Chemical reactivity alkylation-protonation

Compounds with a low HOMO and LUMO (Figure 5.5b) tend to be stable to selfreaction but are chemically reactive as Lewis acids and electrophiles. The lower the LUMO, the more reactive. Carbocations, with LUMO near a, are the most powerful acids and electrophiles, followed by boranes and some metal cations. Where the LUMO is the a of an H—X bond, the compound will be a Lowry-Bronsted acid (proton donor). A Lowry-Bronsted acid is a special case of a Lewis acid. Where the LUMO is the cr of a C—X bond, the compound will tend to be subject to nucleophilic substitution. Alkyl halides and other carbon compounds with good leaving groups are examples of this group. Where the LUMO is the n of a C=X bond, the compound will tend to be subject to nucleophilic addition. Carbonyls, imines, and nitriles exemplify this group. 

The gas-phase reactivity of proton-bound NHC-H-PCyj has been investigated. NHCs are shown to be more basic than PCyj, accounting for the [Ru]-NHC complexes to be more efficient than their PCy3 counterpart. Collision-induced dissociation was shown to result in phosphine alkylation, formation of methylimidazole, and elimination of cyclohexene. Activation of Hj by aminocarbenes has been studied computationally. The energy of every elementary chemical event of the process has been determined, with special attention given to substituent effects on the energy barriers. 

However, there are still important reactivity features which have so far been neglected by the reactivity functions, but yet which must be accounted for even at this stage of development if a sensible overall approach is to result. An important case concerns the special position of the hydrogen atom, and its ion, the proton. Its peculiar role in chemistry is reflected particularly in the way that even weakly basic solvents are able to interact with, and stabilize, it to a degree sufficient to render it a common and feasible independent entity in chemical reactions. This is in marked contrast to simple alkyl group ions, such as the methyl cation, whose electronic properties in many respects are very similar to those of the proton. Our current level of model development does not reflect this difference, and so specific allowance must be made artificially for the proton. 

Rajappa and coworkers20 used isothiocyanates as a probe to examine the enaminic reactivity of nitro-substituted enamines and enediamines. The results were usually consistent with predictions based on the chemical shift of the vinyl proton and on extended Huckel calculations. However, cyclic enediamine 7 was found to be unreactive toward aryl and alkyl isothiocyanates (see Section II.A). Very recently, the same reaction has been re-examined21 and it has been found that cyclic enediamine 7 indeed reacts easily with aryl isothiocyanate to give the addition products 176 in 54-65% yield (equation 68). 

As the relative reactivity, log( 1 /r ) becomes larger, the difference between the chemical shifts of the two /3-methylene protons increases linearly. The geminal coupling constant between the two /3-methylene protons, which can be used as a measure of the. v-character of the C—H bond,378 increases mostly with an increase in the reactivity. These results indicate that the contribution of the resonance form is greatest in methyl vinyl ether, which is the least reactive, and is least in /-butyl vinyl ether, which is the most reactive resonance stabilization in alkyl vinyl ether may play some important role in determining the reactivity.377... 

Thus, the differences in activities of protonic acids are due to the quality of the corresponding anion or to its tendency to form chemical bonds with the carbon cation. If the anion is unable to form such bonds without extensive regrouping or decomposition, the addition of the proton is followed by polymerization. Should the reactivity of the anion be suppressed by solvation, the tendency to polymerize is enhanced. Consequently, the efficiencies of protonic acids depend very much upon the polarities of the media and upon the reaction conditions [12, 13]. Also, the stronger the protonic acid the higher the reaction rate and the resultant degree of polymerization [14]. Generally, hydrogen halide acids do not initiate polymerizations of alkyl-substituted olefins. They may, however, initiate polymerizations of aryl-substituted olefins and vinyl ethers in polar solvents. The same is true of sulfuric acid [15]. 

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