1.2-Aryl shifts

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). 

Chiral dicarboxylic acid (R)-5g (5 mol%, R = Mes) bearing simpler mesityl-substituents at the 3,3 -positions was found to catalyze efficiently the trans-selective asymmetric aziridination of iV-aryl-monosubstituted diazoacetamides 177 and aromatic (V-Boc imines 11 (Scheme 75) [94], In sharp contrast to previous reports on this generally dx-selective sort of aziridination, this method exhibited unique fran -selectivity and afforded exclusively the fran -aziridines 178 in moderate to good yields along with excellent enantioselectivities (<20-71%, 89-99% ee). The 1,2-aryl shift products 179 were observed as side products in varying ratios (178 179= 56 44-90 10). Diazoacetamides were chosen instead of diazoesters. Due... 

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. 

The aromatization of 2-(2 -arylvinyl)ethynylbenzene substrates might be accompanied by a 1,2-aryl shift even though the parent compound 19a failed to sho v such a phenomenon. As sho vn in Scheme 6.12, the 2 -aryl group preferentially under vent a 1,2-shift to the I -vinyl carbon rather than to the terminal alkyne carbon. This 1,2-aryl shift is favored by the electron-donating R groups on the migrating aryl, as reflected by the respective yields 20c (60%) > 20b (35%) > 20a (0%). 

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. 

This transformation has been shown (56JA2278) by 14C-labeIling to involve a 1,2-aroyl migration, rather than a 1,2-aryl shift which would necessitate a carbocation intermediate in which the charge is formally adjacent to the carbonyl group (Scheme 167). 

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,... 

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. 

Some tricyclic 1,3-thiazetidines of type (51) are reported (95CPB63). Selective C-S bond cleavage of 3-aryl-fS-sultams (52) with ethylaluminium dichloride gives aryl ketones or aldehydes by a process involving 1,2-aryl shift, imine formation and hydrolysis of the imine (95TL245). 

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... 

Oxidation of the ethylene acetals of 3-aryl-3-oxo-2-propyl phenyl telluriums, which were prepared from the acetals of 3-aryl-3-oxo-2-bromopropane and arenetellurolates, with 3-chloroperoxybenzoic acid in methanol at 20° produced hydroxyethyl 2-arylpropanoates2. Tellurones were postulated as intermediates that experienced a 1,2-aryl shift and the elimination of an aryltelluro moiety. 

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... 

The rearrangement of acetals of 2-haloalkyl aryl ketones is a well-documented process yielding esters of 2-arylalkanoic acids by 1,2-aryl shift (equation 7). The mechanism of this rearrangement is reminiscent of other semipinacol rearrangements. Loss of the halogen (usually assisted by Lewis acid), yields a carbocation (4), which then undergoes a 1,2-aryl shift with carbonyl group formation. 

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. ... 

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