Pd cats. for Suzuki microwave rxn.

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Discussion Overview

The discussion revolves around the challenges and methodologies associated with Suzuki coupling reactions, particularly focusing on the coupling of alkyl boronic acids to bromo pyridine using microwave conditions. Participants explore various catalyst systems, solvents, and conditions that may influence the success of the reaction.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant inquires about the appropriate palladium catalysts, ligands, bases, temperature, and solvents for their Suzuki coupling reaction.
  • Another participant suggests using palladium acetate or palladium tetrakis(triphenylphosphine) and emphasizes the importance of using an alkali base.
  • A participant reports success by using the alkene version of the boronate ester, achieving near quantitative yields with a specific catalyst system of tetrakis/potassium carbonate/dioxane+water.
  • Concerns are raised about the need for forcing conditions when coupling alkyl groups, with references to using strong non-nucleophilic bases and halide scavengers, resulting in modest yields.
  • One participant shares their experience of consistently needing to use alkene versions for successful couplings, noting the efficiency of their method at high temperatures.
  • A suggestion is made to explore the potential of using Suzuki reactions for making aryl silanes and polymers, highlighting the advantages of continuous processing.
  • Challenges are discussed regarding the removal of triphenylphosphine oxide from products, particularly when basic nitrogens are absent in the molecules being synthesized.
  • There is a mention of potential confusion between the Wittig and Grignard reactions, indicating a need for clarification among participants.
  • A question is raised about the impact of residual catalyst on chromatographic and spectroscopic analyses.

Areas of Agreement / Disagreement

Participants express a variety of experiences and opinions regarding the effectiveness of different approaches in Suzuki reactions, with no clear consensus on the best methods or conditions. Disagreements exist about the necessity of certain conditions and the effectiveness of specific catalyst systems.

Contextual Notes

Limitations include the dependence on specific catalyst systems and conditions, as well as unresolved questions about the impact of residual catalyst on analytical results. The discussion reflects a range of experimental contexts and outcomes.

Who May Find This Useful

Researchers and practitioners in organic chemistry, particularly those focused on coupling reactions and synthetic methodologies, may find this discussion relevant.

gravenewworld
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I was wondering if any pro chemists on here have a decent amount of experience doing suzuki coupling reactions. Currently I am trying to couple alkyl boronic acids to a bromo pyridine in the microwave, but seem to be having a lot of trouble. The only thing that I am observing so far is removal of the bromo, but no addition of my alkyl chain to the pyridine ring. I was wondering if anyone has any suggestions for what types of palladium catalysts/ligands/bases/temperature/or solvents I should be using for this type of reaction. I make sure before every reaction that I degas the vial with nitrogen to remove any oxygen from the solvent.
 
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What kind of catalyst system are you using?
What alkyl boronate are you trying to add?

What solvent are you using? Water/THF?

Palladium acetate will work as will palladium tetrakis(triphenylphosphine). don't forget to use the alkali base...
 
Thanks for the reply. We ended up getting around the problem by using the alkene version of the boronate ester and ended up getting around near quantitative yields for the suzuki couplings. Introducing the double bond seemed to work miracles.

we were using tetrakis/potassium carb/dioxane+water as our system.
 
I have heard that coupling alkyl groups by the Suzuki-Miyaura method needs very forcing conditions. I've seen very strong non-nucleophilic bases like t-BuOK used with halide scavengers like silver triflate (obviously no water) applied with only modest yields. Perhaps the pi system is overlapping into empty d orbital of palladium in the unsaturated versions of this reaction and the much higher energy antibonding orbitals are employed in the alkyl systems.
 
chemisttree said:
I have heard that coupling alkyl groups by the Suzuki-Miyaura method needs very forcing conditions. I've seen very strong non-nucleophilic bases like t-BuOK used with halide scavengers like silver triflate (obviously no water) applied with only modest yields. Perhaps the pi system is overlapping into empty d orbital of palladium in the unsaturated versions of this reaction and the much higher energy antibonding orbitals are employed in the alkyl systems.

Interesting. So far I have never had success at coupling alkyl groups via Suzuki rxns. They always have to be the alkene versions of what I want. What is great with the alkene version is that it only usually takes me 15 minutes at 155 C to get quanitative yields. Work up is a cinch too.
 
Suzuki reactions are nice that way. Have you ever thought to try making aryl silanes by this method. Current methods use grignards and is a obviously a batch process. Suzuki conditions lend themselves well to continuous processing. Have you seen anyone use Suzuki conditions to make polymers?
 
Nah I have never seen anyone do Suzukis to make polymers, that is not the type of thing we do where I am. The only draw back that I have come across is the fact that if my molecule doesn't have any basic nitrogens I can exploit to make HCl salts of, getting rid of the triphenylphosphine oxide is a BEAR of a problem. the TPP oxide always sticks to the columns in the LCMS, makes you get terrible chromatograms when doing a silica column, and trace amounts give you terrible looking NMRs. I can remove a lot of the catalyst by dissolving the product up in DCM and filtering out the catalyst that crashes out, but it doesn't get rid of all of it. If a suzuki rxn is needed, usually I have to tailor my compound so that it will have a basic nitrogen in it.
 
You might be confusing Wittig with Grignard...
 
chemisttree said:
You might be confusing Wittig with Grignard...


_______________huh?
 
  • #10
Does the small amount from the catalyst cause all of those problems?
 

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