Subject aryl migration

Murakami and coworkers reported a further use for this rhodium migration [80, 81]. Instead of protonolysis, they noticed that the aryl rhodium species after the vinylic to aryl migration is nucleophilic enough to attack an ester moiety in an intramolecular fashion to afford a cyclic ketone. Thus, an internal alkyne equipped with ester groups at a specific place was subjected to the rhodium-catalyzed hydroarylation conditions (Scheme 21). Indeed, the desired ketone was obtained in an 89% yield. Not only methyl esters can serve as acylation agents ethyl esters and isopropyl esters are also suitable substrates. 

Synthesis of Biaryls. When TBDPS protected 2-bromobenzyl alcohol is subjected to standard radical conditions, the initially formed aryl radical reacts with one phenyl ring of the TBDPS group at its ipso position, and the resulting intermediate further transforms into a silyl radical, which then reduced to the corresponding silane. After this phenyl migration, desilylation by methyl lithium eventually provides the biphenyl product (eq 10). Other phenyl silyl ethers of this type can also experience the same aryl migration and produce biaryls. 

The flash vacuum pyrolysis of alkynes, arynes, and aryl radicals has been reviewed. A discussion of secondary reactions and rearrangements is included. The pyrolysis of cyclopentadienes has also been examined. The rates for the initial C—H bond fission and the decomposition of C-C5H5 have been calculated. A single-pulse shock study on the thermal decomposition of 1-pentyl radicals found alkene products that are formed by radical isomerization through 1,4- and 1,3-hydrogen migration to form 2- and 3-pentyl radicals. The pyrrolysis of f-butylbenzene in supercritical water was the subject of a report. ... 

These ideas will be discussed in the following subsections, where most of the attention will be devoted to the mechanistic smdies with aromatic esters, which have been the subject of an overwhelming majority of the research efforts. Nevertheless, the same reaction mechanism has been shown to be valid for the PFR of anilides, thioesters, sulfonates, and so forth. Furthermore, it is also applicable to the photo-Claisen rearrangement [i.e. the migration of alkyl (or allyl, benzyl, aryl,)] groups of aromatic ethers to the ortho and para positions of the aromatic ring [21,22]. 

One point which can be settled qualitatively, however, is whether singlet exciton migration does in fact occur in the aryl vinyl polymers. It will be shown that available evidence supports energy migration as an important feature of the photophysics of polystyrene (PS), poly(l-vinyl naphthalene) (P1VN), and poly(2-vinyl naphthalene) (P2VN), the homopolymers which are the subject of the majority of the review. 

Aryl benzoates are subject to acyl migration under basic conditions. ... 

To date, palladium migration to vinylic positions remains unknown. Thus, palladium migration to sp2 carbons has been largely limited to aryl carbons. However, in addition to vinylic and aryl carbons, aldehydic and imidoyl carbons also possess sp2 hybridization and carry a C-H bond. This special type of sp2 C-H bond has also been subjected to palladium migration studies and preliminary success has been achieved. Currently, aryl to imidoyl [27] and aryl/alkyl to acyl [72] palladium migrations are known. Migrations from other types of carbons to imidoyl and acyl positions are... 

The mechanisms of these two cyclization reactions appear fairly different. While the xanthone synthesis would appear to follow a simple palladium migration/arylation mechanism, the fluoren-9-one synthesis has been subjected to isotope labeling experiments (Scheme 12), which indicate that more than one mechanistic pathway may apply. It turns out that the deuterium that replaces the iodide after palladium migration comes not only from the imidoyl position, but also from the pendent aromatic ring used to trap the migrated palladium via arylation. Such results can... 

By changing the reaction conditions, Jia and coworkers [10] were able to control the regioselectivity and trap the palladium intermediates with external nucleophiles. For example, when aryl iodide 26 was subjected to the domino Heck/cyanation sequence, it was found that the presence of water completely shut down the palladium migration, affording product 27 exclusively (Scheme 3.6). In the absence of water, product 28 was obtained in good yield via a Heck/C-H activation/Pd migration/cyanation sequence. 

TBDMS-protected compounds and the resultant analogues (141) of (139) were subjected to hydroboration at pH 7 to give the aryl 3-C-glucosides (142). When the oxidation step was carried out at high pH values silyl migration occurred and the isomeric 3-hydroxy-compounds (143) were isolated. Extension of this work afforded (144) and hence, by... 

The mechanism of insertion of carbon monoxide into a metal-alkyl or metal-aryl bond involves migration of the alkyl (aryl) group to the coordinated carbon monoxide with concomitant formation of an acyl ligand. An extensive literature on this subject exists.l" ... 

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