A-Alkyl group

X-ray analysis of an optically active oxaziridine substituted at nitrogen with the 1-phenylethyl group of known configuration led to the absolute configuration (+)-(2R,3R)-2-(5-l-phenylethyl)-3-(p-bromophenyl)oxaziridine of the dextrorotatory compound as expected, C-aryl and A-alkyl groups were trans to each other (79MI50800). 

Although nitrosation of (l-dicarbonyl compounds becomes increasingly more facile upon successive replacement of the a alkyl groups with perfluoroalkyl groups because of the increased ionization of the perfluorinated enolate (equation 7), the stability of the nitrosodiketone tautomers decreases Thus, 1,1,1-trifluoro pentane-2,4-dione and 1,1,1,5,5,5-hexafluoropentane 2,4 dione mtrosate much faster than penta-2,4 dione but yield ketoximes, which decompose upon workup... 

Much more complicated is the course of the reaction if the oxazirane is derived instead of from benzaldehyde from an aliphatic ketone. Here the possibility of an H-transfer does not occur. Further complications are found if the A -alkyl group can be attacked by the radicals. 

The transfer of chirality from sulfur to carbon as well as the high stereoselectivity were explained by a preference of rearrangement through transition state 11a over lib due to steric interference between the p-tolyl and a-alkyl group. 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

Rearrangements of nitrones due to migration of the A-oxide oxygen can be induced both, photochemically and by various reagents, but in specific conditions it can proceed spontaneously. On one hand, such transformations are caused by the O-nucleophilic character of nitrones able to react easily with acid anhydrides, their halo anhydrides, sulfonyl chloride and other agents on the other hand, by a significant CH-acidity of a-alkyl groups. 

Reactions of Halogenation and Nitrosation Nitrones with protons in the a-alkyl group can occur in tautomeric nitrone-hydroxylamine equilibrium (Scheme 2.117) similar to keto-enol and imine-enamine tautomerisms. 

In both of these solvents, there are several examples of a trend in which the elaboration of a alkyl group on an acidic site, in the usual series of methyl, ethyl, isopropyl, and f-butyl, results in a decrease in the solution phase acidity and an increase in the gas phase acidity. First noted for the alcohol acidities, this trend has been observed for a wide variety of acids since then. ... 

These results are consistent with a syn deprotonation of the hydrazone monoanion conformer, in which the a-alkyl group R is anti to the hydrazone moiety (a lower steric interaction in 107 than in 108) during the formation of the dianion (equation 31. ... 

Cationic polymerization is the only route to the polymerization of A-alkylated lactams. Both the hydrolytic and anionic routes require that a lactam have a hydrogen on the nitrogen. However, there are no commercial applications for A-alkylated polyamides, probably because their lack of hydrogen bonding results in lower melting points than for polyamides without an A-alkyl group. 

As was already shown in Scheme 11, benzothiazine ylides have a nucleophilic nitrogen that was protonated with loss of the A-alkyl group even when heated in dimethyl sulfoxide (DMSO). Reacting the ylides 84, 183, and 184 with acid results in the same transformation (Equation 4) <1982J(P1)831>. Compound 114 and the 4-chlorophenyl derivative 185 could also be protonated with perchloric acid but in this case S-dealkylation did not occur and a salt was obtained (Equation 5) <1986LA1648>. 

The data in DMSO also allow for an evaluation of the effect of a-alkyl groups on the stability of a carbanion. Because alkyl groups are generally categorized as electron donors, one might expect that they would destabilize a carbanion, but the data indicate that the situation is not so simple. In Table 3.7, and Af/acid values are ... 

P-Diketonesfrom %/i-epoxy ketones. 2 In the presence of this Pd(0) complex a,/ -epoxy ketones isomerize to /3-dikelones. An added ligand is usually necessary to avoid precipitation of palladium. The most satisfactory adjunct is l,2-bis(diphenyl-phosphino)ethane (dpe). The reaction is conducted in toluene at 80-140° for 10 100 hours. The isomerization is facile with strained epoxides it is sluggish with epoxides bearing an a-alkyl group. 

A-Alkyl groups in neutral azoles can rearrange thermally to carbon. For example, 2-alkylimidazoles can be prepared in this way in a reaction which is irreversible, uncatalyzed, intramolecular and does not involve radicals (80AHC(27)24l). 

In this sequence a radical, possibly a btradical derived from unpairing the electrons of the oxazirane oxygen-nitrogen bond, abstracts the a-hydrogen atom of the A -alkyl group to form (XXI) which subsequently isomerizes to (XXII). Alternatively the formation of (XXII) may take place directly by a concerted, reaction. In either event the iminoaikoxy radical (XXII) carries the chain. The ammonia which is formed presumably comes from aldol-like condensations of the imine (XXIV). The fact that vapor-phase pyrolysis does not take this course simply reflects tbo low probability of a chain reaction in the vapor phase. 

When 2-benzopyrylium cations have a substituent with a fairly mobile hydrogen atom (a-alkyl group or heteroatom group in any other position of the cation), deprotonation of such a substituent occurs even under mild conditions by an acid-base interaction as the primary step (Section III,A). Although deprotonation in both cases leads to compounds whose structures can be depicted by two resonance formulas, either with charge separation (betaines) or without (anhydrobases), on discussing products of C-deprotonation, the term (and the corresponding formula ) anhydro-base is more often used, whereas products of O-deprotonation are called betaines. ... 

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