Let’s take our attention away from the pericyclic reaction for a moment and discuss one of the reactions mentioned in the big book:
This reaction, which appears on page 808 of the larger book, is an example of the Kornblum reaction, which is formally equivalent to DMSO oxidizing an alkane halide to a carbonyl compound. The reaction mechanism is very similar to Swern:
As you can see, the C-H of the substrate is activated, the sigma bond electrons are “pulled” away by the formed onium ion during the reaction, and finally the onium ion has gained a pair of electrons, which are reduced to dimethyl sulfide. The R in the substrate halon can be a carbonyl group, an ester, a carboxylic acid, or an aromatics activated by an electron-withdrawing group, and the halogen atom can be Cl, Br, I. Under optimized conditions, the substrate can also be extended to aliphatic halides:
The addition of silver ions will cause the halogen atoms on the substrate to leave the system in the form of silver halide precipitation, resulting in a carbon-positive center, which is easier to bind to the nucleophilic oxygen atoms of DMSO. Here a stronger base such as triethylamine may be used to remove the active hydrogen, where C-H is less active because the substrate is an aliphatic halide, but is more likely to form a thioylide as Swern does, and a migration [2,3] occurs to remove the thioether:
Under such conditions, the reaction needs to be heated to a fairly high temperature to occur:
Under such conditions, the reaction needs to be heated to a fairly high temperature to occur:
It is now more useful to oxidize allyl halides with amines instead of DMSO to prepare unsaturated carbonyl compounds, which, if done under the conditions of the traditional Kornblum reaction, need to be heated to very high temperatures at which the substrate may be destroyed:
Let’s go back and look at where DMSO’s oxidation capacity comes from:
DMSO itself has a resonance, exists in an equilibrium form in the reaction system, its oxygen has a certain nucleophilic ability, can be combined with Lewis acid. The sulfur atom is then attacked by a weak nucleophile, such as a halide ion, to form a sulfonium ion with the ability to oxidize. Here Lewis acid can be TFAA or oxaloyl chloride, if you use TFAA for Lewis acid reaction the temperature is usually around -30°C, if you use oxaloyl chloride instead of TFAA, the temperature needs to be reduced to -78°C. There are also reports of the use of pyridine · sulfur trioxide complex, DCC, acetic anhydride, X2, etc. It is worth mentioning that the by-product urea in the product system after the use of DCC is difficult to deal with, if you are not familiar with the way DCC participates in the reaction, you can refer to the Fukuyama Formation A007, here do not repeat.
Based on a similar reaction mechanism, DMSO can also oxidize such compounds as epoxides and unsaturated carbon-carbon bonds:
