The first thing I will say in this post is: you won’t find me using O2 in a microwave reaction…..well with recent developments utilizing better methods of introducing reactive gases to microwave systems…see SRC microwave synthesis, I find that we are seeing more controlled and safer ways to use H2 and O2 in a microwave reactor…..which is great because it is greener and the atom economy approach will help broaden reagents that are useful for these powerful transformations (see scheme below)

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A recent publication (love the open access journals btw, although I can appreciate the difficulty in making sure these stay on the up-and-up) on the conversion of a variety of alcohols to aldehydes and ketones indicates that simple Pd/C in the presence of MeOH, base and molecular oxygen (gas addition to SynthWAVE) converts alcohols into esters with ease. That find was impressive, but what stands out in the article is how they arrived there. Going back several posts I introduced the SRC design and how people are starting to use it — in this research, they  really went into screening mode: what Pd reagent, what base and what combination would work best for the transformation — If I am doing DOE or feasibility studies, I want to quickly get to the best set of conditions possible.

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SRC Cartoon of reaction sequence

Below I have placed the first set of conditions looking for microwave conditions with a few Pd catalysts.

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The reaction chamber was loaded with oxygen (2.5 bar) and pressurized with nitrogen (up to 20 bar) [57]. Reactions were performed in the presence of benzylalcohol, K2CO3 in methanol and numerous different reaction conditions were screened

Without going too heavily into his paper — you have to read it — I have included a table of the bases and catalysts which were screened.

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Enjoy the material — another example of running a few sets of microwave conditions to arrive an an optimized route to a variety of esters. One thing to note — pay attention to the utility of using O2 in a N2 blanket inside the microwave reactor — moving us forward in what is possible using microwave technology for synthetic operations