Alkylation proton

Determining the degree of substitution using standard proton nmr refles on the integral ratio between the ceUulosic ring protons ( i 5.0-2.96) and the ester alkyl protons ( i 1.26 for butyryl and propionyl and i 2.06 for acetyl methyl groups). This simple procedure is used extensively to determine the extent of esterification and is currently the fastest, easiest way for determining the DS of mixed cellulose esters. 

Alkyl groups attached to pyridopyrimidines adjacent to a nitrogen are activated , i.e. they are readily deprotonated and react with electrophilic reagents as their anions, or resonance stabilized equivalents, e.g. (64). This ready deprotonation, of course, leads to facile exchange of the alkyl protons for deuterium (Sections 2.15.2.2.1, 2.15.4.2), but, in... 

Finally, a brief word about aldehydes. They are included at the end of this group for convenience only and should be spotted easily. Aldehydes bound to aromatic rings give sharp singlets at 10.2-9.9 ppm, whilst in alkyl systems, they give sharp signals at 10.0-9.7 ppm, which couple to adjacent alkyl protons with a relatively small coupling constants (2-4 Hz). 

Table 5.8 Estimation of chemical shifts for alkyl protons.

Note both show coupling to neighbouring alkyl protons. 

The authors point out that the dependence of the site of electrophilic attack on the ligand trans to the hydride in the model systems may be important with respect to alkane activation. If the information is transferable to Pt-alkyls, protonation at the metal rather than the alkyl should be favored with weak (and hard ) a-donor ligands like Cl- and H20. These are the ligands involved in Shilov chemistry and so by the principle of microscopic reversibility, C-H oxidative addition may be favored over electrophilic activation for these related complexes. 

O3yhydroperoxides. Peroxides of the oxyhydro type are obtained by the addition of hydrogen peroxide to ketones. High yields of alkyl radicals are then often obtained by reaction with ferrous salts. 1-Meth-oxycyclohexyl hydroperoxide is easily obtained from cyclohexanone and hydrogen peroxide in methanol. It gives rise to the 5-(methoxy-carbonyl)-pentyl radical, which has been used to alkylate protonated heteroaromatic bases in high yield [Eq. (6)]. 

Oxidation of tertiary alcohols by lead tetraacetate gives alkyl radicals by /3-scission of the initially formed alkoxy radicals. The reaction has been used to alkylate protonated heteroaromatic bases using 1-methyl-cyclohexanol. (Scheme 4). 

One of the interesting features of these reactions is the noten-tial for comparing the stereochemistry of oligomerization with that of alkylation protonation or other organic reactions of general interest. For instance, Table 1 shows that the methyla-tion of the Li and Na salts of anion [2a] is highly stereoselective. 

In another related study (using hexadecyltrimethyl ammonium bromide micelles), isopropyl benzene was solubilized, and the chemical shifts of aromatic and alkyl protons were observed. The results suggest that the isopropyl benzene molecules are oriented such that the isopropyl groups are buried more deeply in the core of the micelle, while the benzene ring is in the more hydrated palisade layer. This plus the conclusion of Item 3 is consistent with the description presented in Section 8.3, which located the benzene in a relatively polar portion of the micelle. 

Because of the greater acidity of a vinylic than an alkyl proton, vinyl halides, RHC=CRX, are more likely than alkyl halides to undergo EjcB elimination. However, when the proton is not rendered even more acidic by a vicinal electron-withdrawing group, and when the basic catalyst is not too strong, E2 reaction obtains. Then anti elimination is much the preferred pathway. 

It is likely that many of the reactive intermediates discussed in this chapter are nearing the tight ion pair —> covalent adduct end of this continuum (e.g. reactions of >C=0 with Sm+2, R3Sn., etc.). Our operational criterion for inclusion of a reaction as involving a radical anion centers on whether the paramagnetic intermediate involved in the transformation is sufficiently anionic so as to have nucleophilic properties (i.e. can it be alkylated protonated ). 

Dimethylbenzene has two types of protons those attached directly to the benzene ring and those of the methyl groups. Aryl protons are significandy less shielded than alkyl protons. As shown in text Table 13.1 they are expected to give signals in the chemical shift range 8 6.5-8.5 ppm. Thus, the... 

Other methods for obtaining complexes of ethylene and other alkenes include ligand substitution reactions, reduction of a higher valent metal in the presence of an alkene, and synthesis from alkyl and related species [reductive elimination, of an allyl or hydride, for example hydride abstraction from alkyls protonation of sigma-allyls from epoxides (indirectly)] [74a],... 

As shown in Table 4 the 3-alkyl protons in the NMR spectra of sydnones are considerably further downfield than those of the 4-alkyl protons. The strong deshielding effect of the positively charged N-3 atom accounts for this shift. Protons in other five-membered rings, e.g. thiophene or 1,2,4-oxadiazole, have chemical shifts near those of six-membered aromatic rings (57-8 p.p.m.) but the sydnone protons are upheld (5 6.2-6.8 p.p.m.) (63CI(L)1926) (see also Chapter 4.01, Tables 9 and 17). 

Substituent Sydnone proton Chemical shifts (5, p.p.m.) 3-Alkyl 4-Alkyl protons protons Aryl protons... 

---