[7726-95-6] · C26H24Br4P2 · 1,2-Bis(diphenylphosphino)ethane Tetrabromide · (MW 718.04)
(reagent capable of effecting the conversion of alcohols and tetrahydropyranyl ethers into bromides rapidly and efficiently in the presence of other sensitive functional groups1)
Solubility: sol CH2Cl2.
Preparative Method: generated in situ by treatment of 1,2-bis(triphenylphosphino)ethane with 2 equiv of bromine in dry dichloromethane at 0 °C under an inert atmosphere.1
Handling, Storage, and Precautions: the usual care should be taken to avoid contact of the skin with liquid bromine.
A frequently required synthetic transformation is the conversion of alcohols or protected forms thereof into the corresponding bromides or iodides. Although a myriad of reagents are available for performing this interconversion,2 many are too harsh to operate successfully on highly sensitive molecules. Some time ago, Triphenylphosphine Dibromide was recognized to be capable of effecting the formation of bromides directly from tetrahydropyranyl ethers.3,4 More spectacular still is the ability of 1,2-bis(diphenylphosphino)ethane tetrabromide to effect either of the preceding chemical changes in rapid fashion under extraordinarily mild conditions (eqs 1-3).1 As can be seen in these examples, double and triple bonds, t-butyldimethylsilyl ethers, and carboxylate esters are stable in the presence of this reagent.
Usefully, the phosphorus-containing byproducts of this halogenation are not soluble in a solvent system composed of pentane/ether/dichloromethane (2:1:0.5). In the instances where the bromide product dissolves in this mixture, simple dilution and filtration suffice for isolation purposes.1
Although the inertness of the allylic silyl ether moiety demonstrated in eq 3 is noteworthy, the extraordinary mildness of this brominating agent is best reflected in its capability to function as advertized on the sensitive furanocembranoid precursors exemplified in eqs 4 and 5.5,6 Comparable efficiency was not at all closely approximated by other means.
If bromine is replaced by iodine, 1,2-bis(diphenylphosphino)ethane tetraiodide is generated in situ. This reagent has given indication of being equally effective in delivering iodides (eqs 6 and 7).1
Inversion of configuration has been shown to operate in these interconversions, (R)-(-)-2-octanol giving rise to (S)-(-)-2-bromooctane of high optical purity.1
Leo A. Paquette
The Ohio State University, Columbus, OH, USA