Hexatrienes, fluorinated

The acyclic fluorinated 1,3,5-hexatriene system 454 is also resistant to 6 r-electron electrocyclic ring closure at temperatures up to 200 °C while the analogous hydrocarbons cyclize easily at 160 °C (equation 171)232. 

Theoretical studies have indicated that m-bcnzync is monocyclic with a C(l)-C(3) distance of 2.0 A whereas in tetrafluoro-w-benzyne the increased eclipsing strain between fluorine atoms stabilizes the bicyclo[3.1.0]hexatriene form with a C(l)-C(3) distance of 1.75 A.56 Computational studies coupled with gas-phase experimental studies show that appropriate substituents can be used to tune the reactivity of 1,3-arynes. Thus the presence of NH+ at C(5) makes (13) mildly carbocationic whereas the addition of OH at C(4) in (14) gives a highly reactive (bi)radical.57... 

E.Z -photoisomerization of the fluorinated l-phenyl-4-carboalkoxybutadienes 21 proceeds regioselectively about the phenyl-substituted double bond, consistent with the Dauben intermediate . Saltiel and coworkers have reported a particularly interesting example of solvent- and substituent-dependent regioselectivity in the E.Z-photoisomerization of , , -l,6-diaryl-l,3,5-hexatrienes (22) the quantum yields for isomerization about the central double bond in the parent molecule (22 X = H) are insensitive to solvent polarity, while those for isomerization about a terminal bond are enhanced seven-fold in acetonitrile compared with hydrocarbon solvents". Isomerization of the substituted derivatives proceeds regioselectively to yield the 1-Z and 3-Z isomers, and in both cases the quantum yield for terminal bond isomerization is enhanced dramatically in polar solvents. The results were explained in terms of competing torsional relaxation about the central and terminal bonds via biradicaloid and zwitterionic twisted intermediates, respectively. For the parent and cyano derivatives, the latter is polarized in the same sense proposed by Dauben and Ritscher. Interestingly, the polarization appears to be reversed in the case of the methoxy derivative. 

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). 

In this chapter, we focus on the effect of fluorine as a substituent in a simple polymeric system, polyacetylene. Polyacetylene, of course, has several potentially practical uses because of its conducting and optoelectronic properties (15) and we are interested in studying how F substitution might influence these properties. Our model systems are butadiene and hexatriene, and we discuss both partially fluorinated and perfluorinated materials. Because we discovered that CF - HC hydrogen bonding is important in these systems, we also present results on the nature of the intramolecular hydrogen bond between the CF and OH groups in alcohols and enols. Related results on intramolecular coordination of alkali metals to C-F bonds in fluoroenolates are briefly described. 

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