Carbonate group migration

As the bromine leaves, the nitrogen becomes electron deficient, so the carbon group migrates to the nitrogen. The nitrogen uses a pair of electrons to stabilize the carbonyl carbon. 

The mechanism of the Fries reaction is not known with certainty. One mechanism regards it as a true intramolecular rearrangement in which the acyl group migrates directly from the oxygen atom to the carbon atoms of the ring. Another scheme postulates that the ester is cleaved by the reagent... 

With electrons flowing from ethylene to zirconium the Zr—CH3 bond weakens the carbons of ethylene become positively polarized and the methyl group migrates from zirconium to one of the carbons of ethylene Cleavage of the Zr—CH3 bond is accom panied by formation of a ct bond between zirconium and one of the carbons of ethylene m Step 3 The product of this step is a chain extended form of the active catalyst ready to accept another ethylene ligand and repeat the chain extending steps... 

Substituted aromatics, eg, aLkylbenzenes, sometimes experience attack at the substituent position by NO/ (7). A cyclohexadienyl cation is formed it is unstable and the nitro group migrates on the ring to a carbon atom that is attached to a hydrogen. Loss of the proton results in a stable nitroaromatic. 

In most cases, the cleavage of a carbon-carbon bond causes rearrangements of the carbon skeleton Ring contraction, ring expansion, and alkyl group migration are observed under different conditions These transformations proceed in most cases in the presence of catalysts at elevated temperatures Examples where only temperature causes rearrangements will be discussed m the next section... 

Step 3 The methyl group migrates from zirconium to one of the carbons of the ethylene ligand. At the sane time, the tt electrons of the ethylene ligand are used to fonn a a bond between the other carbon and zirconium. 

In the well-known Brook rearrangment, silyl groups migrate from oxygen to carbon, but the following example is less obvious and not necessarily predictable ... 

Recently, it was shown that the attack of CN on [FeCp(C6H5Cl)]+ PFortho-position. In the intermediate cyclohexadienyl complex, the CN group migrates to the ipso-carbon, whereas Cl is displaced. The monosubstituted benzonitrile complex is subjected to a second ortho-CN- attack but hydride is not removed spontaneously to give back an arene complex (Scheme XIX). Removal of the hydride is achieved by oxidation using DDQ (2,3-dichloro-... 

A 1,2-shift has been observed in radicals bearing an OCOR group at the p-carbon where the oxygen group migrates as shown in the interconversion of 36 and 37. This has been proven by isotopic labeling experiments and other mechanistic explorations. A similar rearrangement was observed with phosphatoxy alkyl radicals such as 38. ... 

If the reaction is carried out with ketone labeled in the C=0 group with the first pathway predicts that the product will contain all the " C in the C=0 carbon, while in the second pathway the label will be in the a carbon (demonstrating migration of oxygen). The results of such experiments have shown that in some cases only the C=0 carbon was labeled, in other cases only the a carbon, while in still others both carbons bore the label, indicating that in these cases both pathways were in operation. With a-hydroxy aldehydes and ketones, the process may stop after only one migration (this is called the a-ketol rearrangement). 

The reaction can also be done thermally. The stereochemistry of the thermal rearrangement of the acetoxy epoxides involves inversion at the carbon to which the acetoxy group migrates,143 and reaction probably proceeds through a cyclic TS. 

Treatment of the cycloadduct 83a with sodium methoxide afforded a pyrrole derivative 86. It was proposed that the key step involved an amino group migration to the ylide carbon of amino sulfonium ylide intermediate 84 as depicted in Scheme 11 <1999TL95>. 

The ruthenium acyloxymethyl complex produced by step 6 of Scheme 1 could, of course, eliminate the methyl ester product, but it also has the possibility of leading to a two-carbon product via alkyl group migration to coordinated CO (eq. 2). 

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