2-Fluoroacetaldehyde enol

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). 

The nature of the hydrogen bond in 2-fluoroacetaldehyde enol is quite different from that in the alcohol. Among the four structures shown below, the cis-syn structure is the most stable (Table V). The difference in energies of the cis-syn and cis-anti structures gives an estimate of the hydrogen bond energy. At the MP-2 level, this is 3.53 kcal/mol and corrections for zero-point effects yield a value of 3.21 kcal/mol. (The... 

Table V. Relative Energies (kcal/mol) of 2-Fluoroacetaldehyde Enol Structures...

Table VI. Relative Energies and Bond Lengths of 2-Fluoroacetaldehyde Enolate Isomers...

Stabilization of the lithium enolate by intramolecular chelation with fluorine of the C—F bond has been demonstrated by calculation. The (Z)-enolate (49) of fluoroacetaldehyde is more stable than the (E)-enolate (50) [24]. Likewise, the double fluorine-chelated (Z)-enolate (51) of 4,4,4-trifluorobutanal is the most stable among the lithium enolates (51-54) [25] (see Table 3.4). 

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