REACTIONS OF GAMMA-ARYLALKANOLS VIA ARYL RADICAL-CATION AND ALKOXYL RADICAL INTERMEDIATES .3. REACTIONS OF 3-ARYLPROP-1-YL HYDROPEROXIDES WITH IRON(II) IN THE PRESENCE OF COPPER(II)

A strategy for comparing the 1,5- and 1,6-cyclisation reactions of 3-p henylpropan-1-oxyl radicals is described. Iron(II)-catalysed reduction of 3-(p-methylphenyl)prop-1-yl hydroperoxide and its para-chloro and par-methoxy-substituted analogues, carried out in the presence of copp er(II), has been found to give in each case the appropriate para-subst ituted 3-phenylpropan-1-ol, 3-phenylpropanal and a low yield of a mixt ure of isomeric 6- and 7-substituted chromans, The alcohols are Propos ed to form via reduction of either the hydroperoxide or the resulting alkoxyl radical or its cyclised intermediates, and the aldehydes as a result of rearrangement of the alkoxyl radical to an alpha-hydroxy alk yl radical which subsequently undergoes oxidation. The 7-substituted c hromans, which arise directly from 1,6-cyclisation of the alkoxyl radi cal, were found to dominate the 6-substituted isomers which result fro m rearrangement of 1,5-cyclised intermediates. This effect is attribut ed to inefficient interception of the 1,5-cyclised radical, intermedia te-which permits equilibration to the thermodynamically more stable 1, 6-cyclised radical isomer to occur. The effect of pH on the reactions has been investigated and although no products typical of the intermed iacy of aryl radical cations were detected (even under highly acidic c onditions), the formation of such intermediates cannot be excluded. Se miempirical MO calculations have been carried out (at the PM3 level of approximation) on a series of model compounds, yielding results which have clarified our understanding of the effect of substituents on the stabilities of the various intermediates arising from the cyclisation reactions or 3-phenylpropan-1-oxyl radicals. Furthermore, these calcu lations have supported our assumptions regarding the probability and s pecificity of rearrangements of the spirodienyl intermediates.