The Anionic Phospho-Fries Rearrangement

The prototypical anionic phospho-Fries rearrangement involves the conversion of an aryl phosphate ester [ArOP(=O)(OR)2] to an ortho-hydroxyarylphosphonate [o-OH-Ar-P(=O)(OR)2]. This review looks in detail at this 1,3-(O→C) migration, from its accidental discovery about 20 years ago to applications in the current literature. From a synthetic perspective, the rearrangement gives rise to a phenol with an ortho C-P bond. Prior to discussion of the details of the reaction, we briefly review the utility of these arylphosphonate compounds. The transposition of the phosphoryl moiety is initiated by formation of an aryl anion adjacent to the phosphate substituent. The phosphate group itself directs metallation to the ortho-position (i.e., via metal-hydrogen exchange), although direct metal-halogen exchange at the ortho position is a more efficient and reliable approach to anion formation. Methods for the synthesis of suitable precursor substrates, approaches to anion generation, and mechanistic aspects of the rearrangement are summarized. Factors which influence the outcome of the rearrangement include: reaction temperature, existing stereochemical elements, the nature of the substituents at phosphorus (phosphites and phosphoramidates have also been employed), existing substituents on the aromatic ring, and the propensity for further reactions. One of the most important issues in the case of unsymmetrical substrates is the regiochemical outcome of the reaction. The limited information available on this, as well as the compatibility of the reaction with other functionalities are discussed. We conclude with miscellaneous examples in relatively complex molecules, illustrating the potential for the reaction to be utilized more widely in synthetic applications.