Hydrogen bonding, conjugate addition determination

The first examples of nitrofluorination of alkenes were demonstrated by the reaction in a solution of 10% nitric acid in hydrogen fluoride at — 10 C.204 Further investigation has shown that the rate of this conjugate addition is determined predominantly by the nature of the tr-bond (electron density, polarity).205 As a rule, the reaction is carried out at temperatures between - 60 C (i.e., 1,1-dichloro-or 1,1-difluoroethene) and + 20 C. However, nitrofluorination of hexafluoropropene is only successful at higher pressure and temperature, i.e. if the alkene rc-bond is less polar the regioselectivity of the addition is also unsatisfactory,205 e.g. reaction of 1 and 2.206... 

The rate-determining step in the ionic hydrogenation reaction of carbon-carbon double bonds involves protonation of the C==C to form a carbocation intermediate, followed by the rapid abstraction of hydride from the hydride source (equation 45). ° There is a very sensitive balance between several factors in order for this reaction to be successful. The proton source must be sufficiently acidic to protonate the C—C to form the intermediate carbocation, yet not so acidic or electrophilic as to react with the hydride source to produce hydrogen. In addition, the carbocation must be sufficiently electrophilic to abstract the hydride from the hydride source, yet not react with any other nucleophile source present, i.e. the conjugate anion of the proton source. This balance is accomplished by the use of trifluoroacetic acid as the proton source, and an alkylsilane as the hydride source. The alkene must be capable of undergoing protonation by trifluoroacetic acid, which effectively limits the reaction to those alkenes capable of forming a tertiary or aryl-substituted carbocation. This essentially limits the application of this reaction to the reduction of tri- and tetra-substituted alkenes, and aryl-substituted alkenes. 

Woodall et al. suggested that the different reactivities of the carotenoids against free radicals can be partly attributed to the differences in electron distribution along the polyene chain of different chromophores that would alter the susceptibility of free radical addition to the conjugated double bond system. However, other factors must be also considered. Stearic hindrance, hydrogen abstraction from the allylic position to the polyene chain (C-4 of P-carotene and its derivatives, end of lycopene), would reduce radical scavenging activity. In addition, the stability of the polyene radical is important in determining the rate of the loss of carotenoids and hence it affects their antioxidant activity. 

The chemical reactivity of resin acids is determined hy the presence of hoth the double- bond system and the COOH group [5], The carboxylic group is mainly involved in esterification, salt formation, decarboxylation, nitrile and anhydrides formation, etc. These reactions are obviously relevant to both abietic- and pimaric-type acids (Rgs 4.1 and 4.3, respectively). The olefinic system can be involved in oxidation, reduction, hydrogenation and dehydrogenation reactions. Given the conjugated character of this system in the abietic-type acids, and the enhanced reactivity associated with it, much more attention has been devoted to these stractures. In terms of industrial applications, salt formation, esterification, and Diels-Alder additions are the most relevant reactions of resin acids. 

There have been two general approaches to determining the amount of stabilization that results from aromatic delocalization. One is to use experimental thermodynamic measurements. Bond energies, as was mentioned in Chapter 1, are nearly additive when there are no special interactions between the various bond types. Thus, it is possible to calculate such quantities as the heat of combustion or heat of hydrogenation of cyclohexatriene by assuming that it is a compound with no interaction between the conjugated double bonds. For example, a very simple... 

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