Radical carbonylation

Treatment of 2,2-dimethylselenazolidines with reactive aromatic aldehydes gives exchange of carbonyl radicals (Scheme 7Ti. 

The ketones are a group of compounds with the general formula R-C-R. The -C-functional group is known as the carbonyl group or carbonyl radical it appears in many different classes of hydrocarbon derivatives. There are only a few important ketones, and they are all extremely hazardous. 

Yu et al. [31] pointed out that hydroxymethene radical will react with Ce(lV) ion to form a carbonyl group very quickly, and then it can further oxidize to form carbonyl radical. Therefore, the grafting polymerization would take place on the carbonyl radical as the following ... 

Bimolecular substitution and oxidation reactions of 17-electron pentacoordinate metal carbonyl radicals. A. Poe, Transition Met. Chem. (Weinheim, Ger.), 1982,7, 65-69 (41). 

Cobalt carbonyl radicals and radical reactions of cobalt carbonyl. G. Palyi, F. Ungvary, V. Galamb andL. Marko, Coord. Chem. Rev., 1984, 53, 37-53 (90). 

Some metal carbonyl radical ions have been generated from neutral carbonyls, in situ in... 

The hypothesis that the cobalt carbonyl radicals are the carriers of catalytic activity was disproved by a high pressure photochemistry experiment /32/, in which the Co(CO), radical was prepared under hydroformylation conditions by photolysis of dicobalt octacarbonyl in hydrocarbon solvents. The catalytic reaction was not enhanced by the irradiation, as would be expected if the radicals were the active catalyst. On the contrary, the Co(C0)4 radicals were found to inhibit the hydroformylation. They initiate the decomposition of the real active catalyst, HCo(C0)4, in a radical chain process /32, 33/. 

Nickel carbonyl radicals show an even greater tendency than cobalt carbonyls to cluster in a krypton matrix. Three binuclear nickel carbonyls have been detected by EPR spectroscopy in the products of y-irradiated Ni(CO)4 in Kr, yet no mononuclear species has been positively identified (65). 13C hyperfine structure has... 

Surprisingly little preparative work has been done on the anodic oxidation of enols and enolates, although the resulting a-carbonyl radicals are important intermediates in synthetic organic chemistry and biological systems [94]. Due to... 

V(CO)6 is the only homoleptic metal carbonyl radical, comparable with Re(CO)j and Mn(CO)3L2 radicals, which however are more difficult to study. For the substitution reaction in hexane solution ... 

Two possible mechanisms are proposed. Primarily the enol radical cation is formed. It either undergoes deprotonation because of its intrinsic acidity, producing an a-carbonyl radical, which is oxidized in a further one-electron oxidation step to an a-carbonyl cation. Cyclization leads to an intermediate cyclo-hexadienyl cation. On the other hand, cyclization of the enol radical cation can be faster than deprotonation, producing a distonic radical cation, which, after proton loss and second one-electron oxidation, leads to the same cyclo-hexadienyl cation intermediate as in the first reaction pathway. After a 1,2-methyl shift and further deprotonation, the benzofuran is obtained. Since the oxidation potentials of the enols are about 0.3-0.5 V higher than those of the corresponding a-carbonyl radicals, the author prefers the first reaction pathway via a-carbonyl cations [112]. Under the same reaction conditions, the oxidation of 2-mesityl-2-phenylethenol 74 does not lead to benzofuran but to oxazole 75 in yields of up to 85 %. The oxazole 75 is generated by nucleophilic attack of acetonitrile on the a-carbonyl cation or the proceeding enol radical cation. 

Calculations indicate that the unpaired electron density in the cation-radical of tetrathiafulvalene resides principally on sulfur, hut with the internal carbon being the site of second highest density. The product of coupling of an a-carbonyl radical to sulfur, an a-carbonyl-sulfonium salt would be destabilized by the adjacent dipoles. The transition state would be expected to mirror this, thus slowing down the C-S coupling and permitting the observed coupling to the carbon of tetrathiafulvalene. 

Useful sources of these radicals were the hydrogen abstraction from alkyl formate and especially the decomposition by ferrous salts of mixtures of hydrogen peroxide and a-keto esters. The alkoxy-carbonyl radicals appear to be less nucleophilic than carbamoyl radicals. 

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... 

The cyclization process can be promoted by using a single electron transfer mediator. Electron transfer from the mediator generates the carbonyl radical-ion away from the electrode surface so that cyclization can occur before there is opportunity for a second electron transfer. Thus reduction of 16, R = Me, in dimethyl-forraaraide at mercury in the presence of tetraethylammonium fluoroborate leads only to conversion of the ketone function to the secondaiy alcohol. However addition of a low concentration of N,N-dimethyl pyrrolidinium fluoroborate alters the course of reaction and the cyclized tertiary alcohol is now formed. This pyrrolidinium salt is reduced at -2.7 V vs. see at mercuiy to yield a complex DMP(Hg5) which is thought to act as a single electron transfer mediator [94]. Cyclization can... 

Linear sweep voltammetry of ben2x yl chloride in acetonitrile containing tetra-ethylammonium fluoroborate shows Ep == -1.4 V vs. see and for heptanoyl chloride Ep = -2,2 V vs. see [169] The one-electron reduction of acid chlorides at these potentials is a source of carbonyl radicals. Reduction of benzoyl chloride at the peak... 

The carbon-nitrogen double bond in imines is reduced at less negative potentials than the corresponding carbonyl function. Also imine radical-anions are more basic than carbonyl radical-anions. Imines with at least one phenyl substituent on the carbon-nitrogen double bond are sufficiently stable for examination in aprotic solvents and reversible one-electron reduction of benzaldehyde anil [179] or benzophenone anil [ISO] can be demonstrated with rigorous exclusion of moisture. 

Although the mechanism of the photo-induced generation of mono- and bimetallic metal clusters, except for the photographic application (Section 20.6), has been studied with considerably less detail than for the radiolytic route, some stable clusters, mostly of noble metals (Ag, Au, Pt, Pd, Rh), have also been prepared by UV excitation of metal ion solutions [129-141]. Generally, halides and pseudo-halides counter anions are known to release, when excited, solvated electrons, which reduce the metal ions up to the zerovalent state. Oxalate excitation yields the strong reducing carbonyl radical COO [30]. Photosensitizers are likewise often added [142]. Metal clusters are photo-induced as well at the surface of photo-excited semiconductors in contact with metal ions [143,144]. 

Useful comparisons may be made here with the corresponding alkoxy-carbonyl radicals, C02Me and C02Et, for which values (6) for D4 of around —10 kcal. per mole have been advanced. These radicals C02R,... 

Compounds of type 43 and 44 tend to undergo radical cleavage readily. Carbonyl radicals formed are relatively stable and do not decarbonylate provided that an aromatic nucleus in the -position exerts a stabilizing effect. Minimal amount of by-products were observed when at least one of the substituents Rx or R2 was phenyl. The photochemical conversion 43 44 is... 

Although the mechanism of the reaction is unknown, Moore and Waters188 have shown that addition of a carbonyl radical to a carbonyl double bond occurs when benzaldehyde is irradiated in the presence of phenanthraquinone, yielding, ultimately, the hydroquinone monobenzoate. 

Fig. 34. Schematic presentation of the reaction scheme for heterogeneous electrocatalysis of the second kind catalytic protonation of carbonyl-radical anions by an electrosorbed proton donating surfactant (STR, strychninium).

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