Tetramethylammonium Dimethyl Phosphate

[756-77-4]  · C6H18NO4P  · Tetramethylammonium Dimethyl Phosphate  · (MW 199.22)

(dehydrobromination of bromo ketones, bromo aldehydes, and bromoalkanes)

Physical Data: mp 215 °C.

Solubility: sol acetonitrile, DMF, HMPA.

Preparative Methods: prepared1,2 by reacting an excess of Trimethylamine (139 g) with Trimethyl Phosphate (210 g) in acetone solution (400 g) in a sealed glass reactor at ambient temperature. The crystalline product is recovered by filtration in greater than 90% yield.

Analysis of Reagent Purity: the purity is assessed by mp.

Handling, Storage, and Precautions: no data available.

Elimination of Hydrogen Bromide.

Steroidal a-bromo ketones undergo elimination of HBr with tetramethylammonium dimethyl phosphate (1) in acetonitrile or DMF at 110 to 150 °C (eq 1) in an autoclave to give a,b-unsaturated ketones in 45-60% yields.3 Alternative procedures, for example Lithium Chloride/Lithium Carbonate in DMF, are reported to give superior yields.4

Reaction of less complex a-bromo ketones with (1) leads to variable results. 2-Bromocyclopentanone yields exclusively 2-cyclopentenone (60%), whereas 2-bromocyclohexanone and 2-bromocycloheptanone (eq 2) give mixtures of a,b-unsaturated ketones and ketophosphates.5,6 Additional a-substitution causes elimination to become the predominant pathway; thus 2-bromo-2-methylcyclohexanone gives 2-methyl-2-cyclohexenone in 62% yield.

Reaction of (1) with a-bromo aldehydes2 leads predominantly to a-aldophosphates, whereas a-alkyl substituted a-bromo aldehydes7 give a,b-unsaturated aldehydes by elimination (eq 3). The reaction of alkyl bromides is exemplified by 2-bromopentane in DMF, which forms (E)- and (Z)-2-pentene, 1-pentene (total alkene 42%), and 2-pentyl dimethyl phosphate (19%).8

Surprisingly, the steroidal diaxial dibromide (eq 4) gave a quantitative mixture of the D5,7-diene and the D4,6-diene (1.3:1) upon treatment with (1) in HMPA at 130 °C.9

1. Carayon-Gentil, A.; Thanh, T. N.; Gonzy, G.; Charbrier, P. BSF 1967, 1616.
2. Kraus, J. L.; Sturtz, G. BSF 1971, 4006.
3. Kraus, J. L.; Sturtz, G. BSF 1970, 2551.
4. Joly, R.; Warnant, J.; Nominé, G.; Bertin, D. BSF 1956, 366.
5. Sturtz, G.; Raphlen, A. TL 1970, 1529.
6. Sturtz, G.; Raphlen, A. BSF 1971, 2962.
7. Kraus, J. L.; Sturtz G. BSF 1971, 4012.
8. Sturtz, G.; Rio, M. BSF 1974, 2180.
9. Freeman, D.; Acher, A.; Mazur, Y. TL 1975, 261.

Christopher D. Spilling

University of Missouri-St. Louis, MO, USA

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