Selective skeletal rearrangement

Takahashi T, Xi Z, Fischer R et al (1997) Intermolecular coupling reaction of alkynes with vinyl bromide with selective skeletal rearrangement. J Am Chem Soc 119 4561-4562... 

The synthesis was carried out as shown in Scheme 13.24. A diol was formed and selectively tosylated at the secondary hydroxy group (Step A-4). Base then promoted the skeletal rearrangement in Step B-l by a pinacol rearrangement corresponding to 23-11 => 23-III in the retrosynthesis. The key intramolecular Michael addition was accomplished using triethylamine under high-temperature conditions. 

Open chain hydrocarbons, skeletal rearrangements, 29 298-302 aromatic selectivities, 29 302 atomization, 29 298-302 cyclization, 29 298-302 Open sequence in reaction mechanisms, 32 ... 

Experiments with selective scavengers for cations and radicals have indicated that both types of intermediate are involved in the formation of isobutane and isobutene. Skeletal rearrangements of ions should occur much more readily than of radicals, although excitation may affect this generalization. 

Bromine trifluoride is used to selectively substitute fluorine for bromine in brominated alkanes and esters. The reactions are carried out by gradual addition of bromine trifluoride to a solution of the substrate in CFC-113 or CFC-112 at 10-20nC. The bromine-fluorine exchange in mono-bromohaloalkanes is nonstereoselective and accompanied, in some eases, by skeletal rearrangements, hydride shifts, and halogen migrations. All three fluorine atoms in bromine trifluoride are involved in the fluorination reaction. Chlorine atoms in the substrate molecules remain intact.109... 

Selective double-bond migration of linear and cyclic alkenes without skeletal rearrangement is achieved by treatment with basic reagents ([Eq. (1.29)] see also Chapter 4) ... 

Oxidation with lead tetraacetate is a far less selective process.490,491 Studied mainly in the oxidation of cycloalkenes, it gives stereoisomeric 1,2-diol diacetates, but side reactions (allylic acetoxylation, skeletal rearrangement) often occur. A change in reaction conditions in the oxidation of cyclopentadiene allows the synthesis of different isomeric mono- and diesters.492... 

To examine selectivity relationships, the following classification of o-ethyltoluene products is proposed for simplification ethylbenzene, toluene, and benzene are dealkylation products p- and m-ethyltoluene are isomerization products. Formation of m- and p-xylenes must have involved skeletal rearrangements to have been produced from o-ethyltoluene. Accordingly, these compounds are counted as isomerization products. In all cases studied, toluene was the major dealkylation product, and m- and... 

Selective activation of alkyne functions of enynes to give products either of alkoxy-cyclization or of exo- and endo-skeletal rearrangement can be achieved by using alkynophilic cationic gold(I) complexes. The endocyclic cyclization catalysed by gold(I) proceeds via a mechanism different from those known for Pd(II), Hg(II), or Rh(I) catalysts.118... 

The site selectivity switch, that is the site to which BF3 coordinated carbonyl rearrangement occurs, in the skeletal rearrangements of cyclobutene-fused diarylhomoben-zoquinones (43) is reported to result from higher order complexation. Thermodynamic... 

Figure 16 Mechanisms proposed for the skeletal rearrangement of C6 hydrocarbons, showing bond shift (BS), cyclic mechanism (CS), selective ring opening (S), and non-selective ring opening (NS)...

Skeletal rearrangement reactions over Pt single crystals have been studied for methyl cyclopentane, 2- and 3-methylpentane350 and for n-hexane.3sl One conclusion351 is that whereas aromatization reactions are very sensitive to surface structure [Pt(l 11)> Pt(100)], isomerization, Cs-cyclization, and hydrogenolysis reactions display little dependence on structure. Temperature and H2 pressure are more important in affecting the selectivity. 

To illustrate this method of approach, results of three of the experiments by Corolleur et al. are shown in Tables XIII and XIV. In the examples selected here, a 13C-labeled 3-methylpentane and a labeled 2-methylpentane are reacted, in turn, and distribution of 13C is shown for the methylpentane products in Table XIII and for the n-hexane products in Table XIV. Distributions expected for a pure cyclic mechanism (C) and for a methyl shift (T) are indicated. Detailed discussion by the authors of abnormal products (i.e., those not predicted by the single-stage purely carbocyclic mechanism) led to the conclusion that these are formed on a second, less numerous, type of surface site by the action of which a succession of several rearrangements, according to a cyclic or a bond-shift mechanism, takes place. In Table XIV it can be seen that assumption of a simple skeletal rearrangement of the type... 

Carbonyl-derived RhCo bimetallic catalysts exhibit high selectivity toward skeletal rearrangement of methylcyclopentane (MCP) (to a mixture of 2- and 3-methylpentanes), whereas on the conventional counterpart hydrocracking... 

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