Dimedone, enolization

Dimedone enol acetate, reaction with TeCl4, 58, 96... 

Normal yS-dicarbonyl compounds such as ethyl acetoacetate 723 a or acetylace-tone 723 b are converted, as the free yS-dicarbonyl compounds or as their sodium salts, by TCS 14, 14/pyridine, or HMDS 2/TCS 14 into their enol silyl ethers 724a [216, 217, 219] and 724b [218]. Yet treatment of / -triketones such as 2-acetyl-dimedone 725 with HMDS 2 results, via the corresponding 2-enol trimethylsilyl... 

Cycloaddition Reactions with Other Nucleophiles The anodic two-electron oxidation of catechol affords o-quinone that may react with the enolates of 4-hydroxycoumarine or 5,5-dimethyl-1,3-cyclohexanedione (dimedone). The resulting adducts undergo a second anodic oxidation leading to benzofuran derivatives in good yields (90-95%) (Scheme 53) [75, 76]. 

Solid enols such as dimedone (255) are able to substitute one of the hydroxyl groups of ninhydrin (254) upon milling and give a quantitative yield of the... 

The potential of carbon-13 NMR in the analysis of keto-enol tautomerism has been demonstrated for 2,4-pentanedione (acetylacetone) and dimedone [293]. Quantitative evaluation of equilibrium concentrations is possible by application of the inverse gated decoupling technique illustrated in Fig. 2.23. 

Enolates. Another class of activators for photoreducible dyes was discovered by Chaberek (24,45). Enolates of diketones such as acetylacetone or dimedone (5,5-dimethyl-l,3-cyclohex-anedione) were shown to be effective reductants for the excited states of MB and to polymerize monomers such as acrylamide. Chaberek, Shepp, and Allen discussed the mechanism of this process in a series of papers (46,47). 

These masked enolate complexes also react smoothly with carbonyl compounds, in reactions surprisingly reminiscent of the reaction of dimedone with formaldehyde (Fig. 5-13). The dinuclear products are of some interest as models for the dinuclear sites of some metalloproteins. 

The photochemical addition of cyclic 1,3-diones such as dimedone, 1,3-cylohexandione 62, or their respective silyl enol ethers leads to the formation of two fused furanylfullerenes, (1) achiral 63 and (2) chiral 64 [244], The latter having an unusual bis-[6,5] closed structure. In the initial step of this reaction, [2 + 2] photocycloaddition across a [6,6] bond to form cyclobutanols or the corresponding TMS ethers is involved (Scheme 26). Oxidation with 02 yields in the formation of the radical 65a. Cleavage to 66a followed by cyclization gives furanyl radical 67a. H abstraction by 102 or a peroxy radical finally leads to product 63. In competition, formation of fullerene triplets by absorption of a... 

An early synthetic application of such intermolecular enone + alkene photocycloadditions was presented by de Mayo in 1964. 5,5-Dimethyl-3-hydroxycyclohex-2-enone (1), i.e., the enol of dimedone, adds to e.g., cyclohexene, and the primarily formed cyclobutane 2 undergoes a... 

Some ylides from cyclic precursors are thermally labile and isomerize to iodo-enol ethers for example, the ylide of dimedone afforded 2-iodo-3-phenoxy-5,5-dimethyl-2-cyclohexanone [41], A similar reaction involving cleavage of the I-Cphenyi bond occurred with triethylphosphite 2-iodo-3-ethoxy-5,5-dimethyl-2-cyclohexen-l-one was the main product (65%) [42],... 

An enol is exactly what the name implies an ene-ol. It has a C=C double bond and an OH group joined directly to it. Simple carbonyl compounds have enols too—in the margin is the enol of cyclohexanone (just dimedone without the extras). 

In the case of dimedone, the enol must be formed by a transfer of a proton from the central CH2 group of the keto form to one of the OH groups. 

When we were looking at spectra of carbonyl compounds in Chapter 15 we saw no signs of enols in IR or NMR spectra. Dimedone is exceptional—although any carbonyl compound with protons adjacent to the carbonyl group can enolize, simpler carbonyl compounds like cyclohexanone or acetone have only a trace of enol present under ordinary conditions. The equilibrium lies well over towards the keto form (the equilibrium con-... 

Did you notice when we were looking at the NMR spectrum of dimedone (p. 522) that the two CH2 groups in the ring seemed to be the same, though they are different (a and b) and the delocalization we have just looked at does not make them the same This must mean that the enol is in rapid equilibrium with another identical enol. This is not delocalization—a proton is moving—so it is tautomerism. 

The first enol you saw at the start of Chapter 21 was the stable enol of dimedone , 5,5,-dimethyl-cydohexa-l,3-dione. This six-membered ring is made by a close analogue of the Robinson annelation. The only difference is in the cyclization step, which is a Claisen ester condensation rather than an aldol reaction. 

This intermediate will exist as a stable enolate under the reaction conditions. Now aqueous KOH is added to the reaction mixture, which is refluxed to hydrolyse the remaining ester. On acidification with HC1 decarboxylation occurs and dimedone is released. 

The whole operation is conducted in one flask, just as for the Robinson annelation, and dimedone is isolated as the crystalline enol in 84% yield. This reaction has not enjoyed such wide application as the Robinson annelation but it has been used to make an aromatic compound that is a starting material for the synthesis of maytensine, which we discussed at the end of Chapter 22. 

Beryllium chemistry includes its S-diketonate complexes formed from dimedone (9), acetylacetone and some other S-diketones such as a,a,a-trifluoroacetylacetone. However, unlike the monomeric chelate products from acetylacetone and its fluorinated derivative, the enolate species of dimedone (9) cannot form chelates and as the complex is polymeric, it cannot be distilled and is more labile to hydrolysis, as might be expected for an unstabilized alkoxide. However, dimedone has a gas phase deprotonation enthalpy of 1418 9 kJmoD while acetylacetone enol (the more stable tautomer) is somewhat less acidic with a deprotonation enthalpy of 1438 10 klmoD Accordingly, had beryllium acetylacetonate not been a chelate, this species would have been more, not less, susceptible to hydrolysis. There is a formal similarity (roughly 7r-isoelectronic structures) between cyclic S-diketonates and complexes of dimedone with benzene and poly acetylene (10). The difference between the enthalpies of formation of these hydrocarbons is ca... 

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