[lp3090]

Hi?everyone?
I am looking for a way of introducing a bromine (or alternatively chlorine) at the alpha-carbon in 2,6-di-substitued-4-methyl-phenol. I've been trying with bromine?or NBS? and AIBN as initiator. It’ll introduce more than one bromine atoms on the para-methyl site.

By using the following methods,it also got bis-brominated products.

1.  2.5 gm of 2,6-di-tert-butyl-4-methyl phenol and dissolved in 25 ml of dry carbon tetrachloride was kept in oil-bath at 85.deg. 0.3 ml of bromine was dissolved  in 25 ml of dry carbon   tetrachloride was added, drop-by-drop.
Ref: US20040192969
2.  the best conditions are 1.5 eq NBS, 0.1 eq AIBN in CCl4, 3-4 hours at 70-80 deg
 
3.  use more NBS (up to 6 equivialents) in CCl4 in presence of benzoyl peroxide and then in a second step a reaction with diethylhypophsphite and ethyl-diisopropy
l-amine in THF.
Ref: Synthesis 2001, No. 14

4. another procedure: sodium bromate/sodium hydrogensulphate in water/organic solvent (exothermic rx).
   Ref: Journal of Organic Chemistry 1998, 63, page 6023.

Does anybody know How to increase the mono-brominated products?
Thanking you in advance.
ROC LIU