1,2-Aryl shifts reactions

Three possible mechanisms may be envisioned for this reaction. The first two i.e. 1) Michael addition of R M to the acetylenic sulfone followed by a-elimination of LiOjSPh to yield a vinyl carbene which undergoes a 1,2 aryl shift and 2) carbometallation of the acetylenic sulfone by R M followed by a straightforward -elimination, where discarded by the authors. The third mechanism in which the organometallic reagent acts as an electron donor and the central intermediates is the radical anion ... 

Treatment of benzaldehydes with ethyl diazoacetate and a catalytic quantity of the iron Lewis acid [ -CpFe(CO)2(THF)]+BF4 yields the expected homologated ketone (80). However, the major product in most cases is the aryl-shifted structure (81a), predominantly as its enol tautomer, 3-hydroxy-2-arylacrylic acid (81b). This novel reaction occurs via a 1,2-aryl shift. Although the mechanism has not been fully characterized, there is evidence for loss of THF to give a vacancy for the aldehyde to bind to the iron, followed by diazoacetate attachment. The product balance is then determined by the ratio of 1,2-aryl to -hydride shift, with the former favoured by electron-donating substituents on the aryl ring. An alternative mechanism involving epoxide intermediates was ruled out by a control experiment. 

It has been shown that the lead tetraacetate-mediated 1,2-aryl shift of various meta-substituted / -cyclohexyl aryl ketones, e.g. (10), results in excellent yields of the corresponding rearranged esters (11). A unique reaction, providing 3-hydroxy-2-arylacrylic acid ethyl esters (14), has been observed between aryl aldehydes and ethyl diazoacetate in the presence of the iron Lewis acid [rj — (C5H5)Fe(CO)2(THF)BF4], It appears that the enol esters are formed by an unusual 1,2-aryl shift from a possible intermediate (13), which in turn is formed from the reaction of the iron aldehyde complex (12) with ethyl diazoacetate (see Scheme 4). 

The possible pathways for the transformations 323 -> 324 and 323 - 325 are outlined in Scheme 84. The first step that is common to these reactions involves the electrophilic attack of the I(III) species on the enol form of 323 at the face of the molecule anti to the C(2)-aryl ring to provide intermediate 328. Routes (a) and (a ) involving a 1,2-aryl shift lead to isoflavones 324. Route (b), involving Sn2 attack of X /XH at the C(3)-position of intermediate 328, leads to 325 via 329. The nucleophilicity of X XH plays a deciding role in affecting the course of the reaction. 

In this cyclodecarbonylation reaction, a ketene species is unlikely to be the reaction intermediate as added alcohols produce no esters. As shown in Scheme 6.26, the ruthenium acyl species 72 is likely to be the intermediate [25], which is prone to decarbonylationto give ruthena-cyclohexadiene 73 this species undergoes subsequent reductive elimination to form 2H-indene. Addition of proton or Ru to species 74 generated the benzylic cation 75, which after a 1,2-aryl shift gave the observed products. 

Reaction of aldehydes with ethyl diazoacetate normally results in the formation of [5-oxo esters. When a cationic Fe(II) Lewis acid is used as the catalyst, an unexpected 1,2-aryl shift results in the formation of a-formyl arylacetic acid ester 23, which is isolated as its enol tautomer (Scheme 8.6) [28]. The catalyst of this reaction,... 

Finally, Bickelhaupt has found that 10-aryl-9-arsaanthracene 6b is formed from the pyrolysis of 9-aryl- 10-benzyl-9,10-dihydro-9-arsaanthracenes 86110). The reaction has been shown to occur by homolysis to radicals, followed by a 1,4-aryl shift and hydrogen abstraction 11 u. 

Flavanones - Isoflavones. Although Koser s reagent (1) is known to effect a-tosyloxylation of ketones,1 the reaction with flavanones (2) results in an oxidative 1,2-aryl shift to provide isoflavones (3) in 74-80% yield.2 This conversion has been effected previously with thallium salts. 

Semipinacol rearrangement of a-haloalkyl aryl acetates has produced good yields of 2-aryl alkanoic esters. The acids formed on hydrolysis are important anti-inflammatory agents (such as Ibuprofen) and the reaction has been reviewed in some detail1010. In a similar process, oc-bromoalkyl aryl acetals thermally rearrange in protic solvents (under neutral or slightly basic conditions) via a 1,2-aryl shift (equation 193)1010,1011. 

In suitable vinylic halides, nucleophilic photosubstitution occurs intramolecular- 295-297,324,330-332 por jnstance irradiation of 113 in methanol only yields benzofuran 114, and no vinyl ether product or 1,2-aryl rearrangement products are formed (equation 84a)296. Apparently, the reaction with an internal nucleophile is faster than the 1,2-aryl shift and also faster than the reaction with an external nucleophile. With the less stabilized vinyl cations derived from the a-methyl- and a-H-analogues of 113-OMe, 1,2-aryl shifts do... 

