The reaction of azoalkanes with radical cation salts

Abstract

While azoalkanes are normally employed as convenient sources of radicals and biradicals, radical cation salts such as tris-(4-bromophenyl)aminium hexachloroantimonate and thianthrenium perchlorate oxidize azoalkanes under mild conditions to products characteristic of alkyl cations. A survey of N\sb2 yields for oxidation of twenty azoalkanes was conducted to compare their reactivity. Detailed product studies were conducted on selected azoalkanes in order to derive a general mechanism and to rationalize the effect of azoalkane structure on reactivity. A slower secondary decomposition observed for both azo-t-octane and 3,3,6,6-tetramethyl-1,2-diazacyclohexene was attributed to an oxidant containing the anion of these radical cations. Replacement of these "oxidizing" anions (SbCl\sb{6\sp-}, ClO\sb{4\sp-}) with "non-oxidizing" anions (SbF\sb{6\sp-}, BF\sb{4\sp-}) prevented the secondary decomposition of these azoalkanes. Tertiary azoalkanes that form an unstrained radical and cation reacted with two equivalents of radical cation salt affording a quantitative yield of N\sb2 and two equivalents of cation-derived products. For bridgehead azoalkanes, the N\sb2 yield decreased as the strain in the incipient radical and cation increased, an effect attributed to diversion of the more reactive radicals from oxidation to hydrogen abstraction. Cis to trans isomerization of azo-1-norbornane was observed with a catalytic amount of radical cation, but no N\sb2 was lost even when the radical cation was in two-fold excess. Replacement of an "oxidizing" anion with a "non-oxidizing" anion in the reaction of a radical cation with azo-1,1\sp\prime-adamantane warrants the reinvestigation of this reaction which involved the "oxidizing" anion. Rather than losing N\sb2, 2,3-diazabicyclo (2.2.2) oct-2-ene formed a red salt with the radical cation, but replacing both bridgehead hydrogens with methyl groups again led to N\sb2 loss. The analogous compound with only one bridgehead methyl group exhibited both N\sb2 loss and adduct formation. The reactivity of an azoalkane was shown to depend upon its structure as well as the structure of its incipient alkyl cation. An electrophilic mechanism was favored over a single electron transfer mechanism because oxidation of azoalkanes by these radical cation salts appears to be endothermic