2-Carboxyethyltriphenylphosphonium Perbromide

[55985-85-8]  · C21H20Br3O2P  · 2-Carboxyethyltriphenylphosphonium Perbromide  · (MW 574.93)

(bromine transfer agent for selective a-bromination of ketones1 and ketone acetals2 in the presence of double bonds,1-3 activated aromatic rings,2 esters1 or another free enolic position1,4)

Alternate Name: 2-carboxyethyltriphenylphosphonium tribromide.

Physical Data: mp 139-41 °C.

Solubility: sol THF (25 g/100 mL), dichloromethane; insol alcohol.

Form Supplied in: orange solid; not widely available; purity &egt;98%.

Analysis of Reagent Purity: identification in phosphonium salt mixtures by TLC/SIMS.5

Preparative Methods: Ph3P (26.2 g, 0.1 mol), acrylic acid (7.2 g, 0.1 mol) and 49% HBr (50 mL) were stirred at rt until a homogeneous solution was obtained, then heated at 95-100 °C for 10 min, whereupon the mixture was cooled and glacial HOAc (240 mL) was added; bromine (9.8 mL) was added slowly with stirring to this mixture at 0 °C; after 0.5 h, the product was filtered and recrystallized from MeCN to give the perbromide (52.5 g, 91%);1 a two step procedure involving bromination of the isolated intermediate monobromide can also be used.

Handling, Storage, and Precautions: irritant and moisture sensitive solid; store in a dry place. This reagent should be handled in a fume hood.

Saturated Ketones.

With this reagent, saturated ketones undergo a-bromination to give mainly the thermodynamically more stable bromo ketone (eq 1).1,4 No reaction was observed in the case of bicyclic ketones or camphor, or with esters under the above conditions.1

Unsaturated Ketones.

These are brominated without affecting the double bond to afford the desired a-bromo ketone (eq 2).1,3

Aryl Alkyl Ketones.

Variable results are obtained with these compounds, depending on the activation level of the aromatic ring.1,2 Either the nature of the solvent (dichloromethane, THF with or without H2O) (eq 3) and/or the use of the one-pot acetalization-bromination procedure2 also play a critical role in determining the distribution of the brominated products; these effects have not yet been optimized.

Note that recovery and reuse of the monobromide byproduct (after treatment with bromine) is possible.

For a-bromination of ketones this reagent often compares favorably with other ammonium or phosphonium salts.2 Reagents for similar transformations include Pyridinium Hydrobromide Perbromide,6 Phenyltrimethylammonium Perbromide,7,8 2-Pyrrolidone Hydrotribromide9 and the phosphonium reagents described by Cristau et al.2

1. Armstrong, V. W.; Chishti, N. H.; Ramage, R. TL 1975, 373.
2. Cristau, H.-J.; Torreilles, E.; Morand, P.; Christol, H. PS 1985, 25, 357.
3. Jeganathan, A.; Richardson, S. K.; Watt, D. S. SC 1989, 19, 1091.
4. Ohuchida, S.; Hamanaka, N.; Hayashi, M. T 1983, 39, 4257.
5. Duffin, K. L.; Busch, K. L. J. Planar Chromatogr. 1988, 1, 249 (CA 1989, 111, 89 533g).
6. Djerassi, C.; Scholz, C. R. JACS 1948, 70, 417.
7. Marquet, A.; Jacques, J. BSF(2) 1962, 90.
8. Johnson, W. S.; Bass, J. D.; Williamson, K. L. T 1963, 19, 861.
9. Awang, D. V. C.; Wolfe, S. CJC 1969, 47, 706.

Jean-Robert Dormoy & Bertrand Castro

SANOFI Chimie, Gentilly, France

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