Carbonyl radical anions, formation

Radical additions to alkenes and aromatic systems are well known reactions. The trapping in this manner of radicals obtained by reduction of the aliphatic carbonyl function has proved to be a versatile electrochemical route for the formation of carbon-carbon bonds. Such reactions are most frequently carried out in protic solvents so that the reactive species is a o-radical formed by protonation of the carbonyl radical-anion. Tlie cyclization step must be fast in order to compete with further reduction of the radical to a carbanion at the electrode surface followed by protonation. Cyclization can be favoured and further reduction disfavoured by a... 

Both oxetanes were formed with exclusively the exo-phenyl configuration. The regio- and diastereoselectivity observed are in accord with the assumption of a PET process involving the oxidation of the ascorbic acid derivatives and the formation of the carbonyl radical anions. In these special instances 1,4-biradical and 1,4-zwitterion stabilization result in similar product regiochemistry. The relative configuration of the products favors the assumption of a PET-process. 

Quantitative determination of proton-transfer rates to radical anion EGBs derived from carbonyl compounds (Sec. III.B.2) is problematic using voltammetric methods, since the protonated EGB is not undergoing further reduction at the potential of radical anion formation (cf. Sec. III.B.2). this may lead to reversibility of the proton transfer. 

The first step (1) involves electron abstraction from the alkylamino radical by the ground state ketone to give a carbonyl radical anion and an immonium ion while the second step (2) involves the formation of an addition product. 

Electron-transfer-catalyzed (ETC) nucleophilic substitution in polynuclear metal carbonyls promises to become a useful synthetic procedure for a wide variety of compounds. Scheme 6 shows the general reaction with a nucleophile L and Scheme 7 gives some mechanistic details (nonessential ligands omitted). For the ETC process to be efficient the radical-anion formation must be reversible, and the rate of electron transfer must be fast... 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

Spectroscopic evidence (44,45) has been adduced for the formation of electron-gain centres upon y-irradiation of the binuclear carbonyls Mn2(C0)lo and Re2(C0)lo. A study (45) of a single crystal of irradiated Mn2(CO)10 has shown that the radical anion contains two equivalent 55Mn nuclei whose hyperfine tensors lie 118 apart. This has led to the suggestion that the anion radical contains a bridging CO and that its correct formulation is Mn2(C0)9 . The observation of a bridged Mn2(C0)9 species in u.v.-photolyzed material lends some support to this hypothesis (46). 

The radical-anions from enecarboxylates show nucleophilic reactivity on the p-carbon atom. Intramolecular carbon-carbon bond formation occurs when a suitably placed alkyl bromide, see p. 58 [26], or carbonyl [128] function is available. Related reactions are shown by the radical-anions from enenitriles [128]. The... 

The catalytic substitution reactions of metal carbonyl clusters, including [M3(CO)i2] (M = Fe, Ru, or Os), [Ru4H4(CO)i2], [Rh6(CO)i6], and [Co3(CO)9(/it-CCl)], with isocyanides or Group V-donor ligands may be induced by either electrochemical or chemical (benzophenone ketyl) reduction. The most favorable conditions for efficient substitution include (1) the formation of a radical anion with a significant lifetime and (2) the use of a ligand which is not reduced by [Ph2CO], and which is less of a tt acid than CO (166). 

Cyclopropyl ketone 222 (Scheme 55) was prepared to probe the mechanism of the cleavage reaction [39,40]. Isolation of 224 where the cyclopropyl ring is intact (Scheme 56) suggests cleavage proceeds via formation of radical 227 rather than ketyl radical anion 226, formed by single-electron transfer to the ketone carbonyl, as cyclopropylmethyl radical anions are known to undergo facile fragmentation. 

Attempts to extend this work to the keto-oxime substrate 54 derived from D-glucosamine with an JV-phthalimido group resulted in the formation of a completely different product (Scheme 40). In this case, cyclization was initiated by reduction of the phthalimido carbonyl group to its corresponding ketyl radical anion followed by cyclization onto the ketone, providing an a-hydroxylactam 55 which was proposed to be a potentially useful scaffold for diversity-oriented synthesis. 

---