1,3-Dicarbonyl anions

For instance, the (pentaphenylcyclopentadienyl) cobalt dicarbonyl anion-radical complex [(q-C5Ph5)Co(CO)2] has (n + 1) metal orbital populated with an unpaired electron, according to calculations by Connelly et al. (1986). In contrast, reduction of (bpy)Cr(CO)4 (bpy = 2,2 -bipyridyl) to its anion radical is known to occur without any major change in its structure or composition. 

Since DBP can react with a variety of p-dicarbonyl anions, a wide assortment of furans and cyclopentenones is available. In addition to its ease of removal,13 the pendant sulfone also offers a convenient and versatile site for further elaboration (via alkylation14 or Julia coupling1 ). This strategy toward furans and cyclopentenones Can clearly be applied to more complex targets. 

The cyclopentadienyliron dicarbonyl anion was reacted with an optically active chlorosilane giving rise to an optically active iron complex58 (equation 8) which was isolated in 98% optical purity. On the basis of the known stereochemistry of nucleophilic displacement of chlorine nucleofuge, the reaction was assumed to occur with inversion of configuration at silicon. 

The irreversible removal of one product is a powerful method of thermodynamic control when all products are in equilibrium. This type of control is commonly exercised through dehydration to give enones, e.g. (81), product ionisation to give stable 3-dicarbonyl anions, e.g. (82), or decarboxylation (page 108), e.g. to give (83). ... 

Phase-transfer catalysis has also been found to be superior to NaK or Na/Hg for generation of the cyclopentadienyliron dicarbonyl anion from bis(dicar-bonylcyclopentadienyliron), [C5H5Fe(CO)a]a.i Cetyltrimethylammonium bromide, Ci6H33N(CH3)a Br , and 18-crown-6 are about equally satisfactory as catalysts. The anion was used in a synthesis of fulvenes from thiobenzophenones (equation II). 

The interconversion of P-dicarbonyl anions through activated cyclopropanes has been demonstrated. Thus (213) and (214) yield the same dihydrofuran on intramolecular cyclization, and it is not surprising to find a preference of (213) over (214) (Scheme 32). 

Reactions of thioketones with nucleophilic reagents have been studied to some extent. Thiobenzophenone (20) and its 4,4 -substituted derivatives react with cyclopentadienyliron dicarbonyl anion and cyclopentadienyl-molybdenum or -tungsten tricarbonyl anion at room temperature to give fulvenes. Phase-transfer catalysis conditions improve the yields. An electron-transfer mechanism has been proposed for this desulphurization and coupling reaction. An electron-transfer mechanism has also been proposed for the reduction of thiopivalophenone (54) by 1-benzyl-1,4-dihydronicotinamide in acetonitrile. 2,3-Diphenylcyclopropene-thione (55) reacts with 7V-methylarylamines to give the bicyclic thioamides (56). The formation of a thioketen as an intermediate has been suggested. A similar reaction scheme has also been proposed for the reaction of (55) with 3,4-dihydroisoquinoline. ... 

The rules and limitations governing Sn2 reactions apply to the alkylation steps. Thus, tertiary haloalkanes exposed to j8-dicarbonyl anions give mainly elimination products. However, the anions can snccessfuUy attack acyl hahdes, a-bromoesters, a-bromoketones, and oxacyclopropanes. 

Dicarbonyl Anion Chemistry Michael Additions CHAPTER 23... 

In Summary -Dicarbonyl anions, like ordinary enolate anions, undergo Michael additions to a,/8-unsaturated carbonyl componnds. Addition of a /8-ketoester to an enone gives a diketone, which can generate six-membered rings by intramolecular aldol condensation (Robinson annulation). 

The Claisen condensation is driven by the stoichiometric generation of a stable /3-dicarbonyl anion in the presence of excess base. 

Reaction of the cyclopentadienyliron dicarbonyl anion, readily prepared from commercially available cyclopentadienyliron dicarbonyl dimer and sodium amalgam, with epoxides gives olefin complexes (XXXI) in good yields. The olefin ligand can be liberated from the complex using iodide ion (Giering et aU 1972). The reaction occurs with net retention of configuration and was proposed to occur via intermediates XXIX and XXX. 

