Fluorination enol structures

Current understanding of the reaction suggests that an unprecedented mechanism is operating. Unlike in classical Lewis acid catalysed reactions [28], the metal complex does not activate the carbonyl moiety but is understood to enhance the degree of enolisation and thus create the necessary nucleophilic enol structure for reaction with the fluorinating agent [29]. 

Metal enolates of carbonyl compounds are important nucleophiles in C—C bondforming reactions for the synthesis of nonfluorinated compounds. However, the metal enolates of fluorinated carbonyl compounds have been severely limited to a-F metal enolates, which can be stabilized by chelate structures containing the M—F moiety. In sharp... 

A -dien-3-ol ethers gives rise to 6-substituted A" -3-ketones. 6-Hydroxy-A" -3-ketones can be obtained also by autooxidation.Structural changes in the steroid molecule may strongly affect the stability of 3-alkyl-A -ethers. Thus 11 j5-hydroxyl and 9a-fluorine substituents greatly increase the lability of the enol ether/ while halogens at C-6 stabilize this system to autooxidation and acid hydrolysis. 

Preliminary fluorination experiments using optically active A -fluoro compounds ( —)-2b and ( + )-2c show that there is reaction with various metal enolates (Table 16) generated under standard reaction conditions to give the anticipated a-fluoro carbonyl compounds with enantiomeric excesses depending strongly on the structure of the metal enolate.119... 

Fluorination and methylation have been used to synthesize975 related fluorinated nitronium ions [Eq. (4.231)]. The trifluoromethyl(methyl)nitronium ion ON(Me) CF3 + exits in the keto form in solution, but X-ray crystal structure data indicate that the enol form HON(CH2)CF3+ exists in the solid state stabilized by a hydrogen bond between the enolic OH group and one of the fluorines of the counterion. 

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... 

The oxophilicity and coordination ability of the lanthanide elements turned out to be crucial for the attraction and activation of oxygenated functions which display pivotal components in important condensation and addition reactions [101]. Orga-nometallic systems such as fluorinated )5-diketonate and alkoxide complexes which contain highly polarized Ln-O-C linkages and are soluble in non-oxy-gen-containing solvents seem to be predestined for this type of homogeneous transformation (Structures 20-24). It must be assumed that other precatalysts (Ln"-derivatives or Ln -alkyls) underlie in situ formation of catalytically active Ln-O(alkoxide) moieties such as enolates when substrates such as ketones or aldehydes are involved. 

Molecular orbital calculations on fluorinated butadienes and hexatrienes were used to model the effects of fluorination on the properties of poly(acetylene). Like poly(acetylene), "head-to-head" poly(fluoro- acetylene), (-CH=CF-CF=CH-), is predicted to adopt a planar, all trans structure, but poly(difluoro-acetylene) favors a non-planar skewed chain conformation. "Head-to-tail" poly(fluoroacetylene), (-CH=CF-CH=CF-) is predicted to favor a nearly planar cis structure stabilized by intramolecular CF-HC hydrogen binding. Calculations on 2-fluoroethanol and on both 2-fluoroacetaldehyde enol and its alkali metal (Li, Na, K) enolates reveal moderately strong intramolecular CF—HO hydrogen bonds(1.9 and 3.2 kcal/mol, respectively) and even stronger intramolecular coordination of CF to alkali metal cations (9-12 kcal/mol). 

Huorinated etiolates are generally difficult to form. Ishihara and coworkers used fluorovinyl phosphates, which can be prepared from a-fluoro ketones and sodium diethyl phosphite. Reaction of these fluorinated enol phosphates with a reagent prepared from lithium aluminum hydride (LiAIH4> and cop-per(II) bromide, zinc(II) chloride, tin(II) chloride or bromine afforded the enolate (Scheme 34).The reaction of the enol phosphate with the reagents mentioned above suggests that the metal cation of the enolate is an aluminum species, though its actual structure is not known at present. 

Dioxocane (106), the only nonbenzannelated dioxocin derivative for which H NMR spectral data was reported, displays no unusual chemical shifts <88JOM(349)43>. 1,5-Benzodioxocin-2,4-dione (107) displays resonances for the methylene groups at 3.30 and 4.43 ppm, quite similar to those seen for the oxazocine analogues and consistent with the proposed structure apparently no enolization is seen (in DMSO) <92G485>. Fluorinated benzodioxocin (108 X = H) displays, in addition to aromatic resonances, a fluorine-coupled doublet J = 56 Hz) for the unique benzylic hydrogen F data was also reported for (108) <8UFC(18)447>. 

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