Electron-rich haloarenes

Aryl exchange occurs before transmetallation. Thus, the transmetal-lation rate constant which is low for steric and electronic reasons results in increasing the coupling product of phosphine-bound aryls (Eq. 51). Transmetallation is slowed down when electron-rich haloarenes and weak bases are used and accelerated with electron-deficient haloarenes because the transmetallation shown in Eqs. 16 and 17 involves nucleophilic substitution of Pd-X. The reported equilibrium ratio of 3/5 is 4/96 at 60 °C when Ar is p-methoxyphenyl. Thus, it is quite reasonable that the reaction accompanies a large amount of 6 when the rate constant of transmetallation ifej is lower than ky A strong base, polar solvent, and a sterically less hindered bidentate ligand, such as dppf, increase k. The formation of yields of 9 in p-iodoanisole higher than the bromo derivative... 

Instability of phenyl cation In case of haloarenes, the phenyl cation formed as a result of self-lonlsatlon will not be stabilised by resonance and therefore, S l mechanism Is ruled out. Because of the possible repulsion. It Is less likely for the electron rich nucleophile to approach electron rich arenes. Replacement by hydroxyl group... 

DPPF-ligated palladium provided nearly quantitative yields for amination of aryl halides with anilines (Eq. (7)). Electron-rich, electron-poor, hindered or unhindered aryl bromides or iodides all participated in the amination chemistry, with only a few exceptions. Nitro haloarenes gave no amination product with aniline substrates,... 

In the presence of an electron rich donor molecule an alternative to direct fission or reaction via an excimer is the formation of an exciplex or radical anion/radical cation ion pair (Eq. 2). The radical anion has been viewed as the key intermediate which undergoes fission to aryl radical and halide ion (Eq. 3). With polyhaloarenes there is an additional option. A polychloroarene radical anion, for example, has two possible modes for bond fission (a) fission to produce aryl radical and chloride ion or (b) fission to form an aryl carbanion and chlorine atom (Scheme 6). The options for fragmentation of a haloarene radical anion... 

Although the ligandless catalysts often achieve significantly fast coupling in aqueous media, complete conversion can not be always possible, especially for the slow reactions of electron-rich and sterically hindered haloarenes. The addition of more than two phosphines is generally recommended to avoid the pre-... 

One of the key problems associated with the Suzuki cross-coupling was the lack of reactivity of aryl chlorides. This is unfortunate, because chlorinated aromatics are usually much cheaper and more readily available than other haloarenes. Equation 12.68 shows a solution to this problem where the addition of the base Cs2C03 and P(t-Bu)3, a sterically hindered and electron-rich phosphine, led to successful coupling of an aryl chloride and arylboronic acid.142... 

Cobalt complexes have been used to catalyze the carbonylation of chloroarenes to the corresponding carboxylic acids and their esters (Sect. 3.3). Some complexes of cobalt in the oxidation state -1 activate the Ar-Cl bond via an SRN1-type mechanism [2] involving single electron transfer from the metal to chloro-arene, followed by elimination of Cl . The simplest Co(-I) carbonyl species, [Co(CO)4] , is not electron-rich enough to react with haloarenes. However, its reactivity has been shown to enhance tremendously in the presence of Caubere s complex bases, mixtures of NaH and NaOAlk [23,66,67]. For instance, the stoichiometric carbonylation of chlorobenzene has been performed with the... 

In addition to heterocycles that were successfully arylated under first-generation procedure, the reaction scope is broadened to include bis(aryl)imidazoles and triazoles, but benzothiazole was found to be incompatible. As previously observed, 3,4-dihydroquinazoline is a reactive coupling partner but only the dehydrogenated product 2-arylquinazoline is obtained. Furthermore, in addition to iodoarenes, bro-moarenes are reactive under these reaction conditions. The reaction is tolerant of a large number of functional groups and haloarene substituents, including nitrile, chloro, primary amide, ketone, and ester. Both electron-poor and electron-rich bro-moarenes showed good reactivity. Both para- and meta-substituents were well tolerated, but ortao-substituents (OMe, CF3) shut down the reaction. 

Choice of Substituents on the Haloarene. The active catalyst of type C is an electron-rich species, and as a consequence, oxidative addition, i.e., the formation of D (from C and an aromatic halide), is dramatically influenced by the nature of the substituents Y on the aromatic nucleus. The more electron-withdrawing Y is, the faster its oxidative addition to the electron-rich Pd° proceeds. Consequently, an electron-with-drawing substituent Y on the halide improves both the rate and yield of these coupling reactions. Ortho-and para-positioned acceptor substituents are more efficient than ones placed in the mefa-position. 

The direct photolysis of alkyl or aryl halides in solution to form carbon-centered radicals is rarely used in organic synthesis." Alkyl iodides usually afford mixtures of radical and ionic products, while alkyl bromides can produce radical-derived products but in low yield. A notable exception is the photocycliza-tion of haloarenes, which has been shown to produce carbon-centered radicals that can add to aromatic rings. A similar reaction has recently been observed on irradiation of iodoheterocycles, with substituted benzenes or electron-poor alkenes, to form arylated or alkylated heterocycles in good yield. Related reactions have also been reported on irradiation of 4-chloroanilines in the presence of (electron-rich) alkenes, although in this case, the alkylations appear to involve the formation of a phenyl cation. An alternative approach to form carbon-centered radicals is to irradiate the alkyl iodide or bromide in the presence of triethylamine this is proposed to form an amine-haHde exciplex, which cleanly breaks down to give a carbon-centered radical and a halide anion. Cossy and co-workers have shown this to be a fast, convenient, and chemoselective method of radical generation, which has recently been used to prepare the bicyclic core of ( )-bisabolangelone (Scheme 1). ... 

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