An amusing interplay of a- and vr-bonded complexes involving similar cationic complexes has been shown to exist (Eq. 13) by Green and Nagy 97-100). A hydride ion may be abstracted from the cr-bonded n-propyl and isopropyl complexes (LXXXIa and b) produced from the dicarbonyl anion (LXXXIV) to generate the 7T-olefin complex (LXXXII). This cation reacts with NaBH4 to give only the isopropyl complex back. In addition the er-allyl complex (LXXXIII) can be protonated to form the same 7r-olefin cation (LXXXII). 

The obvious diseonnection on a 1,4-dicarbonyl compoimd gives us a logieal nucleophilie synthon (an enolate anion) A but an illogieal electrophilic synthon B ... 

The most general methods for the syntheses of 1,2-difunctional molecules are based on the oxidation of carbon-carbon multiple bonds (p. 117) and the opening of oxiranes by hetero atoms (p. 123fl.). There exist, however, also a few useful reactions in which an a - and a d -synthon or two r -synthons are combined. The classical polar reaction is the addition of cyanide anion to carbonyl groups, which leads to a-hydroxynitriles (cyanohydrins). It is used, for example, in Strecker s synthesis of amino acids and in the homologization of monosaccharides. The ff-hydroxy group of a nitrile can be easily substituted by various nucleophiles, the nitrile can be solvolyzed or reduced. Therefore a large variety of terminal difunctional molecules with one additional carbon atom can be made. Equally versatile are a-methylsulfinyl ketones (H.G. Hauthal, 1971 T. Durst, 1979 O. DeLucchi, 1991), which are available from acid chlorides or esters and the dimsyl anion. Carbanions of these compounds can also be used for the synthesis of 1,4-dicarbonyl compounds (p. 65f.). 

The use of a vinylphosphonium salt as the source of the QQ fragment instead of the more commonly employed 1,2-dicarbonyl substrate is illustrated by the pyrrole synthesis in Scheme 79b (8UOC2570). A particularly interesting feature is the intramolecular Wittig reaction with an amide carbonyl group. A very useful synthesis of pyrroles depends upon the addition of the anion of p-toluenesulfonylmethyl isocyanide (TOSMIC) to a,/3-unsatur-... 

One-electron reduction of a-dicarbonyl compounds gives radical anions known as setnidiones. Closely related are the products of one-electron reduction of aromatic quinones, the semiquinones. Both semidiones and semiquinones can be protonated to give neutral radicals which are relatively stable. 

The mechanism of the Feist-Benary reaction involves an aldol reaction followed by an intramolecular 0-alkylation and dehydration to yield the furan product. In the example below, ethyl acetoacetate (9) is deprotonated by the base (B) to yield anion 10 this carbanion reacts with chloroacetaldehyde (8) to furnish aldol adduct 11. Protonation of the alkoxide anion followed by deprotonation of the [i-dicarbonyl in 12 leads to... 

The anion of A-nitromethyIphthalimide 17 also acts as a formyl anion equivalent with Michael acceptors to afford 1,4-dicarbonyl compounds 18 in good to excellent yields, although difficulty was experienced in isolating the products under the conditions required for removal of the phthalimide group (77CJC2919). 

In general the reaction is conducted in aqueous solution under basic conditions—e.g. in the presence of KOH. The 1,3-dicarbonyl substrate is deprotonated to give the corresponding anion 5, which then couples to the arenediazonium species 1, to give the diazo compound 3 ... 

The 1,4-addition of an enolate anion 1 to an o ,/3-unsaturated carbonyl compound 2, to yield a 1,5-dicarbonyl compound 3, is a powerful method for the formation of carbon-carbon bonds, and is called the Michael reaction or Michael addition The 1,4-addition to an o ,/3-unsaturated carbonyl substrate is also called a conjugate addition. Various other 1,4-additions are known, and sometimes referred to as Michael-like additions. 

The enolate anion 1 may in principle be generated from any enolizable carbonyl compound 4 by treatment with base the reaction works especially well with /3-dicarbonyl compounds. The enolate 1 adds to the a ,/3-unsaturated compound 2 to give an intermediate new enolate 5, which yields the 1,5-dicarbonyl compound 3 upon hydrolytic workup ... 

Although this method is aot a geaeral procedure, bemg specific for ct-nitroketoues, k has several merits to avoid the use of toxic reageuts such as organodn compounds Functionalized ketones have been prepared by this denitration reaction, in which functionalized nitroalkanes are used as alkyl anion synthons For example, 3-nitropropanal ethylene acetal can be used as synthon of the 3-oxo-propyl anion and 1,4-dicarbonyl compounds are prepared, as shovm In Eq 7 88... 

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