Reaction mechanisms phenyl migrations

The mechanism of the reaction is unknown. The stereospecificity observed with (E)- and (Z)-l-methyl-2-phenylethylene points to a one-step reaction. The very low Hammett constant, -0.43, determined with phenylethylenes substituted in the benzene ring, excludes polar intermediates. Yields of only a few percent are obtained in the reaction of aliphatic alkenes with (52). In the reaction of cyclohexene with (52), further amination of the aziridine to aminoaziridine (99) is observed. Instead of diphenylazirine, diphenylacetonitrile (100) is formed from diphenylacetylene by NH uptake from (52) and phenyl migration. 

The proposed reaction mechanism is shown in Scheme 9.15. Starting from the phenyl-rhodium complex 87, alkyne rearrangement is expected to furnish the phenyl-vinylidene complex 88. Migration of a phenyl ligand onto the vinylidene moiety of 88 must occur such that the vinyl Rh-C bond and the enone tether of the resultant complex (89) attain a cis-relationship to one another. Intramolecular conjugate... 

Intermolecular.- cis-trans-Isomerization of the enone (31) occurs on irradiation and this is followed by formation of the adducts (32) and (33) by a thermal cycloaddition of the trans-isomer of (31) to a ground state cis-isomer. Such a mechanism involving cis-frans-isomerization is supported by the fact that the reaction does not show a solvent dependency and can be brought about equally efficiently in t-butanol or benzene.The enone 31) does not undergo 1,2-phenyl migrations of the type associated with the di-Tc-methane rearrangement of 4,4-diphenyl eye 1 ohexenone derivatives. The regiospecific... 

Cyclic enones, such as substituted cyclohex-2-enones or cyclohexa-2,5-diones, also undergo sigmatropic photorearrangement to form bicyclo[3.1.0]hexanones (lumiketones) or bicyclo[3.1.0]hex-3-en-2-ones, respectively, for which both concerted and stepwise (biradical) reaction mechanisms have been proposed.640,641,770 For example, a [l,2]-shift concurrently with the ring contraction (termed the type A reaction) is observed upon irradiation of the methylphenyl derivative 159 in polar solvents, whereas phenyl migration (termed the type B reaction) predominates in nonpolar solvents (Scheme 6.70).771,772 The reactions are believed to proceed via both the n,n and n,Tt triplet ketone states. In the presence of alkenes, cyclic enones may readily undergo a competitive photocycloaddition reaction (Section 6.1.5). 

A particularly exciting result in our solid-state photochemistry was the formation of a new type product, bicyclic ketone 46, in addition to the anticipated type-B product. The reaction, termed Type involves a 5 to a phenyl migration, and an overall mechanism is given in Scheme 8.12, the evi-... 

The basic hydrolysis of the bisphosphonium salt (80) gave a phosphonium salt (81) and a phosphine oxide (82) in a ratio which depended upon the acidity of the solvent used. The reaction is presumed to proceed via a carbanion from which the phosphonium salt arises by protonation by the solvent and the phosphine oxide by phenyl migration from phosphorus. The mechanism of the unexpected formation of 2-methyl-2 T-benzopyran from the phosphonium salt (83), under normal Wittig conditions, has been discussed in some detail. ... 

Olah et al. (1992) studied the reaction of triphenylcarbenium tetrafluoroborate (10 mmol) with diphenyldiazomethane in dry dichloromethane hoping to detect the 1,1,2,2,2-pentaphenylethyl cation (9.25). The reaction yielded, however, tetraphenyl-ethene (79%) and a small amount (< 0.2%) pentaphenylethane. Using perdeuterated triphenylcarbenium salt or C-enriched diphenyldiazomethane, the authors demonstrate, by analysis of the labeled products, that the results are consistent with the mechanism (9-17), i.e., with the 1,1,2,2,2-pentaphenylethyl cation as steady-state intermediate, which is expected to undergo 1,2-phenyl migration via a phenonium ion and subsequent phenyl group scrambling. 

Quantum mechanical calculations on the structure of the triplet intermediates in the photochromic reactions of phenoxy quinones show that photochemically induced phenyl migration occurs non-adiabatically with generation of the spiro form of the triplet biradical. Details of a range of new photochromic compounds have appeared in both the scientific and patent literature. ... 

Important contributions toward understanding the mechanism of phenyl migration come from work on 2,2-diphenylpropylalkali metal compounds. 2,2-Diphenylpropyllithium 14 in ethereal solution was obtained (100) either by reaction of lithium with l-chloro-2,2-diphenylpro-pane at 0°C or by reaction of lithium with bis(2,2-diphenylpro-pyl)mercury. The solution upon heating at reflux for 3 hours gave 1-methyl-l,2-diphenylethyllithium 15 as the major organolithium product. 

A more detailed study of the dehydration of 2-phenyl-1-propanol was reported and a radio tracer technique using C was used to delineate the mechanism. Of the reaction products, the allyl benzene formed was labelled only on the benzylic carbon and was thus produced by y hydrogen abstraction and concerted phenyl migration. In the a-methylstyrene, the label was equally distributed between the I and 3 carbon atoms and this product arose either through a symmetrical cyclopropane intermediate or a tertiary carbonium ion. viz-... 

The lactonization of 4-phenyl-4-pentenoic acid (345) upon treatment with PhI(OAc)2 has been reported (Scheme 3.137) [432]. The mechanism of this reaction includes electrophilic lactonization induced by the addition of the iodine(III) electrophile to the double bond of substrate 345 followed by 1,2-phenyl migration leading to the final rearranged lactone 346. 

In 1968, the aforementioned reaction was repeated and found to produce a rearranged product (11). This was the first report of aryl migration with the Pictet-Gams conditions. The expected product was l-methyl-3-phenyl isoquinoline, but only 1-methyl-4-isoquinoline (11) was observed. Interestingly, the authors did not suggest a mechanism for the formation of the isolated product. 

The decomposition of the peroxide IX, which gives acetophenone when the conditions are such as to favor the radical mechanism, gives methyl ethyl ketone if the reaction is run in acid. This is because of the superior migration aptitude of phenyl groups in real or incipient oxygen cations.112... 

When the two groups of the ketone are para-substituted phenyls, the product of the Schmidt reaction is usually a nearly equimolecular mixture of the two isomeric amides.806 This result, obtained instead of a preferential migration of the more electron-releasing group, is consistent with a mechanism in which the formation of the intermediate rather than its rearrangement is rate determining. 

In unsubstituted phenyl ethers, the enolisation is faster than the Cope reaction. This is why the product is predominantly ortho isomer. When both the ortho positions are substituted, the allyl group undergoes a second migration via a concerted sigmatropic mechanism giving a para substituted phenol. This is called para Claisen condensation. 

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