|Latest from the Baran laboratory: Olefin hydroamination. (Gui et al. DOI) Key ingredients: an alkene and a nitro arene together with triphenylsilane (2 eq.), ferric acetylacetonate (30 mole%), zinc, HCl and ethanol. The product: the alkylated amine. Product optimisation and substrate scope are all accounted for but where is the reaction mechanism? Relax, it is hidden in the supplementary info (350 pages!). |
Any mechanism involving iron is insanely complex but here is this blog's attempt at some understanding. Phenyl silane transfers a proton to ferric acetylacetonate to form a ferric hydride species Fe-H. This species reduces the nitro arene to the Ar-NO nitroso compound and at the same time it reacts with the alkene to the alkyl radical. Both intermediates then react to form the hydroxylamine Ar-N(OH)-R or the N,O adduct Ar-N(R)-OR. The hydroxylamine is then converted to the amine by a another iron species and the N,O adduct is reduced by zinc. Replacing iron by cobalt or manganese killed the reaction by the way so iron is really key.
Drawbacks: the alkene is added in large excess. Side-products are the amine as direct reduction product and the N,O adduct. Typical yields are in the range of 50-60% which sounds like not a lot but the article notes that isolating amines from a reaction product in general is a challenge. Nitroalkanes perform badly. Ketone, alcohol, boronic acid and amine functional groups are tolerated.
And how about that 350 page supplimentary information? It also includes a photographic guide with lots of experimental-setup close-ups. Those Baran people really dislike keeping the lab tidy!. Also included is a handy Q&A: how exactly do you monitor a reaction?
new CRD entry: