Aryl halides regioselectivity

Unlike normal all-carbon aryl halides, regioselective Negishi coupling reaction of 2,3-dibromofuran 27 with 2-furylzinc chloride was achieved at C(2) to afford bisfuran 28 [35]. 

Unlike normal all-carbon aryl halides, regioselective Negishi couplings of dihalofu-rans and trihalobenzofurans are synthetically useful. Bach and coworkers have studied both systems extensively and use a variety of cross-coupling reactions to produce functionalized furans and benzofurans. In one example of a selective Negishi reaction,... 

The regioselective arylation occurs when the reaction of 2-arylpyridines with aryl halides is conducted with the aid of the ruthenium(ii)-phosphine complex as catalyst (Equation (66)).81 The ortho-position to the 2-pyridyl group is arylated predominantly. The aromatic imines are also arylated with the same catalytic system.82... 

Unlike simple aryl halides, a regioselective Suzuki coupling of 2,4-dibromofuran may be achieved at C(2) [46]. The coupling between 2,4-dibromofuran and pyrimidylboronic acid 40 provided furylpyrimidine 41, which was then hydrolyzed to 5-substituted uracil 42. 

Palladium chemistry of heterocycles has its idiosyncrasies stemming from their different structural properties from the corresponding carbocyclic aryl compounds. Even activated chloroheterocycles are sufficiently reactive to undergo Pd-catalyzed reactions. As a consequence of a and y activation of heteroaryl halides, Pd-catalyzed chemistry may take place regioselectively at the activated positions, a phenomenon rarely seen in carbocyclic aryl halides. In addition, another salient peculiarity in palladium chemistry of heterocycles is the so-called heteroaryl Heck reaction . For instance, while intermolecular palladium-catalyzed arylations of carbocyclic arenes are rare, palladium-catalyzed arylations of azoles and many other heterocycles readily take place. Therefore, the principal aim of this book is to highlight important palladium-mediated reactions of heterocycles with emphasis on the unique characteristics of individual heterocycles. 

Under copper catalyzed conditions azoles (i.e. imidazoles) couple not only with aryl halides but also with arylboronic acids. The reaction, run in the presence of oxygen, follows a unique path (for details see Chapter 2.5.). From the synthetic point of view, the arylation of imidazole proceeds in good yield, although the regioselectivity in the arylation of 4-substituted imidazoles is only moderate (6.70.),102... 

Unlike all-carbon aryl halide substrates, regioselective Stille coupling of 2,3-dihalofurans has been observed. Taking advantage of C(2) activation, regioselective Stille coupling was achieved using 2,3-dibromofuran 27 to afford 72 [35]. 

On the basis of the reaction of conjugated dienes with unsaturated halides in the presence of external nucleophiles, an elegant intramolecular version leading to a-alkylidene-y-lactams, has been developed (Scheme 8.19). Starting with an aryl halide, the regioselective insertion of an arylpalladium halide to the triple bond of acyclic compound 42 gives the c-vinylpalladium intermediate 43. Subsequent intramolecular carbopalladation of the diene affords a re-allyl palladium intermediate... 

Satoh, T. Kawamura, Y. Miura, M. Nomura, M. Palladium-catalyzed regioselective mono-and diarylation reactions of 2-phenylphenols and naphthols with aryl halides. Angew. Chem. Int. Ed. 1997, 36, 1740-1742. 

Highly regioselective radical addition of the indium hydride reagent HInCl2 to alkynes has been achieved. Various functionalities are tolerant under the reaction conditions. The reaction proceeds with complete // -stereoselectivity. Alkenylindium compounds obtained by this hydroindation can be employed for the subsequent cross-coupling reactions with aryl halides in one pot (Scheme 109).374,375... 

Since halogen-substituted benzo[. ]furans play an important role in the transition metal-catalyzed coupling of benzo[. ]furans with other substrates, synthetic methods to regioselectively synthesize substituted benzo[ ]furan halides have become very critical routes. Several syntheses of benzo[ ]furan based aryl halides are described here. 

For the alkylation and alkenylation of C-H bonds, olefins and acetylenes are used as reactants. This type of coupling protocol is not applicable to arylation. Recently, a nitrogen-directed arylation of aromatic C-H bonds, leading to biaryl compounds has been developed. In 2001, Oi demonstrated that ruthenium(II)-phosphine can be used as a catalyst in the regioselective arylation of 2-arylpyridines using aryl halides (Eq. 9.34) [4 ]. The predominant ortho selectivity indicates that the coordination of... 

The classic SsAi substitution of activated aryl halides by protected cyanohydrin anions provides substituted benzophenones. Another procedure for the arylation of protected cyanohydrin anions involves the use of aromatic substrates activated as their rr-chromium tricarbonyl complexes. - Addition of the anion of (32) to the 1,3-dimethoxybenzene complex, for example, leads principally to the meta-suh-stituted isomer (33 equation 20). Preferential meta regioselectivity is also noted with other -rr-chromium tricarbonyl complexes of arenes. Other arylations of cyanohydrin anions include interesting but synthetically limited additions at the a-position of quinoline N-oxides. In a similar manner, cyanohydrin carbonates of aromatic aldehydes react with A -oxides of quinoline and isoquinoline. ... 

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