of Lindlar catalyst was used but quinoline was added in a double quantity. Although the reaction rates become sluggish (~50% conversion after 24 h), selective formation of cis-alcohol (<3% trans-alcohol) was found. 
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kiwi

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Re: Partial reduction of alkynes to cis-alkenes by using Lindlar catalyst
« Reply #3 on: October 25, 2007, 12:44:17 AM »

that reminds me, you could scope out other cis reductions too. diimide is one that springs to mind.
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