Andy Extance at the Chemistry World blog has picked up on a fascinating article[cite]10.1021/jz401578h[/cite] on the dimer of SF ₂ . This molecule has three F atoms on one S, and only one on the other; FSSF ₃ . But all four S-F bonds are of different length.

According to Herges[cite]10.1021/ci00017a011[/cite],[cite]10.1039/c2cc34026g[/cite] the mechanism of single-step (concerted) reactions can be divided into three basic types;

The concept of a “ hidden intermediate ” in a reaction pathway has been promoted by Dieter Cremer[cite]10.1021/ar900013p[/cite] and much invoked on this blog. When I used this term in a recent article of ours[cite]10.1021/jo401146k[/cite], a referee tried to object, saying it was not in common use in chemistry. The term clearly has an image problem.

A reader asked me about the mechanism of the reaction of 2-picoline N-oxide with acetic anhydride to give 2-acetoxymethylpyridine (the Boekelheide Rearrangement[cite]10.1002/ejoc.201000936[/cite]). He wrote “ I don’t understand why the system should prefer to go via fragmentation-recombination (… the evidence being that oxygen labelling shows scrambling) when there is an easy concerted pathway available (… a

A little while ago, I set out some interpretations of how to push curly arrows.

This is a follow-up to comment posted by Ryan, who asked about isocyanide’s role (in the form of the anion of tosyl isocyanide, or TosMIC): “In Van Leusen, it (the isocyanide) acts as an electrophile”. The Wikipedia article (recently updated by myself) shows nucleophilic attack by an oxy-anion on the carbon of the C≡N group, with the isocyanide group acting as the acceptor of these electrons (in other words, the electrophile). In the form shown

Here is another example gleaned from that Woodward essay of 1967 ( Chem. Soc. Special Publications (Aromaticity), 1967 , 21 , 217-249), where all might not be what it seems. Woodward notes that the reaction between the (highly reactive) 1 does not occur.

Lukas, who occasionally comments on this blog, sent me the following challenge. In a recent article[cite]10.1021/jo3021709[/cite] he had proposed that the stereochemical outcome ( Z ) of reaction between a butenal and thioacetic acid as shown below arose by an unusual concerted cycloaddtion involving an S-H bond.

The previous post described how the acid catalysed ring opening of propene epoxide by an alcohol (methanol) is preceded by pre-protonation of the epoxide oxygen to form a “hidden intermediate” on the concerted intrinsic reaction pathway to ring opening.

A recent theme here has been to subject to scrutiny well-known mechanisms supposedly involving intermediates. These transients can often involve the creation/annihilation of charge separation resulting from  proton transfers, something that a cyclic mechanism can avoid. Here I revisit the formation of an oxime from hydroxylamine and propanone, but with one change.