What is (diacetoxyiodo)benzene?

(Diacetoxyiodo)benzene, also known as phenyliodine(III) diacetate (PIDA) is a hypervalent iodine chemical with the formula . It is used as an oxidizing agent in organic chemistry.

Preparation

This reagent was originally prepared by Conrad Willgerodt¹ by reacting iodobenzene with a mixture of acetic acid and peracetic acid:²³

CHI   +   CHCOH   +   CHCOH   →   CHI(OCCH)   +   HO

PIDA can also be prepared from iodosobenzene and glacial acetic acid:⁴

CHIO   +   2 CHCOH   →   CHI(OCCH)   +   HO

More recent preparations direct from iodine, acetic acid, and benzene have been reported, using either sodium perborate⁵ or potassium peroxydisulfate⁶ as the oxidizing agent:⁷

CH   +   I   +   2 CHCOH   +   KSO   →   CHI(OCCH)   +   KI   +   HSO   +   KHSO

The PIDA molecule is termed hypervalent as its iodine atom (technically a hypervalent iodine) is in its +III oxidation state and has more than typical number of covalent bonds.⁸ It adopts a T-shaped molecular geometry, with the phenyl group occupying one of the three equatorial positions of a trigonal bipyramid (lone pairs occupy the other two) and the axial positions occupied by oxygen atoms from the acetate groups. The "T" is distorted in that the phenyl-C to I to acetate-O bond angles are less than 90°.⁹ A separate investigation of the crystal structure confirmed that it has orthorhombic crystals in space group Pnn2 and reported unit-cell dimensions in good agreement with the original paper.¹⁰¹¹ The bond lengths around the iodine atom were 2.08 Å to the phenyl carbon atom and equal 2.156 Å bonds to the acetate oxygen atoms. This second crystal structure determination explained the distortion in the geometry by noting the presence of two weaker intramolecular iodine–oxygen interactions, resulting in an "overall geometry of each iodine [that] can be described as a pentagonal-planar arrangement of three strong and two weak secondary bonds."¹²

Unconventional reactions

One use of PIDA is in the preparation of similar reagents by substitution of the acetate groups. For example, it can be used to prepare (bis(trifluoroacetoxy)iodo)benzene (phenyliodine(III) bis(trifluoroacetate), PIFA) by heating in trifluoroacetic acid:¹³¹⁴

<figure> <img src="PIFA_synthesis_by_exchange.png" title="PIFA_synthesis_by_exchange.png" width="500" alt="PIFA_synthesis_by_exchange.png" /> <figcaption aria-hidden="true">PIFA_synthesis_by_exchange.png</figcaption> </figure>

PIFA can be used to carry out the Hofmann rearrangement under mildly acidic conditions,¹⁵ rather than the strongly basic conditions traditionally used.¹⁶¹⁷ The Hofmann decarbonylation of an N-protected asparagine has been demonstrated with PIDA, providing a route to β-amino-<small>L</small>-alanine derivatives.¹⁸

PIDA is also used in Suárez oxidation, where photolysis of hydroxy compounds in the presence of PIDA and iodine generates cyclic ethers.¹⁹²⁰²¹ This has been used in several total syntheses, such as the total synthesis of (−)-majucin, (−)-Jiadifenoxolane A,²² and cephanolide A.²³

References

Original source: (diacetoxyiodo)benzene. Shared with Creative Commons Attribution-ShareAlike 3.0 License

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