Arylation regiochemistry

The use of diphenyl hydrazone 33 has been used in the synthesis of pyrazoles under modified conditions where the hydrazine is released in situ. Some reversal of regiochemistry is seen in the reaction with unsymmetrical dicarbonyls. With aryl hydrazine and diphenyl hydrazone, the ratio of 41 to 42 is 22 1 and 5 1, respectively. 

The regiochemistry for the substituted t-S with ammonia and DCNB was also studied (equation 43). When there is an alkoxy substituent on the para position, the reaction yields selectively l-amino-2-aryl-l-phenylamine (equation 44). 

Regiochemistry of the obtained product leading to 1,1-disubstituted ethylenes is contrary to that observed in the addition of sodium butyltellurolate to aryl acetylenes, which produces exclusively the Z isomer resulting from an anti addition. ... 

The selective arylation of azoles, including imidazole, was achieved by Sames. Under the developed conditions, including the use of magnesium oxide as base, imidazole was arylated in the 4-position selectively, while the addition of a stoichiometric amount of copper(I) iodide led to the reversal of the regiochemistry, resulting in the selective formation of 2-phenylimidazole (6.89.),120... 

The associative step (equation 3) determines the nature of the product, since in this step the synthetically important bond formation between the aromatic moiety and the nucleophile takes place. The rates for the association of a number of nucleophiles with a variety of aryl and heteroaryl radicals has been measured in electrochemical studies29-31 and competitive product studies.32-35 The range of nucleophiles, classified according to the atom which becomes directly bonded to the aromatic ring, and, where the nucleophile is ambident, the regiochemistry of the association reaction shown in equation (3) are detailed in Section 2.2.3. The final propagating step (equation 4), that returns the chain-propagating electron from product radical anion to another molecule of substrate, is essential if a chain reaction is to continue. 

The regiochemistry of arylation of ketones has not been studied systematically, but there are sufficient data in the literature to allow reasonable predictions to be made. With enolates having one r-alkyl group attached to the carbonyl (e.g. "Cl COBu ) or with those derived from aryl alkyl ketones, clearly no prob-... 

Very little work has been done on reactions involving nucleophiles formed from hydrocarbons.124-142 The limitation on basicity of the carbanion, so that it does not react with solvent, has led to use of conjugated hydrocaibons, such as dienes or alkenes conjugated with aromatic rings. When initiated by dissolving alkali metal in liquid ammonia, complex mixtures are often produced on account of reduction processes,124 and regiochemistry and multiplicity of arylation in conjugated systems also create prob-... 

A study of the photo-sensitized ring-opening reactions of the radical cations (76) of arylcyclopropanes (75) with methanol, water, and cyanide nucleophiles suggests a three-electron 5k2 mechanism (Scheme 11).185 The isolated products are methyl propyl ethers, derived from nucleophilic attack of methanol on the radical cation (76). They were detected by UV-VIS spectroscopy and shown to react with nucleophiles by transient kinetic methods. The benzyl radical (77) reacts with the DCB radical anion to afford monoaromatic ether (78) by oxidation and protonation or the disubstituted ether (79) by addition of DCB. Regio- and stereo-selectivity of the substitution were complete regiochemistry and rate constant were profoundly effected by the electronic nature of the aryl substituents.186 Elsewhere, a combined ab initio and CIDNP study... 

Neither the regiochemistry nor the stereochemistry is clear cut in reactions of aryl derivatives of hexafluoropropene [100-102] (Scheme 44). 

We have observed that 6-methyl-2-phenyl-l,3-oxazin-4-one (74) photocycloadds regiospecifically to 1,1-dimethoxyethene68 and to furan. The regiochemistry is the same as observed with 3-ethoxyisoindolenone (50) and 2-aryl-2-oxazolinone (65). The oxazinone (74) like the aryloxazolinones is not reactive with other olefins. 

Nwokogu, G.C., Wong, J.-W., Greenwood, T.D. and Wolfe, J.F. (2000) Photostimulated reactions of phenylacetic add dianions with aryl halides. Influence of the metallic cation on the regiochemistry of arylation. Organic Letters, 2, 2643—2646. 

In Bennett and Wenger s study68 of the factors that control the two insertions in the naphthalene forming sequence, they concluded that (1) the regiochemistry of the first insertion step is sterically controlled with the substituted carbon bonding to Ni (e.g., 152 favored over 151) (2) the second insertion occurs between Ni and vinyl rather than Ni and aryl and is sterically controlled with electron-rich alkynes and (3) with electron-deficient alkynes, both steric and electronic factors influence the regiochemistry of the second insertion step. 

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