An exciting Angewandte paper this week from the Wirth lab at Cardiff University describes the chemistry of hypervalent bromine reagents, the violent cousins of hypervalent iodine compounds. The reaction that intrigues me the most is C-H insertion by vinylcarbenes, which are generated from aryl alkynyl bromanes via “Michael-esque” nucleophilic attack.
Vinylcarbenes can do C-H insertion, cyclopropanation…basically everything a normal carbene can do. If Nu is an alkyne (usually an alkynylstannane), they can even rearrange to give diynes in what amounts to an uncatalyzed coupling reaction! Too bad it’s stoichiometric in bromane…hopefully that will change in the future.
ArBrF2 has been shown to mediate what I like to call the “super-Wacker” oxidation of a terminal alkyne to a conjugated ketone. It oxidizes ethanol to acetaldehyde, which then [2+2]’s with the alkyne to give an oxetene that retro [2+2]’s to the conjugated product.
Halogens with more than one bond…imagine that!