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 Fe^II–S₂O₈^2–˙ and thermolysis of S₂O₈^2–]. 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 Cu^II ions which are proposed to oxidise the intermediate cyclised radicals. Reaction of the alcohol (1) with Ag^I–S₂O₈^2– and HO˙[from Ti^III–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).