BECKMANN Rearrangement or fragmentation

BECKMANN Rearrangement or fragmentation Acid catalyzed rearrangement of oximes to amides or cleavage of oximes to nitnies... 

BECKMANN Rearrangement or fragmentation 30 Beech 43 Beirut 155 Belleau 135... 

The reaction that normally occurs on treatment of a ketoxime with a Lewis or proton acid is the Beckmann rearrangement (18-17) fragmentations are considered side reactions, often called abnormal or second-order Beckmann rearrangements. Obviously, the substrates mentioned are much more susceptible to fragmentation than are ordinary ketoximes, since in each case an unshared pair is available to assist in removal of the group cleaving from the carbon. However, fragmentation is a side reaction even with ordinary ketoximes and, in cases where a particularly stable carbocation can be cleaved, may be the main reaction. ... 

Fragmentation of the intermediate or concerted formation of nitriles from the activated oxime (Scheme 9, pathway 3) this is the Beckmann fragmentation. In some circumstances this pathway becomes dominant, particularly when there are quaternary carbons adjacent to the oxime. This transformation has found particular utility in ring-cleavage processes (sometimes called abnormal or second-order Beckmann rearrangements). 

In some cases, the fragmentation product is largely predominant, or even the only product observed. The Beckmann fragmentation process will be analysed briefly in this text as it can limit the utility of the Beckmann rearrangement. 

Beckmann rearrangements that go with fragmentation are sometimes called anomalous or second-order1 Beckmann rearrangements. You should not use the second of these names and, in any case, Beckmann fragmentation is much better than either. 

An unexpected reductive rearrangement was observed in the reaction of the dioxime (33) with diisobutylaluminum hydride in dichloromethane, leading to the diamine (34 equation 21). It should be noted that attempted reaction of (33) under various Beckmann rearrangement conditions resulted either in extensive decomposition or fragmentation. 

This reaction was first reported by Semmler in 1892 and subsequently studied by Wolff in 1902 and Schroeter in 1911. It is the rearrangement of an oxime of an a,p unsaturated cyclohexenone (or known as the o, )0-unsaturated cyclohexenyl ketoxime) to an aromatic amine in a mixture of acetic anhydride and acetic acid saturated with HCl or HBr. Therefore, this reaction is generally known as the Semmler-Wolff aromatization," Semmler-Wolff rearrangement, or Semmler-Wolff reaction. Occasionally, it is also referred to as the Semmler-Wolff-Schroeter reaction. It should be pointed out that the Beckmann Rearrangement,and the fragmentation to nitriles often compete with the Semmler-Wolff aromatization. In addition, the Semmler-Wolff aromatization is obstructed by the substituents close to the reaction center. ... 

A variation in the Beckmann rearrangement occurs if one of the groups R or R can give rise to a relatively stable carbonium ion. In this circumstance, fragmentation of the molecule occurs, as will be discussed more fully in Section 8.4. 

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