O-Bond migration

The numbering is written by the order i, j written in a bracket. The letter / and j denote the number of atoms across which the o bond migrates. Let us take the case of cope-rearrangement of 1, 5 hexadiene. 

As for the cyclopropane rearrangement, the reaction probably involved the formation of a cationic silver cationic species followed by regioselective o-bond migration, leading preferentially to the more stable carbocation intermediate that is finally intramolecularly trapped by the organosilver species (Scheme 3.25).39... 

A special term ( dyotropic reaction see also Ref 222.374) proposed to denote the noncatalyzed processes in the course of which two o-bonds migrate simultaneously and intramolecularly, and a general theory of such reactions was developed In accordance with the definitions given in Ref. the rearrangement of ion (77) by the concerted route should be considered as a dyotropic reaction of type I. 

Scheme 7.46. The formation of camphene hydrochloride (ex< -2-chloro-233-trimethylbicyclo[2.2.1]heptane and its rearrangement to isobomyl chloride (ex<7-2-chloro-l,7,7-trimethylbicyclo[2.2.1]heptane).The addition of a proton to the carbon-carbon double bond of camphene 3,3-dimethyl-2-methyienebicyclo[2.2.1]heptane is shown as accompanied by o-bond migration to produce a singie ion with partial bonding to two sites (called, variously, nonclassical or bridged ) or a pair of rapidly equilibrating ions. The classical ions are shown, leading to the observed products. Debate raged over a period of years about the nature of the ion or ions lying between the starting materials and products. Additional discussion is provided in Chapter 8.

When groups of atoms migrate from one site to another, the O-bonded atom that leaves one site is different than the one that forms a O bond at the second site. In such reactions, both ends of a O bond migrate. For example, in the rearrangement of 3-methyl-1,5-hexadiene, atom 3 of the three-atom migrating group eventually bonds to atom 3 of the second part of the molecule. 

The rearrangement (automerization) of Dewar thiophene 5-oxide (61), observed by NMR, occurs so much more rapidly than that of the corresponding episulfide that special mechanisms have been invoked. The one which involves a zwitterionic intermediate (Scheme 108) is favored over a pseudopericyclic sulfur-walk mechanism in which the electrons of the carbon-sulfur o--bond and the pair of electrons on sulfur exchange places as the sulfur atom migrates around the ring (80JA2861). 

Rearrangement involving cleavage of the C—O bond is also observed with the phenyl isocyanate adduct (59). In place of an O-acylated nitrone its product (60) of acyl migration is isolated (67JPR(36)86). 

Direct proof of an oxaziridine intermediate was achieved in photolysis experiments in an organic glass at 77 K (80JA5643). Oxaziridine (75), formed by photolysis of A/-oxide (74) and evidenced by UV spectroscopy under the above conditions, decomposed at higher temperature to form the imino ether (76) by N—O bond cleavage and C -> O migration of an aryl group. 

Kim and coworkers introduced silyl radical mediated addition of alkyl radical to silyloxy enamine 76. The silyloxy enamine moiety is readily accessible from a variety of functionalities. The mechanistic concept is illustrated in the Scheme 12 and involves the addition of R radical to 76 to give the radical adduct 77 and the subsequent homolytic cleavage of N-O bond to yield the desired product 78 and a silyloxy radical 79. The latter undergoes 1,2-phenyl migration to give the silyl radical 80 that abstracts halogen from the alkyl halide to regenerate the R radical. 

The stereochemistry of the C(3) hydroxy was established in Step D. The Baeyer-Villiger oxidation proceeds with retention of configuration of the migrating group (see Section 12.5.2), so the correct stereochemistry is established for the C—O bond. The final stereocenter for which configuration must be established is the methyl group at C(6) that was introduced by an enolate alkylation in Step E, but this reaction was not very stereoselective. However, since this center is adjacent to the lactone carbonyl, it can be epimerized through the enolate. The enolate was formed and quenched with acid. The kinetically preferred protonation from the axial direction provides the correct stereochemistry at C(6). 

Complex (5) undergoes methyl migration after oxidative addition of CH3I to afford the acyl complex (6) containing two Rh—O bonds. Heating (6) in the presence of CO results in the reductive elimination of Acl, which upon hydrolysis is transformed to AcOH.12... 

MR donor bonds, is o-bond metathesis. Alternatively, as the Lewis-acid strength of M increases, the tendency toward agostic or bridging interactions can finally result in H—H bond scission and formal migration of hydride to the metal atom,... 

In this case the o bond which has migrated is to carbon atom 3 and 3 and the original bond was between C-l and C-l Therefore, this shift is represented as [3, 3], So here / = 1 and 7 = 1. The numbering always starts from the original termini of the o bond. Each of the original termini is given the number 1. 

There will be the homolytic cleavage of the o bond giving two free radicals. So one electron remains with the original migrating group and the other is associated with the K framework. 

Fig. 16 The migration of the proton along an O-H- - O bond in a co-crystal urea-phosphoric acid (1 1), as the temperature is increased from 150 K (top) to 335 K (bottom), becoming essentially centred (neutron study) [55]... 

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