Rearrangement reactions 1.3 shifts

In a rearrangement reaction, a group moves from one atom to another in the same molecule. Most are migrations from an atom to an adjacent one (called 1,2 shifts), but some are over longer distances. The migrating group (Y) may move with its... 

The most common rearrangement reaction of alkyl carbenes is the shift of hydrogen, generating an alkene. This mode of stabilization predominates to the exclusion of most intermolecular reactions of aliphatic carbenes and often competes with intramolecular insertion reactions. For example, the carbene generated by decomposition of the tosylhydrazone of 2-methylcyclohexanone gives mainly 1- and 3-methylcyclohexene rather than the intramolecular insertion product. 

Hydride and 1,2-alkyl shifts represent the most common rearrangement reactions of carbenes and carbenoids. They may be of minor importance compared to inter-molecular or other intramolecular processes, but may also become the preferred reaction modes. Some recent examples for the latter situation are collected in Table 23 (Entries 1-10, 15 1,2-hydride shifts Entries 11-15 1,2-alkyl shifts). Particularly noteworthy is the synthesis of thiepins and oxepins (Entry 11) utilizing such rearrangements, as well as the transformations a-diazo-p-hydroxyester - P-ketoester (Entries 6, 7) and a-diazo-p-hydroxyketone -> P-diketone (Entry 8) which all occur under very mild conditions and generally in high yield. 

Charge density-NMR chemical shift correlation in organic ions, 11, 125 Charge distribution and charge separation in radical rearrangement reactions, 38, 111 Chemically induced dynamic nuclear spin polarization and its applications, 10, 53 Chemiluminesance of organic compounds, 18, 187 Chiral clusters in the gas phase, 39, 147... 

As explained in the Introduction, alkene oxides (10.3) are generally chemically quite stable, indicating reduced reactivity compared to arene oxides. Under physiologically relevant conditions, they have little capacity to undergo rearrangement reactions, one exception being the acid-catalyzed 1,2-shift of a proton observed in some olefin epoxides (see Sect. 10.2.1 and Fig. 10.3). Alkene oxides are also resistant to uncatalyzed hydration, thus, in the absence of hydrolases enzymes, many alkene oxides that are formed as metabolites are stable enough to be isolated. 

Reaction pathways for the addition of ethylene to butadiene radical cation involving H-shifts have been investigated at the coupled cluster UCCSD(T)/DZP//UMP2(fc)/DZP-b ZPE level of theory.Several rearrangement reactions have been found to occur below the energy limit of separated ethylene and butadiene radical cation. The cyclopentenyl cation ( 5117)+ in the gas phase may originate from various pathways. 

Concerning the possible rearrangement of the lithiooxirane into the alkoxy carbene 155, calculations have also shown that the activation energies of the 1,2-H shifts (to cyclopentanone enolate or cyclopentenol) are extremely high (at least 23 kcalmol" ) from 155, whereas they are much lower (between —0.4 kcalmol" and 8.8 kcalmol" ) from carbene 154. This is explained by a strong intramolecular stabilization of the carbene by the alcoholate moiety, as depicted in Scheme 66. This stabilization could signify that the formation of a carbene from the carbenoid is a disfavored process, and that the carbenoid itself is involved in the rearrangement reaction. 

Rearrangement reactions have turned out to be very convenient and the most used methods for the synthesis of butatrienylidene complexes. 1,4-H shift reactions have been used by Bruce et al. [19, 20] and Winter et al. (e.g.. Scheme 3.6) [21, 22] for generating butatrienylidene ruthenium complexes. Analogously, a butatrienylidene iron complex tvas obtained from [Cp (dppe)Fe-Cl] (dppe = Ph2PCH2CH2PPh2), Mc3Si-C = CC = CH, and NaBPh4 in methanol via 1,4-H shift (Scheme 3.7) [23, 24]. 

In addition to the migration of hydrogen atoms in sigmatropic rearrangements, alkyl shifts also take place. A large number of such reactions occur... 

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... 

Shifts of atoms or groups in fluorine-containing aromatic or heteroaromatic compounds are covered in this section. Shifts due to valence isomerization (benzene-Dewar benzene-prismane-benzvalene), however, are covered in Section 5.3.4.3. A major review1 on polyfluoroaromatic and heteroaromatic compounds has been written by G.M. Brooke, one of the principal contributors in this area, covering the literature until the end of 1995 (see pp 56-59 of ref 1 for the section on rearrangement reactions). 

Hoffmann and Pete [106] have irradiated D-alk- 3 -e n y I salicylates and obtained products which result from rearrangement reactions of primary ortho adducts (Scheme 31). The authors realized that the linear tricyclic dienes, formed by ring closure of the cyclooctatriene derivatives, are enol ethers which can be converted into acetals by acid-catalyzed addition of an alcohol. This shifts the... 

All rearrangement reactions in Table 1 can be categorized as an apparent intramolecular 1,2-shift of a hydrogen and an electronegative X group (OH, three examples NH2, five examples C framework, four examples) [1,9,10],... 

These rearrangement reactions are interpretable in terms of [2.3] sigmatropic shifts of the intermediate ylides. A number of such rearrangements of open-chain systems have been described, involving sulfonium ylides [43] [44] [45], ammonium ylides [46] [57], anions in a-position to oxygen (Wittig rearrangement) [48] [49], and fluorenyl carbanions [50]. 

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