[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.
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
Christopher D. Spilling
University of Missouri-St. Louis, MO, USA