Phase Transfer Catalyst Promoted Halogenation of Alkenes with Hydrohalic and Acid Hydrogen Peroxide
Addition of molecular halogens especially that of gaseous chlorine, presents a potential environmental hazard and their quantitative utilization is often problematical. Chlorination of sulfanilamide with a mixture of aqueous hydrochloric acid and hydrogen peroxide has been reported, and recently the combination has been used for converting phenol to chloranil. These reactions might not have involved nascent molecular chlorine, but the in situ formation of a positive halogen species. In the light of this consideration, it seemed to be of interest to study the efficiency of the halogenation of olefins by this reagent system.
We wish to report the results of such an investigation using conditions similar to those of Lübbecke and Boldt. Furthermore, our investigations showed that the presence of a cosolvent (carbon tetrachloride) and catalytic quantities of phase transfer agent, such as benzyltriethylammonium chloride is beneficial, as the formation of chlorohydrins can be largely suppressed. These conditions are applicable to the preparation of dichlorides and dibromides, and give good, frequently better yields than the conventional halogen addition reactions.
It is noted that the ease of handling and regulation may render the method viable for laboratory halogenation, especially chlorination. Both gaseous chlorine and sulfuryl chloride could induce side reactions involving free radicals, which are not observed under our ionic conditions.
Halogenation of Alkenes; General Procedure:
To an ice-cooled, magnetically stirred suspension of the alkene (50 mmol), calcium halide (50 mmol), conc, hydrohalic acid (10 ml), carbon tetrachloride (10ml), and benzyltriethylammonium chloride (100 mg) is added dropwise 30% hydrogen peroxide (6 ml). The mixture is then allowed to warm up to room temperature while stirring is continued for 20 min. Dilution with petroleum ether, washing with water, drying and removal of solvent, gives the crude dichloride or dibromide which is devoid of starting alkene as shown by infrared spectral analysis. Purification of the product by vacuum distillation furnished by sample identical in physical and spectral properties to the authentic compound.
Chlorination: cyclohexene (76%), 1-octene (56%)
Bromination: cyclohexene (95%), 1-octene (92%)
Reference: Synthesis 676 (1977)
