Palladium-Catalyzed Regioselective Direct Arylation of Benzofurazans at the C-4 Position (original) (raw)
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Studies on Palladium-Catalyzed Benzylic Arylation
2009
Contents General Introduction Chapter 1 Palladium-Catalyzed 2-Pyridylmethyl Transfer from 2-(2-Pyridyl)ethanol Derivatives to Organic Halides by Chelation-Assisted Cleavage of Unstrained sp 3 C-sp 3 C Bonds Chapter 2 Palladium-Catalyzed Direct Arylation of Aryl(azaaryl)methanes with Aryl Halides Providing Triarylmethanes Chapter 3 Palladium-Catalyzed Benzylic Arylation of N-Benzylxanthone Imine Chapter 4 Palladium-Catalyzed Benzylic Direct Arylation of Benzyl Sulfone Publication List Acknowledgment tol tolyl Xantphos 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene Halogen-magnesium exchange Cross-homo scrambling Scheme 6. Br Br m 1 mol% Pd(PPh 3) 4 + Br m = MgCl, Et 2 O, reflux, 20 h m = ZnBr, THF, 50 ˚C, 16 h 20% 68% Scheme 7. X Mg metal MgX + Undesirable dimerization Scheme 5. General Introduction derivative in 22% yield. Because organoboron compounds are generally synthesized via hydroboration, benzylboron compounds are less readily available and are uncommon coupling partners in Suzuki-Miyaura cross-coupling reaction. Flaherty et al. reported the palladium-catalyzed benzylation of aryl halides with commercially available benzylboron compounds (Scheme 9). 11 As described above, benzylzinc derivatives appear to be the most favorable benzylmetals in all respects for palladium-catalyzed benzylation. Although palladium-catalyzed cross-coupling reactions of benzylzincs display high reactivity and chemoselectivity, the need still exists for preparation of benzylzinc reagents by treatment of benzyl halides with a stoichiometric amount of Zn powder. Furthermore, synthesis of benzyl halides often requires several steps. An alternative attractive route for the palladium-catalyzed benzylation of aryl halides is the use of stable organic compounds bearing no metallic atom as nucleophilic partners (Scheme 10).
RSC Advances, 2012
Aryldiazonium tetrafluoroborates have been successfully employed in the direct and regioselective arylation of heteroaromatics such as indole, benzofuran and benzothiophene. The cationic aryl palladium intermediates derived from the aryldiazonium salts act as effective electrophiles in the process. These arylating reactions display operational simplicity and provide the arylated heterocycles regioselectively in moderate to excellent yields.
Chemistry – A European Journal, 2011
A new ferrocenyl triphosphane ligand associated to palladium was found to be an efficient catalyst for the direct coupling of highly congested, functionalised aryl bromides with a variety of heteroarenes. These coupling reactions can generally be performed by using a low‐loading (0.1–0.5 mol %) of the catalyst. The present protocol tolerates important and useful functional groups, which allows for further elaboration into more sophisticated heterocyclic molecules. The straightforward arylation of heteroaromatic compounds with congested ortho‐substituted aryl bromides may permit further convergent syntheses of diverse ligands, biologically active molecules and molecular materials in only a few steps.
Regioselective Palladium-Catalyzed Arylation of 4-Chloropyrazoles
Organic Letters, 2010
A highly regioselective Pd-catalyzed arylation of N-methylpyrazoles with aryl bromides is described. This transformation was studied extensively via automated reaction screening. A Design of Experiments (DoE) approach for optimizing the critical parameters was applied, resulting in excellent conditions for preparing selectively 5-arylpyrazoles in moderate to excellent yields under mild conditions. Heteroaryl derivatives, including aryl pyrazoles, are important building blocks in organic synthesis due to their biological properties. 1 Although a number of synthetic methods have been developed to construct pyrazoles possessing an aryl group on the ring, 2 many of these approaches are based on time-consuming multistep procedures or structure-limited ring transformation reactions. Direct introduction of an aryl group onto the pyrazole nucleus via palladium-mediated cross-coupling reaction has been demonstrated via Suzuki, 3 Stille, 4 and Negishi 4,5 reactions. Metal-catalyzed arylation and heteroarylation of C-H bonds have been shown to be a
Direct Arylation of Benzo[b]furan and Other Benzo-Fused Heterocycles
The direct arylation of benzo[b]furan, benzo[b]thiophene, and indole has been studied by using aromatic bromides as the aryl source. The protocol employing common reagents and a Pd catalyst has led to the regioselective arylation of these heterocycles at the 2-position. A range of functional groups were tolerated, providing quick access to a variety of arylated benzo-fused heterocycles that would be accessible more elaborately using classical synthetic strategies. This is the first systematic study of the direct arylation of benzo[b]furan