migratory aptitude in beckmann rearrangement


On the basis of this and other evidence, it has been concluded that for a 1,2-shift of hydrogen or methyl to proceed as smoothly as possible, the vacant p orbital of the carbon bearing the positive charge and the sp3 orbital carrying the migrating group must be coplanar,29 which is not possible for 20. Creative Commons Attribution-Sharealike 3.0 Unported CC BY-SA 3.0. Most nucleophilic 1,2-shifts are intramolecular. Several cyclic α,β-unsaturated ketoximes, or their tosylates were subjected to the Beckmann rearrangement. 17.C), so that the choice of migrating group is largely determined by which group is in the right place in the most stable conformation of the molecule.31 However, in some reactions, especially the Wagner–Meerwein (18-1) and the pinacol (18-2) rearrangements, the molecule may contain several groups that, geometrically at least, have approximately equal chances of migrating, and these reactions have often been used for the direct study of relative migratory aptitudes. Other possibilities are (1) that the ions have geometrical structures that are twisted in opposite senses (e.g., a twisted 32 might have its positive. When the group α to the oxime is capable of stabilizing carbocation formation, the fragmentation becomes a viable reaction pathway. Different reaction conditions can favor the fragmentation over the rearrangement. Certain conditions have been known to racemize the oxime geometry, leading to the formation of both regioisomers.

The 1,2 shift of phenyl group is faster than that of alkyl groups. It is due to formation of phenonium ion. The Beckmann fragmentation is a reaction that frequently competes with the Beckmann rearrangement. The rearrangement of aldoximes occurs with stereospecificity in the gas phase and without stereospecificity in the solution phase. In these instances, it may be said that R assists in the removal of the leaving group, with migration of R and the removal of the leaving group taking place simultaneously. On closer inspection, however, it is clear that several factors are operating. (function(){for(var g="function"==typeof Object.defineProperties?Object.defineProperty:function(b,c,a){if(a.get||a.set)throw new TypeError("ES3 does not support getters and setters. In above case, the R' group is assumed to possess greater migratory aptitude and hence only one product is formed preferentially. Cyclic oximes and haloimines yield lactams. In the pinacol rearrangement, there is the additional question of which OH group leaves and which does not, since a group can migrate only if the OH group on the other carbon is lost. In the transition state leading to the imidate, the water oxygen atom is coordinated to 4 other atoms. 15.B.iv). The Beckmann rearrangement (Reaction 18-17) provides an example. Likewise, in for example the pinacol rearrangement, the most stable cation controls the reaction pathway, as opposed to the migrating group. For example, in superacid solution, the ions 17 and 19 are in equilibrium. However, exceptions are known, and which group is lost may depend on the reaction conditions (e.g., see the reaction of 59). Comment: The migration of hydrogen is seldom observed. In other instances, there are two or more potential migrating groups, but which migrates is settled by the geometry of the molecule. In the other, W goes from A to B in the same molecule (intramolecular rearrangement), in which case there must be some continuing tie holding W to the A–B system, preventing it from coming completely free. The first step therefore is creation of a system with an open sextet. [3] The Beckmann fragmentation is another reaction that often competes with the rearrangement, though careful selection of promoting reagent and solvent conditions can favor the formation of one over the other, sometimes giving almost exclusively one product.

In strong acids, the reaction proceeds with the protonation of the OH group of the oxime with subsequent loss of water to yield(A) with electron deficient nitrogen. Formation of a Nitrene: The decomposition of acyl azides is one of several ways in which acyl nitrenes (3) are formed (see Sec.

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