The effect of α-(N,N-dimethylcarbamoyl) and α-(N,N-dimethylthiocarbamoyl) substituents on the rate constants for partitioning of α-substituted 1-(4-methoxyphenyl)ethyl carbocations between nucleophilic addition of 50:50 (v:v) MeOH/H2O (k s, s-1) and deprotonation by this solvent (k e, s-1) have been investigated. The data show that these α-amide and α-thioamide substituents result in 80-fold and ≥30 000-fold decreases, respectively, in k s for capture of the 4-methoxybenzyl carbocation by solvent, but that they lead to much smaller changes in k e for deprotonation of the corresponding α-substituted 1-(4-methoxyphenyl)ethyl carbocations by solvent. The large effect of the α-thioamide substituent on the partitioning of α-substituted 1-phenylethyl carbocations between formation of the products of solvolysis and elimination is therefore due primarily to the effect of this α-substituent on k s for capture of the carbocation by solvent. The results of experimental and computational studies are consistent with the conclusion that the relative magnitude of the rate constants k s and k e for partitioning of α-substituted 1-phenylethyl carbocations is strongly controlled by the relative thermodynamic stabilities of the neutral products of these reactions.

Bisheteroatom-substituted amides (R-CO-NXY) can undergo a novel rearrangement of the more electronegative atom from nitrogen t o the carbonyl carbon producing acyl derivatives (R-CO-X) and substituted nitrenes (N-Y). Such reactions have been observed chemically. A M 1 molecular orbital calculations o n N-substituted acetamides support the concerted nature of this process and predict that amino substituents (Y = NR,)promote the rearrangement of similarly or more electronegative nitrogen substituents (X = NR, , CI, OR). Migration appears to be driven b y an anomeric effect involving interaction between the lone pair on Y and the X-N G* orbital. Favourable transiton states display a significant increase in N-Y double bond character, negative charge on the migrating substituent and little RCO-N heterolysis. G* ab initio calculations on migration of the hydroxy group in N-amino-N-hydroxyformamide largely accord with the A M 1 findings for this model compound.N-Alkoxy-N-aminobenzamides are intermediates in the bimolecular reaction of mutagenic N-acetoxy-N-alkoxybenzamides and arylamines. S,2 reaction at the amide nitrogen and displacement of acetate results in the formation of N-anilino-Nalkoxybenzamides (1) which are unstable and undergo concerted migration of the alkoxy group to form a benzoate ester and a 1,l-diazene (Scheme l).' Ester formation is an / ' Ph< + k h l e A r -ArMeN-N=N-NMeAr OMe Scheme 1intramolecular process as shown by a crossover experiment and the process is a rapid one since the intermediates 1 cannot be observed by 'H NMR spectroscopy. Only starting material, ester and the products from dimerisation of 1 1 -diazenes, tetrazenes, can be detected during the course of the reaction. The rearrangement may be typical of N-aza-N-oxo substituted amides. We have recently shown that the thermal decomposition of N,N'-diacyl-NJV'-dialkoxyhydrazines (2) to 2 Scheme 2 esters and nitrogen occurs predominantly through two threecentre rearrangements rather than the previously proposed four-centre process (Scheme 2).3 These dimers are analogues of 1 where the N-amino group bears both an acyl and an alkoxy group.The reaction may not be confined to N-aza-N-oxo systems however. Earlier results indicate that N-alkoxy-N-hydroxy amines (3) formed as intermediates in the acid-catalysed solvolysis of N-acetoxy-N-alkoxybenzamides undergo a similar OR J ..

Abstract:The origin of the HERON reaction is reviewed from a historical perspective and shown to have its foundation in the unusual properties of bisheteroatom-substituted amides, so-called anomeric amides. The reaction involves migration of anomerically destabilized oxo-substituents on an amide nitrogen to the amide carbon and dissociation of the amide bond. Computational work providing a theoretical basis for the reaction is presented, together with physical organic measurements that support results therefrom. The rearrangement has been observed in a number of chemical transformations of N-alkoxy-N-aminoamides, reactions of 1-acyloxy-1-alkoxydiazenes, N-alkoxy-N-aminocarbamates, Nalkoxyhydroxamic acids, as well as in the gas-phase reactions of N-acyloxy-N-alkoxyamides.Key words: HERON reaction, anomeric amides, rearrangements, hindered esters, concerted reactions. Résumé :On a fait une revue historique de l'origine de la réaction HERON et on montre que sa base réside dans les propriétés inhabituelles des amides portant des substituants bishétéroatomiques, les amides dits anomères. La réaction implique la migration de substituants oxo anomériquement déstabilisée de l'azote d'un amide vers le carbone de l'amide accompagnée d'une dissociation de la liaison amide. On présente un ensemble de calculs théoriques qui sert de base pour expliquer la réaction ainsi que des mesures de chimie organique physique qui supportent les résultats de ces calculs. Le réarrangement a été observé dans un certain nombre de transformations chimiques de N-alkoxy-N-aminoamides, dans les réactions de 1-acyloxy-1-alkyoxydiazènes, de N-alkoxy-N-aminocarbamates et d'acides N-alkoxyhydroxamiques ainsi que dans les réactions en phase gazeuse de N-acyloxy-N-alkoxyamides.

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