Migration group

Translocations studied, mainly those of methyl or benzyl groups, have included movement from ring atom to ring atom, exocyclic atom to ring atom, and ring atom to exocyclic atom in all cases both donor and recipient atoms are heteroatoms. 

Heating a solution of 3-benzylpurine hydrobromide (140748 hr) in di-methylformamide gives initially a mixture of the 1-, 7-, and 9-benzyl isomers. With further heating interconversion of the 1-benzyl component occurs. 

1- and 3-benzylpurines arise as a result of a kinetically controlled reaction whereas subsequent conversion to the more stable 7- and 9-benzyl isomers is thermodynamically dependent. Migrations of this type also occur with iV-methylated purines by thermal means. In dimethylformamide the 1,3-dimethylhypoxanthinium iodides (44) isomerize to mixtures of the 1,7- and 

The best-known example is the Dimroth rearrangement of 1-alkylade-nines to 6-alkylaminopurines, the parameters and mechanism of which have been established and confirmed by the results of recent mass spectral investigations with deuterated 1-alkyladenine derivatives. The effect of various substituent groups on the rate of rearrangement has been examined. In addition to studies with 1-alkyladenines the kinetics of the analogous isomerization with 1-alkoxyadenines have been determined and compared.  

An unrelated reaction mechanistically, but falling under this heading, is the thermal transformation of 3-benzyladenine, under pressure conditions (1207autoclave), in low yield, into 6-benzylaminopurine. Results of a detailed investigation, involving magnetic resonance and mass spectrometry, point 

54 (1975). 

Kawakatsu, and T. Fuji, Chem. Pharm. Bull. 23, 2643 (1975). 

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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... 

In the alkyl aryl ketones, the aryl groups migrate preferentially, yielding V-aryl amides. 

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

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... 

The reaction is stereospecific the alkyl group migrates with retention of configuration O O... 

The presence of water in the carbonylation mixture makes it possible to halt the reaction after the second group migration (step 2). Oxidation of the intermediate boraglycol yields the corresponding ketone, and alkaline hydrolysis affords the secondary alcohol, R HOH. A blocking group of low... 

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. 

Bake sulfonation is an important variant of the normal sulfonation procedure. The reaction is restricted to aromatic amines, the sulfate salts of which ate prepared and heated (dry) at a temperature of approximately 200°C in vacuo. The sulfonic acid group migrates to the ortho or para positions of the amine to give a mixture of orthanilic acid [88-21-1] and sulfanilic acid [121 -57-3] respectively. This tendency is also apparent in polynuclear systems so that 1-naphthylamine gives 1-naphthy1amine-4-su1fonic acid. 

When alkoxypyridazine 1-oxides are heated alone or in the presence of p-toluenesulfonic acid the methyl group migrates from the methoxy group to the A-oxide group. In this manner, 4-methoxypyridazine 1-oxide rearranges to l-methoxypyridazin-4(l//)-one, 5-methoxypyridazine 1-oxide to 2-methylpyridazin-5(2//)-one 1-oxide and substituted 3,6-dimethoxypyridazine 1-oxides to l,3-dimethoxypyridazin-6(l//)-ones. 

A well-known example of non-prototropic tautomerism is that of azolides (acylotropy). The acyl group migrates between the different heteroatoms and the most stable isomer (annular or functional) is obtained after equilibration. In indazoles both isomers are formed, but 2-acyl derivatives readily isomerize to the 1-substituted isomer. The first order kinetics of this isomerization have been studied by NMR spectroscopy (74TL4421). The same publication described an experiment (Scheme 8) that demonstrated the intermolecular character of the process, which has been called a dissociation-recombination process. 

When an alkyl group migrates, there is an additional stereochemical feature to consider. The shift can occur with retention or inversion at the migrating center. The analysis of sigmatropic shifts of alkyl groups is illustrated in Fig. 11.7. The allowed processes include the suprafacial 1,3-shift with inversion and the suprafacial 1,5-shift with retention. Sigmatropic rearrangements of order [3,3] are very common ... 

Sometimes reduction of a ketone by NaBH4 is accompanied by hydrolysis of an ester elsewhere in the molecule. Norymberski found that a 20-keto-21-acetoxy compound with NaBH4 in methanol at 0° for 1 hour gives the 20/ ,21-diol. 50 % aqueous dimethylformamide has been used as the solvent in an attempt to prevent acetate hydrolysis, but sometimes under these conditions the 21-acetoxy group migrates to the 20-position. The rearrangement is favored by addition of the 20-acetate as seeds or by addition of... 

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. 

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