Hi.
The following is a very interesting excerpt from
March's Advanced Organic Chemistry, 5th Ed. pg.1599-1560. After the excerpt I will begin to discuss an obvious idea that came to me after reading this, and would love to get some feedback on it.
Reductive Coupling: Bimolecular Reduction of Aldehydes and Ketones to 1,2-Diols2 RCOR ==Na/Hg==> [R
2C(OH)]
21,2-Diols (pinacols) can be synthesized by reduction of aldehydes and ketones with active metals such as sodium, magnesium, or aluminum. Aromatic ketones give better yields than aliphatic ones. The use of Mg-MgI
2 mixture has been called the
Gomberg-Bachmann Pinacol Synthesis. As with a number of other reactions involving sodium, there is a direct electron transfer here, converting the ketone or aldehyde to a ketyl, which dimerizes.
Other reagents have been used, including SmI
2, Yb, activated Mn, In with ultrasound, VCl
3(thf)
3/Zn, and a reagent prepared from TiCl
4 and Mg amalgam (a low-valent titanium reagent). Dialdehydes have been cyclized by this reaction (with TiCl
3) to give cyclic 1,2-diols in good yield. Unsymmetrical coupling between two different aldehydes has been achieved by the use of a vanadium complex, while TiCl
3 in aqueous solution has been used to couple two different ketones. Two aldehydes have also been coupled using magnesium in water. Coupling leads to a mixture of syn- and anti-diols. “Syn-selective” reagents are Cp
2TiCl
2/Mn and NbCl
3.
Then it goes on into photochemical combination. The same reaction, but using photons to induce a singlet state that facilitates the same reaction. But that is another subject.So, I’m wondering to myself if one could react benzaldehyde with an excess of acetaldehyde, to form 1-phenyl-1,2-dihydroxypropane, which also happens to be the diol of propenylbenzene, not allylbenzene. Yum, you know what that means. Pinacol rearrangement with dilute H
2SO
4 like usual will afford the phenyl-2-propanone. If piperonal were used instead of benzaldehyde, the product would, after Pinacol rearrangement, be MDP2P of course.
I do realize the drawback of needing things like sodium metal to perform the original reaction, but some of the reagents don’t seem that tough, like the Mg/H2O combination for example. This is a very cool possibility.
I also wanted to bring up the question of using the H
2SO
4 rearrangement on ethylene glycol to prepare acetaldehyde. Ethylene glycol is sold as common antifreeze for making coolant for automobiles, and is widely available in good purity. Get a brand without a dye added to it. Prepare the acetaldehyde any way you want, or buy it, but I’m pretty interested if this is possible, because ethylene glycol is very cheap in the USA, and widely available, as is sulfuric acid.
Then react your acetaldehyde with your benzaldehyde of choice, and rearrange the formed diol to the phenyl-2-propanone of your desire. Who needs nitroethane anyway?
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