Trimethylsilyl Polyphosphate

(can activate functional groups containing an oxygen atom; cyclodehydration agent for synthesis of heterocycles)

Alternate Names: polyphosphoric acid trimethylsilyl ester; PPSE.

Solubility: sol dichloromethane, chloroform, 1,2-dichloroethane, benzene, toluene, xylene, 1,2-dichlorobenzene.

Form Supplied in: colorless or pale yellow, viscous liquid; commercially available.

Preparative Methods: prepared by heating a mixture of Phosphorus(V) Oxide and Hexamethyldisiloxane (2-3.5 moles per mole P4O10) in a solvent such as dichloromethane or without solvent (eq 1).1,2

PPSE consists mainly of three tetraphosphoric acid trimethylsilyl esters together with a small amount of tetrakis(trimethylsilyl) pyrophosphate.3

Handling, Storage, and Precautions: PPSE is hygroscopic, and should be stored strictly in the absence of moisture.

Transformations of Functional Groups Containing an Oxygen Atom.

PPSE is capable of activating functional groups containing an oxygen atom by interaction with a phosphorus atom or a trimethylsilyl moiety. These reactivities of PPSE in conjunction with its aprotic nature and high solubility in organic solvents render this reagent useful in transformations of some functional groups. Thus amides are smoothly dehydrated by PPSE at reflux in chloroform to give nitriles (eq 2).4 Ketoximes are readily subjected to the Beckmann rearrangement to yield amides.1 Acetophenone oximes possessing OH, NH2, or SH groups at ortho positions are converted to 2-methylbenzoxazole, 2-methylbenzimidazole, and 2-methylbenzothiazole, respectively, via the Beckmann rearrangement and cyclization (eq 3).3 On the other hand, aldoximes are dehydrated by PPSE to afford nitriles.5 It is noted that aromatic nitriles are conveniently prepared from aromatic aldehydes by refluxing with Hydroxylamine hydrochloride and PPSE (eq 4).4

Primary and secondary alcohols are converted to the corresponding alkyl iodides by treatment with the Sodium Iodide-PPSE reagent system (eq 5).6 The reaction of optically active secondary alcohols results in the formation of virtually racemized alkyl iodides. Carboxylic acids are activated by PPSE, undergoing condensation with two equivalents of amines to afford amidines in high yields (eq 6).7

Reagent for Cyclodehydration.

PPSE can be used as a condensation agent for the preparation of such heterocycles as 1,3-dioxane derivatives (eq 7),2 9H-selenoxanthen-9-ones and related sulfur and oxygen congeners (eq 8),8 1,3-oxazin-6-ones,9 2H-1,2,4-benzothiadiazine derivatives,10 and benzoxazoles (see also Polyphosphoric Acid (PPA) and Polyphosphate Ester (PPE)).11,12

1. Imamoto, T.; Yokoyama, H.; Yokoyama, M. TL 1981, 22, 1803.
2. Imamoto, T.; Matsumoto, T.; Yokoyama, H.; Yokoyama, M.; Yamaguchi, K. JOC 1984, 49, 1105.
3. Yamamoto, K.; Watanabe, H. CL 1982, 1225.
4. Yokoyama, M.; Yoshida, S.; Imamoto, T. S 1982, 591.
5. Aizpurua, J. M.; Palomo, C. NJC 1983, 7, 465.
6. Imamoto, T.; Matsumoto, T.; Kusumoto, T.; Yokoyama, M. S 1983, 460.
7. Kakimoto, M.; Ogata, S.; Mochizuki, A.; Imai, Y. CL 1984, 821.
8. Berman, E. M.; Showalter, H. D. H. JOC 1989, 54, 5642.
9. Yokoyama, M.; Hatanaka, H.; Sakamoto, K. CC 1985, 279.
10. Imai, Y.; Mochizuki, A.; Kakimoto, M. S 1983, 851.
11. Aizpurua, J. M.; Palomo, C. BSF(2) 1984, 142.
12. Flouzat, C.; Guillaumet, G. S 1990, 64.

Tsuneo Imamoto

Chiba University, Japan

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