The question arises whether there is any need for formation of the cyclopropyldicarbinyl diradical in the aryl di- ir-methane rearrangement. In the event that this intermediate is indeed formed, one has to keep in mind that the aromaticity of the aryl moiety has to be sacrificed along the reaction coordinate. Alternatively, a 1,2-aryl shift with direct formation of the 1,3-diradical may operate, which irreversibly cyclizes to the di- ir-methane product. In Scheme 3 these mechanistic alternatives are illustrated for ben-zonorbomadienes, which have been studied in great detail by Paquette et alP... 

Alkyl (Eq. 11.59) [99] and aryl (Eq. 11.60) [99, 110, 111[ nitro compounds can be reduced to the corresponding amines in high yields under the water-gas shift reaction. 

The indirect reduction of many organic substrates, in particular alkyl and aryl halides, by means of radical anions of aromatic and heteroaromatic compounds has been the subject of numerous papers over the last 25 years [98-121]. Many issues have been addressed, ranging from the exploration of synthetic aspects to quantitative descriptions of the kinetics involved. Saveant et al. coined the expression redox catalysis for an indirect reduction, in which the homogeneous reaction is a pure electron-transfer reaction with no chemical modification of the mediator (i.e., no ligand transfer, hydrogen abstraction, or hydride shift reactions). In the following we will consider such reactions and derive the relevant kinetic equations to show the kind of kinetic information that can be extracted. 

Another reaction of the lithiated l,3-dithiole-2-thione 103 with aryl carboxaldehydes, followed by acidic quenching of the resulting oxyanions, afforded bisalcohol products 109. In the presence of perchloric acid, a 1,4-aryl shift was observed to furnish new formylated derivatives 112 and 113. Phenyl and 2-methoxyphenyl diols gave dihydrofurans 110 and 111, respectively (Scheme 7) <2001CC369>. 

The steroidal dienone (170) can be induced to undergo the relatively unusual reorganization to the 2-hydroxy-4-methylandrosta-l,3,5-triene (171), by irradiation in refluxing acetic acid although only 48% of the acetate (171) was isolated, the related 17-ketone (36%) was also formed. The p-dienones (172) and (173) are restructured on irradiation in cyclohexane with net relocation of methyl and t-butyl groups to afford the phenols (174) and (175) in 55% and 73% yield, respectively. A photo-induced aryl shift in the benzylisoquinoline dienone (176) was employed to prepare the aporphine (177 44%) en route to ( )-boldine. The p-dienones (178 X = Cl, X = Ns) aromatize on irradiation to yield the alkylated p-hydroxybenzoates (179 e.g. X = Ns, 57%) as well as minor quantities, of their isomers (180). Mechanistic aspects of these reactions are discussed in the literature cited. 

Three-membered heterocycles. Decomposition of diazo compounds by the iron complex in the presence of imines leads to aziridines. An analogous reaction of diazoalkanes with aldehydes gives some epoxides and the rearrangement products (ketones) owing to the Lewis acidic nature of the catalyst. Ethyl diazoacetate behaves differently, as 1,2-aryl shift occurs during the reaction. ... 

Many rearrangements have been invoked to explain the fragmentation pattern of molecular ions22 (It should be remembered that the fragmenting species are radical cations. Their radical functionality may open to them reaction paths which are not available to simple carbocations). Only a few illustrative examples of well-established alkyl and aryl shifts are given here. The mass spectrum of camphor displays the base peak at m/e 95, corresponding to the loss of ketene and a methyl radical. As shown... 

The photolysis of triarylsulphonium salts yields diarylsulphides and products of lateral nuclear shift reactions which are ortho, meta, and para aryl substituted diaryldisulphides such as (342). A by-product in these reactions is a proton because of this these reactions have been applied to photoinitiation of cationic polymerisations. A full paper describing a detailed study of the reaction mechanism has been published.In addition, the product distribution obtained by photolysis of triphenylsulphonium salts in films of the polymer of 4-(tert -butoxycarbonyloxy)styrene has been compared with that obtained in solution.The synthesis of some new triarylsulphonium salts and their application for photoinitiation of cationic polymerisation has also been reported.The formation of the products arising from lateral nuclear shifts in sulphonium salts occurs under direct photolysis but not under triplet sensitisation. 

Although it is known that the presence of electron-donor substituents on the aromatic ring greatly enhances the efficiency of the transposition leading to 2-arylpropanoates, we have chosen for our study the unsubstituted phenyl group, which shows a low migratory aptitude. This will provide us a deeper understanding of the influence of the physicochemical parameters of the zeolite on the side reactions competitive to the 1,2-aryl shift. 

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