[4207-56-1] · C9H14Br3N · Phenyltrimethylammonium Perbromide · (MW 375.94)
Alternate Names: phenyltrimethylammonium tribromide; PTAB; PTT.
Physical Data: mp 115.5-116.5 °C.
Solubility: very sol THF, CH2Cl2; sol acetic acid.
Form Supplied in: orange crystals; widely available.
Preparative Methods: prepared in situ by reaction of equimolar amounts of phenyltrimethylammonium bromide and Bromine in THF (eq 1).5,6,8 The stable reagent, suitable for storage, is prepared by quaternization of N,N-dimethylphenylamine with Dimethyl Sulfate in toluene, followed by bromination with bromine in aqueous hydrogen bromide (eq 2).6
Handling, Storage, and Precautions: the reagent is highly stable but should be protected from moisture as it is slightly hygroscopic. For some applications, phenyltrimethylammonium perbromide must be strictly anhydrous in order to achieve maximum selectivity.
Phenyltrimethylammonium perbromide has the same properties as Pyridinium Hydrobromide Perbromide without the disadvantages.1
This reagent is widely used for the a-bromination of ethylene acetal derivatives of ketones. Phenyltrimethylammonium perbromide in THF converts 3b-acetoxy-20-ethylenedioxypregna-5,16-diene into 3b-acetoxy-20-oxo-21-bromopregna-5,16-diene;1 various other ethylene acetals have been transformed into the corresponding brominated acetals, which are readily hydrolyzed to a-bromo ketones (eqs 3 and 4).1-3,5
This conversion has also been performed as a one-pot procedure starting with a ketone, ethylene glycol, and PTAB in THF (eq 5).4 Anhydrous THF is important to the selectivity of this process because of the stability of Br3- in this solvent. Moreover, THF acts as a buffer by reacting with liberated HBr, and is therefore very useful for reactions of substrates bearing acid-sensitive functions.
Ketones are selectively monobrominated at the a-position with PTAB in anhydrous THF. Small amounts of water can greatly retard the rate of bromination and can result in decreased selectivity. a-Monobromination of methyl aryl ketones with PTAB occurs smoothly, and a,a-dibromination is also possible (eq 6).6
a-Bromo ketones have been selectively obtained in substrates containing carbon-carbon double bonds3 and cyclopropane rings.9-11 Thus trans-dihydroaristolone can be a-brominated and dehydrobrominated to (±)-aristolone (eq 7), a sesquiterpene from Aristolochia debilis,9 while methyl cis-2-oxo-7,7-dimethylnorcarane-4-carboxylate has been a-brominated and further converted into methyl cis-chamic ester (eq 8).10 a-Bromination of cyclic ketones with PTAB has been used for the construction of anti-bishomoquinone12 and triasteranedione13 from cyclooctane-1,5-dione and bicyclo[3.3.1]nonane-3,7-dione, respectively (eqs 9 and 10).
Tosylazoalkenes are obtained from the reaction of tosylhydrazones and PTAB, followed by treatment with base (eq 11).7,14 The reaction fails with bromine and N-bromosuccinimide.14
Norbert De Kimpe
University of Gent, Belgium