Free radicals ketyl

The ketyl radicals (R1R2C OH) have a weak O—H bond and are very active reducing agents. They react as donors of hydrogen atoms with carbonyl compounds, dioxygen, and free radicals. 

Metal ketyls are ion-radicals analogous to semiquinone ion radicals and may be considered either oxygen or carbon free radicals. They are readily prepared by treating aromatic ketones with alkali metals in dry ether, benzene, or liquid ammonia under an inert atmosphere.124 125 Benzophenone potassium has been shown to be paramagnetic in the solid state.126... 

The previous chapter covered radical cation cyclization reactions that were a consequence of single-electron oxidation. In the following section, radical anion cyclization reactions arising from single-electron reduction will be discussed. In contrast to the well documented cyclization reactions via carbon-centered free radicals [3, 4], the use of radical anions has received limited attention. There are only a few examples in the literature of intramolecular reductive cyclization reactions via radical anions other than ketyl. Photochemi-cally, electrochemically or chemically generated ketyl radical anions tethered to a multiple bond at a suitable distance, have been recognized as a promising entry for the formation of carbon-carbon bonds. 

Luminescence is seldom observed from free radicals and radical ions because of the low energy of the lowest excited states of open-shell species, the benzophenone ketyl radical being however a noteworthy exception. There are few reports of actual photochemical reactions of free radicals, but the situation is different with biradicals such as carbenes. These have two unpaired electrons and can exist in singlet or triplet states and they take part in addition and insertion reactions (Figure 4.90). 

In general, N20 is rather inert towards attack by free radicals. It is, however, a good electron acceptor, and efficiently reacts with such ketyl radical ions as are depicted in reaction 33. 

In a third report Zilkha and associates (140) regarded metal ketyls as species having both a free radical and an anion in the same molecule. 

Although less common, ketyl anions can also be generated by removal of an a-hydrogen from an alkoxide (Figure 1, reaction 3). An interesting example where a ketyl anion is formed as an intermediate in this manner is provided by the electrochemically-initiated reduction of an aryl halide by an alkoxide anion via the free radical chain process illustrated in Scheme l6. 

Finally, radical anions of carbonyl-containing compounds are often produced unexpectedly in reactions involving nucleophiles or easily oxidized free radicals. In a seminal 1964 report, Russell, Janzen and Strom noted that ketyl anions could be detected by ESR... 

Another fundamental reaction of >C=0 involves its reactivity as a base. In the Brpnsted sense, >C=0 - may react with a proton donor to produce a neutral ketyl radical (>C(.)OH, Figure 2, reaction 2). This is an important process when the reduction of a carbonyl compound is carried out under acidic conditions or in a protic media (e.g. elec-trochemically, with less reactive reducing reagents such as Mg or Zn, or when >C=0"-is produced via PIET and R3N"+ has available a-protons). The follow-up chemistry of >C(.)OH is that of a neutral free radical (dimerization to form pinacols, addition to unsaturated compounds, fragmentations/ring-openings, etc.), and thus beyond the scope of this chapter. 

The nugor reaction pathway via the ketyl free-radical... 

Allylic O-stannyl ketyl, a resonance-stabilized radical species, is produced under mild free-radical conditions by the reaction of a conjugated aldehyde or ketone with a tributyltin radical. The formed radical abstracts a hydrogen from... 

Free radical rearrangements 6 Nitrogen-centered radicals 7 Nitroxyl radicals 7 Aryloxy radicals 8 Sulfur-centered radicals 8 Ketyl radicals 9 Radical cations and anions 10... 

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