[7789-60-8] · Br3P · Phosphorus(III) Bromide · (MW 270.67)
(brominating agent for conversion of alcohols to bromides)
Alternate Name: phosphorus tribromide.
Physical Data: mp 41.5 °C; bp 168-170 °C/725 mmHg; d 2.85 g cm-3.
Solubility: sol acetone, CH2Cl2, CS2.
Form Supplied in: widely available as liquid and 1.0 M solution in CH2Cl2.
Preparative Method: from Bromine and red phosphorus.1
Purification: generally used without purification; can be distilled under N2 at atmospheric pressure.
Handling, Storage, and Precautions: corrosive. The colorless, fuming liquid has a very penetrating odor. It has a vapor pressure of 10 mmHg at 48 °C. The reagent is stable if kept dry, but reacts violently with water. It is extremely destructive to tissue of mucous membranes, upper respiratory tract, eyes, and skin. This reagent should only be used in a fume hood.
The conversion of alcohols (ROH) into bromides (RBr) using PBr3 is very general. Reaction conditions for this transformation are quite varied. Each of the bromine atoms in the reagent is available for reaction with an alcohol. The reagent can be used to prepare chiral bromides from chiral alcohols (eq 1).2 It is generally important to carry out the conversion under
relatively mild conditions (between 4 °C and rt). Addition of HBr at the end of workup increases both the optical purity and the isolated yield. An example is provided by eq 2. A polyhydroxylic compound has been converted to a polybrominated product (eq 3).3
The reaction of an allylic alcohol with PBr3 in ether at 0 °C leads to both stereoselective and regioselective replacement of the hydroxy group by bromine.4 Examples of this transformation are given in eqs 4 and 5.5,6 The latter example shows that the reaction can be run in the presence of significant unsaturation. There are many examples of similar reactions.7
Silverstein8 followed an older procedure9 that results in rearrangement of a secondary allylic alcohol to a terminal bromide (eq 6). In a similar fashion, the conversion of propargylic alcohols to allenyl bromides has been noted (eq 7).10
Remote p-bond participation provides important stereochemical control in the reaction. Heathcock (eq 8)11 has reported retention of stereochemistry due to p-bond participation, in contrast to a similar system without the p-system (eq 9).12
The reagent will convert an alcohol to a bromide in the presence of an ether (eq 10).13 Acetals are also stable to the bromination procedure,14 although an interesting reaction of a cyclopropanone acetal has been reported (eq 11).15
Replacement of two alcohol groups by bromine, followed by zinc dehalogenation, provides an interesting alkene synthesis methodology (eq 12).16 The preparation of 1,3,5-hexatriene is readily accomplished via a PBr3 step (eq 13).17 In the presence of DMF, PBr3 has been used to eliminate a hindered diallylic tertiary alcohol (eq 14).18 See also Phosphorus(III) Bromide-Copper(I) Bromide-Zinc.
A simple one-step preparation of 2-bromo-2,3-dihydro-1,3,4,2-oxadiazaphospholes has been reported (eq 15).19 An interesting preparation of 1,5-dibromopentane from piperidine is available (eq 16).20
A number of related reagents have been used for the conversion of alcohols to bromides. Other reagents for this purpose covered in this encyclopedia include Hydrogen Bromide, Triphenylphosphine Dibromide, Triphenylphosphine-Carbon Tetrabromide, Bromine-Triphenyl Phosphite, and 1,2-Bis(diphenylphosphino)ethane Tetrabromide.
Bradford P. Mundy
Colby College, Waterville, ME, USA