Elimination lone pair assisted

Ad 2 (Addition, Electrophilic, Bimolecular), A -i- An and Hetero Ad Z, p.t. + AdN El (Elimination, Unimolecular), Dn + Dg and Lone-Pair-Assisted El, Ep + p.t. Se2Ar Electrophilic Aromatic Substitution, Ag + Dg ElcB (Elimination, Unimolecular, Conjugate Base), p.t.-i-Ep AdN2 (Addition, Nucleophilic, Bimolecular), AdN + P f ... 

Pyrazolines substituted at position 4 or 5 with hydroxy or amino groups readily eliminate a molecule of water or amine yielding pyrazoles. The 4-substituted derivatives are relatively more stable than the 5-substituted ones, because for the last group the lone pair at N-1 assists the elimination (407) -> (408) -> (409). The sulfonyl group at position 1 is also easily eliminated and this property is taken advantage of in Dorn s elegant synthesis of 3-aminopyrazole (Section 4.04.3.3.1). 

This has been mentioned at various points in this paper and may involve either a direct acid-base reaction of nitrene and nucleophile or, in some instances, reaction of the nitrene precursor with the nucleophile (or 1,3-dipolarophile) followed by loss of nitrogen. For example, the reaction of benzenesulphonyl azide with pyridine to give 31 (Ar=Ph) 69> could either involve a free nitrene or a concerted process in which the lone pair on the pyridine nitrogen atom assists the elimination of molecular nitrogen. That some free nitrene can be involved in these reactions is clear from the isolation of some 3-benzenesulphonamido-2,6-lutidine... 

Although the dominant reactivity pattern for carbonyl compounds in reactions with nucleophiles is the direct nucleophilic attack at the carbonyl k, eliminations promoted by assistance from the lone pair of oxygen are also possible in those rare cases when X is a strong acceptor with a low energy o (Figure 7.38). There is no stereoelectronic bias in this system the lone pair of oxygen is positioned well to overlap well with both carbonyl substituents and subsequent reactivity is determined by the leaving ability of X vs. R. 

The product is a ketal, which will undergo dehydration, by elimination of water under aqueous acidic conditions the loss of water is assisted by one of the lone pairs of the ketal oxygens. Deprotonation of the oxonium ion gives the product. 

All that remains now is leaving group departure. Protonation of the amine followed by a 1,2-elimination assisted by the neighboring lone pairs on oxygen gives an intermediate that is simply a proton transfer away from the correct product. 

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