Heteroatoms as Neighboring Groups

Other groups such as esters, silylethers, and imides are also successfully incorporated through ADMET depolymerization with 14 (Fig. 8.21).49 For an ester functionality, at least two methylene spacer units must be present between die olefin site and die functional group in order to achieve depolymerization. This is due to die negative neighboring group effect, a deactivation of the catalyst by coordination of the functionality heteroatoms to die catalyst.50 By physically... 

As it has been already mentioned vide Sec. 3.3), neighboring groups containing electron-rich heteroatoms play a significant role during the one-electron oxidation since they stabilize the forming sulfide radical cations. In oligopeptides, very often heteroatoms in peptide bonds are the only nucleophiles present in the vicinity of the sulfide radical cation site Met(S ). 

The influence of heteroatom substituents in directing reactions to one face or another has been noted in this as well as several preceding chapters. In Chapter 3, a neighboring group effect influenced delivery of oxygen to the double bond of an allylic alcohol, described by Henbest and by Sharpless (secs. 3.4.B, 3.4.C), which is... 

In the series of dihetero-tricyclodecanes, substitutions and rearrangements involving neighboring group participation of heteroatoms (onium ions) especially of oxygen (oxonium ions) sulfur (episulfonium ions, thiiranium ions) and nitrogen (ammonium ions, aziridinium ions) as well as in one case also of selenium (epi-selenium ions), were studied on adamantanes (G1), isotwistanes (G 2), twistanes (G 2) and homotwistbrendanes (G 4). 

Involving neighboring group participation of the heteroatom X(2) by intramolecular nucleophilic substitution at the R -substituted carbon atom, an identical onium ion G 46 is formed starting from isotwistanes G 44 as well as from twistanes G 45. 

With respect to the orientation of the leaving group no isotwistanes, twistanes and homotwistbrendanes (see 3.1.) are known so far as having both leaving groups anti to the heteroatom Y, which could exhibit neighboring group participation. 

In its simplest form, the study of neighboring group effects notes that a heteroatom with a pair of nonbonding electrons can act as a nucleophile in an intramolecular displacement reaction, just as external nucleophiles can participate in intermolecu-lar S]v2 reactions. In an example we have seen (p. 317), oxiranes can be formed from halohydrins in base (Fig. 21.2). 

Now let s look at more heteroatoms with nonbonding electrons that might participate in neighboring group reactions. For example, nitrogen bears a lone pair of electrons, and so can act as an internal nucleophile. Let s look at the reactions of the two bicyclic amines in Figure 21.14. Treatment of the endo chloride with ethyl alcohol leads to the endo ether, but the exo chloride gives neither the exo nor endo ether under the reaction conditions. Now we ll see why. 

The neighboring group effect becomes particularly apparent within the SaS series relative to the oxidation of a simple sulfide. The generally lower oxidation potentials for the selenium- and phosphorus-containing compounds not only prove participation of these heteroatoms but also reflect their lower ionization potentials compared with sulfur. As discussed before, the best orbital overlap for formation of the 2(r/l(r three-electron bond is obtained for the five-membered ring (n = 3), evidenced by the lowest oxidation potentials and the most blue-shifted absorptions. (Remember that absolute optical transition energies may... 

The density domain analysis of several molecules has indicated that the appearance of individual DD s of hydrogen nuclei and their Joining with the DD of a neighboring atom or functional group follow a trend, especially, if electronegative heteroatoms are involved. This trend, called the "Last-First Rule", can be stated as follows. 

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