Diketones Dimerization

It was shown (262) that whereas the tris-/3-diketonates are monomeric with hexacoordinated aluminum, the monoalkoxide bis-/3-diketonates dimerize with alkoxide bridges and can be represented by [(/3-dik)2Al(/i-OR)2Al(/3-dik)2], in which both aluminum atoms are hexacoordinated. The dialkoxide mono-/3-di-ketonate, instead of adopting a symmetric structure [(RO)(/3-dik)Al( -OR)2Al(/3-dik)(OR)] with both aluminum atoms in the five-coordinate state shows a structure that can be represented by the unsymmetric [((3-dik)2Al( -OR)2Al(OR)2], in which one of the aluminum atoms is six coordinate while the other is tetra-coordinate. Although higher (74) coordination for aluminum was reported in a number of glycol derivatives (263), the preferential formation of [ 3-dik)2Al(/i-0-i-Pr)2Al(0-/-Pr)2] may be considered to arise from the stability of Al(0-i-Pr)4 moiety, a very large number of derivatives of which were already described (4-8, 14-20, 25-30, 38, 39). 

The occurrence of two competing pathways was also observed in the reaction of some enol silyl ethers with diphenyliodonium fluoride (19), which afforded the mono- or the di-phenylated products in moderate to good yields. This C-phenylation reaction is likely to occur by a ligand coupling process. However, with some hindered silyl enol ethers, for example (20), diketone dimers (21) were also isolated, indicative of a free-radical component in the overall mechanistic picture. ... 

Trimethylsilyl enol ethers of acetophenone derivatives (8) react with lead tetraacetate in benzene at room temperature to give unstable phenacyllead triacetate intermediates (9), which decompose to the a-acetoxyacetophenones (10) in high yields (90-95%), when a 1 1 ratio of the enol ether to lead tetraacetate is used. However when a 2 1 ratio is used and the reaction performed in CH2CI2 or in THF at - 78°C, only small amounts of the a-acetoxyacetophenone derivatives (11) are formed, the main product being the 1,4-diketone dimers of acetophenone (40-60%). 

Furthermore, steric bulk increase around the metal center has been proven useful to promote ligand-centered radical reactivity. Several examples of ligand-centered reactivity in metalloporphyrin systems have been previously discussed. The (TMP)RhCO (TMP = tetramesitylporphyrin) couples through the CO ligand to form the diketone dimer.90... 

The corresponding enol ether from cyclohexane>l,2 dione is obtained in the same way from the hydrated diketone dimer as obtained from the nitric acid oxidation of cyclo-hexanol this enol ether was used in a Refoimatski reaction... 

When a bidentate phosphine is used as a ligand for the reaction of J-keto esters or /i-diketones, no dimerization takes place. Only a 2-butenyl group is introduced to give 68[49,62], Substituted dienes such as isoprene, 1,3-cyclohexa-diene, and ocimene react with carbon nucleophiles to give a mixture of possible regio- and stereoisomers of 1 1 adducts when dppp is used as a ligand[63,64]. 

Diketones are intermediates for synthesis of perfumes and natural products, and several preparative methods have been developed (327) in the simplest preparative methods, ketone enolates ate oxidatively dimerized (328) ... 

AEyl chloride reacts with sodamide in Hquid ammonia to produce benzene when sodamide is in excess, hexadiene dimer is the principal product, with some trimer and tetramer (C24, six double bonds). AEylation at carbon atoms alpha to polar groups is used in the preparation of a-aEyl-substituted ketones and nittiles. Preparation of P-diketone derivatives, methionic acid derivatives, and malonic ester, cyanoacetic ester, and P-keto-ester derivatives, etc, involving substitution on an alpha carbon between two polar carbonyl groups, is particularly facEe. 

On the other hand y-pyrones or 1,3-diketones could be obtained from the reactions of ketone derived enamines with diketene 423-426). The addition of dimethyl ketene dimer to aldehyde or ketone derived enamines produced cyclohexanediones 425,426). 

For the mechanistic course of the reaction the diketone 5 is assumed to be an intermediate, since small amounts of 5 can sometimes be isolated as a minor product. It is likely that the sodium initially reacts with the ester 1 to give the radical anion species 3, which can dimerize to the dianion 4. By release of two alkoxides R 0 the diketone 5 is formed. Further reaction with sodium leads to the dianion 6, which yields the a-hydroxy ketone 2 upon aqueous workup ... 

