(Diethoxymethyl)diphenylphosphine Oxide

[20570-20-1]  · C17H21O3P  · (Diethoxymethyl)diphenylphosphine Oxide  · (MW 304.32)

(readily accessible Horner-Wittig reagent for the conversion of aldehydes and ketones into homologous O,O-ketene acetals1)

Physical Data: mp 77-79 °C; NMR: 31P d -25.5 ppm (H3PO4 = 0 ppm); 13C d(CH) 104.7 ppm; 1H d(CH) 4.98 ppm.

Solubility: sol benzene, THF, and ether.

Analysis of Reagent Purity: 31P NMR; mp.

Preparative Method: the reagent (1) can be conveniently prepared by reaction of Chlorodiphenylphosphine with Triethyl Orthoformate.2 Adduct (2) is formed in a fast, exothermic, step. Upon heating, an Arbuzov reaction occurs to give the phosphine oxide (eq 1). Other O,O-acetals of formyldiphenylphosphine oxide can be prepared equally well.2

The usual procedure is for chlorodiphenylphosphine (88.2 g, 0.4 mol) to be added dropwise to triethyl orthoformate (59.3 g, 0.4 mol) over a period of 45 min. Subsequent heating of the reaction mixture (2 h, 110 °C) leads to formation of 24 g (93% of theory) of ethyl chloride. Upon cooling, a practically quantitative yield of (1) is obtained, mp 74-77 °C. Addition of some solvent, when (1) starts precipitating, avoids the formation of a solid cake that is hard to remove from the reaction flask. Crystallization from benzene/petroleum ether (80-110) affords the pure phosphine oxide.

Handling, Storage, and Precautions: infinitely stable at rt. It should be protected from moisture.

Synthesis of Ketene O,O-Diethyl Acetals.

Aldehydes and ketones, aliphatic as well as aromatic and a,b-unsaturated, can be converted into their homologous ketene O,O-diethyl acetals by Horner-Wittig reaction with the anion of (1).1,3 This conversion represents a considerable improvement over existing methods.4 The metalation of (1) with Lithium Diisopropylamide as well as subsequent reactions of the anion should be carried out at below -90 °C as the anion is unstable at higher temperatures. Best results are obtained if the intermediate adducts (3) are isolated and subsequently treated with a potassium base to bring the Horner-Wittig reaction to completion (eq 2). By applying a water-free workup procedure, the ketene O,O-diethyl acetals (4) are obtained in the yields listed in Table 1. Also included in this table are some characteristic 13C and, where applicable, 1H NMR data of the ketene acetals. These products are sufficiently pure for most applications. Analytically pure ketene acetals can be obtained by distillation, but only at appreciable loss of product.

In contrast to ketene S,S- and S,O-acetals,5 the possibilities to prepare ketene O,O-acetals by Horner-Wittig or Peterson alkenation reactions appear to be restricted to phosphine oxides. Attempts to lithiate the corresponding phosphonates failed.5 Horner-Wittig synthesis of ketene O,O-acetals is not restricted to the diethoxy compounds (4). Other (dialkoxymethyl)phosphine oxides perform equally well. The use of phosphine oxide (5), prepared in analogy to (1) from 2-ethoxy-5,5-dimethyl-1,3-dioxane,2 can be especially advantageous because the corresponding ketene acetals have higher melting points. The reported examples (corresponding to entries 1, 10 and 13 in Table 1) are solids that can be conveniently purified by crystallization.1

General Procedure.

To a solution of (1) (3.04 g, 10 mmol) in 200 mL of THF/diethyl ether (3:1), cooled to -100 °C, is added a solution of LDA (1.17 g, 11 mmol) in 25 mL of THF.1,3 After stirring for 1 min, a solution of 10 mmol of the carbonyl compound in 25 mL of THF is added to the deeply colored solution at such a rate that the temperature of the reaction mixture is kept at all times below -90 °C. After stirring for an additional 5 min at this temperature, the mixture is quenched by the addition of 40 mL of water. After warming to rt, solid Potassium Carbonate is added and the organic layer is separated and dried. Evaporation of the solvents at reduced pressure affords the adducts (3) in almost quantitative yields. To a solution of the adduct in 50 mL of THF, Potassium t-Butoxide (1.23 g, 11 mmol) in 50 mL of THF is added in one portion. After stirring for 1 h at rt, the solvent is removed at temperatures not exceeding 40 °C. The residue is taken up in 100 mL of hexane. After centrifugation, the solution is decanted. Upon evaporation of the solvent, the ketene acetal is obtained as a pale yellow oil.

1. van Schaik, T. A. M.; Henzen, A. V.; van der Gen, A. TL 1983, 24, 1303.
2. Dietsche, W. LA 1968, 712, 21.
3. Scheeren, J. W. MOC 1993, E-15/2, 1674.
4. McElvain, S. M.; Starn, R. E., Jr. JACS 1955, 77, 4571.
5. Mikolajczyk, M.; Grzejszczak, S.; Zatorski, A.; Mlotkowska, B.; Gross, H.; Costisella, B. T 1978, 34, 3081.

Arne van der Gen

Leiden University, The Netherlands

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