Black rearrangement

What is Black rearrangement?

Black first described the reaction now known as the Black rearrangement in 1986. This reaction is catalyzed by 4-N,N-dimethylamino-pyridine (DMAP) and involves the [1,3] migration of an ester group, such as formate, from an oxygen atom to the adjacent α-carbon atom of the carbonyl group for the carbonate esters of benzofuran enolates.

Black rearrangement
Black rearrangement
  • R = alkyl, aryl
  • R’ = alkyl, aryl, vinyl, etc.

Similar to the competition between O-alkylation and C-alkylation in the reaction of enolates with alkylating agents, the O-acylation and C-acylation also compete with each other in the reactions of enolates with acylating reagents. Although C-acylations are often favored under certain conditions, such as when acyl chlorides are used instead of anhydrides as acylating agents, or when the enolate is added inversely to acyl chlorides at low temperatures, or when enolates of divalent countercations are used instead of alkali, the O-acylation may still prevail for specific types of substrates. For instance, the acylation of benzofuran enolates under the aforementioned conditions typically yields O-acylated products as the preferred product, with maximal C-acylation up to 30 %. In particular, only O-acylated products were obtained for the enolate of diaryl acetic acid esters. This high preference for O-acylation is likely due to the high electron density located at the negative oxygen atom, resulting in kinetically favored products.

The formation of quaternary chiral centers in the benzofuran series is a restricted application of this reaction.

References

Black, T. H.; Arrivo, S. M.; Schumm, J. S. and Knobeloch, J. M., “A novel oxygen-to-carbon ester migration catalysed by 4-(N,N-dimethylamino)-pyridine in the benzofuranone ring systemJ. Chem. Soc., Chem. Commun., 1986, 1524-1525
DOI: 10.1039/C39860001524

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