Cyclisation of 3-phenylpropan-1-ol via alkoxyl radical and aryl radical-cation intermediates. A product and e.s.r. study
Abstract
A comparison is made of the cyclisation reaction of the 3-phenyl-1-propoxyl radical (2) generated by metal-catalysed reduction of 3-phenylpropyl hydroperoxide, with that of the aryl radical cation [(Ph(CH2)3OH]+˙(3) obtained by oxidising 3-phenylpropan-1-ol (1) with SO4–˙[from FeII–S2O82–˙ and thermolysis of S2O82–]. Significant yields of the cyclised product 3,4-dihydro-2H-1-benzopyran (7) were obtained from the radical (2) and radical cation (3) only in the presence of CuII ions which are proposed to oxidise the intermediate cyclised radicals. Reaction of the alcohol (1) with AgI–S2O82– and HO˙[from TiIII–H2O2] was also investigated, as were e.s.r. spectra of intermediates which formed from its reaction with SO4–˙ and HO˙.
The results are interpreted in terms of the reaction of the 3-phenyl-1-propoxyl radical (2)via competing 1,2-hydrogen shift and cyclisation [to give, ultimately, the six-membered ring product (7)]; generation of the radical cation [Ph(CH2)3OH]+˙(3)(from either SO4–˙ or HO˙/H+) is followed by cyclisation via nucleophilic attack to give a cyclic radical intermediate which can undergo one-electron oxidation to give (7), ring-open to give the alkoxyl radical (2), or at low pH revert to the radical cation (3) to give the benzylic radical PhĊHCH2CH2OH (15).