The condensation of dihydrazones of aryl-substituted 1,2-diketones with aryl-substituted 1,2-diketones has also been described, albeit with rather low yields (up to 15 %).20 Subsequently, it was explicitly shown that this reaction leads to the same products as the dimerization of benzil monohydrazones, i.e. the 1,2,5,6-tetrazocine system, is not produced, rather the bicyclic tetraazapentalene system is obtained,21... 

As in the [22]porphyrin(3.1.3.1) series, the cyclohexene moieties being part of the bridges originate from the preparation of the dimers from dimethylpyrrole and a hydronaphthalene diketone by acid-catalyzed condensation. The synthetic approach developed by Franck2b has the advantage over the foregoing method that more stable conventional pyrroles and dipyrryl-methanes can be used to form the macrotetracycle in a stepwise manner. 

Under different conditions (in aqueous electrolyte) the selectivity of the cleavage reaction may be perturbed by the occurrence51-53 of a dimerization process. Thus, while the major process remains the two-electron reductive pathway, 20% of a dimer (y diketone) may be isolated from the cathodic reduction of PhC0CH2S02CH3. The absence of crosscoupling products when pairs of / -ketosulphones with different reduction potentials are reduced in a mixture may indicate that the dimerization is mainly a simple radical-radical coupling53 and not a nucleophilic substitution. 

By the radical pathway l, -diesters, -diketones, -dienes or -dihalides, chiral intermediates for synthesis, pheromones and unusual hydrocarbons or fatty acids are accessible in one to few steps. The addition of the intermediate radicals to double bonds affords additive dimers, whereby four units can be coupled in one step. By way of intramolecular addition unsaturated carboxyhc acids can be converted into five raembered hetero- or carbocyclic compounds. These radical reactions are attractive for synthesis because they can tolerate polar functional groups without protection. 

Silyi enol ethers can be dimerized to symmetrical 1,4-diketones by treatment with Ag20 in DMSO or certain other polar aprotic solvents." The reaction has been performed with R , R = hydrogen or alkyl, though best yields are obtained when r = r = H. In certain cases, unsymmetrical 1,4-diketones have been prepared by using a mixture of two silyi enol ethers. Other reagents that have been used to achieve either symmetrical or cross-coupled products are iodosobenzene-Bp3-Et20," ceric ammonium nitrate," and lead tetraacetate." If R =0R (in which case the substrate is a ketene silyi acetal), dimerization with TiCU leads to a dialkyl succinate (34, r =0R)." ... 

In a similar reaction, lithium enoiates, RC(OLi)=CH2, were dimerized to 1,4-diketones, RCOCH2CH2COR, with CUCI2, FeCl3, or copper(II) triflate, in a nonprotic solvent." ... 

Another mechanism, involving addition of the ketyl to another molecule of ester (rather than a dimerization of two ketyl radicals), in which a diketone is not an intermediate, has been proposed Bloomfield, J.J. Owsley, D.C. Ainsworth, C. Robertson, R.E. J. Org. Chem., 1975, 40, 393. 

An interesting use of the diketone type of ketene dimer (34) is reduction and cleavage to B,y-unsaturated aldehyde (35),... 

Pt/Al2C>3-cinchona alkaloid catalyst system is widely used for enantioselective hydrogenation of different prochiral substrates, such as a-ketoesters [1-2], a,p-diketones, etc. [3-5], It has been shown that in the enantioselective hydrogenation of ethyl pyruvate (Etpy) under certain reaction conditions (low cinchonidine concentration, using toluene as a solvent) achiral tertiary amines (ATAs triethylamine, quinuclidine (Q) and DABCO) as additives increase not only the reaction rate, but the enantioselectivity [6], This observation has been explained by a virtual increase of chiral modifier concentration as a result of the shift in cinchonidine monomer - dimer equilibrium by ATAs [7],... 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Diketones. Potassium enolates of some ketones can dimerize with incorporation of a methylene group from DMF to give 1,5-diketones. This novel reaction requires a rert-alkyl or phenyl group attached to the carbonyl group.2... 

With the bulky metallo-organic Pd(II) catalyst 98, on the other hand, selective formation of 99 was possible here functional groups are tolerated that would react with an Ag(I) catalyst (for example, terminal alkynes, alkyl chlorides, alkyl bromides and alkyl iodides) [59]. With l,n-diallenyl diketones (100), easily accessible by a bidirectional synthesis, up to 52-membered macrocycles (101) could be prepared in an end-group differentiating intramolecular reaction (Scheme 15.26) [60], For ring sizes lager than 12 only the E-diastereomer is formed overall yields of the macrocydes varied between 17 and 38%. Only with tethers shorter than 11 carbon atoms could the Z-diastereomer of the products be observed, a stereoisomer unknown from the intermolecular dimerization reactions of 96. 

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