o-Bromo-p-methoxyphenyl ethers. Protecting/radical translocating (PRT) groups that generate radicals from C-H bonds beta to oxygen atoms

The o-bromo-p-methoxyphenyl ether group is introduced as a new protecting/radical translocating (PRT) group. This group protects an alcohol both before and after its use as a translocating group to generate a radical from alpha C-H bond beta to the protected alcohol. All prior PRT groups generate radicals a to the functional group that they protect. The group is introduced by Mitsunobu reaction or Williamson ether synthesis, and removed by oxidation with eerie ammonium nitrate. The efficiency of the radical translocation reaction has been studied by isotopic labeling experiments with tributyltin deuteride. These results were used to design and execute a series of tandem radical translocation/cyclization reactions that illustrate the potential usefulness of the PRT group in synthesis. Secondary radicals are generated with about 50% efficiency due to slow 1,5-hydrogen transfer and competing 1,6-hydrogen transfer. Tertiary radicals are generated with efficiencies of about 80%. Modified PRT groups with added ortho substituents (Br, Me) can increase the efficiency of radical generation up to about >80% and >90%, respectively. The results provide only limited support for a rate and selectivity analogy based on radical cyclizations that was used to design the groups.