MoOPH explained

MoOPH, also known as oxodiperoxymolybdenum(pyridine)-(hexamethylphosphoric triamide), is a reagent used in organic synthesis.^[1] It contains a molybdenum(VI) center with multiple oxygen ligands, coordinated with pyridine and HMPA ligands, although the HMPA can be replaced by DMPU.^[2] It is an electrophilic source of oxygen that reacts with enolates and related structures, and thus can be used for alpha-hydroxylation of carbonyl-containing compounds.^[3] Other reagents used for alpha-hydroxylation via enol or enolate structures include Davis oxaziridine, oxygen, and various peroxyacids (see Rubottom oxidation). This reagent was first utilized by Edwin Vedejs as an efficient alpha-hydroxylating agent in 1974 and an effective preparative procedure was later published in 1978.

Synthesis

MoOPH is synthesized from molybdenum trioxide by oxidation with hydrogen peroxide and addition of the HMPA and pyridine ligands:^[4] $$\backslash ce\; \backslash xrightarrow\; \backslash ce\; \backslash xrightarrow\; \backslash ce\; \backslash xrightarrow\; \backslash ce$$

Reactivity

Due to MoOPH's steric bulk, preferential attack at the O–O bond occurs from the less hindered enolate face in the absence of stereoelectronic factors.^[5] In addition, nitriles with acidic alpha protons can be converted directly to cyanohydrins; however, in the case of branched nitriles, this reaction directly affords the ketone.^[6] In the case of sulfones, alpha-hydroxylation leads directly to the ketone or aldehyde.^[7] Common byproducts of the alpha-hydroxylation tend to include overoxidation to the corresponding dicarbonyl or intermolecular aldol reaction of the starting material. Procedures to prevent side reactions include the inverse addition of the enolate to MoOPH or careful control of the temperature (-78 to -20 °C). Notable miscellaneous reactions include MoOPH’s ability to oxidize alkylboranes directly to the alcohol with net stereo-retention.^[8] MoOPH has also been shown to oxidize N-trimethylsilyl amides directly to the hydroxamic acid.^[9]

Notes and References

1. Book: Oxodiperoxymolybdenum(pyridine)(hexamethylphosphoric triamide) . Edwin Vedejs . Encyclopedia of Reagents for Organic Synthesis . April 15, 2001 . 10.1002/047084289X.ro022 . E-EROS Encyclopedia of Reagents for Organic Synthesis. 978-0471936237 .
2. Paquette, Leo A.; and Koh Dongsoo (14 Sep 1992). "Explosion with MoO₅-DMPU complex" (letter to the editor) in Chemical and Engineering News, p. 2.
3. 10.15227/orgsyn.064.0127. HYDROXYLATION OF ENOLATES WITH OXODIPEROXYMOLYBDENUM(PYRIDINE)(HEXAMETHYLPHOSPHORIC TRIAMIDE), MoO5·Py·HMPA(MoOPH): 3-HYDROXY-1,7,7-TRIMETHYLBICYCLO[2.2.1]HEPTAN-2-ONE. Organic Syntheses. 64. 127. 1986.
4. Vedejs. E.. Engler. D. A.. Telschow. J. E.. 1978-01-01. Transition-metal peroxide reactions. Synthesis of α-hydroxycarbonyl compounds from enolates. The Journal of Organic Chemistry. 43. 2. 188–196. 10.1021/jo00396a002. 0022-3263.
5. Yuan. Changxia. Jin. Yehua. Wilde. Nathan C.. Baran. Phil S.. 2016-07-11. Short, Enantioselective Total Synthesis of Highly Oxidized Taxanes. Angewandte Chemie International Edition. en. 55. 29. 8280–8284. 10.1002/anie.201602235. 1521-3773. 4972021. 27240325.
6. Vedejs. E.. Telschow. J. E.. 1976-02-20. Synthesis of cyanohydrins from cyanides. Transition metal peroxide reactions. The Journal of Organic Chemistry. 41. 4. 740–741. 10.1021/jo00866a048. 0022-3263.
7. Little. R.Daniel. Myong. Sun Ok. Oxidative desulfonylation. Phenyl vinyl sulfone as a ketene synthetic equivalent. Tetrahedron Letters. en. 21. 35. 3339–3342. 10.1016/s0040-4039(00)78683-7. 1980.
8. Midland. M. Mark. Preston. Scott B.. 1980-10-01. Stereochemistry of molybdenum peroxide oxidation of organoboranes. The Journal of Organic Chemistry. 45. 22. 4514–4515. 10.1021/jo01310a054. 0022-3263.
9. Matlin. S. A.. Sammes. P. G.. 1972-01-01. A new method for the preparation of hydroxamic acids from secondary amides. Journal of the Chemical Society, Chemical Communications. en. 22. 1222. 10.1039/c39720001222. 0022-4936.